spiral microstrip antenna with finite ground plane

hii,
i am working in HFSS v.10 and i am designing a spiral microstrip antenna with finite ground plane. However, i am getting most of the radiation below ground plane. Please help me detecting the problem with my design. i knw i can use finite ground plane but its impractical. i have to fabricate this antenna. so every dimension should be finite. no using sheets which has zero thickness coz again its impractical please help its urgent. thank you in advance.

A spiral antenna is a balanced antenna (like dipoles). In such cases, there normally is no ground plane. I do not understand how you have setup a ground plane for this antenna? Maybe you can post a photo ?

A spiral antenna has a bidirectional radiation pattern with opposite polarization in the top and bottom hemispheres. If you place a metal sheet below your spiral, then it will result in reflection of the backward waves. However, I have observed such sheets to cause a lot of impedance mismatch in the spiral.

Absorber loaded cavities are generally used to stop the backward radiation of the spiral antenna. However, this results in a drop in efficiency.

it is a microstrip rectangular spiral so it has to be on the substrate and ground plane. here is the image.

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it is a microstrip spiral, so it is reactangular spiral patch fed from center, on a substrate with a ground plane

I'm not a subject matter expert, but your ground plane looks very small in comparison to the size of the antenna. Can you make it any bigger, say, double the size? This may help drastically.

You might want to take a look at
http://newport.eecs.uci.edu/rfmems/p...C076-APS03.pdf
and
http://www.ece.uci.edu/rfmems/public...077-APMC03.pdf