Ground plane of patch antenna - RF Cafe Forums

 Post subject: Ground plane of patch antenna

Posted: Wed Jan 13, 2010 4:10 pm 

Hello everyone,

I would like to ask some help from experts about ground plane of a 2.4GHz patch antenna. I have a 2.4GHz antenna 50 x 10 x 0.5 mm. When I attach an antenna to an abs plastic enclosure, the operating frequency reduce around 0.1 GHz (S11 parameter changes). So I think I should put a ground plane to an antenna. Or anyone has an idea to solve this problem. Please recommend me. I appreciate every comments.

Thank you.
Bill

 Post subject: Re: Ground plane of patch antenna

Posted: Wed Jan 13, 2010 10:35 pm 

Hi Bill_SC

The shift in frequency is due to the radome effect on the antenna. Enclosing the antenna in plastic loads the radiating element - in effect, changes the effective dielectric constant of the antenna substrate/structure.

There are 2 possible ways to overcome this. One would be to re-tune the antenna to compensate for the added dielectric. The antenna element will be have to be made smaller (to shift the center frequency higher by 0.1GHz). Or, another approach would be to move the ABS plastic away from the radiating element. Moving the plastic away by a quarter wavelength (~30mm @2.45GHz) would be a good starting point. If size is a constraint, you can reduce that spacing till you get to a point where the frequency shift is negligible/acceptable/tolerable as the case may be

Karthik

 Post subject: Re: Ground plane of patch antenna

Posted: Thu Jan 14, 2010 10:55 am 

Thank you very much Kathik.

From your first suggestion, the operating frequency shifts too much, so I can't tune it to match at 50 ohms. Maybe this choice doesn't work.

For you second suggestion, I have tried making an air gap between a plastic and an antenna, and it works. The problem is it may take too much space, and it is too difficult to make it in mass production. So from what I know to kill the effect of enclosure, making another ground plane attaches to an antenna is another choice.

Does anyone have any recommendation about it. I appreciate every comments.

Bill

 Post subject: Re: Ground plane of patch antenna

Posted: Thu Jan 21, 2010 5:48 am 

Hi Bill
a third suggestion is to replace the antenna area ABS with a low density foam instead, which will have a permittivity close to 1, and thus hardly detune the antenna at all. The rest of the box that remains in ABS should have little influence on the frequency.
The big problem with changing the ground plane dimensions is the detrimental effect on the antenna pattern. For sure it will influence S11 and improve (or degrade) it, but the effect on radiation pattern and gain can be catastrophic.

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 Post subject: Re: Ground plane of patch antenna

Posted: Thu Jan 21, 2010 1:23 pm 

Thank you very much nubbage. Your suggestion helps me a lot. I have two types of antenna. One works well on plastic with air gap. Another doesn't work well like the first case, so the next step I am working on it is finding a ground plane like metal, abs plastic, or etc to get a good performance. I am working on a 2.4 GHz antenna to attach in metal or plastic enclosure, but I have no idea how to get the best performance of it before i do the matching thing. Do you have any resource or suggestions so I can research it? Thank you in advance. I appreciate it.

Bill