Previous Article in Journal
Alleviating the Communication Bottleneck in Neuromorphic Computing with Custom-Designed Spiking Neural Networks
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
JLPEA
Volume 15
Issue 3
10.3390/jlpea15030051
jlpea-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Design of Tri-Mode Frequency Reconfigurable UAV Conformal Antenna Based on Frequency Selection Network

by
Teng Bao
1,
Mingmin Zhu
1,*,
Zhifeng He
2,
Yi Zhang
3,
Guoliang Yu
1,
Yang Qiu
1,
Jiawei Wang
1,
Yan Li
1,
Haibin Zhu
4 and
Hao-Miao Zhou
1,2
1
Key Laboratory of Electromagnetic Wave Information Technology and Metrology of Zhejiang Province, College of Information Engineering, China Jiliang University, Hangzhou 310018, China
2
College of Metrology Measurement and Instrument, China Jiliang University, Hangzhou 310018, China
3
Key Laboratory of Wireless Power Transmission of Ministry of Education, College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China
4
Jiaxing Key Laboratory of Flexible Electronics based Intelligent Sensing and Advanced Manufacturing Technology, Institute of Flexible Electronics Technology of THU, Jiaxing 314006, China
*
Author to whom correspondence should be addressed.
J. Low Power Electron. Appl. 2025, 15(3), 51; https://doi.org/10.3390/jlpea15030051
Submission received: 4 August 2025 / Revised: 5 September 2025 / Accepted: 8 September 2025 / Published: 10 September 2025
Browse Figures
Versions Notes

Abstract

With the rapid growth of unmanned aerial vehicles (UAVs) and IoT users, spectrum resources are becoming increasingly scarce, making cognitive radio (CR) technology a key approach to improving spectrum utilization. However, traditional antennas are difficult to meet the lightweight, compact, and low-drag requirements of small UAVs due to spatial constraints. This paper proposes a tri-mode frequency reconfigurable flexible antenna that can be conformally integrated onto UAV wing arms to enable CR dynamic frequency communication. The antenna uses a polyimide (PI) substrate and has compact dimensions of 31.4 × 58 × 0.05 mm3. A microstrip line-based frequency-selective network is designed, incorporating PIN and varactor diodes to realize three operation modes, dual-band (2.25~3.55 GHz, 5.6~6.75 GHz), single-band (3.35~5.3 GHz), and continuous tuning (4.3~6.1 GHz), covering WLAN, WiMAX, and 5G NR bands. Test results show that the antenna maintains stable performance under conformal conditions, with frequency shifts less than 4%, gain (3.65~4.77 dBi), and radiation efficiency between 67.2% and 82.9%. The tuning ratio reaches 38.8% in the continuous mode. This design offers a new solution for CR communication in compact UAV platforms and shows promising application potential.

1. Introduction

As an emerging economic model, the low-altitude field, with its distinctive features and broad market prospects, is gradually becoming an important engine driving national economic growth. Unmanned aerial vehicles (UAVs), as typical representatives of military–civilian integration technologies, play a pivotal role in this economic form [1,2]. However, with the rapid growth in the number of UAVs and the increasing demands for high-speed and high-reliability wireless communication, the UAV communication network inevitably faces a new problem—the scarcity of spectrum resources [3]. To improve the utilization efficiency of spectrum resources, Cognitive Radio (CR) communication technology has been proposed [4]. A CR system needs to be able to sense the electromagnetic environment in real time, accurately identify spectrum holes not occupied by primary users, and authorize secondary users to temporarily use these idle frequency bands for communication. When a primary user is sensed to access the channel, the secondary user will quickly switch to other frequency bands [5]. Figure 1 shows the schematic diagram of CR technology application, where a UAV user, after sensing the target spectrum, enters the spectrum holes of three different frequency bands, f1, f2, and f3, at different times. This innovative spectrum management mode significantly improves the utilization efficiency of spectrum resources, and the introduction of this technology will effectively alleviate the current shortage of spectrum resources in UAV communication systems [6].
CR communication is an intelligent communication technology with adaptive functions, whose core function is frequency-reconfigurable dynamic access technology, enabling rapid and dynamic switching of communication channels [7]. The antenna is a core component in the physical layer of a wireless communication system. However, the operating frequency of a traditional antenna is fixed once its configuration is determined. Therefore, to realize the CR dynamic communication function, it is necessary to design an antenna with frequency reconfigurability. At present, there are many schemes to realize antenna frequency reconfiguration. Among them, the mechanical control scheme changes the operating frequency by switching different radiating elements [8,9]. Reference [10] successfully realized four discrete operating frequency bands by rotating the antenna to four specific angles and connecting four radiating elements with different geometric shapes, respectively. Although this method avoids the complexity caused by the DC bias circuit required for electronic control, it has a slow reconfiguration speed and a large physical size. Reference [11] successfully realized the reconfigurable function in three frequency bands, 2.8~3.5 GHz, 3.8~4.2 GHz, and 5.1~5.7 GHz, by integrating MEMS switches. This method has the advantages of excellent isolation performance and low power loss, but it also has technical bottlenecks such as slow tuning speed and vulnerability to environmental disturbances. Reference [12] directly embeds photoconductive switches into the antenna structure and realizes frequency switching by controlling the on and off states of the switches. This technical scheme does not require additional metal connecting wires, thus showing excellent anti-electromagnetic interference ability and excellent signal isolation characteristics. However, the need to build an independent optical control system significantly increases the overall implementation cost. At present, the most mainstream frequency reconfiguration technology scheme is to directly embed electronic control components (PIN diodes, varactor diodes) into the antenna structure. Frequency reconfiguration is realized by controlling the on–off state of PIN diodes to change the surface current mode or adjusting the capacitance value of varactor diodes (VDs) to change the electrical characteristics of radiating elements [13,14,15,16,17,18,19,20]. This scheme is widely used in the field of reconfigurable antennas due to its fast frequency switching capability and excellent integration performance. Taking the design reported in Reference [21] as an example, researchers integrated four groups of PIN diodes into the slot structure of an H-shaped antenna and precisely controlled their switch combinations with a DC bias network, successfully realizing the discrete reconfigurable function in three operating frequency bands: 900 MHz, 1800 MHz, and 2100 MHz. On the other hand, the design requirements for UAV-borne antennas include miniaturization, light weight, and low cost. Traditional rigid antennas are relatively large in volume and need to occupy additional space [22,23]; most of them weigh more than 100 g [24,25], which may affect flight performance and are therefore not suitable for UAV communication systems.
Based on the above design requirements, this paper proposes a flexible monopole CR communication antenna based on a reconfigurable frequency-selective network (FSN). The antenna has a thickness of only 0.05 mm and a weight of only 0.5173 g, and it can withstand a deformation with a minimum bending radius of 10 mm, which can be conformally deployed in the UAV wing arm, solving the problem of difficult deployment of antennas for small UAVs. By loading two groups of PIN diodes, the antenna realizes the gating of three working areas, and varactor diodes are loaded in the microstrip line reconfigurable feeding network, realizing three different working modes, dual-frequency (2.4/5.8 GHz), single-frequency (3.35~5.3 GHz), and continuous frequency modulation (4.3~6.1 GHz), which is especially suitable for the frequency reconfigurable dynamic communication requirements in CR technology. In addition, even when conformal inside the UAV wing arm, the antenna can still realize three working modes (dual-frequency 2.4~3.6 GHz and 5.55~7.05 GHz, single-frequency 3.15~5.15 GHz, continuous frequency regulation 4.05~6 GHz), with a gain range of 3.45~4.77 dBi and a radiation efficiency of 67.2%~85.9%. This design provides an effective solution for the design of CR communication antennas based on small UAVs.

