Compact Broadband Circularly Polarized Filtering Patch Antenna With Multi-Element Coupling Structure

This paper presents a compact broadband circularly polarized (CP) filtering patch antenna with multi-element coupling structure. The antenna consists of two substrate layers. The upper substrate layer incorporates a main radiating patch with an etched rectangular slot, a pair of unequal-sized L-shaped parasitic stubs placed along the + 45&#x00B0; diagonal of the main patch, and surrounding microstrip patches. The lower substrate features a ground plane etched with an asymmetric U-shaped slot on its top surface and a microstrip feedline on the bottom. The signal enters the port, propagates along the microstrip feedline, and couples to the main radiating patch through the asymmetric U-shaped slot, generating CP radiation. Further coupling between the main patch and the surrounding microstrip patches produces two additional CP modes, enabling wideband CP operation. Without the need for additional filtering circuits, the antenna achieves bandpass filtering characteristics by introducing two controllable radiation nulls near 2.0 GHz and its second harmonic at 4.0 GHz through the unequal L-shaped parasitic stubs. A third radiation null near 3.6 GHz is generated by the rectangular slot etched on the main patch. The proposed design maintains a compact profile (<inline-formula> <tex-math notation="LaTeX">$0.54 \lambda_0 \times 0.54 \lambda_0 \times 0.07 \lambda_0$ </tex-math></inline-formula>, where <inline-formula> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> corresponds to the lowest operating frequency) while realizing integrated wideband CP and filtering performance. The measured results demonstrate an impedance bandwidth of 23.1% (2.64&#x2013;3.33 GHz), an axial ratio (AR) bandwidth of 11% (2.8&#x2013;3.125 GHz) and a stable in-band gain of approximately 7 dB. The out-of-band suppression levels reach 17 dB and 24 dB in the lower and upper stopbands, respectively, confirming good CP filtering performance.