Spiral antennas for ESM

In the absence of any published mathematical analyses of the cavity-backed spiral antennas that have become popular in Electronic Support Measures systems, the functions and operating principles of the antenna's spiral radiator, backing cavity, and balun transformer components are treated by means of elementary reasoning. As an illustration of the performance quality currently achievable with these antennas, an evaluation is conducted for the cases of a reduced size square antenna for the 0.4-4.0 GHz frequency band, a 52-mm diameter antenna covering the standard 2.0-18.0 GHz band, and an in-line, 16-mm diameter antenna and integral radome for the 18.0-40.0 GHz band, where the output is by means of a double ridge waveguide.