2. Design and Analysis of Proposed CR Antenna

The overall size of the antenna is 31.4 × 58 × 0.05 mm3, and its width is designed as half the circumference of a cylinder with a radius of 10 mm, aiming to achieve conformal fitting with the wing arm of small UAVs with a minimum bending radius of 10 mm. This antenna uses polyimide (PI) as the flexible substrate, which has a relative dielectric constant of 3.5 and a loss tangent of 0.008, and this material has the advantage of balanced mechanical and electrical performance [26].
In this work, frequency reconfigurability of the antenna is realized by loading PIN diodes and varactor diodes. Compared with other frequency reconfigurability methods, PIN diodes have nanosecond-level switching speed, enabling dynamic and rapid switching of communication frequencies [27]. On the other hand, the on-state equivalent resistance of PIN diodes is small, which reduces antenna loss; compared with other adjustable components, their cost is lower, which is conducive to large-scale deployment of UAV products. Varactor diodes, on the other hand, can achieve continuous frequency adjustment under certain conditions, making the frequency modulation mode more flexible [28].

2.1. Evolution of CR Communication Antenna Design

Figure 2a shows the design evolution process of the tri-mode frequency reconfigurable antenna designed in this work. The prototype of Antenna 1 (ANT1) is a monopole patch antenna, which is fed by a microstrip line. The size of the microstrip line is designed to be very close to 50 Ω to ensure good matching performance. Antenna 1 has dual-band operating characteristics, where operating Band 1 is 2.21 GHz to 3.76 GHz, and operating Band 2 is 5.1 GHz to 7.41 GHz, which can cover common applications such as WLAN (2.4/5.8 GHz) bands, as shown in the simulation results in Figure 3a.
To achieve frequency reconfigurability, the radiation structure of the antenna is split in this paper, and the original radiation unit is divided into upper and lower working areas (Part I, Part II), which are connected by a PIN diode and a 100 pF capacitor, as shown in Figure 2b. The function of the capacitor is to isolate the influence of DC voltage on the antenna. The PIN diode selected in this work is BAR50-02L from Infineon. It has the advantage of small size, and after being installed on the flexible substrate, it has little impact on the flexibility of the antenna. As shown in Figure 2c, when the PIN diode is forward-conducting, it is equivalent to a series of a 3 Ω resistor and a 0.4 nH inductor; in the off state, it can be regarded as a 0.1 pF capacitor in parallel with a 5 kΩ resistor and then in series with a 0.4 nH inductor; the bias circuit of the PIN diode is a DC power supply in series with a 1 kΩ current-limiting resistor and a 49 nH AC blocking inductor. When the PIN diode is in the on state, Antenna 2 is in Mode 1 (M1); at this time, both the upper and lower working areas are activated, and Antenna 2 still works in dual-frequency mode, with operating frequencies of 2.2 GHz to 3.65 GHz and 5.35 GHz to 6.45 GHz, respectively. When the PIN diode is in the off state, Antenna 2 enters Mode 2 (M2); at this time, only the lower part of the antenna is activated, and the operating frequency range is 3.45 GHz to 6.35 GHz. As shown in Figure 3b, the two operating modes of Antenna 2 can cover the full frequency band from 2.2 GHz to 6.45 GHz.
To improve the flexibility of dynamic frequency reconfigurability of the CR antenna, a reconfigurable frequency-selective network based on microstrip line ground slots is designed in Antenna 3. Combined with PIN diodes and varactor diodes, the equivalent size of the floor slots is adjusted to achieve continuous reconfigurability of the operating frequency. The detailed working principle of this structure will be introduced in the next section. This structure is finally integrated into the microstrip line of Antenna 2, which not only does not affect the original operating mode but also expands the frequency adjustment range of the antenna, improving its applicability in different application scenarios. The last part of Figure 2a clearly shows the classification of different working areas and PIN diode switch groups of Antenna 3, which is convenient for accurately describing the working state of this multi-mode antenna. Antenna 3 uses two groups of PIN diodes (PIN_A and PIN_B) to control the gating of the three working areas (Part I, Part II, Part III), thus realizing three different working modes, as shown in Table 1.
When both groups of diodes PIN_A and PIN_B are conducting, the antenna works in dual-band mode, with Part I and Part II activated and involved in operation. As shown in Figure 3c, in Mode 1, operating Band 1 covers 2.05 GHz to 3.9 GHz, and operating Band 2 covers 5.45 GHz to 6.55 GHz. When PIN_A is in the off state and PIN_B is conducting, the antenna works in single-band mode (Mode 2); at this time, Part I and Part III are in the inactive state, only Part II is involved in operation, and the operating frequency band is 3.4 GHz to 6.5 GHz. When both PIN_A and PIN_B are in the off state, the U-shaped slot network of Part III is activated. By adjusting the reverse bias voltage of the varactor diode, continuous adjustment of the operating frequency of the CR antenna is realized, which can achieve continuous frequency modulation from 4 GHz to 6.25 GHz, with a tuning ratio of 43.9%, as shown in Figure 3d.

