The spiral antenna

A simplified description of the radiation mechanism of the spiral antenna is given, and the effects of the various parameters on the characteristics of the cavity-backed Archimedian spiral are discussed. Experimental data illustrating the effect of cavity diameter, cavity depth, conductor loading, and spiral rate of growth are offered as design guides. Finally, a spiral antenna and balun feed are described. Over an octave frequency range, ellipticity ratios of less than 3 db have been maintained over azimuth angles greater than
\pm60
degrees, boresight errors have been virtually eliminated, and standing wave ratios of less than 2:1 have been realized.