Millimeter-Wave Ultrawideband Circularly Polarized Planar Array Antenna Using Bold-C Spiral Elements With Concept of Tightly Coupled Array

An ultrawideband circularly polarized (CP) planar array antenna built by new radiation elements, the bold-C spiral elements, in millimeter-wave band is presented in this article. Using the tightly coupled array (TCA) technique, the embedded CP bold-C spiral element in the infinite array has achieved both impedance bandwidth and 3 dB axial ratio (AR) bandwidth of 50.7% covering 18.1-30.4 GHz for the fixed broadside beam. Then, a feeding network is designed by using the dielectric-based inverted microstrip gap waveguide (DIMGW) technology in order to have a compact layout. An 8×8 CP planar array based on the proposed radiating element and the feeding network is designed and prototyped. The measured results show that the impedance bandwidth of the whole array for the reflection coefficient below -10 dB is 49% from 18.2 to 30 GHz, and 3 dB AR bandwidth is 44.9% covering 19-30 GHz.