Multi-octave cavity-backed four-arm slot spiral for multi-mode operation

A design of a wideband multi-mode four-arm spiral antenna is presented. In order to achieve a unidirectional and efficient mode 1 and 2 radiation, the spiral is cavity-backed and resistively-loaded. The resistive loading is implemented as a necessary compromise to maintain reasonable pattern and impedance match with the reflective backing over the operating bandwidth. Numerical simulations are conducted to assess the VSWR, axial ratio, and gain performances of the antenna for different resistor tapers and modes of operation. Guidelines to design a multi-mode spiral with VSWR <; 2.5, axial ratio <; 3 dB, and nominal gain of 6 dBic over multi-octave bandwidth is highlighted.