Spiral antennas for ESM

The reason for the popularity of the cavity-backed spiral antenna in ESM systems is discussed, together with the surprising lack of supporting analysis and detailed understanding of its operation. In the absence of any published mathematical analysis of the complete antenna, the function and principle of operation of the three main antenna components¿¿the spiral radiator, backing cavity and balun transformer¿¿ are described using elementary reasoning. Finally, as an illustration of the quality of performance presently achievable from these antennas, the performances of three particular spiral antenna designs are discussed; a reduced-size square antenna covering the frequency band 0.4¿¿4.0 GHz, a 52 mm-diameter antenna covering the standard band 2.0¿¿18.0 GHz, and an in-line 16 mm-diameter antenna and integral radome for the 18.0 to 40.0 GHz band, where the output is in double ridge waveguide.