Design and Performance Evaluation of a Compact Frequency-Reconfigurable Coplanar-Waveguide-Fed Slotted Patch Antenna for Multi-Band Wireless Communication

Abstract

A miniaturized and low-profile planar antenna is a crucial part of any wireless communication system. To cover additional narrowband services and to reduce system complexity, antennas in portable devices should offer several operating bands. In this paper, we propose a coplanar-waveguide-fed (CPW-fed), flexible, and compact slotted patch frequency reconfigurable antenna with a compact size of 20 × 24 mm${}^2$. The designed antenna employs a low-cost Rogers 5880 substrate with a thickness of 0.127 mm. This choice of substrate ensures cost-effectiveness while preserving the desired performance of the antenna. The antenna radiates through five distinct frequency bands, including 5.58–6.25 GHz, 6.05–8.81 GHz, 8.79–9.7 GHz, 9.7–10.22 GHz, and 10.48–15 GHz, depending on the switch on/off condition, thereby enabling the antenna to span a broader range of frequencies for WLAN, C-UWB, sub-6 GHz, and X-band communications. The designed antenna is fabricated and tested in both the on and off states. The measured results closely match the simulated outcomes.

1. Introduction

The role of an antenna becomes indispensable as communication systems become increasingly miniature. To accommodate diverse applications in a single device, reconfigurable multi-band and wideband antennas have emerged as essential components. However, designing such antennas is challenging because they need to satisfy a number of stringent criteria, such as the ability to operate in the broadband range, steady gain, consistent radiation pattern, low profile, compactness, lightweight, and ease of fabrication, which can be achieved using inexpensive substrates such as paper, FR-4, or Roger, as well using an inexpensive production technique like inkjet printing [1].

Owing to the diverse requirements of contemporary communication devices, there has recently been an increase in interest in finding antennas that cover numerous frequency bands, such as WLAN (5.15–5.825 GHz), C-band (4–8 GHz), UWB (3.1–10.6 GHz), and X-band (8–12 GHz) [2,3,4,5]. The addition of several antennas for various applications may improve performance, but at the expense of complexity, coupling, and system size, depending on how closely the antennas are placed. A multi-band antenna that operates at several frequencies with optimal gain, improved efficiency, and bandwidth is ideal for consolidating various services. A narrowband antenna can be transformed into a multi-band antenna by strategically incorporating slits/slots or by adjusting the dimensions of the patch at specific positions. This modification significantly impacts the resonance and stopband behavior of the antenna. However, they transmit all resonances regardless of the specific needs or requirements of the end user [6]. In other words, it is difficult to adjust multi-band antennas to operate at a specific frequency. Additionally, when using multi-band antennas, there is a higher risk of interference, and wireless services become unavailable because of inadequate isolation between different frequency bands. Researchers have found potential solutions to these challenges by developing reconfigurable antennas. Reconfigurable antennas can address the issues encountered with both single-band and multi-band antennas, making them promising alternatives. Several types of reconfigurable antennas exist, such as frequency reconfigurable [7,8,9], pattern reconfigurable [10], and polarization reconfigurable [11].

Numerous studies have documented the use of microstrip lines, corporate feedlines, and CPW-fed antennas for wideband/multi-band and reconfigurable applications [9,12,13]. Contrary to microstrip line-fed antennas, CPW-fed antennas are designed with both the radiator and ground plane positioned on top of the substrate, and they almost completely cover the space around the radiator. The presence of a patch and its proximity to the ground make CPW feeding highly advisable for antenna fabrication. These antennas are characterized by low dispersion, low radiation leakage, and large operating bandwidths. In addition, it allows for the simple series and shunt attachment of additional surface components [14]. In [15], a bow-tie antenna was designed using CPW-feeding, and three bands of 2.4–2.7 GHz, 3.4–3.7 GHz, and 5.2–5.8 GHz were observed by introducing slots in the patches. However, it had a large size of 100 × 60 mm${}^2$. In [16], a pliable and reconfigurable CPW-fed monopole antenna was proposed. However, the antenna exhibited certain drawbacks, such as using an expensive substrate leading to relatively high fabrication costs. In [17], a CPW-fed super-wideband antenna was proposed, and the desired bands were achieved by etching the elliptical slots and using a Split ring resonator placed on the ground. A flexible CPW-fed transparent antenna for sub-6 GHz 5G applications was proposed in [18] and was fabricated using a transparent sheet made up of silver tin oxide (AgHT-8). In [19], the author proposed a CPW-fed fractal antenna for various applications.

