A Review of the Structure, Performance, Fabrication, and Impacts of Application Conditions on Wearable Textile GNSS Antennas

Abstract

The advancement of wearable technologies has resulted in significant interest in GNSS-integrated textile antenna development. Although existing literature surveys predominantly concentrate on flexible non-textile antenna systems operating within UHF and 5G frequency spectra, systematic investigations of textile-based antenna configurations in the 1–2 GHz GNSS band have been relatively scarce. Contemporary GNSS textile antenna architectures primarily target GPS frequency coverage, while the global proliferation of BeiDou Navigation Satellite System (BDS) infrastructure necessitates urgent development of BDS-compatible textile antenna solutions. This review methodically examines the structural configurations and radiation characteristics of 1–2 GHz textile antennas, bandwidth enhancement techniques, miniaturization methodologies, and gain optimization approaches, along with material selection criteria and manufacturing processes. Technical challenges persist in simultaneously achieving broadband operation, compact dimensions, and elevated gain performance. Primary manufacturing approaches encompassing laminated fabric assemblies, printed electronics, and embroidered conductive patterns are analyzed, while existing methodologies exhibit limited capacity for seamless garment integration. Despite remarkable progress in conductive material engineering, dielectric property modification studies demonstrate insufficient theoretical depth. Comprehensive mitigation strategies for multifaceted operational environments involving human proximity effects, mechanical deformation, and variable meteorological conditions remain notably underdeveloped. This comprehensive analysis aims to establish a foundational framework for next-generation BDS-oriented textile antenna development.

1. Introduction

The Global Navigation Satellite System (GNSS) is widely applied across various aspects of modern society. As a critical space-based infrastructure, GNSS significantly enhances daily life, promotes societal development, and holds substantial political, military, and economic value. Currently, there are four major global navigation systems: the United States’ Global Positioning System (GPS), Russia’s GLONASS, the European Galileo system, and China’s BeiDou Navigation Satellite System (BDS) [1,2].

The rapid advancement of wearable systems, equipped with sensing [3,4,5,6,7,8,9], signal processing, actuation, communication, energy harvesting [10,11], and storage capabilities, has unlocked transformative applications across diverse fields. By integrating BDS antennas, these systems can be deployed in telemedicine monitoring [12], exercise physiology tracking [13,14], emergency rescue coordination, aerospace operations, and military protection systems [15]. Wearable antennas, serving as the electromagnetic wave transceivers within these systems, function as physical-layer interfaces between electronic devices and wireless channels. They achieve this by enabling bidirectional conversion between guided electromagnetic waves (carried as current or voltage signals) and free-space electromagnetic waves [16], positioning them as critical components in wearable technology architectures.

Wearable antennas are classified into two primary categories: textile-based and non-textile-based architectures. Non-textile antennas are fabricated using rigid substrates or polymers, where conductive or dielectric components are constructed from these materials. Such substrates typically exhibit high permittivity, low-loss characteristics, and excellent electrical conductivity. However, rigid materials often compromise wearer comfort and pose safety hazards during physical collisions. Although flexible polymers allow deformation, their limited breathability diminishes user experience under conditions like perspiration. Textile antennas offer advantages including softness, lightweight construction, and enhanced concealment. Nevertheless, the substrate properties of textile antennas, such as low permittivity, high loss, and hygroscopic properties, frequently lead to increased volume, reduced gain, and performance fluctuations, respectively. Despite these limitations, textile antennas remain a strategic focus for wearable systems due to their potential conformation with the human body.

To meet application requirements, wearable textile antennas must possess specific performance characteristics. Wearable textile antennas necessitate an application-specific operational bandwidth to ensure functional compatibility. The growing popularity of ultra-wideband (UWB) antennas arises from their ability to span multiple frequency bands, addressing diverse applications. Concurrently, these antennas must exhibit radiation patterns that comprehensively cover target operational scenarios, as well as sufficiently high gain in critical orientations to maintain signal integrity. A paramount design challenge stems from the role of the human body as a complex dielectric medium, which induces mutual electromagnetic coupling with the antenna. This interaction not only degrades radiation efficiency but also poses potential electromagnetic exposure hazards; therefore, dual countermeasures are mandated, namely high-gain configurations to counteract body-induced detuning and strict adherence to low specific absorption rate (SAR) thresholds to ensure biological safety. During operation of wearable devices, complex body movements induce bending and creases in fabric antennas, necessitating evaluation of deformation effects on antenna performance. Beyond environmental temperature and humidity, there also exist changes in antenna performance induced by perspiration. Furthermore, compact structures for miniaturization, combined with lightweight and low-profile designs, can reduce the wearer’s burden. Collectively, these requirements define a multidimensional framework for advancing textile antenna technology in wearable applications. Thus, as shown in Figure 1, this work reviews GNSS wearable textile antennas in four aspects, including their structure, performance, preparation process, and the influence of environmental application conditions.

Previous studies have systematically reviewed wearable antenna technologies from diverse perspectives. Singh et al. [17] comprehensively analyzed the structural design, materials, fabrication processes, and applications of flexible wearable antennas (including non-textile variants), particularly emphasizing their roles in Wireless Body Area Networks (WBANs) and healthcare technologies. Taher et al. [18] investigated ultra-high-frequency (UHF) conformal flexible antennas for partial discharge (PD) diagnostics, with a focus on five non-textile polymer substrates. Marterer et al. [19] critically reviewed textile-specific materials, textile-forming fabrication techniques, and feedport connection technologies for wearable textile antennas. Ali et al. [20] synthesized advancements in human–antenna interaction mechanisms and the integration of metamaterials to simultaneously reduce SAR values and enhance antenna gain.

With GNSS textile antennas being widely applied, the demand for BDS textile antennas has been gradually increasing; yet, systematic reviews of BDS textile antennas operating in the 1–2 GHz frequency band have been lacking. Accordingly, this work is focused on reviewing BDS antennas within this frequency band, focusing on the structural design approaches for realizing ultra-wideband performance, miniaturization, and high gain. The current materials and textile fabrication techniques used in wearable antenna production and their effects on antenna performance are discussed. Furthermore, the impact of human body proximity, dynamic motion, and operational conditions on antenna operation in wearable contexts is discussed. As the BDS frequency band is categorized within GNSS, while research on BDS antennas is summarized in this work, related studies on GNSS antennas operating in the same frequency band will also be analyzed.

2. Structure and Performance of Wearable Textile Antennas

2.1. Frequency Bands of BDS Antennas and Performance of GNSS Textile Antennas

2.1.1. Frequency Bands of BDS Antennas

The BeiDou Navigation Satellite System (BDS) provides global users with all-weather, all-time, high-precision, and high-efficiency positioning, navigation, timing, and velocity measurement services. High-performance antennas are essential for efficient services [1].

As shown in

Table 1. BDS frequency bands [1].

BDS	Frequency Band	Operating Band
BD1	S	2491.75 MHz ± 4.08 MHz
BD2	L	1615.68 MHz ± 4.08 MHz
	B1	1561.09 MHz ± 2.046 MHz
	B2	1207.52 MHz ± 2.046 MHz
	B3	1268.52 MHz ± 10.23 MHz
BD3	B1	1575.42 MHz ± 2.046 MHz
	B2	1176.45 MHz ± 2.046 MHz
	B3	1268.52 MHz ± 10.23 MHz

, the BDS opened three frequency bands—B1, B2, and B3—for user access. These bands fall within the L-band (1–2 GHz), which features low signal attenuation and strong anti-interference capability, making it suitable for long-distance satellite signal transmission. The B1C band (1575.42 MHz) overlaps with GPS L1 and Galileo E1; the B2 band includes two center frequencies: B2a (1176.45 MHz) and B2b (1207.14 MHz), where B2a aligns with GPS L5 and B2b corresponds to Galileo E5b. This overlap enhances the compatibility and positioning accuracy of the BDS. In addition, the B3 band (1268.52 MHz), as a military-specific frequency, is used for global military positioning and navigation signal transmission.

GNSS and BDS antennas have similar operating frequency bands and performance requirements; thus, this work promotes the development of BDS antennas by summarizing the structure and performance of GNSS antennas. The specific operating frequency bands of GNSS and BDS antennas have both overlaps and differences, so their structural designs need to be tuned to the corresponding operating frequencies to cover the required frequency bands. For single-frequency GNSS antennas, bandwidth design should be moderate; excessively expanding bandwidth may degrade other performance characteristics of the antenna. For GNSS receiving antennas, favorable gain and circular polarization (CP) characteristics are also critical; therefore, based on meeting the requirement of covering the operating frequency, emphasis should be placed on improving gain or maintaining good CP characteristics.

In terms of gain, the basic receiving gain of GNSS antennas generally needs to be ≥2 dBi. However, since the BDS B3 band carries two-way short message communication, which requires support for transmission, its receiving gain needs to be ≥3 dBi to offset the losses caused by the transmit chain. Hence, the gain requirement for BDS B3 antennas is slightly higher than other GNSS antennas. Thus, we summarize common gain improvement methods for GNSS antennas and suggest coordinating them with bandwidth tuning methods to enhance the gain in BDS bands.

SAR is an indicator measuring the rate at which biological tissues absorb electromagnetic energy. Wearable antennas should comply with the SAR standard requirements of their respective nations. For example, in accordance with the European standard EN 62311 [21] and IEEE C95.1 [22], the US standard specifies that the average SAR over 1 g of tissue shall be <1.6 W/kg, and the EU standard requires that the average SAR over 10 g of tissue shall be <2 W/kg. Therefore, all wearable antennas must comply with these requirements. Furthermore, differences exist in application modules, meaning that receiving antennas for different applications need to be equipped with corresponding application modules.

2.1.2. Performance and Dimensions of GNSS Textile Antennas

This work focuses on wearable textile antennas operating in the BDS L-band (1–2 GHz). As the BDS frequency band is categorized within GNSS, while research on BDS antennas is summarized in this work, related studies on GNSS antennas operating in the same frequency band will also be analyzed.

As a fundamental antenna parameter, the bandwidth determines an antenna’s suitability for specific applications. Therefore, it is essential to clarify the specific frequency bands used within GNSS applications. As a key component in wearable systems, the antenna must support device miniaturization and concealment to reduce the physical burden on the wearer. Accordingly, characteristics such as lightweight design and compact size are critical aspects of wearable antennas. Furthermore, excellent radiation performance includes an appropriate radiation pattern to cover the target application scenario and sufficient gain, while also accounting for the electromagnetic coupling effects that arise from integration with the human body and clothing, which form a complex dielectric environment. Therefore, radiation-related performance, such as in gain and radiation efficiency, should also be carefully considered.

In the following sections, the bandwidth, structures, dimensions, and radiation performance parameters of wearable textile antennas designed for various GNSS frequency bands are systematically summarized, as in Table 2.

