In the rapidly evolving field of remote sensing, the integration of synthetic aperture radar (SAR) systems with unmanned aerial vehicles (UAVs) presents a unique set of challenges and opportunities. The inherent limitations of UAV drones, particularly their finite payload capacity, necessitate the development of highly compact, lightweight, and efficient antenna systems. This is especially critical for low-frequency SAR systems, such as those operating in the L-band, where the wavelength inherently dictates a larger antenna footprint. We present our work on a novel slot-coupled dual-polarized microstrip patch antenna designed specifically for UAV-borne SAR applications.
The principal challenge we addressed was the miniaturization and weight reduction of the antenna without compromising its electrical performance. Traditional microstrip patch antennas, while offering advantages like low profile and ease of conformability, suffer from narrow impedance bandwidths, typically in the range of 0.7% to 7%. This limitation renders them unsuitable for high-resolution SAR systems that demand wide operational bandwidths for range resolution. Furthermore, the requirement for dual-polarization capability, which is essential for fully polarimetric SAR applications, adds another layer of complexity, particularly in maintaining high isolation between the two orthogonal ports.
Our proposed antenna overcomes these limitations through a combination of innovative design techniques and advanced materials. The core of the design is an H-shaped slot-coupled feeding mechanism integrated with a dual-layer stacked patch radiator. This configuration creates a multi-resonant structure that significantly broadens the operational bandwidth. The antenna is designed as a 1×2 linear array with an overall dimension of 375 mm x 150 mm x 48 mm, as detailed in the table of design parameters. The inter-element spacing is set at 187.5 mm, a value optimized to balance gain and the suppression of grating lobes.
The operational principle of the antenna is grounded in a multi-layered architecture. Two 50-ohm coaxial feed lines are connected to a microstrip feed network, which is designed to equally split power to two orthogonal H-shaped slots. The electric fields radiated from these slots sequentially couple energy to the two layers of stacked square patches. These square patches support both TM01 and TM10 modes, which are orthogonal to each other, thereby realizing the dual-polarization functionality. The three distinct resonant structures—the H-shaped slot, the lower patch, and the upper patch—are tuned to provide a wide, multi-point impedance match. The H-shaped slot primarily governs the lower frequency response, while the two patches control the mid and high-frequency characteristics, respectively. To enhance port isolation, the two H-shaped slots are arranged orthogonally, and the feed network is physically isolated from the radiating patches by a honeycomb core layer, minimizing parasitic coupling.
The equivalent circuit of a slot-coupled microstrip antenna provides a powerful framework for understanding its impedance characteristics. The input impedance, \( Z_{in} \), can be modeled as a function of the coupling coefficients, patch admittance, and slot admittance. The general formulation for the input impedance of the proposed antenna is given by the following equation, where \( n_1 \) is the coupling coefficient between the slot and the radiating patch, \( n_2 \) is the coupling coefficient between the feedline and the slot, \( Y_p \) is the admittance of the patch, \( Y_a \) is the admittance of the slot, \( Z_{0m} \) is the characteristic impedance of the feedline, and \( \beta_m \) is the phase constant along the stub of length \( S \):
$$
Z_{in} = \frac{n_2^2}{n_1^2 Y_p + Y_a} – j Z_{0m} \cot(\beta_m S)
$$
This equation highlights how the input impedance can be precisely tuned by adjusting the physical dimensions of the coupling slot and the stub length to achieve a wideband impedance match at both ports.
The key structural parameters that define the antenna’s performance are systematically listed in the following table. These values were the result of an extensive optimization process using full-wave electromagnetic simulation software. The parameters are critical for setting the resonant frequencies, coupling levels, and overall impedance bandwidth of the antenna.
| Parameter | Value (mm) | Parameter | Value (mm) |
|---|---|---|---|
| \( L_1 \) | 27.00 | \( L_{s1} \) | 9.00 |
| \( L_2 \) | 6.00 | \( L_{s2} \) | 7.62 |
| \( L_3 \) | 6.00 | \( S \) | 40.00 |
| \( L_4 \) | 41.00 | \( w_{11} \) | 71.00 |
| \( L_5 \) | 7.33 | \( w_{12} \) | 71.00 |
| \( L_6 \) | 31.77 | \( w_{22} \) | 83.00 |
| \( L_7 \) | 39.54 | \( w_{21} \) | 84.00 |
| \( L_8 \) | 7.33 |
A pivotal aspect of our design is the material selection, which is driven by the stringent weight constraints of UAV drones. We replaced traditional metallic substrates with a composite structure that utilizes Kevlar fabric impregnated with resin. This material offers exceptional strength-to-weight and stiffness-to-weight ratios, making it ideal for airborne applications. The two radiating patches are etched onto thin polyimide films and surrounded by this Kevlar composite. The air gaps between the patches, the feed layer, and the ground plane are filled with lightweight honeycomb cores (both aluminum and paper), which provide the necessary structural integrity to withstand the mechanical stresses of flight while maintaining an extremely low areal density of 12.0 kg/m². This multi-layered, lightweight construction is a key innovation that allows the antenna to be integrated onto UAV drones without significantly impacting their endurance or flight altitude. The integrated metal cavity backing the structure further enhances performance by suppressing back-lobe radiation, thereby increasing the directivity and reducing interference.
The performance of the fabricated antenna was validated through extensive measurements, which showed excellent agreement with the simulation results. The measured impedance bandwidth (VSWR < 2.0) was found to be 35.38%, covering a frequency range from 1.05 GHz to 1.51 GHz. This wide bandwidth is crucial for achieving high-range resolution in SAR imaging systems. The measured port-to-port isolation was greater than 33.07 dB over the entire operational band, demonstrating the effectiveness of the orthogonal slot arrangement and the multi-layer architecture in suppressing cross-coupling. This high isolation is a fundamental requirement for dual-polarized SAR systems, as it ensures that the signals received at each port are truly orthogonal and independent, leading to more accurate polarimetric measurements. The peak gain of the antenna was measured at 9.94 dBi, which is a significant improvement over several state-of-the-art designs.
Radiation pattern measurements at the center frequency of 1.3 GHz further confirm the antenna’s suitability for SAR. The antenna exhibits a high front-to-back ratio of -15 dB, indicating excellent directivity. The cross-polarization discrimination (XPD) is better than -29.81 dB at the boresight and remains at a similar level across the main beam for both azimuth and elevation planes. The measured patterns over the frequency band from 1.1 GHz to 1.5 GHz are stable, with no significant squinting of the main beam, which is essential for consistent image formation in a moving platform like UAV drones.
A comprehensive comparison of our antenna with other related works in recent literature is presented in the table below. This comparison highlights our antenna’s balanced performance in terms of bandwidth, gain, isolation, and profile, which is a direct result of the integrated design approach.
| Reference | Polarization | Gain (dBi) | Frequency (GHz) | Isolation (dB) | Thickness (\(\lambda_0\)) |
|---|---|---|---|---|---|
| [24] | Single | 3.48 | 3.70 – 4.30 | – | 0.260 |
| [25] | Single | 2.20 | 3.02 – 3.26 | – | 0.520 |
| [19] | Dual | 4.12 | 2.88 – 3.12 | 25 | 0.031 |
| [20] | Dual | 7.20 | 27.20 – 28.80 | 25 | 0.076 |
| [21] | Dual | 4.90 | 4.85 – 4.95 | 20 | 0.057 |
| This Work | Dual | 9.94 | 1.05 – 1.52 | >32 | 0.208 |
The formula for the realized gain of the antenna array, which is a key performance indicator in SAR, can be expressed in terms of its effective area and efficiency. The gain \( G \) is related to the effective aperture area \( A_e \) and the wavelength \( \lambda \) by:
$$
G = \frac{4\pi A_e}{\lambda^2}
$$
For our array, the effective area is a function of the physical aperture size, the aperture efficiency \( \epsilon_{ap} \), and the element pattern. The calculated gain of 9.94 dBi at 1.3 GHz is consistent with the physical dimensions and measured efficiency.
The ultimate validation of our antenna’s design came from its integration into a complete L-band dual-polarized UAV-borne SAR system. The antenna was successfully mounted on a UAV platform with its array axis aligned with the flight direction. During two flight-test campaigns, the SAR system was used to acquire high-quality images of various terrains, including urban areas and farmland. The resulting SAR images exhibited excellent focus and clarity, with clear delineation of building structures and agricultural fields. The successful acquisition of these images proves that the antenna’s performance is not just a laboratory result but is robust and reliable in a real-world operational environment. The lightweight nature of the antenna, enabled by the use of Kevlar and honeycomb materials, was crucial for this integration, allowing the UAV drones to carry the payload without exceeding their operational limits.
In conclusion, we have successfully designed, fabricated, and flight-tested a novel L-band slot-coupled dual-polarized microstrip patch antenna specifically tailored for the demanding requirements of UAV-borne SAR systems. The key to its success lies in the synergistic combination of a multi-resonant H-slot and dual-patch structure, which provides a wide bandwidth and high isolation, and an advanced lightweight composite construction using Kevlar and honeycomb cores, which addresses the critical payload constraints of UAV drones. The antenna achieves a measured bandwidth of 35.38%, an isolation of over 33 dB, and a peak gain of 9.94 dBi, with a profile of only 0.208 \(\lambda_0\). While the current design has some limitations, such as out-of-band interference at 2.4 GHz, future work will focus on incorporating integrated filtering techniques and scaling the design for larger arrays for higher-resolution, space-borne, or long-endurance UAV missions. This antenna represents a significant step forward in enabling advanced, compact, and lightweight SAR payloads for the rapidly growing fleet of operational UAV drones.