Antenna Plastic Covers vs. potting - Er=4 - Performance

[pcjon]

Hi,

I have potted an antenna for wifi (2.4ghz) and had it in a plastic case (small air gap around antenna). The antenna performance goes way down if it's potted (Er=4).

I'm guessing this is due to the speed of propagation through the dielectric changing the antennas effective length?

If the plastic skin is about 1/4" away it works well. Does this also mess up the antenna somewhat or is the wave already launched at his point?

Thanks,
PCJon

 

[davenn]

Increasing the antenna elements to air capacitance decreases the resonant freq of the antenna. As a result the elements have to be slightly shorter in length to bring them back to resonance once the radome is placed over the antenna.

I'm sure there's formula around for calculating the effects for a given Er of the radome and the freq involved etc. I have never used it. I have always used a 5% shorter "rule of thumb" and havent got into trouble

Dave