High-Efficiency Ka-Band Circularly Polarized Radial-Line Slot Array Antenna on a Bed of Nails

Radial-line slot-array antennas (RLSAs) provide an extremely simple solution to achieve high-gain circularly polarized radiation patterns without the need for complicated feeding networks or polarizers. The dielectric-filled radial waveguide, however, drastically reduces the efficiency that is potentially achievable by RLSA antennas at millimeter waveband. In this article, a novel architecture for an all-metal RLSA is proposed by replacing the dielectric material with a regular bed of metallic nails, thus maintaining the required slow wave characteristic within the radial waveguide. The slot array is efficiently optimized by using an
ad hoc
method-of-moments solver, based on the definition of an equivalent problem in the waveguide region. This accurate optimization process, along with the all-metal nature of the antenna, allows to reach a measured peak total efficiency above 80% at 30 GHz. The fabricated prototype consists of two pieces: the bottom waveguide with the bed of nails and the top slotted plate, which are easily assembled by means of a few screws. Experimental results report a peak gain of 35.0 dBi for a radiation efficiency of 94%, and a wideband matching performance with a very pure axial ratio, below 0.6 dB.