Archimedes spiral with absorber-lined cavity backing

Image of the Archimedes spiral with absorber-lined cavity backing.

A cavity (partially filled with an appropriate absorbing material) is used to obtain unidirectional radiation for multi-octave bandwidth applications. A typical application of this antenna is in EW (Electronic warfare) where it is typically used in RWRs (radar warning receivers) or jammers.

Since the spiral is a fully balanced design, a balun must be used if it is fed using a coaxial or other unbalanced transmission line type. The balun is often designed to also provide impedance matching, and integrated into the cavity.

Typical radiation pattern at minimum, centre and maximum frequency.
Typical S11 performance of the antenna.