2.2. Design of Reconfigurable Frequency-Selective Network Based on Microstrip Line

One of the cores of this work’s design is the reconfigurable frequency-selective network based on the microstrip line ground, and the appearance design of its structure is shown in Figure 4a. The main working characteristics of this frequency-selective network are as follows: (1) Embedded in the original microstrip structure. After Antenna 2 evolves into Antenna 3, the operating bandwidth of Antenna 2 is not significantly affected. (2) By adding varactor diodes, the antenna adds a working mode with continuously adjustable frequency. (3) Since this structure is connected in series in the feeder line, it will not change the radiation direction characteristics of the original Antenna 2.
The frequency-selective structure is constructed on the basis of the 50 Ω microstrip line size. The middle U-shaped slot is composed of three rectangular DC metal islands. The island in the center is placed in the center of the ground under the microstrip line structure. The upper side is connected to the grounding structure through three PIN diodes, and the lower side is connected to the ground through three 100 pF DC blocking capacitors. The central island is connected to the DC power supply through the cable of U2, and a 49 nH AC blocking inductor is connected in series on the connecting line of U2. At the same time, the via structure connects the upper and lower layers, so that this group of PIN diodes can be controlled by U2. A group of varactor diodes and DC blocking capacitors are, respectively, connected between the islands on both sides and the ground slots. The reverse bias voltage of the varactor diodes is adjusted through U1, so as to realize continuous adjustment of the slot capacitance characteristics through changes in their capacitance values.

2.2.1. Equivalent Circuit Analysis

The equivalent circuit diagrams of the reconfigurable frequency-selective structure are shown in Figure 4b–d. As can be seen from the schematic diagram above Figure 4b, the opening and closing of PIN_B control two different working states. A two-port network is used for analysis, where C1 is a 100 pF DC blocking capacitor located at the front end of Port 1; the switch state of PIN_B guides the direction of the radio frequency current passing through the network. When PIN_B is forward-conducting, the structure works in broadband mode, and the current hardly passes through the slot structure. Its working equivalent circuit is shown in Figure 4c, where L1 is the series equivalent inductance with a value of 0.06 nH; RPD is the forward conduction resistance of PIN_B with a value of 3 Ω; C4 is the parasitic capacitance with a size of 3 pF. When PIN_B is off, the current mainly passes through the ground slot frequency-selective network, and the structure works in narrowband continuous tuning mode. In this mode, the slot structure is equivalent to a group of frequency-selective networks, and the parameters of the varactor diode affect the band-pass characteristics of the frequency-selective structure.

2.2.2. Simulation of Surface Current and S-Parameters

To verify the effectiveness of the designed structure, we simulated its two-port network through HFSS (High Frequency Structure Simulator) and synchronously analyzed the frequency response of the equivalent circuit through ADS (Advanced Design System). The forward conduction voltage of the PIN diode BAR50-02L in the on state is 1 V. When the voltage provided by U2 to its network reaches 1 V, the PIN diode presents a low-resistance (conducting) state to the radio frequency signal, and at this time, the energy transmitted by the microstrip line hardly passes through the slot network. In this state, the surface current distribution at 5 GHz is shown in Figure 5a. It can be seen that the current energy bypasses the frequency-selective structure and mainly concentrates on the microstrip line, and the current near the slot structure is almost zero. In the conducting state, the S-parameters of the slot network are shown in Figure 5c, which shows that its operating bandwidth well covers the target frequency band of 2 GHz to 9 GHz, indicating that the structure works in broadband mode. Therefore, in this broadband mode, Antenna 3 can retain the original operating bandwidth of Antenna 2.
When the PIN diode is in the closed state, the network works in narrowband continuous tuning mode. In this mode, the slot structure is equivalent to a group of frequency-selective networks, and the parameters of the varactor diode affect the band-pass characteristics of the frequency-selective structure. The varactor diode selected in this work is model SMV2019-040LF produced by Skyworks, which has the advantages of low series equivalent resistance and low phase noise. Its capacitance adjustment range is from 2.2 pF to 0.3 pF. By adjusting the reverse bias voltage, the adjustable capacitance value CT of the varactor diode can be changed within this range. Figure 5b shows the surface current distribution when the resonant frequency is about 5 GHz when the adjustable capacitance value CT = 0.7 pF, and the S-parameter simulation results of the frequency-selective structure at this time are shown in Figure 5d. When the reverse bias voltage of the varactor diode is 0 V, CT = 2.2 pF, and the structure is in the cut-off state. When the reverse bias voltage gradually increases, CT gradually decreases. When CT = 1.5 pF, the resonant frequency is 3.8 GHz; as the reverse bias voltage increases, the operating frequency also gradually increases; when CT = 0.3 pF, the resonant frequency reaches 5.9 GHz. The tuning ratio of the structure reaches 43.3%. The frequency response of the ADS equivalent circuit is in good agreement with the HFSS results, and the tuning range (3.8~5.9 GHz) covers the Mode 2 frequency band (3.45~6.35 GHz) of Antenna 2, which proves the feasibility of realizing continuous adjustment of the operating frequency of Antenna 3 by integrating the floor slot structure into the microstrip line.

