Antenna radiation pattern problem.

leonlqx

Antenna
Vent holes
X
Y
Z
Question as below:

We have a PIFA antenna mounted at the front of a desktop chassis. The chassis is a metal box with some vent holes, power supply, CD and hard disk opening in the front and back. The whole chassis outside layer is painted with non-conductig paint. The antenna is mounted on the front, with feeding cable inside the chassis. There are also some vent hole with diameter less than 0.5 cm in the vent hole reigon. The vent hole reigon is close to antenna about a few centimeters.

Now we measure the antenna pattern on x-y plane at 802.11bg. With the vent holes open, the front-back ratio is about 15dB. With the vent holes sealed with copper tape (we sanded the vent hole reigon so that the copper tape is conducting with the chassis box.) the front-back ratio is about 10dB, the maximum gain is about the same.

We then tried to leave the copper tape on, but pin throuh the copper tape at the vent hole places so all the vent holes open again but has copper tape on the reigon, the front-back ratio is 15dB again.

It seems that the vent holes are playing a good role here in terms of the pattern. But I can not explain why is that.

Thank you so much for sharing your knowledge!

Leon