spiral antenna gain

It depends...

If the "reflecting cavity" was the 1000 foot diameter dish in Arecibo, then you could get quite a bit of gain...

If the "reflecting cavity" was badly designed, then you could have a deep null where you might normally expect to find gain.

I was thinking in terms of a cavity 2 * dia, approx Lamda / 4 deep.

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?