3. Results and Discussion

3.1. Fabrication and Performance Testing

To verify the actual performance of the designed antenna, we fabricated a multi-mode flexible antenna with the final optimized design. Figure 6a,b show the schematic diagrams of the fabricated antenna, with the front view and back view from left to right, respectively. We installed electronic control devices on the antenna and fabricated the DC bias circuit used in the experiment. The DC bias network for each control line consists of a 1 kΩ current-limiting resistor and a 49 nH RF choke inductor in series. The RF choke inductors and DC blocking capacitors in the bias network effectively isolate the DC sources, preventing any influence on the RF measurement results. The PIN_A group diode consists of two PIN diodes on the left and right, responsible for realizing the gating between Part I and Part II. The PIN_B group diode consists of three PIN diodes in the microstrip line ground slot, which function to control the gating of the U-shaped slot reconfigurable network in Part III. Two varactor diodes SMV2019-040LF are, respectively, deployed on the left and right sides of the slot. One of the major goals of the antenna design conformal to small UAVs is light weight. The measured bare weight of the antenna is only 0.5173 g, and its ultra-light characteristic makes it have little impact on the dynamic balance of the UAV after installation.
Figure 6c shows the connection diagram of the experimental system used for antenna performance testing. The antenna under test is placed in an anechoic chamber, and the DC power supply is connected to the DC bias network. Each DC power supply line is connected in series with a 1 kΩ current-limiting resistor and a 49 nH AC isolation inductor. In the experiment, the antenna is connected to a DC power supply through a bias network to provide the bias voltage required for different working modes. The antenna feed end is measured by a connected vector network analyzer (N5230C, Agilent, Santa Clara, CA, USA) to evaluate the antenna performance parameters. The antenna on the test platform is firmly fixed on foam supports with different bending radii Rb to ensure stability and accuracy during measurement.
Figure 7a,b show the simulation and test results of the reflection coefficients of the CR antenna in working Mode 1 and Mode 2 in the planar state, respectively. When the antenna is in working Mode 1, both PIN_A and PIN_B are in the on state, and the operating frequency band of the antenna is divided into two parts: Band 1 (2.25~3.55 GHz) and Band 2 (5.6~6.75 GHz). It can be seen from the figure that the reflection coefficient curves of the antenna in these two frequency bands change smoothly, and the measured data are in good agreement with the simulation data, indicating that the radiation characteristics of the antenna in this mode are consistent with the design simulation results. The results show that the antenna performs stably in these two frequency bands, suitable for applications such as WLAN (2.4/5.8 GHz) dual-band operation. In working Mode 2, PIN_A of the antenna is off, while PIN_B remains on, which leads to a change in the operating frequency band. In this mode, the reflection coefficient of the antenna is mainly concentrated around 3.35~5.3 GHz, suitable for applications such as WiMAX, 5G Sub 6G, etc. Compared with the simulation results, the measured operating bandwidth is reduced, which is due to errors caused by device welding and DC bias networks.
Figure 7c shows the test results of working Mode 3. Compared with the simulation results, the tuning ratio in this mode is reduced. When the reverse bias voltage is 0 V, the antenna is in the “off” state; when the voltage is 2 V, the operating frequency is 4.3 GHz; as the voltage increases to 6 V, the operating frequency rises to 5.6 GHz, and the tuning ratio at this time is 26.3%. When the voltage is further increased to 10 V, the operating frequency is 6.1 GHz, and the tuning ratio is the largest, reaching 34.6%. In this mode, the operating frequency can be continuously adjusted, with a tuning range of 4.3 GHz to 6.1 GHz. The measured results show that the three working modes are still well achieved.

