Logarithmic spiral with absorber-lined cavity backing

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

To obtain a unidirectional beam for the logarithmic spiral, a cavity backing, partially filled with an appropriate absorbing material, is added. The antenna is suitable for applications that require circularly polarised radiation and stable pattern performance over large bandwidths, and thus is typically used in electronic warfare.

The log spiral antenna has a balanced feed at the centre of the spiral, and thus must be fed with a balanced line. The two arms of the spiral can be fed at the centre point from a coaxial cable bonded to one of the arms to realise an infinite balun. Alternatively, the spiral may be fed by a multi-octave bandwidth balun, such as the Marchand balun, typically incorporated in the cavity.

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