Broadband Compact Circularly Polarized Spiral Antenna Array Fed by Printed Gap Waveguide for Millimeter-Wave Applications

This paper presents a new design for a wideband circularly polarized (CP) spiral antenna array fed by a differential aperture coupling. The differential excitation is realized by two vias located at the lateral sides of a transverse slot etched on the lower substrate. The proposed two-arm spiral antenna has a smaller size compared to conventional spiral antennas with several turns. The presented single radiating element provides the impedance bandwidth of (vertical bar S11 vertical bar < -10$ dB) 40.4% and 3-dB axial ratio (AR) of 23.2% with a peak RHCP gain of 7.2 dBic over the operating frequency band. To further enhance the AR bandwidth and the maximum gain, the presented CP spiral element is employed in a 2x2 sequential feeding arrangement. The sequential feeding network is implemented using the low-loss printed gap waveguide (PGW) line, which is compatible with low-cost printed circuit board (PCB) technology. A prototype of the proposed 2x2 antenna array is fabricated and measured experimentally. Measured results reveal that the designed antenna array has a wide impedance bandwidth of over 48.5%, the 3-dB AR bandwidth of 34.2%, and the 3-dB gain bandwidth of 32.4% with a maximum gain of 11.74 dBic and excellent radiation efficiency of 85% over the entire operating frequency band.