A Millimeter-Wave Wideband Circularly Polarized Planar Spiral Antenna Array with Unequal Amplitude Feeding Network

A 2 x 2 wideband circularly polarized (CP) antenna array operating at millimeter wave (mmWave) band is presented. The array element is a wideband CP Archimedean spiral radiator with special-shaped ring slot. The elements are fed by an unequal amplitude (UA) feeding network based on a microstrip line (MSL) power divider. The side lobe level is improved by this UA feeding network. In addition, a cross slot is employed to isolate the elements for decoupling. A prototype is fabricated, and the measured results show that the proposed array achieves an impedance bandwidth (IBW) of 6.31 GHz (22.5% referring to 28 GHz) and an axial ratio bandwidth (ARBW) of 7.32 GHz (26.1% referring to 28 GHz). The peak gain of the proposed array is 11.3 dBic, and the gain is greater than 9.3 dBic within the whole desired band (from 25 GHz to 31 GHz). The proposed array consists of only one substrate layer and can be built by the conventional printed circuit board technology. Attributed to the characteristics of wide bandwidth, simple structure, low profile, and low cost, the proposed antenna array has a great potential in mmWave wireless communications.