Table 2. Wearable textile antennas applied to GNSS.

Ref.	Freq. (GHz)	Type	Size (mm) Length × Width × Thickness	Gain/Effi.
Sungwoo [23]	1.555–1.625 GPS, GLONASS	Patch antenna	100 × 100 × 1.575	/
Vallozzi [24]	GPS-L1	Patch antenna	/	8.4 dBi (open space) 8.34 dBi (with additional textile layers) 7.52 dBi (on body with firefighter jacket)
Sabapathy [25]	1.538–1.622	Patch antenna	113.75 × 99	/
Ahmad [26]	1.563–1.587 GPS L1 902–928 MHz ISM	Patch antenna monopole antenna	/	9.37 dBi, 80.31% 7.83 dBi, 14.61%
Anbalagan [27]		Slot antenna	48 × 80 × 0.99	0.36 dB
Hassan [28]	1–1.62 GPS L1 L2	/	139.43 × 139.43 × 6	L1: −0.4–2.4 dB L2: 1.1–2.1 dB
Salleh [29]	1.575	Patch antenna	113 × 99 × 2.04	0.2–4.8 dBi 17.1–47.2%
Ahmed [30]	/	Patch antenna	86 × 69 × 1.6	6.33 dBi
Gil [31]	1.575 GPS L1	Patch antenna	115 × 114 × 0.8	−0.4 dBi 15.6%
Nordin [32]	/	Patch antenna	100 × 66 × 1	/
Choi [33]	1.563–1.587 GPS L1 902–928 MHz ISM	Patch antenna monopole antenna	158 × 158 × 5.2	8.26 dBi, 72.61% −10.96 dBi, 6.39%
Monti [34]	1.575 GPS L1 1.710–1.785, 1.805–1.880 GSM-1800	Patch antenna	95.2 × 47.6	7.5 dB
Gil [35]	1.575 GPS L1	Patch antenna	72 × 71 × 1	1.7 dBi 23.1%
Sundarsingh [36]	1.435–1.81 GPS L1, L3 2.755–3.19 4 G LTE	Patch antenna	Patch 53 × 44.35 × 7	96% 88%
Jais [37]	GNSS		Patch r = 40 mm	1.81 dB
Kaivanto [38]	Iridium GPS	Slot antenna	Patch 65 × 65 × 3	−2.5–7.5 dBi

Table 2. Wearable textile antennas applied to GNSS.

Ref.	Freq. (GHz)	Type	Size (mm) Length × Width × Thickness	Gain/Effi.
Sungwoo [23]	1.555–1.625 GPS, GLONASS	Patch antenna	100 × 100 × 1.575	/
Vallozzi [24]	GPS-L1	Patch antenna	/	8.4 dBi (open space) 8.34 dBi (with additional textile layers) 7.52 dBi (on body with firefighter jacket)
Sabapathy [25]	1.538–1.622	Patch antenna	113.75 × 99	/
Ahmad [26]	1.563–1.587 GPS L1 902–928 MHz ISM	Patch antenna monopole antenna	/	9.37 dBi, 80.31% 7.83 dBi, 14.61%
Anbalagan [27]		Slot antenna	48 × 80 × 0.99	0.36 dB
Hassan [28]	1–1.62 GPS L1 L2	/	139.43 × 139.43 × 6	L1: −0.4–2.4 dB L2: 1.1–2.1 dB
Salleh [29]	1.575	Patch antenna	113 × 99 × 2.04	0.2–4.8 dBi 17.1–47.2%
Ahmed [30]	/	Patch antenna	86 × 69 × 1.6	6.33 dBi
Gil [31]	1.575 GPS L1	Patch antenna	115 × 114 × 0.8	−0.4 dBi 15.6%
Nordin [32]	/	Patch antenna	100 × 66 × 1	/
Choi [33]	1.563–1.587 GPS L1 902–928 MHz ISM	Patch antenna monopole antenna	158 × 158 × 5.2	8.26 dBi, 72.61% −10.96 dBi, 6.39%
Monti [34]	1.575 GPS L1 1.710–1.785, 1.805–1.880 GSM-1800	Patch antenna	95.2 × 47.6	7.5 dB
Gil [35]	1.575 GPS L1	Patch antenna	72 × 71 × 1	1.7 dBi 23.1%
Sundarsingh [36]	1.435–1.81 GPS L1, L3 2.755–3.19 4 G LTE	Patch antenna	Patch 53 × 44.35 × 7	96% 88%
Jais [37]	GNSS		Patch r = 40 mm	1.81 dB
Kaivanto [38]	Iridium GPS	Slot antenna	Patch 65 × 65 × 3	−2.5–7.5 dBi

Previous studies have demonstrated diverse application scenarios for GNSS wearable textile antennas. Vallozzi et al. [24] proposed a wearable textile GPS antenna integrated into protective clothing capable of withstanding harsh environmental conditions. Sabapathy [25] combined an on-body positioning tracking system with a wearable GPS antenna to achieve location tracking for patients and elderly individuals within a 1.0 km radius. Anbalagan et al. [27] integrated a GPS module with an embroidered textile antenna for location tracking and border alerts in marine environments within an intelligent wearable system. Gil et al. [31] proposed integrating a GPS L1 embroidered antenna into wearable wireless communication system garments.

It is evident that the main types of GNSS wearable textile antennas are patch antennas and slot antennas. Patch antennas are widely used due to their simple structure, low profile, ease of conformality, and ability to achieve CP. Typical examples include the cotton-based GPS antenna with an e-textile patch proposed by Nordin et al. [32] and the copper foil adhesive denim GPS antenna presented by Gil et al. [35]. Slot antennas operate by creating one or more slots in waveguides or cavity resonators to radiate or receive electromagnetic waves. They share similar characteristics with patch antennas, featuring a planar structure that easily conforms to mounting surfaces. For instance, Kaivanto et al. [38] demonstrated a typical slot antenna by introducing slots in the center of a patch antenna.

Existing GNSS textile antennas are mostly concentrated in the GPS frequency bands, especially GPS L1, with a generally narrow overall bandwidth. Some antennas achieve multiband operation, covering a GNSS band along with other RF frequency bands. For example, Ahmad et al. [26] proposed a dual-function dual-patch wearable antenna integrated into a military beret, where the inner patch operates in the GPS L1 band for outdoor use and the outer ring patch covers the ISM band for RFID indoor applications. Similarly, Choi et al. [33], in 2017, presented a wearable antenna integrated into a military beret, with a corner-cut patch antenna achieving CP at 1.575 GHz GPS L1 for outdoor use and a ring patch antenna operating at 915 MHz for indoor positioning. Both bands are realized by combining patch and monopole antennas for dualband GPS L1 and ISM 915 MHz operation.

Monti et al. [34] employed PIN diodes to enable switching between GPS L1 and GSM-1800 bands, while Jais et al. [37] used coaxial probe feeding for four sectoral radiating elements, controlling antenna polarization through PIN diode switches. Similarly, Salleh et al. [29] achieved frequency reconfigurability by embedding three RF PIN diode switches to modify slot dimensions, allowing switching among six frequencies, with a relatively high gain at 1.575 GHz that decreases with increasing frequency.

Some studies achieved dualband operation by modifying the patch or ground. For instance, Sundarsingh et al. [36] cut a regular rectangular patch into a vertical dumbbell-shaped conductive area to cover GPS L1, L3, and 4G LTE dualbands. Likewise, Kaivanto et al. [38] created dualband satellite Iridium and GPS applications by slotting the patch.

In the 1.0–2.0 GHz frequency range, antenna frequency is closely related to size. Patch antennas in this band typically measure about 50–90 mm with a thickness of 1–7 mm. The radiating area and thickness of the antenna directly affect its radiation performance. For both patch and slot antennas, a more complete radiating area and a thicker substrate usually result in better gain and efficiency. Existing GNSS antennas have gain values ranging from –3 to 10 dBi, with performance varying due to specific structures and material properties. The following sections will summarize common techniques for bandwidth enhancement, miniaturization, and gain improvement.

2.2. GNSS Textile Antenna Bandwidth Enhancement Techniques

Antennas with a bandwidth covering multiple bands can adapt to diverse application scenarios, reducing the number of antennas needed and enabling a single antenna to meet various application requirements. Additionally, multiband operation in user terminals significantly improves positioning accuracy and enhances device security and robustness. Currently, as shown in Table 3, common approaches to enhance the bandwidth of GNSS textile antennas include loading defected ground structure (DGS), slotting, via holes as specific perturbation techniques, along with introducing metasurfaces (MSs) or integrating PIN diodes. Some studies achieve bandwidth enhancement by combining multiple techniques.

In the rigid antenna field, Hussine et al. [39] developed an antenna with a bandwidth covering 1.08–1.69 GHz (43%), including the GPS L1–L5, GLONASS G1, G2, G3, and Galileo E5a, E5b, E6, and E1 bands. However, the thickness of the antenna is relatively large, making it unsuitable for wearable devices. There are also flexible wearable antennas achieving ultra-wideband coverage, such as the UWB wearable patch antenna developed by Ullah et al. [40], which covers the 1.0 to 6.0 GHz frequency range and maintains comparable performance even under bending conditions.

Table 3. GNSS textile antenna bandwidth expansion techniques.

Ref.	Type	Application	Method	Bandwidth (MHz)
Hussine [39]	Patch antenna	GNSS	/	610
Mao [41]	Patch antenna	2.45 GHz ISM	Additional strip on feedline	40 (1.6%) (Before) 120 (4.8%) (After)
Abdulmalek [42]	Slot antenna	5.2 GHz ISM WLAN	Slot the patch	180 (Before) 870 (After)
Jaiswal [43]	Slot antenna	/	“U” slot via open hole pin	/
Mustaqim [44]	Patch antenna	UBW	DGS stepped slits on patch Add notch on ground	2500 (Before) 7500 (After)
Le [45]	Slot antenna	5.85 GHz WBAN	MS	/
Dwivedi [46]	Patch antenna	/	Central hexagonal slot DGS	1635
Soh [47]	PIFA	1.8–3 GHz WBAN	PIFA Short-Circuit Wall Interconnection Slot the patch	/
Hassan [48]	Patch antenna	GPS, UMTS, WiFi, ISM, Bluetooth, WLAN, WiMAX, 5G	Slots on patch DGS	0.58–0.83 GHz 1.39–1.58 GHz 2.40–2.43 GHz 2.88–3.52 GHz 4.93–5.15 GHz
Zaidi [49]	Patch antenna	1.575 GHz GPS	DGS	4.04% (Before) 12.20% (After)
Karimyian [50]	Fractal antenna	GPS, PCS-1900, WiFi, UWB	Non-uniform geometry patch DGS	1.4–20 GHz (18,600)
Ullah [40]	Patch antenna	/	Non-Uniform geometry patch	1–6 GHz (5000)

Table 3. GNSS textile antenna bandwidth expansion techniques.