In [20], a novel reconfigurable antenna was introduced in the shape of a cedar that demonstrated operations across multiple bands, including WiMAX, Bluetooth, GPS, and WLAN. To achieve reconfigurability, the antenna design incorporated both PIN and varactor diodes, with three pairs of varactor diodes used for this purpose. The utilization of multiple varactor diodes from two manufacturers—SMV1211 from SKYWORKS and 1SV325 from TOSHIBA—caused a notable decline in the performance of the antenna. Similarly, [21] proposed L- and U-shaped slots combined with three-pin diodes for LTE and WLAN applications, respectively. The proposed antenna design incorporated antenna elements on both sides of the substrate.

This manuscript illustrates the simulation and measurement results of a compact, slotted patch-shaped, and CPW-fed frequency reconfigurable antenna. The suggested antenna exhibited five distinct resonances, each with an acceptable gain and bandwidth. The resonance can be adjusted by manipulating the operational status of the PIN diode integrated into the patch.

The primary contribution of the proposed design lies in its achievement of practical frequency reconfigurability with a minimalistic approach, offering exceptional versatility and cost-effectiveness. By utilizing a single-switch mechanism for on and off operations, the proposed design simplifies complex antenna systems while maintaining competitive performance across multiple frequencies. This innovation addresses real-world challenges related to frequency band coverage, bandwidth, and return loss, showcasing engineering ingenuity. Furthermore, the design’s comparative advantage stems from its seamless transition across frequencies with acceptable bandwidth and return loss, making it a competitive alternative in scenarios prioritizing simplicity, reliability, and affordability. Its adaptability finds relevance in a wide array of applications, underscoring its real-world implications. Overall, the proposed design’s contribution lies in bridging the gap between efficiency and ease of implementation in the realm of antenna design.

The remainder of the paper is structured as follows: The design and theory of the proposed slotted patch-shaped reconfigurable antenna are discussed in Section 2. The analysis of the simulated and measured outcomes is presented in Section 3. Section 4 provides a comparison between the proposed design and a recently published state-of-the-art design. Finally, Section 5 concludes the study and summarizes the findings and implications of the study.

2. Antenna Configuration

A multi-band slotted patch-shaped reconfigurable antenna is illustrated in Figure 1. The compactness and multi-resonance of the designed antenna are achieved by introducing slots on the patch. To obtain a low profile and wide bandwidth, a single feed CPW has a width of “Wt” and two gaps of “g” are used. This approach reduced the fabrication complexity because the radiator and ground were placed on the common side of the dielectric substrate.

The substrate material utilized is Rogers 5880, which has a thickness of 0.127 mm and possesses a loss tangent of 0.0009 and a permittivity of 2.2. To achieve the targeted impedance matching of 50 $\mathsf{\Omega }$, the gaps (g) between the feedline and ground planes, the width of the feedline (wt), and other dimensions are essential. The antenna has dimensions of 20 × 24 × 0.127 mm${}^3$ and operates in five frequency bands.

Table 1. Attributes of the proposed design.

Parameter	Size (mm)	Parameter	Size (mm)	Parameter	Size (mm)
Ls	24	L7	2	W4	2
L1	15.8	L8	1.2	W5	2.4
L2	12	L9	1	W6	1
L3	7.3	Ws	20	W7	5
L4	8	W1	13	Wt	1.52
L5	6.5	W2	7.8	g	0.55
L6	7	W3	4.84	C	3

lists the optimized dimensions of the designed reconfigurable antenna.

To optimize matching, the feedline impedance can be computed as [22]

$$\begin{array}{c}\hfill Z_o=\frac{30\pi }{\sqrt{{ϵ}_e}}\frac{K\left(k_o^{/}\right)}{K\left(k_o\right)},\end{array}$$

(1)

where ${ϵ}_e$ represents the effective permittivity of the substrate. K($k_o$), K($k_o^{/}$), K($k_1$), and K($k_1^{/}$) are the modulus values of the complete integral, which can be evaluated as given below.

$$\begin{array}{c}\hfill {ϵ}_e=1+\frac{({ϵ}_r-1)}{2}\frac{K\left(k_1\right)K\left(k_o^{/}\right)}{K\left(k_1^{/}\right)K\left(k_o\right)},\end{array}$$

(2)

$$\begin{array}{c}\hfill k_o=\frac{S}{S+2d}\end{array}$$

(3)

$$\begin{array}{c}\hfill k_o^{/}=\sqrt{1-k_o^2}\end{array}$$

(4)

$$\begin{array}{c}\hfill k_1=\frac{sinh(\pi S/4h)}{sinh\pi (S+2d)/4h}\end{array}$$

(5)

$$\begin{array}{c}\hfill k_1^{/}=\sqrt{1-k_1^2}.\end{array}$$

(6)

Similarly, for a specific resonance, the effective antenna length is determined using transmission line theory [9].

$$\begin{array}{c}\hfill L_r=\frac{c}{4f_r\sqrt{\frac{{ϵ}_r+1}{2}+\frac{{ϵ}_r-1}{2}{(1+12\frac{h}{w})}^{-0.5}}}\end{array}$$

(7)

In the aforementioned equation, ${ϵ}_r$ and h represent the substrate’s relative dielectric constant and thickness, respectively.