3.2. Performance Evaluation of Antenna Conformal to UAV

The antenna designed in this work aims to achieve conformal installation with the wing arm of small UAVs, so it is necessary to evaluate its performance in the bent conformal state. For a typical small quadrotor UAV configuration, the minimum bending radius Rb of its wing arm is about 10 mm. First, we conform the antenna to cylinders made of foam materials with different Rb values to evaluate its multi-mode working characteristics in the bent state.
Figure 8a shows the S-parameter test results of Mode 1 with a bending radius Rb = 10 mm in the bent state. Band 1 (2.4~3.6 GHz) and Band 2 (5.55~7.05 GHz) can still cover the target application frequency bands (WLAN 2.4G/5.8G). Figure 8b shows the changes in operating bandwidth (BW) and center frequency (f0) of Antenna Mode 1 as the bending radius gradually decreases from the planar state. The f0 of Band 1 and Band 2 increases from 2.9 GHz and 6.175 GHz in the planar state to 3 GHz and 6.3 GHz, respectively, with a change rate of 3.4% and 2.0%.
Figure 8c,d show the performance changes of the antenna in Mode 2 under different bending conformal radii. The operating bandwidth and center frequency change from 3.35~5.3 GHz and 4.325 GHz in the planar state to 3.15~5.15 GHz and 4.15 GHz, with a change rate of 4.0%. Figure 8e,f show that under the continuous tuning of Mode 3, as the bending radius decreases, the tuning range changes from 4.3~6.1 GHz in the planar state to 4.05~6.0 GHz, while the narrowband operating bandwidth does not change significantly. The measured results show that the S-parameter change rate of the antenna in the conformal state is ≤4%, and multiple operating bandwidths are well preserved, proving that it has strong working stability in the bent state.
Figure 9a,b show schematic diagrams of the anechoic chamber test arrangement of the antenna in this work on a small UAV platform. The antenna is embedded inside the wing arm of the UAV to ensure that the antenna can maximize the use of space during flight and optimize aerodynamic performance. The underlying bias network provides the necessary bias voltage for the antenna to support its multi-working modes. In actual use, these bias modules will be integrated into the antenna control motherboard, which can further improve the compactness and reliability of the overall system. The radiation efficiency and measured gain of the antenna in three different working modes in the UAV conformal state are shown in Figure 9c,d. In the dual-band working state of Mode 1, the maximum gain of the antenna around 2.5 GHz is 3.7 dBi, which is 0.1 dB higher than that in the planar state; the gain at 6 GHz reaches 4.15 dBi, but the radiation efficiency decreases slightly, about 68.15%. In Mode 2, the maximum gain at 4 GHz is 4.77 dBi, and the radiation efficiency is 82.92%. In Mode 3, the gain at 5 GHz is 4.34 dBi, and the radiation efficiency is 69.58%. In the conformal state of the three modes, the antenna gain does not decrease compared with the planar state, while the radiation efficiency decreases slightly. Compared with traditional planar antennas, the frequency-selective network introduced in this design has unavoidable losses, so the antenna radiation efficiency is reduced.
Figure 10a,d show the radiation patterns of the antenna in the planar state and conformal state. The xoz plane presents an “O” shape, while the yoz plane presents an “8” shape. The antenna maintains good directional stability in different modes. Compared with the planar state, the difference in size between the main lobe and back lobe at 6 GHz in Mode 1 and 5 GHz in Mode 3 is obvious. The reason for this change is that after the radiation structure is deformed, the antenna radiation energy shifts to one side of the main lobe, resulting in a narrower beamwidth of the main lobe and increased gain, while the gain in the back lobe direction decreases, and the half-power beamwidth slightly widens. This indicates that the radiation characteristics of the antenna in the conformal state are significantly affected by the deformation of the antenna structure, especially the energy distribution between the main lobe and the back lobe changes, thus affecting the radiation performance.
Table 2 compares the performance of the antenna proposed in this work with similar frequency-reconfigurable antennas. The results show that this antenna is superior in several aspects. First, it has three working modes, showing strong frequency reconfiguration capability. Secondly, it not only supports continuous frequency adjustment but also adopts a flexible design, improving the flexibility and adaptability in practical applications. In terms of tuning ratio, the simulation result reaches 43.9%, and the measured values are 34.6% (planar state) and 38.8% (conformal state), which are the highest among the compared antennas, indicating that it has a wider dynamic frequency adjustment range. The peak gain of the antenna is up to 4.77 dBi, and the radiation efficiency in multi-mode remains 66.9~85.9% (planar state) and 67.2~82.9% (bent state). The antenna volume is only 31.4 × 58 × 0.05 mm3, with a thickness of only 0.05 mm, suitable for space-constrained environments. In addition, the bare weight of the antenna is only 0.5173 g, which can be conformal to the UAV wing arm without affecting the aerodynamic shape, thereby reducing the difficulty of UAV structural design.

4. Conclusions

This paper proposes a tri-mode frequency-reconfigurable flexible antenna loaded with a U-shaped ground slot frequency-selective structure. Through the combination of PIN diodes and varactor diodes, it realizes dual-band (2.25~3.55 GHz and 5.6~6.75 GHz), single-band (3.35~5.3 GHz), and continuous frequency modulation (4.3~6.1 GHz), covering mainstream communication frequency bands such as WLAN (2.4/5.2/5.8 GHz), WiMAX (2.5/3.5/5.5 GHz), and 5G NR (n41/n77/n78/n79). The antenna has a size of only 31.4 × 58 × 0.05 mm3 and a weight of 0.5173 g. It adopts a flexible material to achieve a low-profile configuration and can be conformal to the UAV wing arm. The measured results show that the antenna gain is 3.45~4.77 dBi in different modes, and the radiation efficiency reaches 66.9~85.94%. It maintains stable performance in conformal states with various bending radii, with a frequency offset of <4%. This design excels in air resistance reduction, being lightweight, frequency adjustment, and cost control; has multi-mode and multi-band dynamic communication capabilities; is particularly suitable for UAV CR communication; and can effectively improve the integration and deployment flexibility of small UAV CR antennas.