Ref.	Type	Application	Method	Bandwidth (MHz)
Hussine [39]	Patch antenna	GNSS	/	610
Mao [41]	Patch antenna	2.45 GHz ISM	Additional strip on feedline	40 (1.6%) (Before) 120 (4.8%) (After)
Abdulmalek [42]	Slot antenna	5.2 GHz ISM WLAN	Slot the patch	180 (Before) 870 (After)
Jaiswal [43]	Slot antenna	/	“U” slot via open hole pin	/
Mustaqim [44]	Patch antenna	UBW	DGS stepped slits on patch Add notch on ground	2500 (Before) 7500 (After)
Le [45]	Slot antenna	5.85 GHz WBAN	MS	/
Dwivedi [46]	Patch antenna	/	Central hexagonal slot DGS	1635
Soh [47]	PIFA	1.8–3 GHz WBAN	PIFA Short-Circuit Wall Interconnection Slot the patch	/
Hassan [48]	Patch antenna	GPS, UMTS, WiFi, ISM, Bluetooth, WLAN, WiMAX, 5G	Slots on patch DGS	0.58–0.83 GHz 1.39–1.58 GHz 2.40–2.43 GHz 2.88–3.52 GHz 4.93–5.15 GHz
Zaidi [49]	Patch antenna	1.575 GHz GPS	DGS	4.04% (Before) 12.20% (After)
Karimyian [50]	Fractal antenna	GPS, PCS-1900, WiFi, UWB	Non-uniform geometry patch DGS	1.4–20 GHz (18,600)
Ullah [40]	Patch antenna	/	Non-Uniform geometry patch	1–6 GHz (5000)

Loading specific perturbation structures, such as strips, slots, shorting pins, or the alteration of patch shape, causes changes to the original current and electromagnetic field distributions and modifies the electromagnetic parameters of antennas, like equivalent inductance and capacitance, thereby exciting new resonant modes or adjusting existing ones, improving impedance matching and adjusting radiation patterns. Multiple resonant modes interact, enabling the antenna to meet resonance conditions over a wider frequency range, ultimately achieving bandwidth expansion. For example, Mao et al. [41] introduced two strips perpendicular to the feed line in antenna design, generating additional resonant frequencies that combine with the fundamental mode of the patch antenna to form second-order resonance, effectively extending bandwidth. However, adding complex perturbation structures may increase the radiating surface area of the antenna.

DGS involves creating specific-shaped defects or perturbations on the ground, altering the current and electromagnetic field distribution on the ground plane, causing changes in the characteristics of the resonant and impedance matching. Properly designed DGS produces stopband characteristics at certain frequencies, effectively suppressing harmonics, spurious signals, and surface waves, thereby improving antenna selectivity and bandwidth. DGS does not significantly increase antenna size or complexity; however, it may reduce gain and increase SAR, thereby enhancing the impact on the human body. Common DGS shapes include dumbbell, circular, square, and elliptical forms. Zaidi et al. [49] used DGS to address frequency mismatch, increasing antenna bandwidth from 4.04% to 12.20%, achieving a gain of 1.45 dB and radiation efficiency of 23.75%.

The slotting technique enhances antenna performance by etching specific slot shapes into the conductive part to alter surface current distribution and electromagnetic characteristics. Like DGS, slotting effectively broadens bandwidth and suppresses surface waves without significantly increasing antenna size or complexity. Abdulmalek [42] demonstrated bandwidth enhancement by introducing slots in the patch. Simulation results showed bandwidths of 1023.6 MHz (no slot) and 997.7 MHz (with slot), with only a 2.5% deviation. However, measurements indicated that the slotted CP patch antenna achieved a bandwidth of 870 MHz—an improvement of 690 MHz compared to the 180 MHz of the un-slotted design. Nonetheless, slotting reduces the radiating surface area, which may degrade radiation performance.

MSs are artificially layered materials with thicknesses smaller than the wavelength, composed of subwavelength structures like metallic patches or dielectric pillars. By carefully designing the size, shape, and spacing of the patches and the refractive index of the pillars, control over key electromagnetic properties such as amplitude, phase, and polarization is achieved. Le [45] enhanced the impedance bandwidth of a slotted antenna by loading a 4 × 4 MS on top of the patch, which excited additional surface wave resonances and adjacent high-frequency modes. The MS also acted as a polarization converter, producing orthogonal waves with equal amplitude and 90° phase difference, combining with the low-frequency mode to generate broadband CP and a wider axial ratio bandwidth. The MS lattice configuration was further optimized for the best performance. CP patch antennas integrated with MSs achieved bandwidths up to 34.7% [51], about 26% higher than those without MS integration. Similarly, partially reflective surfaces (PRSs) [52] have been used to broaden CP Fabry–Pérot antenna bandwidths. However, MSs add an extra structural layer, which can hinder the realization of low-profile antenna designs, and also increase the design complexity of the antenna, but can simultaneously improve antenna gain and reduce SAR; relevant content will be further discussed in the next section when addressing gain enhancement methods.

Vias can alter the current distribution and electromagnetic characteristics of antennas, allowing adjustment of input impedance to improve matching with the feed network or transmission line. In multiband patch antennas, via-connected open pins interact with the patch and substrate to modify current paths and field distributions, enabling resonance at multiple frequency bands. The effective design of vias and open pins tailored to specific applications and fabrication constraints, considering shape, size, quantity, and placement, could significantly optimize antenna performance. For example, Jaiswal [43] introduced a U-shaped slot on their patch and added an open-hole via between the patch and ground to extend bandwidth. Similarly, Soh et al. [47] developed a Planar Inverted-F Antenna (PIFA) with a slot in the patch and a shorting wall—rather than pins—at a strategically chosen location, achieving an over 46% bandwidth increase.

Bandwidth could be enhanced by the integration of multiple techniques. For instance, Mustaqim [44] increased bandwidth by reducing the ground size, introducing a central notch in the ground, and embedding symmetrical stepped slots on both sides of the patch. The initial basic rectangular patch antenna, simulated with an infinite ground, only resonated at 7.5 GHz and 10 GHz, failing to cover the entire UWB range. After optimization, the simulated bandwidths on FR4 and denim substrates reached 3.9–10.9 GHz and 3.6–11.3 GHz, respectively. Measured results confirmed that both fabricated UWB antennas fully covered the 3.1–10.6 GHz UWB range. Dwivedi [46] proposed an antenna structure combining nested circular elements, a central hexagonal slot, and DGS. The hexagonal slot enhanced bandwidth and overall performance, while the DGS suppressed unwanted radiation and reduced environmental interference. Parametric analysis showed that shorter ground lengths resulted in lower resonance frequencies. The antenna achieved an impedance bandwidth of 70.7%, covering 1.495–3.13 GHz, suitable for ISM, Wi-Fi, WLAN, and Bluetooth applications, with a peak gain of 3.25 dB. Hassan [48] implemented a five-band antenna using a 50 Ω coplanar waveguide (CPW) feed and incorporated dual rectangular slots and periodic DGS on the radiating patch. Additionally, Karimyian [50] utilized a genetic algorithm to iteratively design a textile fractal antenna based on triangular and circular patches combined with a partially modified elliptical ground, achieving an impedance bandwidth covering 1.4–20 GHz.

2.3. GNSS Textile Antenna Miniaturization Techniques

As the signal transceiver component of wearable terminal devices, the size of the antenna directly determines the overall device dimensions and significantly affects the user experience. Given the requirements for portability and concealment in wearable systems, miniaturization of textile antennas is required. A summary of miniaturization techniques that have been applied to GNSS textile antennas is provided in Table 4. Slotting and meandering as specific perturbation techniques and nested split-ring resonator (NSRR) mechanisms are commonly employed to achieve antenna miniaturization.

Table 4. GNSS textile antenna miniaturization techniques.

Ref.	Freq (GHz)	Type	Method	Area (mm3)
Tekneci [53]	1.568–1.582 GPS 5.15–5.35 WLAN		NSRR	31 × 27 × 0.7 0.187λg × 0.215λg × 0.004λg
Rao [54]	1.57–1.615 GPS, GLONASS	Patch antenna		58 × 58 × 1.57 0.307λo × 0.307λo × 0.008λo
Wen [55]	-	Patch antenna		50 × 50 × 1.6
Ismail [56]	1.575 GPS	Dipole antenna	Meandering Slot	40 × 25 × 0.1016

Table 4. GNSS textile antenna miniaturization techniques.

Ref.	Freq (GHz)	Type	Method	Area (mm3)
Tekneci [53]	1.568–1.582 GPS 5.15–5.35 WLAN		NSRR	31 × 27 × 0.7 0.187λg × 0.215λg × 0.004λg
Rao [54]	1.57–1.615 GPS, GLONASS	Patch antenna		58 × 58 × 1.57 0.307λo × 0.307λo × 0.008λo
Wen [55]	-	Patch antenna		50 × 50 × 1.6
Ismail [56]	1.575 GPS	Dipole antenna	Meandering Slot	40 × 25 × 0.1016

The meandered-line technique increases the effective current path length by designing the conductive trace of the antenna in a bent or folded configuration to lower the resonant frequency, allowing the antenna to achieve a comparable resonant frequency to larger designs while maintaining a smaller physical size, thus realizing miniaturization. For example, Ismail et al. [56] proposed a GPS dipole antenna for wearable tracking devices targeting children and Alzheimer’s patients. Miniaturization is achieved by maintaining the structure size as significantly smaller than the guided wavelength. The resonant frequency decreases as the dipole arm width increases due to the stepped structure introduced at the open ends, which extends the electromagnetic wave path. Simulations reveal that the frequency shift is proportional to the number of steps. Thus, this design reduces the resonant frequency effectively by increasing the number of steps and decreasing the step length, without altering the overall antenna dimensions.

While commonly used to enhance bandwidth, slotting can also contribute to miniaturization and CP. For instance, Rao et al. [54] introduced four asymmetric slots along the diagonal of a square patch, simultaneously achieving CP and miniaturization. Similarly, Wen et al. [55] designed a patch antenna with chamfered corners and four slots, resulting in an approximate 36% size reduction.

NSRRs consist of multiple concentric split-ring resonators, enabling strong mutual coupling among resonant elements and thus broadening the resonant frequency range. This allows the antenna to achieve lower resonant frequencies within the same physical size, facilitating miniaturization. Tekneci et al. [53] implemented the NSRR mechanism to realize antenna miniaturization, introducing more split gaps compared to conventional split-ring-resonator-based designs. The resonant frequency of the proposed antenna can be effectively tuned by adjusting the effective current path length, the number of split gaps, and the spacing between them.

