CST：Impedance of MS antenna

Hi

I want to know how can i measure the impedance of a microstrip antenna.

The problem is :
With a microstrip line if i feed the antenna i don't know its input impedance. And for the feed line i will need to design it for a certain impedance. Now there will be a mismatch.

In IE3D i have a option to define a port on the edge so i can know antenna's input impedance but in CST i used a waveguide port to excite the feeding microstrip line. so if i don't feed i can't exite in CST and if i feed its total hit and trial to get the input impedance that each time i see return loss make changes in feedline width and do it again. Can there be a solution for this?

Also if i use a coaxial cable in feed how do i know its impedance as in CST its just a cylinder. in IE3d it was a 50 ohms by default. If there is a mismatch again its huge problem.

Last Q where i get z matrix in CST ?

Hi, Crazy_dude: For probe-feed patch, the outer conductor radius is not very critical when it is small compared to wavelength. The way handled in IE3D is very accurate. IE3D normalizes s-parameters to 50-ohms for compatibility with industrial standards. However, you can convert it back to the Zc of the coaxial line to get the s-parameters normalized to the Zc. Regards.

Hi crazy_dude

1. There is solution to your problem go to port click left, properties and
Insert the distance from the WG port to the patch that?s the way to get your
Input impedance (dembedding the WG port). You can see it in the examples
In patch antenna if you install them.
2. You can do it also with coaxial input.
3. Go to macro look there.

PL

If I may elaborate on Plasma's tip about de-embedding (using CST) :

1. Go to your Waveguide (WG) port definitions. Set the reference plane distance as your feedline length, you should b able to see a port outline set right on the edge of the MS antenna.

2. Run the simulation as you did before.

3.a. Go to "1D results", check the Smith Chart. Add a marker at the desired frequency. The impedance will appear on the screen.

3.b. Another way is to assume the antenna is a complex load on the end of your feedline (a 50 ohm MS line if I understand correctly). Find S11 amplitude and phase at the desired frequency (10 GHz), and use the formula:

Γ=(Zload-Zo)/(Zload+Zo) , whereas Γ is abs(S11)*exp(j*phase).

Good luck,
P.

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