The planar dual-polarized cavity backed sinuous antenna - a design summary

DuHamel's dual-polarized cavity-backed sinuous antenna was a remarkable invention, which is now widely used in RWR, DF and ELINT systems. The physical principles of especially the four-arm planar sinuous are well understood, including its interaction with the absorbing cavity that is used to achieve essentially frequency independent input impedance and unidirectional radiation patterns over multi-octave bandwidths. The absorber loaded cavity reduces the efficiency of the equivalent bi-directional free-space planar sinuous by around 50%, which is usually acceptable for EW receivers but not in low-noise applications like radio-astronomy where absorbing materials cause havoc with noise-temperature.