Orthogonal Circular Polarized Transmitter and Receiver Antennas for Mitigation of Mutual Coupling in Monostatic Radars

Through-wall radar systems require compact, wideband and high gain antennas for detecting targets. Building walls introduce considerable attenuation on the radar signals. When the transmitted power is raised to compensate the through-wall attenuation, the direct coupling between the transmitter and receiver can saturate the receiver because of which weaker reflections off the target may remain undetected. In this paper, we propose using transmitter and receiver antennas of orthogonal circular polarization to reduce the direct coupling between the transmitter and receiver while retaining the first bounce off the target. In our paper, we demonstrate that the quadrafilar helical antenna (QHA) is a good candidate for this operation since it is characterized by a small size, wide frequency band of operation, high gain and low axial ratio over a wide field of view. We compare the reduced mutual coupling between the transmitter and receiver elements for the oppositely polarized QHA antennas with other commonly used through-wall radar antennas such as the Vivaldi and horn antennas. The system is tested in through-wall conditions.

💡 Research Summary

The paper addresses a critical limitation of through‑wall radar (TWR) systems: the direct coupling between co‑located transmitter and receiver antennas, which can saturate the receiver when transmit power is increased to compensate for wall attenuation. Traditional TWR antennas—microstrip patches, Vivalna, and horn antennas—are linearly polarized and, when used in a monostatic configuration, exhibit strong mutual coupling because they share the same polarization. This coupling limits the usable transmit power and consequently the detection range, especially in scenarios where walls cause several dB of two‑way loss.

To mitigate this, the authors propose using a pair of orthogonal circularly polarized antennas: one left‑hand circularly polarized (LHCP) and the other right‑hand circularly polarized (RHCP). Circular polarization has the property that a single reflection from a planar surface reverses the handedness. Therefore, the signal reflected from a target behind a wall will have the same handedness as the transmitted wave when it returns to the receiver, while the direct line‑of‑sight leakage between the two antennas suffers a polarization mismatch and is strongly attenuated.

The specific antenna selected is the Quadrafilar Helical Antenna (QHA). A QHA consists of four helical windings fed with progressive 90° phase shifts, producing a compact, broadband, high‑gain, and low‑axial‑ratio radiator. The authors fabricated both printed and wire‑wound versions resonant at 3.4 GHz, with a physical height of 4.5 cm, radius 0.5 cm, and a 1 GHz 3‑dB bandwidth. Measured gain is about 4 dBi, axial ratio ≤2 dB over a wide field‑of‑view, and the antenna weight is less than half that of a conventional single‑turn helix at the same frequency.

A feed network employing a rat‑race coupler and Wilkinson divider provides the required 0°, 90°, 180°, and 270° phase shifts to the four ports. The QHA’s compact size makes it suitable for portable TWR platforms, while its broadband nature matches the typical 1–4 GHz operating band identified for through‑wall imaging.

Experimental validation was performed in an anechoic chamber and in a realistic through‑wall scenario. Two QHAs were placed 15 cm apart, one LHCP (transmitter) and one RHCP (receiver). S‑parameter (S21) measurements showed that the orthogonal‑polarization (OP) configuration achieved –31 dB coupling, an 8 dB improvement over the same‑polarization (SP) case (–23 dB). For comparison, a Vivaldi antenna pair exhibited –23 dB coupling, while a broadband horn showed –37 dB but at the cost of large size and weight.

The authors then introduced a trihedral corner reflector as a target behind an 18‑inch concrete wall, with the radar positioned 15 cm from the wall and the target 2 m beyond it. In the SP configuration, the receiver saturated with the direct leakage and the target return was indistinguishable across the 1 GHz measurement band. In contrast, the OP QHA configuration produced a clear S21 peak corresponding to the target, enabling range estimation after background subtraction and Fourier transformation of the frequency‑domain data. Measurements were repeated for antenna separations of 15 cm, 20 cm, and 25 cm, and for target distances from 0.5 m to 3 m; the coupling reduction and target detectability remained consistent, confirming robustness to practical deployment variations.

The paper concludes that orthogonal circular polarization, realized with compact quadrafilar helical antennas, effectively suppresses transmitter‑receiver coupling while preserving the first‑bounce target return. This enables higher transmit powers within FCC limits, improving detection range in high‑loss through‑wall environments without resorting to complex switching or time‑division schemes. The QHA’s combination of high gain, wide bandwidth, low axial ratio over a broad field‑of‑view, and small footprint makes it a strong candidate for next‑generation portable TWR systems.

Future work suggested includes extending the concept to antenna arrays for beamforming, implementing dynamic polarization switching to adapt to varying target orientations, and integrating real‑time background subtraction algorithms to further enhance detection of weak targets in cluttered indoor scenes.