2.4. GNSS Textile Antenna Gain Enhancement Techniques

Wearable textile antennas are typically placed on the human body or embedded in clothing, where strong electromagnetic coupling with the body can degrade antenna performance. Therefore, high gain is essential to mitigate human-body effects and ensure sufficient radiation performance while maintaining a low SAR to ensure user safety. As shown in Table 5, common techniques for enhancing the gain of textile antennas include loading artificial magnetic conductors (AMCs) and electromagnetic band gap (EBG) structures as metamaterials (MTMs), split-ring resonators (SRRs), using high-conductivity materials, and increasing the thickness of the dielectric substrate.

Table 5. GNSS textile antenna gain enhancement techniques.

Ref.	Freq (GHz)	Method	Area (mm3)	Gain (dBi)
Tetik [57]	1.654–2.052 ISM	EBG	76 × 58 × 0.71	2.746
Sabban [58]	0.4	SRR	/	/
Zhu [59]	2.4 5	EBG	Patch: 55 × 55 × 1.1 EBG: 120 × 120 × 2.2	/
Hassan [48]	0.58–0.83 1.39–1.58 2.40–2.43 2.88–3.52 4.93–5.15 GPS, UMTS, Wi-Fi, ISM, WLAN, WiMAX, 5G	Use silver nanoparticles for high electrical conductivity and precise printing	/	12
Joshi [60]	1.575 GPS 2.45 WLAN	AMC	85.5 × 85.5 × 5.79	1.94–1.98
Hassan [61]	GPS L1	Increase thickness of substrate	R = 37 mm H (substrate thickness) = 1–3 mm	H = 1, 1.5, 2, 2.5, 3 mm Gain = −2.7, −1.1, 1.09, 1.7, 2.28

Table 5. GNSS textile antenna gain enhancement techniques.

Ref.	Freq (GHz)	Method	Area (mm3)	Gain (dBi)
Tetik [57]	1.654–2.052 ISM	EBG	76 × 58 × 0.71	2.746
Sabban [58]	0.4	SRR	/	/
Zhu [59]	2.4 5	EBG	Patch: 55 × 55 × 1.1 EBG: 120 × 120 × 2.2	/
Hassan [48]	0.58–0.83 1.39–1.58 2.40–2.43 2.88–3.52 4.93–5.15 GPS, UMTS, Wi-Fi, ISM, WLAN, WiMAX, 5G	Use silver nanoparticles for high electrical conductivity and precise printing	/	12
Joshi [60]	1.575 GPS 2.45 WLAN	AMC	85.5 × 85.5 × 5.79	1.94–1.98
Hassan [61]	GPS L1	Increase thickness of substrate	R = 37 mm H (substrate thickness) = 1–3 mm	H = 1, 1.5, 2, 2.5, 3 mm Gain = −2.7, −1.1, 1.09, 1.7, 2.28

During antenna radiation, electromagnetic waves emitted from different locations may exhibit phase differences, leading to uneven energy distribution in space, thus affecting the antenna gain. MSs enable the phase manipulation of radiated waves, allowing them to constructively interfere in the far-field region, thereby enhancing energy concentration and improving gain. EBG structures, a type of MTM, possess the ability to suppress electromagnetic wave propagation within specific frequency bands. Tetik et al. [57] proposed a wearable microstrip patch textile antenna based on an EBG design, which significantly reduces the SAR and enhances gain. Zhu et al. [59] integrated a dualband EBG with the antenna, functioning as a high-impedance surface (HIS) to suppress back radiation. This integration reduced the SAR by a factor of 20, achieved higher gain in directions away from the torso, improving communication performance, reduced inward body radiation by more than 10 dB, and increased antenna gain by 3 dB.

Using traditional metal planes or MTM-based AMCs to shield the body can effectively reduce SAR and enhance gain. Joshi et al. [60] employed a 3 × 3 unit cell AMC array placed between the patch and ground to suppress back radiation and improve antenna gain. For dipole antennas without a ground, AMCs could also be used to counteract the coupling effect between the human body and the antenna, thereby increasing gain. Zheng et al. [10] designed a wearable fabric-based dipole antenna for energy harvesting, incorporating AMCs in a composite structure to minimize the impact on the human body.

Improving the conductivity of conductive materials can also enhance antenna gain to some extent, as common textile conductive materials are not perfect conductors. Hassan et al. [48] fabricated a high-gain multiband antenna by inkjet printing functional silver nanoparticles on a flexible transparent PET substrate, enabling high conductivity and precise printing; in addition, the design featured two identical Z-shaped patch elements symmetrically arranged with a right-angle ground assembly to converge the otherwise dispersed radiation pattern, resulting in a high gain of 12 dBi and multiband bandwidth.

For patch antennas with a full ground, antenna gain can be increased by adjusting the thickness of the substrate. Shishir et al. [62] reported antenna gains of 0.03, 2.18, and 3.82 dB when the substrate thickness was increased by factors of 1, 3, and 5, respectively. Hassan et al. [61] proposed a novel textile antenna for GPS L1 applications on a hat brim, testing substrate thicknesses of 1, 1.5, 2, 2.5, and 3 mm, with corresponding gains of −2.7, −1.1, 1.09, 1.7, and 2.28 dB.

Sabban et al. [58] proposed a wearable, textile, MTM-based, high-efficiency fractal antenna. Periodic split-ring resonators (SRRs) and metallic post structures were used to engineer materials with effective permittivity and permeability values less than unity, as demonstrated in [63,64,65,66]. The integration of SRRs into the antenna design enhanced gain and directivity by approximately 2.5 dB compared to conventional patch antennas. Moreover, antennas incorporating SRRs exhibit a reduction in resonant frequency by 5% to 10% relative to those without SRRs.

The approaches to achieving miniaturization, wide bandwidth, and high gain for GNSS textile antennas are shown in Figure 2. When attempts are made to simultaneously enhance these three performance metrics, contradictions are often present in a single operation method: for instance, enhancing antenna gain by increasing substrate thickness or adding MTM is detrimental to miniaturization; meanwhile, bandwidth improvement and miniaturization achieved through operations such as slotting and DGS lead to a reduction in antenna gain. Thus, appropriate methods should be selected and parameters optimized in line with target requirements to achieve optimal performance or the best balance.

3. Fabrication of GNSS Wearable Textile Antennas

The fabrication of wearable textile antennas significantly affects their mechanical properties, radiation performance, and flexibility. This section reviews current fabrication techniques in two aspects: textile forming methods and material selection. The two most critical materials determining the overall performance of wearable textile antennas are the conductive materials for radiating elements and the non-conductive substrate. Substrate materials must be carefully selected based on their dielectric properties, deformation tolerance (including bending, wrinkling, twisting, and stretching), compatibility with miniaturization, and environmental durability. Due to their porous, rough, heterogeneous structures and inherent air gaps, textiles typically exhibit low relative permittivity and high dielectric loss [67,68]. A key parameter of conductive materials is electrical conductivity. While perfect electric conductors (PECs) with infinite conductivity are often assumed in simulations, real-world materials have finite conductivities; for example, copper has a conductivity of 5.8 × 10⁷ S/m. Conductive materials used in wearable textile antennas (i.e., electronic textiles) are non-ideal and exhibit anisotropic conductivity, which differs significantly from conventional conductors. Furthermore, the electrical and dielectric parameters of many such materials are not yet fully cataloged in existing material libraries, presenting challenges for antenna engineers [69,70]. The dielectric and conductive layers together support the realization of miniaturization, high gain, and wide bandwidth in wearable textile antenna designs [71,72].

Common fabrication methods for GNSS textile antennas include fabric lamination, embroidery, and surface deposition techniques such as screen printing and inkjet printing. For textile antennas operating in other frequency bands, additional forming methods such as weaving, knitting, and 3D weaving are also employed.

3.1. Fabric Lamination

3.1.1. Fabrication Process

A large number of GNSS textile antennas utilize simple bonding techniques such as direct gluing, spraying, and heat pressing to attach the radiating and substrate layers. These methods enable researchers to quickly validate antenna designs. However, antennas fabricated through direct adhesion often suffer from weak interlayer bonding, resulting in poor mechanical stability and limited durability. Different bonding methods significantly affect antenna flexibility; for example, textile antennas laminated with PA hot-melt fabric via thermal pressing exhibit notable flexibility degradation. After repeated wear or washing, the antenna easily detaches from the fabric surface [73], leading to reduced efficiency and degraded performance due to the detachment of conductive materials from the textile substrate. Additionally, discrepancies exist between simulation models and actual fabrication materials, as practical textiles are not perfect conductors. For instance, Nordin et al. [32] used cotton as the substrate and a conductive textile composed of 36% copper and 64% polyester threads for the radiating patch of a GPS antenna. Due to the non-ideal properties of the textile materials, the measured operating frequency was shifted approximately 30% higher than the simulated result.

3.1.2. Textile Substrate

As shown in Table 6, commonly used dielectric fabrics for GNSS laminated textile antennas include felt, jeans, and denim, which typically exhibit relative permittivities between 1.2 and 1.8 and relatively high dielectric losses. Mustaqim et al. [44] compared antennas fabricated using a rigid FR4 substrate and a denim textile substrate. Due to differences in material properties, the FR4-based antenna measured 31 × 42 × 1.6 mm, with an impedance bandwidth of 7.71 GHz and a gain of 5.12 dBi. In contrast, the denim-based antenna measured 70 × 56 × 1 mm, achieving a slightly wider bandwidth of 7.95 GHz but a lower gain of 3.57 dBi. These results indicate that while the denim substrate enables a broader bandwidth, it sacrifices gain. The higher permittivity of FR4 lowers the resonant frequency, whereas the lower permittivity of denim leads to a higher resonant frequency. This disparity results in differences in antenna size, impedance bandwidth, and gain under identical designs. Additionally, substrate thickness affects the antenna’s effective electrical length, thereby influencing resonant frequency and bandwidth. A thicker FR4 substrate can increase the effective electrical length, leading to a lower resonant frequency and narrower bandwidth.

Hu et al. [74] investigated the influences of woven fabric specifications, including yarn density, count, and weave structure, and yarn constitutions on dielectric properties such as the dielectric constant and dielectric loss tangent in the UHF band, aiming to provide guidance for the design and selection of woven fabrics as electronic substrates in this frequency range. The study reveals that an increase in warp yarn density enhances the fiber volume fraction and fabric compactness, thereby leading to the increase in relative dielectric constant and dielectric loss tangent. The dielectric constant increases approximately linearly with the increase in weft yarn density before reaching 220 picks/10 cm, but after that the dielectric properties decrease due to the straightening of weft yarns and the crimping of warp yarns, which reduces the fabric’s compactness. When the warp yarn count increases, the dielectric constant and dielectric loss tangent gradually increase. The increase in weft yarn count leads to the first increase and then decrease in dielectric properties, because when the weft yarn count exceeds a certain limit, its bending rigidity exceeds that of warp yarns, the crimping degree of warp yarns increases, and the fabric compactness decreases. In terms of weave structure, plain weave fabric has the largest relative dielectric constant and dielectric loss tangent due to having shortest float length, the most interlacing points, and the most compact structure, followed by 3/1 twill, 5/1 twill, and satin fabric, which has the smallest dielectric properties due to its loosest structure and large air volume fraction. The dielectric properties of fabrics with different yarn materials vary significantly, and the crystallinity, hygroscopicity, and impurity content of the fibers are the main influencing factors. For example, fibers with high crystallinity such as cotton have a low dielectric constant, fibers with strong hygroscopicity such as Modal have high dielectric values, and carded cotton fabrics with more impurities have greater dielectric loss.