Author Contributions

Conceptualization, T.B. and M.Z.; methodology, T.B. and Z.H.; hardware, T.B.; validation, T.B. and Z.H.; formal analysis, T.B., Y.Z. and G.Y.; investigation, Y.Q., J.W. and Y.L.; resources, G.Y. and M.Z.; data curation, T.B. and Y.L.; writing—original draft preparation, T.B.; writing—review and editing, M.Z. and H.-M.Z.; visualization, Y.Q., J.W. and Y.Z.; supervision, M.Z.; project administration, M.Z. and H.Z.; funding acquisition, M.Z., H.Z. and H.-M.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key R&D Program of China (Grant No. 2023YFF0616800), the Zhejiang Provincial Natural Science Foundation of China (Grant Nos. LZ23A020002, and LZ22A020006), the National Natural Science Foundation of China (Grant No. 12072323), and the Fundamental Research Funds for the Provincial Universities of Zhejiang (Grant No. 2023YW07).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Wei, Q.; Li, R.; Bai, W.; Han, Z. Multi-UAV-Enabled Energy-Efficient Data Delivery for Low-Altitude Economy: Joint Coded Caching, User Grouping, and UAV Deployment. IEEE Internet Things J. 2025, 12, 27519–27532. [Google Scholar] [CrossRef]
  2. Li, Y.; Wang, W.; Zhang, C.; Huang, Y.; Niyato, D. Joint UAV Deployment and Space-Time-Frequency Resource Allocation for Low-Altitude Economy. IEEE Wirel. Commun. Lett. 2025, 1–1. [Google Scholar] [CrossRef]
  3. Saleem, Y.; Rehmani, M.H.; Zeadally, S. Integration of Cognitive Radio Technology with unmanned aerial vehicles: Issues, opportunities, and future research challenges. J. Netw. Comput. Appl. 2015, 50, 15–31. [Google Scholar] [CrossRef]
  4. Hilal, W.; Gadsden, S.A.; Yawney, J. Cognitive Dynamic Systems: A Review of Theory, Applications, and Recent Advances. Proc. IEEE 2023, 111, 575–622. [Google Scholar] [CrossRef]
  5. Zhao, X.; Riaz, S.; Geng, S. A Reconfigurable MIMO/UWB MIMO Antenna for Cognitive Radio Applications. IEEE Access 2019, 7, 46739–46747. [Google Scholar] [CrossRef]
  6. Wu, J.; Li, P.; Bao, J.; Ge, H. Quick Multiband Spectrum Sensing for Delay-Constraint Cognitive UAV Networks. IEEE Sens. J. 2022, 22, 19088–19100. [Google Scholar] [CrossRef]
  7. Asadallah, F.A.; Khalek, H.A.; Modad, B.A.A.; Hosn, J.A.; Costantine, J.; Kanj, R.; Tawk, Y. Spectrum-Aware Compact Reconfigurable UHF Antenna for Interweave Cognitive Radio. IEEE Open J. Antennas Propag. 2023, 4, 69–73. [Google Scholar] [CrossRef]
  8. Sadiq, A.M.; Liu, M.; Zhang, X.; Luo, Y.; Chen, Y.; Ma, K. Single-Motor Controlled Mechanically Frequency Reconfigurable Unidirectional Antenna Array With Stable Radiation Patterns. IEEE Open J. Antennas Propag. 2024, 5, 1773–1785. [Google Scholar] [CrossRef]
  9. McMichael, I.T. A Mechanically Reconfigurable Patch Antenna With Polarization Diversity. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 1186–1189. [Google Scholar] [CrossRef]
  10. Tawk, Y.; Costantine, J.; Christodoulou, C.G. A frequency reconfigurable rotatable microstrip antenna design. In Proceedings of the 2010 IEEE Antennas and Propagation Society International Symposium, Toronto, Canada, 11–17 July 2010; pp. 1–4. [Google Scholar] [CrossRef]
  11. Kovitz, J.M.; Rajagopalan, H.; Rahmat-Samii, Y. Design and Implementation of Broadband MEMS RHCP/LHCP Reconfigurable Arrays Using Rotated E-Shaped Patch Elements. IEEE Trans. Antennas Propag. 2015, 63, 2497–2507. [Google Scholar] [CrossRef]
  12. Andy, A.; Alizadeh, P.; Rajab, K.Z.; Kreouzis, T.; Donnan, R. An optically-switched frequency reconfigurable antenna for cognitive radio applications. In Proceedings of the 2016 10th European Conference on Antennas and Propagation (EuCAP), Davos, Switzerland, 10–15 April 2016; pp. 1–4. [Google Scholar] [CrossRef]
  13. Dang, Q.H.; Nguyen-Trong, N.; Fumeaux, C.; Chen, S.J. Tuning-Range Extension Strategies for Varactor-Based Frequency-Reconfigurable Antennas. IEEE Open J. Antennas Propag. 2023, 4, 1087–1094. [Google Scholar] [CrossRef]
  14. Dildar, H.; Althobiani, F.; Ahmad, I.; Khan, W.U.R.; Ullah, S.; Mufti, N.; Ullah, S.; Muhammad, F.; Irfan, M.; Glowacz, A. Design and Experimental Analysis of Multiband Frequency Reconfigurable Antenna for 5G and Sub-6 GHz Wireless Communication. Micromachines 2021, 12, 32. [Google Scholar] [CrossRef] [PubMed]
  15. Nguyen-Trong, N.; Fumeaux, C. Tuning Range and Efficiency Optimization of a Frequency-Reconfigurable Patch Antenna. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 150–154. [Google Scholar] [CrossRef]
  16. Subbaraj, S.; Kanagasabai, M.; Alsath, M.G.N.; Palaniswamy, S.K.; Kingsly, S.; Kulandhaisamy, I.; Shrivastav, A.K.; Natarajan, R.; Meiyalagan, S. A Compact Frequency-Reconfigurable Antenna with Independent Tuning for Hand-Held Wireless Devices. IEEE Trans. Antennas Propag. 2020, 68, 1151–1154. [Google Scholar] [CrossRef]
  17. Paramayudha, K.; Chen, S.J.; Kaufmann, T.; Withayachumnankul, W.; Fumeaux, C. Triple-Band Reconfigurable Low-Profile Monopolar Antenna with Independent Tunability. IEEE Open J. Antennas Propag. 2020, 1, 47–56. [Google Scholar] [CrossRef]
  18. Jin, G.; Deng, C.; Xu, Y.; Yang, J.; Liao, S. Differential Frequency-Reconfigurable Antenna Based on Dipoles for Sub-6 GHz 5G and WLAN Applications. IEEE Antennas Wirel. Propag. Lett. 2020, 19, 472–476. [Google Scholar] [CrossRef]
  19. Ma, T.; Dang, Q.H.; Fumeaux, C.; Nguyen-Trong, N. A Low-Profile Frequency-reconfigurable Filtering Patch Antenna. IEEE Trans. Antennas Propag. 2024, 72, 2998–3003. [Google Scholar] [CrossRef]
  20. Shereen, M.K.; Khattak, M.I. A Hybrid Reconfigurability Structure and Improved Gain Characteristics for a Novel 5G Monopole Antenna for Future Mobile Communication. Wirel. Pers. Commun. 2022, 123, 1841–1853. [Google Scholar] [CrossRef]
  21. Nazir, I.; Mukhtar, F.; Rana, I.E. A Miniaturized Frequency Reconfigurable Substrate Integrated Waveguide Antenna for Frequency Jamming Applications. IEEE Access 2024, 12, 127805–127824. [Google Scholar] [CrossRef]
  22. Nosrati, M.; Jafargholi, A.; Pazoki, R.; Tavassolian, N. Broadband Slotted Blade Dipole Antenna for Airborne UAV Applications. IEEE Trans. Antennas Propag. 2018, 66, 3857–3864. [Google Scholar] [CrossRef]
  23. Carpenter, A.; Lawrence, J.A.; Ghail, R.; Mason, P.J. The Development of Copper Clad Laminate Horn Antennas for Drone Interferometric Synthetic Aperture Radar. Drones 2023, 7, 215. [Google Scholar] [CrossRef]
  24. Dubrovka, F.F.; Piltyay, S.; Movchan, M.; Zakharchuk, I. Ultrawideband Compact Lightweight Biconical Antenna with Capability of Various Polarizations Reception for Modern UAV Applications. IEEE Trans. Antennas Propag. 2023, 71, 2922–2929. [Google Scholar] [CrossRef]
