These antennas are mounted on aircraft so it is desirable to have a relatively thin, flat antenna which can be mounted in a conformal patch to a fuselage or wing section.
Spiral antennas are inexpensive, low-profile, wideband radiators.
For spirals operating at microwave frequencies, the cavity is typically one or two inches in depth. The fields in the cavity are dissipated in order to minimize the effects of the cavity on the radiated fields. Although the lossy cavity enhances the wideband performance of the antenna, it lowers the gain of the antenna. A square spiral with either two, four or more paired arms is attached to the dielectric slab. Each arm of the spiral has resistor elements which dissipate excess energy not already emitted through radiation.