Table 6. Materials and properties of textile laminate antenna substrates.

Ref.	Material	Thickness (mm)	εr	Tanδ	Pic.
Abdulmalek [42]	Felt	3	1.44	0.044
Mustaqim [44]	FR4	1.6	4.4	0.019
	Denim	1	1.67	0.01
Le [45]	Felt	1.3	1.4	0.044
	Denim	1	1.7	0.024
Dwivedi [46]	/	/	1.7	0.024
Tetik [57]	Felt	0.65	1.44	/
Sadadiwala [75]	Jeans	3	1.7	0.002
Sabapathy [25]	Felt	1.5	1.22	/
Soh [47]	Felt	6	1.43	0.025
Zhu [59]	Felt	1.1	1.38	0.02
Tekneci [53]	Denim	0.7	1.8	0.03
Ahmad [26]	Felt	5	1.2	0.2

Table 6. Materials and properties of textile laminate antenna substrates.

Ref.	Material	Thickness (mm)	εr	Tanδ	Pic.
Abdulmalek [42]	Felt	3	1.44	0.044
Mustaqim [44]	FR4	1.6	4.4	0.019
	Denim	1	1.67	0.01
Le [45]	Felt	1.3	1.4	0.044
	Denim	1	1.7	0.024
Dwivedi [46]	/	/	1.7	0.024
Tetik [57]	Felt	0.65	1.44	/
Sadadiwala [75]	Jeans	3	1.7	0.002
Sabapathy [25]	Felt	1.5	1.22	/
Soh [47]	Felt	6	1.43	0.025
Zhu [59]	Felt	1.1	1.38	0.02
Tekneci [53]	Denim	0.7	1.8	0.03
Ahmad [26]	Felt	5	1.2	0.2

3.1.3. Conductive Textiles

When using fabric lamination technology to fabricate antennas, conductive textiles are commonly employed as the conductive material. Locher et al. [76] compared the performance of various conductive fabrics and proposed key requirements: low and stable sheet resistance (≤1 Ω/□), uniform resistance across the antenna area, flexibility, and stretchability to allow deformation. The study analyzed a woven fabric coated with silver, copper, and nickel. Although nickel offers excellent corrosion resistance, nickel-plated fabrics are unsuitable for antenna applications because the coating is applied after weaving. As a result, the interlaced fibers are not fully coated, preventing current from flowing continuously along individual fibers. Instead, the current must “hop” through the small overlapping coated areas at fiber intersections, leading to a sheet resistance of about 5 Ω/□ and non-uniform conductivity. The study also examined a knitted conductive fabric made from silver-coated polyamide fibers. However, its excessive elasticity caused dimensional instability and fluctuating resistance, reducing accuracy. Zaidi et al. [49] continued using a Sulzer Textile G6300 rapier weaving machine to produce conductive textiles. They adopted a satin weave technique to interlace conductive and non-conductive yarns, achieving both wearability and comfort.

As illustrated in Figure 3, Ali et al. summarized common conductive textiles such as Zelt, Flectron, Shieldit, and Taffeta samples [20,77], along with materials like Kevlar, nylon, nickel-coated, and silver-coated fabrics [78]. Mechanical deformations such as stretching and rubbing in wearable applications can induce microcracks, which in turn degrade the performance of wearable textile antennas.

As shown in Table 7, Abdulmalek et al. [42] employed ShieldIt Super, a conductive textile manufactured by LessEMF Inc., as the radiating element of their antenna. ShieldIt Super is a ripstop woven polyester fabric coated with copper and nickel. All conductive components were fabricated using this material, which exhibits a conductivity of 118,000 S/m.

Table 7. Materials and properties of textile laminate conductive textile.

Ref.	Thickness (mm)	Conductivity (S/m)	Material
Abdulmalek [42]	0.17	1.18 × 105	ShieldIt Super (ripstop, woven polyester textile plated with copper and nickel)
Le [45]	/	1.18 × 105	Shieldit super-conductive textile
Tetik [57]	0.03	2.5 × 105	Pure copper Polyester taffeta fabrics (PCPTFs) Electrotextile materials (ETMs)
Vallozzi [24]	/	/	Flectron®
Sabapathy [25]	/	/	SheildIt Super
Zhu [59]	0.06	1 × 106	Zelt conducting material High-quality nylon-based substrate plated with copper
Ahmad [26]	0.1	0.02 Ω/□	Shieldex conductive metallized nylon fabric

Table 7. Materials and properties of textile laminate conductive textile.

Ref.	Thickness (mm)	Conductivity (S/m)	Material
Abdulmalek [42]	0.17	1.18 × 105	ShieldIt Super (ripstop, woven polyester textile plated with copper and nickel)
Le [45]	/	1.18 × 105	Shieldit super-conductive textile
Tetik [57]	0.03	2.5 × 105	Pure copper Polyester taffeta fabrics (PCPTFs) Electrotextile materials (ETMs)
Vallozzi [24]	/	/	Flectron®
Sabapathy [25]	/	/	SheildIt Super
Zhu [59]	0.06	1 × 106	Zelt conducting material High-quality nylon-based substrate plated with copper
Ahmad [26]	0.1	0.02 Ω/□	Shieldex conductive metallized nylon fabric

3.2. Printing Technologies

3.2.1. Fabrication Process

As shown in Figure 4, commonly used printing techniques include screen printing, inkjet printing, gravure printing, flexograghic printing, and direct-write printing [79]. Screen printing [80,81], a traditional technique widely applied in the textile and garment printing industry, requires the preparation of a mesh stencil with open areas corresponding to the desired pattern and blocked areas elsewhere. During printing, a squeegee is used to press conductive paste through the open mesh onto the textile surface. The pressure and speed of the squeegee significantly affect the quality of the printed layer, while the overall cost is influenced by the stencil fabrication and the utilization rate of conductive ink [82,83,84]. The quality of screen-printed antennas largely depends on factors such as the surface roughness and surface energy of the textile substrate [85,86], ink–substrate compatibility [87], ink viscosity [88], and printing parameters [89].

Inkjet printing and direct-write methods share the advantage of computer-controlled precision in defining printed patterns. Inkjet printing deposits ink droplets containing fine silver nanoplates or nanorods as the conductive component. Electrohydrodynamic (EHD) direct-write printing [90,91,92] uses an electric field to guide viscoelastic inks onto a moving substrate in a continuous and highly accurate manner. By adjusting the electric field strength, the system can switch between droplet ejection and continuous jetting modes, enabling the fabrication of diverse patterns through fine control of processing parameters [93]. As a non-contact technique, EHD printing overcomes the stencil dependency of screen printing and offers better compatibility with variations in ink viscosity compared to conventional inkjet printing, making it suitable for a wide range of conductive materials and substrates.

Figure 4. Print ink and printing processes for e-textiles: (a) preparation of conductive inks, (b) printing techniques for e-textile fabrication, (c) applications [94].

Figure 4. Print ink and printing processes for e-textiles: (a) preparation of conductive inks, (b) printing techniques for e-textile fabrication, (c) applications [94].

As shown in Figure 5, Guo et al. [95] employed EHD printing combined with UV-curable silver ink and chemical sintering to fabricate textile-based circuits and devices. By tuning the ink viscosity, well-defined conductive patterns were achieved at fabric edges, and adjustable line widths were obtained through pattern stitching. The process offers several advantages, including rapid curing, low processing temperatures, solvent-free operation, high conductivity, and low silver ink loading. In antenna fabrication, strong adhesion between the conductive layer and the substrate, along with good mechanical stability, is critical; thus, thermal curing is often required to enhance bonding performance.

Figure 5. Fabrication process of flexible antenna through inkjet material printing: (a) Hassan [48]; (b) Guo [95].

Figure 5. Fabrication process of flexible antenna through inkjet material printing: (a) Hassan [48]; (b) Guo [95].

The irregular surface topology of textiles—characterized by gaps, undulating textures, and protruding fibers—poses a key challenge for fabricating high-performance conductive coatings in printed textile antennas. Conductivity relies on percolation pathways formed by contact between dispersed conductive particles in the ink; textile surface roughness impedes particle contact, hindering continuous conductive pathway formation and degrading conductivity [96]. Introducing a non-conductive smooth interlayer between the textile and conductive coating mitigates roughness-induced performance degradation. During antenna fabrication, adhesion strength between the conductive coating and substrate, along with the coating’s mechanical properties, is critical. Thermal curing—essential for robust adhesion—volatilizes solvents, reducing coating integrity and increasing susceptibility to cracking and abrasion during bending. Curing temperature/time directly affect the coating’s flexural resistance and conductivity [97], necessitating elastic coatings to resist deformation. Studies [98,99] demonstrate that incorporating elastic particles into conductive inks significantly enhances tensile performance. As conductive layers are fragile and textile antennas are garment-integrated (requiring washing), laundering accelerates failure. Wash durability is vital for printed antennas [100]. A screen-printed thermoplastic polyurethane (TPU) protective layer enables antennas to maintain operational bandwidth functionality after repeated washing despite reduced radiation efficiency [101]. Comparative laundering tests across textiles confirm severe washing-induced damage and validate protective layers for mitigating degradation and delamination [102].

3.2.2. Textile Substrate

The presence of gaps, irregular textures, and prominent fuzz on textile surfaces poses a significant challenge in forming high-performance conductive coatings for printed textile antennas. The conductivity of these coatings relies on the effective contact between conductive particles within the ink. However, the inherent roughness of textiles hinders such particle-to-particle contact, making it difficult to establish continuous conductive pathways and thereby reducing the overall conductivity of the printed layer [103]. Introducing a smooth, non-conductive interfacial coating between the textile substrate and the conductive layer has been shown to effectively mitigate the adverse effects of surface roughness on the electrical performance of the conductive coating [104].