  25. Burr, R.; Schartel, M.; Mayer, W.; Walter, T.; Waldschmidt, C. Lightweight Broadband Antennas for UAV based GPR Sensors. In Proceedings of the 2018 15th European Radar Conference (EuRAD), Madrid, Spain, 26–28 September 2018; pp. 245–248. [Google Scholar] [CrossRef]
  26. Alhamad, R.; Almajali, E.; Mahmoud, S. Electrical Reconfigurability in Modern 4G, 4G/5G and 5G Antennas: A Critical Review of Polarization and Frequency Reconfigurable Designs. IEEE Access 2023, 11, 29215–29233. [Google Scholar] [CrossRef]
  27. Zhou, J.; Cai, J.; Chen, J.X. Contactless Varactor-Loaded Bandwidth-Enhanced Frequency-Reconfigurable Patch Antenna. IEEE Antennas Wirel. Propag. Lett. 2024, 23, 2456–2460. [Google Scholar] [CrossRef]
  28. Khaleel, H.R.; Al-Rizzo, H.M.; Rucker, D.G.; Mohan, S. A Compact Polyimide-Based UWB Antenna for Flexible Electronics. IEEE Antennas Wirel. Propag. Lett. 2012, 11, 564–567. [Google Scholar] [CrossRef]
  29. Deng, J.; Hou, S.; Zhao, L.; Guo, L. Wideband-to-narrowband tunable monopole antenna with integrated bandpass filters for UWB/WLAN applications. IEEE Antennas Wirel. Propag. Lett. 2017, 16, 2734–2737. [Google Scholar] [CrossRef]
  30. Zhang, Z.; Pan, Z. Time domain performance of reconfigurable filter antenna for IR-UWB, WLAN, and WiMAX applications. Electronics 2019, 8, 1007. [Google Scholar] [CrossRef]
  31. Shome, P.P.; Khan, T.; Koul, S.K.; Antar, Y.M. Compact UWB-to-C band reconfigurable filtenna based on elliptical monopole antenna integrated with bandpass filter for cognitive radio systems. IET Microw. Antennas Propag. 2020, 14, 1079–1088. [Google Scholar] [CrossRef]
  32. Ibrahim, A.A.; Mohamed, H.A.; Rizo, A.R.D.; Parra-Michel, R.; Aboushady, H. Tunable filtenna with DGS loaded resonators for a cognitive radio system based on an SDR transceiver. IEEE Access 2022, 10, 32123–32131. [Google Scholar] [CrossRef]
  33. Liu, B.-J.; Qiu, J.-H.; Lan, S.-C.; Li, G.-Q. A wideband-to-narrowband rectangular dielectric resonator antenna integrated with tunable bandpass filter. IEEE Access 2019, 7, 61251–61258. [Google Scholar] [CrossRef]
Jlpea 15 00051 g001
Figure 1. Schematic diagram of CR communication technology.
Figure 1. Schematic diagram of CR communication technology.
Jlpea 15 00051 g001
Jlpea 15 00051 g002
Figure 2. (a) Evolution of antenna structure design (unit: mm). (b) Integration of PIN diodes and bias network. (c) Equivalent circuit of PIN diode.
Figure 2. (a) Evolution of antenna structure design (unit: mm). (b) Integration of PIN diodes and bias network. (c) Equivalent circuit of PIN diode.
Jlpea 15 00051 g002
Jlpea 15 00051 g003
Figure 3. Simulated S-parameter results. (a) ANT1; (b) ANT2; (c) ANT3_M1&M2; (d) ANT3_M3.
Figure 3. Simulated S-parameter results. (a) ANT1; (b) ANT2; (c) ANT3_M1&M2; (d) ANT3_M3.
Jlpea 15 00051 g003
Jlpea 15 00051 g004
Figure 4. (a) Frequency-selective network (FSN) structure (unit: mm). (b) Equivalent circuit of FSN. (c) Broadband mode. (d) Continuous tuning mode.
Figure 4. (a) Frequency-selective network (FSN) structure (unit: mm). (b) Equivalent circuit of FSN. (c) Broadband mode. (d) Continuous tuning mode.
Jlpea 15 00051 g004
Jlpea 15 00051 g005
Figure 5. (a) Surface current distribution with PIN_B: ON. (b) Surface current distribution with PIN_B: OFF. (c) S-parameters under PIN_B ON state. (d) S-parameters under PIN_B OFF state.
Figure 5. (a) Surface current distribution with PIN_B: ON. (b) Surface current distribution with PIN_B: OFF. (c) S-parameters under PIN_B ON state. (d) S-parameters under PIN_B OFF state.
Jlpea 15 00051 g005
Jlpea 15 00051 g006
Figure 6. (a) Front view. (b) Back view. (c) Connection schematic of experimental setup for reflection coefficient.
Figure 6. (a) Front view. (b) Back view. (c) Connection schematic of experimental setup for reflection coefficient.
Jlpea 15 00051 g006
Jlpea 15 00051 g007
Figure 7. Measured S-parameters of the antenna under multiple modes (flat state). (a) Mode 1; (b) Mode 2; (c) Mode 3.
Figure 7. Measured S-parameters of the antenna under multiple modes (flat state). (a) Mode 1; (b) Mode 2; (c) Mode 3.
Jlpea 15 00051 g007
Jlpea 15 00051 g008
Figure 8. S-parameter measurement under bent condition. (a) M1: Rb = 10 mm; (b) M1: bandwidth and center frequency (f0) vs. Rb; (c) M2: Rb = 10 mm; (d) M2: bandwidth and f0 vs. Rb; (e) M3: Rb = 10 mm; (f) M3: tuning range and f0 vs. Rb.
Figure 8. S-parameter measurement under bent condition. (a) M1: Rb = 10 mm; (b) M1: bandwidth and center frequency (f0) vs. Rb; (c) M2: Rb = 10 mm; (d) M2: bandwidth and f0 vs. Rb; (e) M3: Rb = 10 mm; (f) M3: tuning range and f0 vs. Rb.
Jlpea 15 00051 g008
Jlpea 15 00051 g009
Figure 9. (a) Conformal mounting. (b) Measurement setup. (c) Measured radiation efficiency. (d) Measured gain.
Figure 9. (a) Conformal mounting. (b) Measurement setup. (c) Measured radiation efficiency. (d) Measured gain.
Jlpea 15 00051 g009
Jlpea 15 00051 g010
Figure 10. Typical radiation patterns at (a) M1: 2.5 GHz; (b) M1: 6 GHz; (c) M2: 4 GHz; (d) M3: 5 GHz.
Figure 10. Typical radiation patterns at (a) M1: 2.5 GHz; (b) M1: 6 GHz; (c) M2: 4 GHz; (d) M3: 5 GHz.
Jlpea 15 00051 g010
Table 1. Description of multiple operating modes of the CR antenna.
Table 1. Description of multiple operating modes of the CR antenna.
Mode TypeApplicationsFrequency Range (GHz)PIN_APIN_BPart IPart IIPart III
M1: Dual-bandWLAN (2.4G/5.8G)
5G NR (n41/n77/n78)
ISM (2.45G/5.8G)		Band 1: 2.09–3.9
Band 2: 5.45–6.55		ONONONONOFF
M2: Single-bandWiMAX (3.5G)3.4–6.5OFFONOFFONOFF
M3: Continuous tuningWLAN (5.2G)
5G NR (n79)		4.0–6.25OFFOFFOFFONON
Table 2. Reconfigurable antenna performance comparison with related literature.
Table 2. Reconfigurable antenna performance comparison with related literature.
ReferenceSubstrateModes SupportedTuning Ratio (%)Peak Gain (dBi)Efficiency (%)Size (mm)
[29]FR-4202.150–8245 × 40 × 2
[30]FR-430<2.0NA80 × 90 × 1.6
[31]FR-4202.0>7642 × 39 × 1.6
[32]Roger 4003C129.8NANA60 × 80 × 0.813
[19]F4B221.910>65114 × 114 × 3
[33]FR-4215.34.69NA24 × 32 × 5
this workPI3flat: 34.6
bent: 38.8		flat: 4.57
bent: 4.77		flat: 66.9–85.9
bent: 67.2–82.9		31.4 × 58 × 0.05
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Bao, T.; Zhu, M.; He, Z.; Zhang, Y.; Yu, G.; Qiu, Y.; Wang, J.; Li, Y.; Zhu, H.; Zhou, H.-M. Design of Tri-Mode Frequency Reconfigurable UAV Conformal Antenna Based on Frequency Selection Network. J. Low Power Electron. Appl. 2025, 15, 51. https://doi.org/10.3390/jlpea15030051

