Cavity back spiral antennas have been requested by a number of Antenna Magus users.
The Cavity backed log spiral is typically used in electronic warfare applications. Unlike the Equiangular spiral (included in the first release of Antenna Magus) the cavity-backing results in a unidirectional radiation pattern with +- 8 dB gain. Its gain is about 3dB higher than the non-cavity-backed antenna with lower achievable bandwidth. The air-filled Cavity-backed log spiral is sufficient for medium bandwidth applications and much simpler to design and model than spirals backed with cavities containing absorbing layers.