Zheng et al. [81] investigated the effects of three polyester woven fabrics with different structures—plain weave, 2/1 right-handed twill, and 5/3 warp-faced satin—on the performance of screen-printed conductors and antennas. As shown in Figure 6, the study revealed that plain weave fabrics, featuring low surface roughness and small pores, enabled screen-printed conductors to exhibit high edge clarity, minimal width deviation, and the lowest sheet resistance (1.362 Ω/□). Due to their smooth surface and tight structure, ink diffusion and penetration were well-controlled, leading to optimal antenna impedance matching. In contrast, satin weave fabrics with long float lengths resulted in conductors with poor edge sharpness and the largest width deviation among the three structures, primarily because the extended-float yarns caused structural looseness. This research clarifies the key influence mechanism of fabric structural characteristics on the performance of printed electronic components, providing a theoretical basis for material selection and the structural design of textile-based antennas.

As shown in

Table 8. Materials and properties of printed antenna substrates.

Ref.	Type	Thickness (mm)	εr	Tanδ	Material
Mao [41]	Screen printing	2	1.7	0.0095	Evolon (30 wt% polyamide and 70 wt% polyester) (0.3 mm), polyurethane (PU) (0.057 mm)
Sungwoo [23]	Screen printing	1.575	2.1	0.0472	Leather
Ha [105]	Screen printing	1.5	1.71	0.02	Polyester 66.2% Cotton 33.8%
Hassan [48]	Inkjet printing	0.1	3	0.002	Transparent flexible PET

, dielectric materials for printed GNSS textile antennas include polymers, common textiles, and chemical fiber textiles. Mao et al. [41] selected Evolon nonwoven as the base substrate, composed of 30 wt% polyamide and 70 wt% polyester, with a basis weight of 95 g/m² and a thickness of approximately 0.3 mm. A 2 mm thick dielectric layer was formed by stacking multiple Evolon layers bonded with a porous 0.057 mm thick polyurethane (PU) web. Sungwoo et al. [23] used leather as the dielectric substrate, Ha et al. [105] used a common textile fabric composed of 66.2% polyester and 33.8% cotton, and Hassan et al. [88] used transparent flexible PET.

3.2.3. Conductive Inks

When fabricating wearable textile antennas using inkjet printing, screen printing, and similar techniques, achieving highly conductive patterns with conductive ink is crucial for optimal antenna gain, efficiency, and bandwidth. Additionally, the printed conductor must resist performance degradation caused by mechanical deformation. Currently, silver and copper nanoparticle inks are commonly used in flexible antenna manufacturing due to their high conductivity. Compared to copper nanoparticle inks, silver nanoparticle inks are preferred because of their slower oxidation rate [106]. As shown in

Table 9. Materials and properties of printed antenna conductive inks.

	Type	Thickness (mm)	Conductivity (S/m)	Material
Mao [41]	Screen printing	0.037	1.3 × 106	Dupont 5064H silver
Sungwoo [23]	Screen printing	-	2.22 × 106	EM-271S silver paste
Ha [105]	Screen printing	-	-	Silver paste: mixture of silver powder and acrylic resin
Hassan [48]	Inkjet printing	-	0.28 × 107	Adhered Ag nanoparticles (AgNPs)

, Mao et al. [41] used a Dupont 5064H silver conductor to form the patch and ground layer patterns, owing to its high conductivity and flexibility. A PU mesh was laminated over the conductive layer as an encapsulation, with gaps left for connector access. Sungwoo et al. [23] employed EM-271S silver paste as the conductive material. Hassan et al. [48] utilized additive manufacturing with inkjet printing via Fujifilm DMP-2850 to fabricate antenna structures. The prototype antenna was made by the precise piezoelectric inkjet printing of silver nanoparticles (AgNPs) on a 100 μm thick transparent flexible PET substrate. The AgNPs exhibited better adhesion to the substrate surface. Parameters such as layer count, droplet spacing, nozzle stepping, and substrate temperature influenced the antenna’s radiation performance.

3.3. Embroidery Technology

3.3.1. Fabrication Process

Embroidery, as a mainstream fabrication method for textile antennas, enables the formation of conductive radiating elements directly on the substrate using conductive threads. The size of the embroidered antenna elements is limited by the dimensions of the embroidery machine, and due to limited penetration force, the substrate thickness must also fall within a certain range. During the embroidery process, the fabric must be tensioned, requiring the substrate to have sufficient tensile strength. The conductive threads commonly used in embroidered antennas are synthetic fibers coated with conductive layers. Since these threads undergo rapid stretching and are subjected to considerable tensile force during embroidery, they must possess high tensile strength and the coating must exhibit adequate durability [107,108].

3.3.2. Textile Substrate

For GNSS textile antennas fabricated using embroidery techniques, cotton is a commonly used substrate. As shown in

Table 10. Materials and properties of embroidered antenna substrates.

Ref.	Material	Thickness (mm)	εr	Tanδ
Anbalagan [27]	cotton	0.33	1.6	0.04
Gil [31]	cotton	0.4	1.3	0.058

, both Anbalagan [27] and Gil [31] employed cotton cloth in their embroidered GNSS antenna designs. The fabric typically has a thickness ranging from 0.3 to 0.4 mm, with a relatively low dielectric constant and high dielectric loss. Cotton offers excellent comfort and moisture absorption, making it suitable for wearable applications.

3.3.3. Conductive Yarns

When preparing antennas using embroidery technology, conductive yarns can be directly fixed onto the fabric surface through computer-aided techniques or manual sewing. This process eliminates the need for adhesives or other bonding materials, effectively ensuring the stability of antenna performance [109]. As the core conductive components, conductive yarns are influenced by parameters such as stitch length, spacing, and type when sewn onto fabrics [110,111]. These parameters directly affect the arrangement density of the yarns, thereby altering the resistance values of conductive yarns and ultimately impacting the signal transmission performance of antennas.

As shown in

Table 11. Materials and properties of embroidered antenna conductive yarns.

Ref.	Material	Conductivity
Wang [112]	Flexible silver-coated Amberstrand fibers	0.8 Ω/m
Anbalagan [27]	Embroidering Zari threads (silver-coated conductive threads with silk core) 0.125 mm	6.2 × 107 S/m
Gil [31]	Commercial Shieldex 117/17 dtex 2-ply (99% pure silver-plated nylon yarn 140/17 dtex)	30 Ω/cm

, Wang [112] proposed a conformal antenna utilizing embroidered conductive metal–polymer fibers (E-fibers) on polymer–ceramic composites. These E-fibers, consisting of high-strength, flexible polymer cores coated with conductive metal layers, are precisely embroidered onto standard textile fabrics and assembled onto a polymer substrate. The realized E-fiber antenna exhibits a measured gain of only 0.3 dB lower than its simulated copper counterpart.

Anbalagan [27] embroidered the patch and ground plane separately onto two substrates to prevent electrical contact between them. The layers were then stitched together at the edges, and an SMA connector was soldered onto the feed line using conventional soldering techniques to complete the antenna structure.

Gil [31] implemented an embroidered antenna using a Singer Futura XL-550 embroidery machine (Singer Futura, Nashville, TN, USA) with a contour-filled stitching pattern. This technique involved curved fill stitches along the patch outline, forming rows of stitches across the rectangular patch. The conductive yarn used was commercial Shieldex 117/17 dtex 2-ply, composed of 99% pure silver-plated nylon (140/17 dtex) with a linear resistance below 30 Ω/cm. The patch antenna was fabricated on a two-layer cotton substrate, with the ground plane realized using commercial WE-CF adhesive copper foil.

Figure 7 shows the common dielectric materials, conductive materials, and process advantages and disadvantages of the three fabrication techniques—printing, embroidery, and lamination. In other frequency bands, textile antennas have adopted knitted spacer fabric structures [76,113,114,115,116,117,118], as well as weaving methods such as woven [119,120,121] and 3D-woven techniques [122,123]. These integrated weaving processes enable deep integration with clothing, yet challenges remain in resolving their performance stability issues. Future research could explore the development of GNSS-band textile antennas using integrated knitting or woven techniques.

3.4. Environmental Stability of Fabrication Processes

The environmental stability of wearable textile antennas is a core challenge restricting their practical application. This stems from the need for wearable antennas to adapt to complex working conditions in the long term—including washing, repeated bending, mechanical wear, sweat corrosion, environmental temperature and humidity fluctuations, and long-term use. These conditions act on different textile materials and affect antenna performance through the characteristic weaknesses of different preparation processes such as embroidery, inkjet printing, and lamination. Issues such as conductive layer fracture, dielectric layer deformation, yarn fiber aging, and fabric structure delamination are prone to occurring, ultimately leading to increased antenna resistance, resonant frequency shift, and gain reduction. Therefore, reliability and durability verification are crucial for the practical application of textile wearable antennas. Currently, research on their reliability focuses mainly on the impact of environmental conditions and working conditions on performance, such as washing [124] reliability, corrosion resistance reliability, and strain and bending reliability.

The embroidery process directly embroiders antenna patterns onto fabric substrates using conductive yarns. Its stability mainly depends on the yarn–substrate interface bonding strength—affecting the continuity of the conductive path—the fatigue resistance of the yarn itself, the ability to resist repeated deformation, and friction and oxidation resistance during long-term use. Relevant studies [125] fabricated embroidered dipole antennas using various woven fabrics as substrates and found that after 30 washing cycles, the antenna resonant frequency changes by 6–9%, the size shrinks by 1.8–3.8%, and the dielectric constant changes by less than 10%; after 55 wear cycles, the resonant frequency changes by 6–15%. Moreover, substrates with higher crimp rates have better stability due to stronger structural buffering, while low-crimp structures containing elastic fibers have higher performance variability due to low deformation redundancy. Another study [126] conducted durability tests on two electronic textiles: NC (nickel–copper-yarn-coated nylon tear-resistant fabric) and SJ (silver-yarn-coated cotton/polyester knitted fabric). The results showed that pilling causes NC to completely lose conductivity, significantly increases the longitudinal and transverse resistance of SJ, and leads to the disappearance of resonant characteristics and a gain reduction of nearly 20 dB after pilling; after 40 wear cycles, the resistivity of NC increases, while the longitudinal resistivity of SJ decreases; under sweat erosion, the resistivity of NC increases and that of SJ decreases; wrinkling increases the resistance of both, with a gain reduction of 2 dB; humidity and washing have little effect on the resistivity of both, but cause a decrease in resonant frequency and a gain reduction of 7–10 dB, which can partially recover after drying; vertical stretching reduces the resonant frequency and causes a gain reduction of 5 dB, while horizontal stretching increases the resonant frequency with insignificant gain change.