2.1. Design Methodology

Figure 2 and Figure 3 presents the step-by-step procedure for obtaining the desired CPW-fed slotted patch multi-band frequency reconfigurable antenna with its corresponding $S_{11}$, which indicates the degree of impedance matching achieved between the antenna and the connected transmission line. A perfect match has a reflection coefficient of zero (linear scale), whereas a mismatch has a reflection coefficient with some nonzero values. As observed in the simulations, the corresponding $S_{11}$ is less than −10 dB in all the operating bands. An investigation of the antenna through a parametric study is conducted based on $S_{11}$. Initially, a single C-shaped resonator is designed along with a CPW feedline and resonates at 11.5 GHz. By adding another strip, the frequency shifts to 7.9 GHz. A central patch is then added in between the C-shaped resonator, which operates at two wide bands, namely, 6.12–8.72 GHz and 11.4–15 GHz. All these variations and their corresponding S-parameters are shown in Figure 3.

The final intended design was obtained by introducing a PIN diode at a precise location. A 1 mm slot was allocated for the switch in the upper part of the radiator. The PIN diode commonly exhibits characteristics of a variable resistor within the RF frequency spectrum. Nonetheless, the transition between its on and off states entails intricate circuit configurations. The on and off states of the PIN diode can be adequately represented by equivalent circuit models comprising an inductor (L) and a resistor (R). When the diode is forward-biased, the inductor and resistor (R) are linked in a series configuration. Conversely, in the off state, the inductor is paired with a parallel arrangement of a resistor (R) and a capacitor (C). The dynamic behaviors of the on and off states of the PIN diode are effectively analyzed through the framework of RL and RLC circuits, respectively [23].

The RL circuit, governed by a lower (R) value, facilitates current flow between radiating components. On the other hand, the RLC circuit, characterized by a higher RC value, impedes current propagation amidst radiating elements. For the sake of simplification, we have chosen to model our PIN diode using the RL circuit paradigm in our simulations. The inductance (L) is held at a constant value, while the resistor (R) is set to 1 $\mathsf{\Omega }$ in the on state and 5 M$\mathsf{\Omega }$ in the off state of the diode. A biasing voltage (V${}_B$) of 3 V is applied for the switch on condition, while 0 V is applied for the switch off condition in the circuitry. This resulted in resonance in the five frequency bands. Figure 4 presents a flowchart outlining the various steps required to obtain the desired results.

2.2. Parametric Analysis

The parametric analysis of various parameters of the proposed antenna is performed to assess the impact of different parameters on the antenna performance. The presented antenna is analyzed using the parameter W6, L6, and C. It can be observed from Figure 5a by varying the parameter W6, this changing the higher frequency. The higher band changes from 12.1 GHz to 12.7 GHz; however, the effect on the center frequency is negligible. Thus, it is concluded that the higher frequency can be controlled by the parameter W6. Similarly, by varying the parameter L6, the reflection amplitude of the higher frequency varies, while it has a negligible effect on the first bands; however, a small variation is observed at the center resonance. Figure 5b presents the simulated $S_{11}$ versus frequencies for different values of L6. Figure 5c presents the S-parameter versus frequencies plot for varying the parameter C. It is demonstrated that the parameter C is effective in shifting the center bands. By increasing the value of C from 1.5 mm to 4 mm, the center band shifts toward the higher frequency. Thus it is concluded from the parametric analysis that the center band can be controlled by varying the parameter C.

3. Results and Discussion

The proposed slotted-shaped frequency reconfigurable antenna was designed and analyzed using HFSS simulation software. A prototype was fabricated to test the antenna’s performance. Figure 6 illustrates the prototype of the fabricated design, biasing circuit for the PIN diode, and measurement setup used to capture the scattering parameters. The S-parameters of the design were measured using a network analyzer model N5227B, which operated within the frequency range of 10 MHz to 67 GHz, while the radiation pattern was measured using an anechoic chamber.

