question about Archimedean spiral antenna

Hi everyone,
There is a question in Archimedean spiral antenna.I want to enhance gain of the antenna,a cavity was added on the back of the spiral and about 1/4 labuta from it.I had simulated the antenna in hfss and found that the axial ratio of the antenna increased too(from 1 dB to 7dB).I consulted lots of papers about it ,but they didn't mention the phenomena.
How to solve this question?
Thanks and regards!
caocao

Hi
I have such a problem with my spiral!
your Thread was for a long time ago.I think you had already solved the problem.
if you had solved it please help me.
how can I decrease The Axial ratio in Spiral antennas and ehat are the parameters here?
Thank you

whit abs you can reduce the axial ratio

what do you mean by "with abs ..."?
my question is "what are the important parameters on axial ratio of a spiral antenna".
regards

see this paper
send your file to my email: ferdows_j@yahoo.com