The printing process forms circuits on the fabric surface using conductive inks. Its stability is limited by the adhesion between the ink and the substrate and the adaptability to substrate deformation. Performance degradation mainly results from ink–fabric interface peeling, chemical corrosion—such as from electrolytes in sweat—and crack propagation inside the ink layer caused by repeated deformation. Relevant studies [127] on the strain reliability of passive tag antennas showed that in the initial state, the maximum readable distance of both tags is approximately 9.5 m; under an incremental strain of 0–20% and cyclic stretching, the readable distance decreases to 8 m and recovers to 8.5 m after release, with the frequency showing a downward trend with strain; among materials, high-elastic fabrics have more significant changes in frequency and amplitude under strain due to their larger deformation, but the readable distance still exceeds 6.5 m and is close to the initial value after recovery.

The lamination process composites conductive fabrics with non-conductive fabrics through adhesives. These composites’ durability and reliability are mainly affected by interface delamination caused by adhesive degradation or swelling in hot and humid environments, and substrate deformation. A study [128] fabricated wearable antennas using 0.5 mm thick denim as the substrate and three types of viscous nonwoven conductive fabrics. After multiple stress cycle tests, where each cycle included 10 min of cold water washing and 45 min of natural drying, it was found that after eight cycles, the antenna gain decreases by 0.95–1.97 dB with a slight shift in resonant frequency; after four cycles, the feed lines of all three antennas show degumming. Ironing can reduce the impact of interface peeling by heating to activate the viscosity of the remaining adhesive.

Wearable textile antennas with different materials and preparation methods have significant differences in their resistance to the above external factors, which are closely related to substrate selection, structural optimization, and process parameters. For most wearable textile antennas, when the number of washing cycles reaches a certain threshold [129], or when they come into contact with complex and corrosive solutions and body fluids, the resistance characteristics of the textile substrate and the radiation efficiency of the antenna itself will be significantly affected. Therefore, printing protective coating materials on the antenna surface has become a feasible approach to improving reliability [130], but some textile adhesives often fail to provide ideal protection as conformal coatings due to insufficient compatibility with fabrics. In addition, the instability in connection strength and bending resistance of the connecting wires between fabric antennas and testing equipment, as key links for signal transmission, also affect the overall performance of the system. Therefore, in addition to the above typical reliability issues, more scenario-specific reliability studies need to be carried out in combination with specific application scenarios to enhance the reliability of textile antennas.

4. Influence of Operational Conditions on Antenna Performance of Wearable Textile Antennas

4.1. Human Body Effects

The use of textile-based wearable flexible antennas [47,67,76,131] near the human body introduces various challenges. Key antenna performance parameters, such as reflection coefficient, bandwidth (BW), gain, efficiency, and radiation characteristics, are expected to be affected by electromagnetic coupling and absorption due to the human body [59,132,133,134,135]. Additionally, potential health risks from electromagnetic exposure must be considered. SAR quantifies the rate of electromagnetic energy absorbed by biological tissue, and it should comply with regulatory limits—less than 1.6 W/kg averaged over 1 g of tissue as per U.S. standards and below 2 W/kg averaged over 10 g of tissue according to EU guidelines.

As shown in Figure 8, most studies reproduce realistic human-body application scenarios by placing antennas near multilayer phantoms that simulate human tissues. In specific simulation processes, FEM software can be used to first construct a simulated human tissue model, which is typically a three-layer skin–fat–muscle structure in the form of planar plates or cylinders. Then, the wearable textile antenna is placed at different relative positions or distances from the model, and the safety of antenna electromagnetic waves for the human body is evaluated by analyzing changes in SAR. For example, Le et al. [45] positioned the antenna 5 mm above a tissue-equivalent phantom with dimensions of 160 × 160 × 50 mm³, composed of four layers—skin, fat, muscle, and bone—with respective thicknesses of 1.5, 8.5, 27.5, and 12.5 mm. SAR simulations were performed with an input power of 500 mW. The simulated SAR values at 5.85 GHz were 0.39 W/kg (U.S. standard) and 0.139 W/kg (EU standard), while at 7.0 GHz, they were 0.30 W/kg and 0.10 W/kg, respectively. These results indicate that the proposed antenna can be safely used in proximity to the human body.

Pinapati et al. [136] conducted simulations to evaluate antenna performance when placed 1 mm from human tissue and when located 3–5 mm beneath a simulated finger. The results showed minimal change in resonant frequency in the first scenario. However, in the second scenario, the resonant frequency shifted upward. This shift occurs because, in the unloaded condition, the electric field radiates freely into the upper half-space, whereas in the presence of a load, the field becomes more confined. When such confinement occurs in the antenna’s near-field region, the antenna’s effective electrical size is reduced, leading to an upward shift in the resonant frequency.

Some studies have jointly considered the effects of bending and proximity to the human body. Tetik et al. [57] analyzed the combined impact of these factors. Mao et al. [41] conducted practical tests by placing their antenna on relatively flat body areas such as the abdomen, back, and chest, where the gain and radiation efficiency showed only slight reductions compared to free-space conditions. However, when the antenna was placed on the arm, significant structural deformation led to a reduction in gain by about 1 dB and an 8% drop in efficiency. These findings indicate that severe bending has a considerable impact on antenna performance.

Some studies have investigated whether the gap between the antenna and the human body affects antenna performance or human safety. Ahmad et al. [26] developed a six-layer human head model with varying layer thicknesses and electrical properties to analyze the SAR at different distances between the antenna and the head. The results showed that as the distance increased, the SAR decreased.

Common methods to reduce SAR values in the literature include providing a complete ground plane to reflect electromagnetic waves, thereby reducing the electromagnetic wave absorption by the human body, and using metamaterial periodic structures such as EBG and AMC structures to reduce SAR values. For example, Zhu [59] integrated a dualband EBG with their antenna, which functions as a high-impedance surface (HIS) to suppress back radiation. After integration, the SAR value was reduced by a factor of 20, higher gain was achieved in directions away from the torso to improve communication performance, inward body radiation was reduced by more than 10 dB, and the antenna gain was increased by 3 dB. Reference [137] points out that a full ground plane on the back of a patch antenna can effectively reduce the impact of nearby objects and significantly lower the SAR. Based on this principle, as shown in Figure 9, Sabapathy [25] employed a circularly polarized square antenna topology with corner truncation and a full ground-plane design, effectively weakening the coupling between the human body and the antenna. Hertleer [67] found that the presence of the arm and an overlying fabric layer led to a downward shift in the antenna’s resonant frequency and return loss characteristics toward lower frequencies. Additionally, a larger ground plane was shown to improve gain in the broadside direction and enhance radiation efficiency, while reducing local SAR.

4.2. Dynamic Effects

The human body and garment surfaces often exhibit curved geometries. Antennas fabricated from textile materials possess exceptional elasticity and flexibility, allowing them to conform closely to the contours of the human body or clothing. In terms of structural precision, nonwoven and woven fabrics offer superior geometric stability for antenna implementation. However, due to the inherent deformability of textiles, antennas integrated into garments, flexible substrates, or wearable systems are subject to various mechanical stresses such as bending [138,139,140], wrinkling [27,141], compression [135], shear, and stretching [36]. These deformations can alter the antenna’s electromagnetic properties and consequently degrade its performance. A number of studies have systematically characterized these effects to evaluate their influence on antenna behavior.

4.2.1. Bending

The majority of existing research assesses the impact of bending on antenna performance by mounting antennas onto cylindrical surfaces of varying diameters, simulating typical wearable locations on the human body such as the arms, legs, and waist. For example, Zhu [59] employed polystyrene formers with diameters of 80 mm and 140 mm to approximate the curvature of the human arm and thigh, respectively. Their results indicated negligible changes in beamwidth or gain. Ferreira [142] conducted both simulations and experimental tests with the chest as a planar surface, the upper arm as a cylinder with a 35 cm circumference, and the wrist with an 18 cm circumference. The findings demonstrated that when the wearable textile antenna was positioned on the wrist (corresponding to a smaller bending radius), the gain was reduced by 2–4 dB and the front-to-back ratio decreased with increasing curvature. The bandwidth remained stable, while bending along the antenna’s width led to a downward shift in resonant frequency [44].

Several studies have focused on the effects of bending along different directions. As shown in Figure 10a, Tekneci et al. [53] investigated antenna performance under bending along both the short edge (x-axis) and the long edge (y-axis) at three different radii. The results showed negligible changes in frequency response, and the impedance bandwidth still covered the dual-target frequency bands. Mao [41] examined bending along the 0° axis (feedline direction) and 45° axis (diagonal direction), revealing that slight bending (radius > 150 mm) had minimal impact. However, when the bending radius was ≤90 mm, the bandwidth decreased from 100 MHz to 50 MHz, and back radiation increased by approximately 4 dB. Bending at 45° showed better impedance matching due to the symmetry of the dual-port design.

Some researchers also analyzed the effect of extremely small bending radii. As shown in Figure 10b, Dwivedi [46] evaluated antenna performance when bent around cylindrical surfaces with radii of 1, 2, and 3 cm. The results demonstrated the excellent flexibility of the antenna: the bandwidth remained unchanged at 1 and 3 cm, with slight fluctuations in impedance matching, while a rightward shift in bandwidth was observed at 2 cm.

4.2.2. Wrinkling

When the human body assumes different postures, flexible antennas positioned near joints are subject not only to bending but also to wrinkling. Thus, evaluating antenna performance under both bending and wrinkling conditions is essential [119]. Anbalagan [27] simulated single and severe wrinkles using cross-sectional lengths of 30 mm and 15 mm, respectively. Although the operating frequency bands fluctuated slightly, the S₁₁ values remained below −10 dB at 1.575 GHz. Bai [141] induced wrinkling by compressing the antenna between two complementary Rohacell formers and studied the impact of three different degrees of wrinkling in both the Y-Z and X-Z planes. Their experimental results showed that Y-Z plane wrinkles caused minimal changes in resonant frequency, with reflection coefficients exhibiting slight oscillations. However, significant reductions in radiation efficiency were observed due to altered spacing between the top radiating element and the ground plane. In contrast, wrinkles in the X-Z plane led to a substantial upward shift in resonant frequency. Although radiation efficiency remained relatively high, the reflection coefficient degraded, resulting in antenna detuning and a shift in the resonant peak out of the ISM band.

4.2.3. Compression

Lilja [135] investigated the impact of mechanical compression on fabric antennas using Rohacell rods of 10 mm and 120 mm in diameter applied at specific locations. The compression reduced the antenna thickness by 0.7 mm. Initially, compression was applied only to the substrate edge, sparing the radiating patch, since the electric field is primarily concentrated beneath the radiating element. Under this condition, return loss remained nearly unaffected. When the edges of the patch radiator were compressed, however, significant shifts in the resonance frequency were observed. This behavior is attributed to the antenna’s dual vertical resonant dimensions—compression along the top or bottom edges reduces the coupling of the higher-order resonance, whereas compression along the left or right edges affects the lower-order resonance similarly. In all these cases, the resonance frequency shifted downward, suggesting a localized increase in the substrate’s effective relative permittivity due to compression.

When compression was applied to the center of the patch, a combination of upward and downward frequency shifts occurred. This asymmetrical behavior is likely due to the non-uniform deformation relative to the feed region. As the compressive force was gradually released, the resonance condition progressively returned to its initial state. The most critical finding from this study was that compression at the edges of the radiating element resulted in the most pronounced impact on antenna resonance. This highlights the high sensitivity of resonance behavior to perturbations in the high-electric-field regions located at the patch edges.

4.2.4. Shear and Tensile Deformation

As shown in Figure 11, Sundarsingh [36] investigated the effects of shear and tensile deformation on textile antennas by bending their antenna into cylindrical shapes with diameters of 5 cm, 10 cm, and 15 cm, corresponding to typical dimensions of the human wrist, arm, and thigh, respectively. In addition, the antenna was subjected to three shear angles (0°, 10°, and 20°) and three tensile states: a compressed length of 10 cm and a stretched length of 14 cm. The results showed that both bending and stretching led to a reduction in antenna bandwidth, primarily due to the decrease in substrate height during deformation. Conversely, increasing the shear angle resulted in a slight improvement in bandwidth, which may be attributed to the marginal increase in antenna width induced by the applied shear force.

4.3. Environmental Effects

4.3.1. Humidity

Textile antennas used in wearable devices are inevitably exposed to liquid media such as sweat and rain, and daily relative humidity varies dynamically, with fluctuations between 60% and 95%. Humidity and moisture can alter the swelling degree of textile fibers, dielectric constant, and loss tangent, while affecting the conductivity of conductive materials, thereby interfering with the impedance matching and signal transmission stability of antennas. Investigating this influence mechanism is of key significance for improving the adaptability and reliability of antennas in complex environments.

Hertleer [143] investigated five textile substrates with different moisture regains—including natural fibers, blends of natural and synthetic fibers, pure synthetic fibers, and flexible polymer foams—using woven copper-coated-nylon fabric for both the patch and ground plane. Fifteen antenna prototypes were preconditioned at 23 °C under different relative humidity levels (10% to 90%) for 24 h, after which their return losses were measured using a network analyzer. By adjusting the simulated dielectric constants to match measurements, they found that increased humidity elevated the relative permittivity and loss tangent, which in turn reduced the resonant frequency and broadened the reflection coefficient curve. The relationship between relative permittivity and humidity was quadratic, with high-moisture-regain materials being more susceptible. As a result, materials with moisture regain below 3% were recommended for substrate use.

Some researchers have examined antenna behavior in direct contact with water, providing compensation coefficients within the tuning range of reconfigurable antennas. As shown in Figure 12, Pinapati [136] modeled a patch antenna with a water layer above it, showing that the high permittivity of water leads to a leftward shift in the resonant frequency. In practical tests, water was sprayed near the antenna in five cycles, with measurements taken after each spray and gentle wiping. Resonant frequency deviations were recorded, showing frequency ratios between dry and wet states of 0.5%, 3%, and 5% for the first, third, and fifth cycles, respectively—aligning with trends in related studies. Based on these results, it is suggested that wearable reconfigurable textile patch antennas include at least ±5% frequency compensation within their tuning range to accommodate resonance shifts caused by moisture.

Under identical relative humidity conditions, textile materials exhibit varying moisture regain rates. When textile fibers absorb moisture, they undergo axial expansion, causing the fabric to contract, which affects dimensional stability and consequently alters antenna performance. This moisture absorption is an exothermic process; fibers with higher moisture regain release more heat, which can further influence the electromagnetic properties of the fabric. Therefore, environmental changes affecting air or moisture content can lead to fluctuations in antenna behavior. Based on this, materials with low moisture regain are more suitable as antenna substrates [101]. Increased moisture content modifies the dielectric properties of textiles. While the relative permittivity of most textile materials ranges from 1 to 3, distilled water exhibits a relative permittivity of 76.7 at 3 GHz. Hence, water absorption can significantly lower the resonant frequency and introduce additional dielectric losses, leading to reduced radiation efficiency and detuning. A deep understanding of the interaction between moisture content and textile properties is essential for optimizing antenna designs under various environmental conditions, ensuring functionality and reliability in humid settings, and enhancing adaptability to humidity variations [46].

4.3.2. Combined Effects of Temperature and Humidity

Ambient temperature and humidity often interact synergistically to affect the properties of textile materials. Moreover, water exhibits different relative permittivity values at varying temperatures.

When textile antennas operate under extreme weather conditions, exposure to ultra-low-temperature water or ice, as well as high-temperature steam, can significantly impact antenna performance.

Kaija [68,144] developed textile antennas integrated into soldiers’ uniforms that are often exposed to harsh environmental conditions, where reliable communication among soldiers is critical. The research compared the antenna performance under dry, wet, icy, and snowy conditions, as well as when bent or covered with waterproof textile pouches, using communication SNR as the metric. The results showed that antennas fully submerged in water maintained performance comparable to dry antennas. Slightly damp antennas showed a minor SNR decrease. Severe bending and proximity to the human body affected radiation characteristics and degraded performance but did not prevent successful communication tests. A thin water layer formed by multilayer wet tissue atop the antenna significantly reduced performance. Pure water’s relative permittivity is approximately 81, while that of ice ranges from 2.6 to 4.5. Upon freezing, water’s dielectric behavior drastically changes, dropping to about 3.15 [144], shifting the antenna’s center frequency from 1.6 GHz (dry) to 1.23 GHz, with a notably lower average SNR. When ice melts to liquid water with high permittivity, the center frequency shifts to 882 MHz, and receivers fail to compute SNR from multiple satellites. Drying restores antenna performance. Snow’s electrical properties closely resemble ice, acting as a mixture of air and ice. Snow density and impurities affect wave attenuation, though its impact on return loss is minimal. Waterproof fabric covers effectively preserve antenna functionality in harsh conditions; uncovered antennas absorb moisture, degrading performance, with center frequencies reduced by factors of 0.96 and 0.77 for covered and uncovered antennas, respectively.

Fabric evaporation rates significantly influence electrical material properties under high humidity. Rising temperature increases saturated vapor density, decreasing relative humidity at constant water mass, while falling relative humidity accelerates evaporation. Lilja [135] investigated the combined effects of high temperature and humidity on textile antenna performance. Measurements revealed that the dielectric constant and loss tangent of non-conductive fabrics are humidity-dependent: dielectric constant changes highly correlate with relative humidity, whereas the loss tangent shows a stronger correlation with vapor density.

5. Conclusions and Future Perspectives

5.1. Conclusions

As GNSS textile antennas are widely utilized, demand for BDS textile antennas has been progressively increasing. However, BDS textile antennas remain scarce in both research and application domains. Since the BDS falls under the GNSS category, relevant studies on GNSS antennas operating within the same frequency band were analyzed, encompassing antenna structures, performance optimization strategies, fabrication techniques, and the influence of operational conditions on the antenna performance of wearable textile antennas.

From a structural perspective, patch, slot, and dipole antennas are predominantly employed in GNSS textile antennas due to their ease of low-profile realization. Performance optimization techniques, such as the incorporation of perturbation structures, MTMs, and advanced composite materials, have enabled significant progress in bandwidth expansion, dimensional miniaturization, and gain enhancement. However, contemporary research priorities emphasize structural innovation and performance metrics, while having comparatively limited considerations of textile substrate materials and architectures. The critical challenge of ensuring simultaneous achievement of superior electromagnetic performance and mechanical stability in textile-based antenna systems remains an essential research direction requiring comprehensive investigation.

In terms of fabrication, conventional lamination techniques provide notable advantages in operational efficiency and manufacturing convenience, yet frequently exhibit structural deficiencies including interfacial cracking and delamination. Approaches such as embroidery and printing encounter technical limitations manifested through inconsistent electromagnetic performance and mechanical failure modes involving the delamination of conductive layers or substrate deformation. Consequently, the optimization of manufacturing coupled with the systematic parameterization of critical design variables to ensure consistent antenna functionality remains a pivotal research focus. Furthermore, the integration of traditional textile engineering techniques, particularly warp-knitting and weft-knitting processes, demonstrates substantial potential for achieving the seamless structural incorporation of antenna elements within garment architectures.

Regarding operational impact factors, current research predominantly focuses on human-body, bending, wrinkling, temperature, and humidity effects on GNSS antenna performance, while textile antenna fatigue mechanisms and their cumulative impact on antenna durability remain underexplored. Although hygroscopic environmental effects on antennas have been discussed, the development of systematic mitigation strategies to address these performance-degrading phenomena merits further attention.

5.2. Future Perspectives

The in-depth development of AI technology is driving a shift in traditional antenna design, transitioning from parameter optimization towards intelligent inverse design [145,146]. This shift overcomes key pain points such as reliance on trial-and-error methods, multi-parameter coupling, and difficulties in reconciling performance indicators. Compared to traditional algorithms like the Method of Moments (MoM) and Finite Element Method (FEM), AI-assisted design, by integrating generative models [147], surrogate optimization algorithms [148], and Physics-Informed Neural Networks (PINNs) within multi-objective optimization frameworks, not only enables the inverse generation of antenna structures and coordinated multi-parameter optimization, effectively resolving parameter conflicts, but also accelerated parameter optimization, enhanced multi-objective balancing capabilities, and reduced reliance on traditional simulations. Ultimately, it achieves substantial computational cost and efficiency advantages, shortening design cycles and elevating performance limits. Therefore, when designing future GNSS textile antennas, employing an AI-assisted design approach should be considered. Leveraging its advantages in addressing multi-parameter coupling, improving design efficiency, and enhancing performance will propel the intelligent advancement of GNSS antenna designs.

Electrostatic spinning technology is increasingly being applied in components such as wave-absorbing structures, flexible sensors, and conductive layers, offering advantages over traditional textile processes like weaving and knitting in terms of microstructural control and performance co-optimization [149]. This technology constructs nanoscale fiber networks, leveraging their high specific surface area, high porosity, and excellent flexibility. The multiple scattering between fibers and interfacial polarization effects enhances the electromagnetic wave attenuation capability of wave-absorbing materials. Concurrently, its ease of facilitating multi-component compounding—such as loading functional nanoparticles [150] or constructing gradient structures—enables the preparation of dielectric layers with targeted dielectric properties. Furthermore, by compounding metallic nanowires, hybrid structures incorporating magnetic particles, or carbon-based magnetic fibers [151,152], highly conductive and stretchable antenna radiating layers can be fabricated. However, electrostatic spinning is still predominantly applied in wave-absorbing materials [153] and is rarely directly used for preparing antenna substrate materials. Nonetheless, in the future, with further optimization of spinning parameters and material systems, electrostatic spinning is expected to overcome current process limitations, providing a stronger impetus for the high-performance and integrated development of flexible electronic devices like GNSS textile antennas.