Input impedance of a patch antenna

Hello,

I've a Coax-fed patch antenna designed with HFSS.I want to find it's input impedance how do i proceed?

help please it's urgent.

From S11 find the Z11 using transformations.

Hi kspalla,

You mean that in HFSS i can use the Z parameters option in" results report" to get the input impedance of the antenna.Am i right ?!

correct

how about in FEKO, do you knows how to get the input impedance of a patch after request the s-parameters?

Concerning FEKO you can go to the PostFeko window after the simulation; Then you have to choose 'Add a source data graph'.
Take a look on the document below.
Don't forget to choose the option substract loading and to fill the value of the reference that you want to normalize. Example on the document for 50ohm.
Cheers

what is your resonant frequency? Im trying to get reflection coefficient lower than -20dB for a 150 ohm input feed patch antenna, but still hopeless, my resonant frequency is 15e9 hz, er = 2.35 while the width can not exceed 1 cm, the lowest i can reach is -12dB. Please give me some ideas

if you want to use a coaxial feed, you cannot really design a matching network, so you have change the location of the feed point. Another method is adjusting the width of the antenna. This should not change the resonance frequency by a lot but it will change the impedance of the patch. If these dont work, you can try making some cuts in your patch.

-12 dB is good enough for most applications anyway, as long as you have a good bandwidth. Anything below -9 or -10 db is ok. It is a dB scale so your losses at -13 dB are 5 percent and at 20 dB they are 1 percent. So one would rather have a better bandwidth than a low S11. The losses will be much higher than 1 percent in practice in any case.

To get better S11 level (<-20dB) you can use a microstrip feed than:
- do recessed structure (see details in Balanis book)
- move the position of microstrip feed line
- play on the dimensions of the patch
- add slots in the patch
- add a network of two microtrip lines
- add microstrip lines close to the patch to add resonant frequencies (this method have bad x-polar levels)