AMA Style

Bao T, Zhu M, He Z, Zhang Y, Yu G, Qiu Y, Wang J, Li Y, Zhu H, Zhou H-M. Design of Tri-Mode Frequency Reconfigurable UAV Conformal Antenna Based on Frequency Selection Network. Journal of Low Power Electronics and Applications. 2025; 15(3):51. https://doi.org/10.3390/jlpea15030051

Chicago/Turabian Style

Bao, Teng, Mingmin Zhu, Zhifeng He, Yi Zhang, Guoliang Yu, Yang Qiu, Jiawei Wang, Yan Li, Haibin Zhu, and Hao-Miao Zhou. 2025. "Design of Tri-Mode Frequency Reconfigurable UAV Conformal Antenna Based on Frequency Selection Network" Journal of Low Power Electronics and Applications 15, no. 3: 51. https://doi.org/10.3390/jlpea15030051

APA Style

Bao, T., Zhu, M., He, Z., Zhang, Y., Yu, G., Qiu, Y., Wang, J., Li, Y., Zhu, H., & Zhou, H.-M. (2025). Design of Tri-Mode Frequency Reconfigurable UAV Conformal Antenna Based on Frequency Selection Network. Journal of Low Power Electronics and Applications, 15(3), 51. https://doi.org/10.3390/jlpea15030051

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop