Our research team has dedicated significant effort to addressing the challenges inherent in designing high-performance antennas for unmanned aerial vehicle (UAV) synthetic aperture radar (SAR) systems. The limited payload capacity of a typical UAV drone imposes stringent constraints on the size and weight of all onboard equipment. For low-frequency SAR systems, such as those operating in the L-band, the inherently large dimensions of the antenna pose a formidable obstacle to achieving the necessary miniaturization and light-weight design. To overcome these challenges, we have designed, simulated, fabricated, and tested a novel dual-polarized L-band microstrip patch antenna based on an H-shaped slot-coupled feeding mechanism. Our design specifically caters to the rigorous demands of UAV drone-based SAR imaging, successfully achieving wide bandwidth, high isolation, low profile, and reduced weight.
The core of our antenna design lies in its ability to reconcile the conflicting requirements of broad impedance bandwidth, high port isolation, and compact physical dimensions. We achieved this through a synergistic combination of advanced feeding techniques and multi-layered structural design. The H-shaped slot-coupled feed, in conjunction with two stacked radiating patches, effectively expands the operational bandwidth. This configuration leverages the coupling between the feeding microstrip line, the slot, and the two patches to create multiple resonant modes, which coalesce to form a wide impedance bandwidth. The dual-polarized operation is realized by incorporating two orthogonal H-shaped slots, each fed by a separate microstrip network, which inherently enhances the isolation between the two polarization ports.
In our pursuit of a solution suitable for UAV drone integration, we prioritized both electromagnetic performance and mechanical robustness. The antenna’s architectural design is built upon a multi-layer structure: a ground plane, a microstrip feed network layer with integrated H-shaped slots, two stacked radiating patches, and a supporting back cavity. The material selection was critical for weight reduction without compromising structural integrity. We replaced traditional heavy substrates with a sandwich structure composed of Kevlar fabric impregnated with resin for the outer skins and a honeycomb core. This composite construction provides the necessary strength and rigidity while achieving a remarkably low areal density. The entire antenna assembly measures 375 mm × 150 mm × 48 mm, demonstrating a successful miniaturization effort that is crucial for mounting on a UAV drone.
The following table summarizes the key parameters that define our antenna’s geometry:
| Parameter | Description | Value (mm) |
|---|---|---|
| L1 | Length of the H-shaped slot arm | 27.00 |
| L2 | Width of the H-shaped slot arm | 6.00 |
| L3 | Width of the H-shaped slot central bar | 6.00 |
| L4 | Length of the ground plane slot | 41.00 |
| L5 | Matching stub length | 7.33 |
| L6 | Microstrip line length to first branch | 31.77 |
| L7 | Microstrip line length to second branch | 39.54 |
| L8 | Matching stub width | 7.33 |
| Ls1 | Length of the short side of the H-slot | 9.00 |
| Ls2 | Length of the long side of the H-slot | 7.62 |
| S | Distance from slot to open circuit stub | 40.00 |
| w11 | Length of the lower radiating patch | 71.00 |
| w12 | Width of the lower radiating patch | 71.00 |
| w21 | Length of the upper radiating patch | 84.00 |
| w22 | Width of the upper radiating patch | 83.00 |
The operational principle of our dual-polarized antenna can be understood through its equivalent circuit model. The input impedance, $Z_{in}$, is a function of the impedances of the open-circuit stub, the slot on the ground plane, and the radiating patches. The coupling between these elements is represented by ideal transformers. The input impedance seen from the feeding port can be expressed as:
$$
Z_{in} = \frac{n_2^2}{n_1^2 Y_p + Y_a} – j Z_{0m} \cot(\beta_m \cdot S)
$$
where $n_1$ and $n_2$ are the turn ratios representing the coupling between the slot and the patch, and the feed line and the slot, respectively. $Y_p$ and $Y_a$ are the admittances of the radiating patch and the slot, $Z_{0m}$ is the characteristic impedance of the microstrip line, $\beta_m$ is the propagation constant, and $S$ is the length of the open-circuit stub. This model allows for a systematic optimization of the antenna’s parameters to achieve the desired impedance match over a broad frequency range.
To rigorously evaluate the design, we conducted extensive simulations using a full-wave electromagnetic solver. The results demonstrated exceptional performance, which we then validated through measurements on a fabricated prototype. The measured results confirmed a voltage standing wave ratio (VSWR) of less than 2.0 over a relative bandwidth of 35.38%, covering a frequency range from 1.05 GHz to 1.51 GHz. This wideband characteristic is essential for high-resolution SAR systems deployed on a UAV drone.
The measured isolation between the two polarization ports was excellent. Across the entire operating bandwidth, the isolation was consistently greater than 33.07 dB. This high degree of isolation is a direct result of the orthogonal arrangement of the two H-shaped slots and the inherent decoupling provided by the aperture-coupled feeding mechanism. Such performance is critical for maintaining polarization purity in fully polarimetric SAR applications from a UAV drone.
| Performance Metric | Value |
|---|---|
| VSWR Bandwidth (< 2.0) | 35.38% (1.05 – 1.51 GHz) |
| Port Isolation | > 33.07 dB (over bandwidth) |
| Gain | 9.94 dBi (at center frequency) |
| Cross-polarization Level | < -29.81 dB (at center frequency) |
| Front-to-Back Ratio | 15 dB |
| Overall Size | 375 mm × 150 mm × 48 mm |
| Areal Density | 12.0 kg/m² |
| Electrical Thickness | 0.208 λ₀ |
A comparative analysis against other state-of-the-art designs further highlights the advantages of our antenna for UAV drone applications. The table below compares the key performance characteristics of our designed antenna with several antennas from recent literature.
| Antenna | Polarization | Gain (dBi) | Frequency (GHz) | Isolation (dB) | Thickness (λ₀) |
|---|---|---|---|---|---|
| Yang et al. [24] | Single | 3.48 | 3.70 – 4.30 | N/A | 0.260 |
| Sun et al. [25] | Single | 2.20 | 3.02 – 3.26 | N/A | 0.520 |
| Yang et al. [19] | Dual | 4.12 | 2.88 – 3.12 | 25 | 0.031 |
| Cai & Lin [20] | Dual | 7.20 | 27.20 – 28.80 | 25 | 0.076 |
| Wang et al. [21] | Dual | 4.90 | 4.85 – 4.95 | 20 | 0.057 |
| This Work | Dual | 9.94 | 1.05 – 1.52 | 32 | 0.208 |
Our antenna demonstrates a clear advantage in terms of gain and isolation, achieving a gain of 9.94 dBi and isolation greater than 32 dB, which are superior to the other dual-polarized designs. While the electrical thickness is larger than some, it is a necessary trade-off for achieving the significantly wider operating bandwidth required for high-performance SAR on a UAV drone. Furthermore, the physical thickness is minimized through our material selection.
The radiation patterns of the antenna were also characterized in a planar near-field measurement facility. The measured co-polarization and cross-polarization patterns for both H and V ports showed excellent agreement with simulations across the entire band. At the center frequency of 1.3 GHz, the cross-polarization level remained below -29.81 dB in both the azimuth and elevation cuts. The front-to-back ratio was measured to be 15 dB, thanks to the integrated metallic back cavity, which effectively suppresses rearward radiation and enhances the antenna’s directivity. This pattern shape is well-suited for side-looking SAR geometry on a UAV drone.
To validate the practical utility of our design, the fabricated antenna was integrated into a complete L-band dual-polarization SAR payload and mounted onto a UAV drone platform. Two separate flight campaigns were conducted over different terrain types, including agricultural fields and built-up areas. The UAV drone successfully acquired SAR data, which was then processed to form high-resolution images. The resulting SAR images exhibited excellent focusing quality, with distinct and clear features of ground objects, such as buildings and farmland boundaries. This successful flight test unequivocally confirms that our antenna meets all the stringent system-level specifications required for operational UAV drone-based SAR imaging.
In conclusion, we have successfully demonstrated a novel L-band, dual-polarized, slot-coupled microstrip patch antenna that is exceptionally well-suited for integration into UAV drone SAR systems. Our key contributions include:
- A wide impedance bandwidth of 35.38% and high port isolation exceeding 33.07 dB, achieved through a combination of H-shaped slots, dual-stacked patches, and an orthogonal feeding configuration.
- Significant miniaturization and weight reduction, with a total thickness of only 48 mm and an areal density of 12.0 kg/m², made possible by using lightweight materials like Kevlar fabric and honeycomb core.
- Excellent radiation characteristics with high gain, low cross-polarization levels, and a favorable front-to-back ratio.
- Real-world validation through successful SAR imaging flights, providing high-quality, focused imagery that confirm the antenna’s operational readiness for UAV drone missions.
While this design has proven highly effective, future work will focus on enhancing its out-of-band rejection, particularly around 2.4 GHz, to mitigate potential interference. We will also explore the scalability of the design for larger planar arrays suitable for high-end airborne and spaceborne platforms. The current antenna provides a robust and high-performance solution for the growing demands of UAV drone-based remote sensing.