3.1. Switch On

Figure 7 shows the simulated and measured S-parameters for the switch on condition. When the switch was turned on, the current followed a prolonged path, resulting in full radiation. In this scenario, the antenna radiated in three distinct bands. The initial frequency band spans from 6.05 to 9.74 GHz, with a central frequency of 7.9 GHz and a bandwidth of 2730 MHz. This range is crucial within the X-band spectrum that is commonly used for wireless communication. It encompasses the military requirements for satellite uplink and the mobile satellite sub-band from 7.9 GHz to 8 GHz, catering to naval and land mobile satellite Earth stations. For Earth exploration satellite purposes, the military requirements for downlink fall within the frequency range of 8 GHz to 8.4 GHz. The second band of the antenna spans from 9.74 GHz to 10.22 GHz, with a center frequency of 9.95 GHz and a bandwidth of 270 MHz. This band specifically covers the X-band used for satellite communication. Finally, the third band extends from 10.7 GHz to 15 GHz, with a center frequency of 12.29 GHz and a bandwidth exceeding 4300 MHz. This wideband covers various applications, such as 5G communication from 12.2 GHz to 12.7 GHz, and satellite internet providers Starlink, and SpaceX use the 12 GHz band. The proposed design shows $S_{11}$ of −38 dB, −14 dB, and −21.65 dB at 7.9 GHz, 9.9 GHz, and 12.29 GHz, respectively, guaranteeing perfect matching. A close agreement was observed between the measured and simulated results.

Similarly, the simulated and measured peak gain and radiation efficiency plots for the SW ON mode are shown in Figure 8. Efficiency values of 64%, 59.7%, and 62.5% are observed for 7.9, 9.9, and 12.2 GHz, respectively. Similarly, peak gains of 4.33, 2.95, and 4.89 dBi are observed for 7.9, 9.9, and 12.2 GHz, respectively, making the proposed design an efficient candidate for various wireless applications.

The 3D radiation patterns of the proposed slotted-patch multi-band frequency reconfigurable antenna for the SW ON conditions are shown here. It can be observed from Figure 9 that the presented design has a good radiation pattern along the +z and -z axes with minimum side lobes. Maximum gains of 4.1, 3, and 4.8 dB were observed for the three desired frequency bands of 7.9, 9.9, and 12.3 GHz, respectively. At higher frequencies, the radiation near the port is highly energized and radiates more power, which then decreases toward lower frequencies.

Figure 10 illustrates the simulated and measured 2D radiation patterns for the E and H fields of the antenna. In this instance, with the switch turned on, the antenna resonates at three distinct frequency bands spanning from 6.05 to 15 GHz. Figure 10a shows the E-plane radiation pattern of the multi-band frequency reconfigurable antenna at frequencies of 7.9 GHz, 9.9 GHz, and 12.3 GHz. The antenna has a four-lobe configuration at 9.9 GHz and bidirectional radiation patterns at 12.3 and 7.9 GHz. Figure 10b illustrates the H-plane of the proposed antenna at the three resonances. The antenna design exhibits nearly identical radiation patterns across different frequency bands. This makes it a highly suitable candidate for insertion into portable electronics intended for wireless applications.

The summarized results for the switch on conditions are presented in

Table 2. Summarized results for SW ON.

Frequency (GHz)	7.9	9.9	12.3
Return loss (dB)	−38	−15	−21
Bandwidth (GHz)	2.76	0.48	5.3
Gain (dBi)	4.1	3	4.8
Efficiency (%age)	64.4	59.7	62.5

.

3.2. Switch Off

The simulated and measured S-parameters of the switch-off condition are shown in Figure 11. In this mode, the antenna operates at three different resonances. The first frequency band ranges from 5.58 GHz to 6.25 GHz, with a center frequency of 5.84 GHz and a bandwidth of 670 MHz. This range covers the upper portion of the WLAN band, specifically the frequencies around 5.9 GHz and 6 GHz. The second frequency band spans from 8.79 GHz to 9.7 GHz, with a center frequency of 9.18 GHz and a bandwidth of 910 MHz. This band falls within the X-band spectrum. The third band is from 10.4 GHz to 15 GHz, with a center frequency of 13.75 GHz and a bandwidth of more than 4600 MHz. This band covers various applications for 5G communications from 12.2 GHz to 12.7 GHz, and satellite Internet providers Starlink and SpaceX use the 12 GHz bands. The proposed design shows $S_{11}$ of −35 dB, −18 dB, and −27 dB at 5.8 GHz, 9.1 GHz, and 13.75 GHz, respectively, guaranteeing perfect matching. A close agreement is observed between the measured and simulated results.

