How much more gain could I expect to get by placing a spiral above a reflecting cavity as opposed to a cavity backed spiral where the back lobe is absorbed?
I'm not sure about the gain difference, however the patterns and operational bandwidth will change. A spiral in a metal cavity will have an approximate octive bandwidth. When the caity is loaded the bandwidth is greatly increased. The max gain is typically acheived when the cavity is .25*lambda (wavelength). Think in terms of an image (or dipole over a ground plane).
Reference: "Antenna Engineering Handbook" second edition by Johnson and Jasik, chapter 14.
Thank you photistor, What gain would I expect from a spiral lamba/4 above a cavity as oposed to having an absorbing cavity. Would you expect a cavity to give 3 dB enhancement or 6 dB due to voltage squared being induced for doubling of power due to cavity?