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# spiral antenna gain

## spiral antenna gain

(OP)
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?

### RE: 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.

### RE: spiral antenna gain

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

### RE: spiral antenna gain

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.

### RE: spiral antenna gain

(OP)
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?

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