The simulated and measured peak gain and radiation efficiency plots for the SW OFF mode are illustrated in Figure 12. Efficiency values of 99% 84% and 86% are observed at 5.8 GHz, 9.2 GHz, and 13.75 GHz, respectively. Similarly, peak gains of 2.62, 3.27, and 3.99 dBi are observed for 5.8 GHz, 9.2 GHz, and 13.75 GHz, respectively, making the proposed slotted patch reconfigurable antenna a better candidate for wireless communication.

The 3D radiation patterns of the proposed reconfigurable antenna for the SW OFF condition are presented. It can be observed from Figure 13 that the presented design has a perfect radiation pattern along the +z and -z axes with minimum side lobes. Maximum gains of 2.4, 3.5, and 3.8 dB can be observed for the three desired frequency bands of 5.8, 9.1, and 13.7 GHz, respectively. At higher frequencies, the radiation near the port is highly energized and radiates more power.

Similarly, Figure 14 illustrates the simulated and measured 2D radiation patterns for both the E and H fields for the SW OFF condition. The antenna radiates at three different frequency bands, from 5.58 GHz to 15 GHz. Figure 14a presents the E-plane of the proposed reconfigurable antenna at 5.8 GHz, 9.1 GHz, and 13.7 GHz. The antenna exhibits a bidirectional radiation pattern at 13.7 GHz and an omnidirectional radiation pattern at 9.1 GHz and 13.7 GHz. Figure 14b illustrates the H-plane of the proposed antenna at the three resonant frequencies. The design exhibited almost the same radiation pattern for all the desired frequencies. This makes the antenna an excellent option for incorporation into portable electronics for wireless applications.

The summarized results for the SW OFF conditions are presented in

Table 3. Summarized results for SW OFF.

Frequency (GHz)	5.85	9.18	13.75
Return loss (dB)	−35	−18	−27
Bandwidth (GHz)	0.67	0.9	5.6
Gain (dBi)	2.4	3.5	3.8
Efficiency (%age)	99	84	86

.

4. Comparison with State-of-the-Art Designs

To emphasize the novelty of this study, the designed antenna is compared with state-of-the-art designs. The comparison results are summarized in

Table 4. Performance comparison with other designs.

Ref.	[20]	[24]	[25]	[26]	[27]	[28]	[29]	[30]	Proposed Work
Dimensions (mm${}^2$)	3900	900	4000	600	875	900	352	1225	480
Material used	FR4	FR4	FR4	Polyimide	Roger 5880	Neltec	FR4	FR4	Roger 5880
Height (mm)	1.55	0.8	0.6	0.2	0.254	0.762	1.6	1.6	0.127
No. of resonance	6	1	3	3	3	4	2	5	5
No. of switch	6	N/A	2	N/A	1	2	2	3	1
Bandwidth (MHz)	1400 to 4600	1210	N/A	690	980; 2170; 1200	790; 100 330; 620	N/A	0.3 to 3.06	2760; 480; 5300; 670; 900

, which presents the key findings and highlights the advancements achieved by the proposed antenna. In [20], a frequency-reconfigurable antenna was presented using a low-cost Fr-4 substrate, and a wide bandwidth was achieved at the cost of several switches and the antenna size. In addition, [24] presented a circular polarized rotated L-shaped antenna using defected ground structure; however, the proposed design operates on a single frequency band. Reference [25] reports the design of a triple band reconfigurable antenna for IoT applications; by utilizing two switches, three different bands are achieved. The proposed design has a better efficiency of more than 90% but has a gain of only ≤2 dBi for all three operating bands. A miniaturized 20 × 30 mm${}^2$ antenna for sub-6 GHz 5G applications was proposed in [26], which can operate from 3.05 to 3.74 GHz. The performance is observed under two different bending conditions; later on, an eight port MIMO antenna is designed with a common ground plane. Considering all the state-of-the-art designs presented in Table 4, it can be concluded that the proposed slotted patch multi-band frequency reconfigurable antenna is a better candidate for wireless applications.

5. Conclusions

In this study, a slotted patch multi-band frequency reconfigurable antenna was designed to cater to various wireless communication applications, including WLAN, X-band, and SpaceX. Its versatile design enables its operation across multiple frequency bands, making it suitable for a wide range of wireless communication applications. Frequency agility was achieved by introducing a switch in the radiator to alter the effective electrical length. The proposed design was studied under switch-on and switch-off conditions and in terms of important parameters, including the reflection coefficient, gain, efficiency, and 2D and 3D radiation patterns. A close agreement was observed between the simulated and measured reflection coefficients. Efficient radiation was observed in the five desired frequency bands, which makes the proposed design a good choice for future communication systems.