How can I measure antenna impedance in HFSS?

[ifelix]

edge input impedance hfss

Hi,
I've a problem that I've to solve as soon as possible.
I've modeled a printed antenna without ground plane which is feeded by a microstrip line grounded. The ground plane ends some space before the antenna edge so this antenna couldn't be considered a patch...moreover the last part of the feeding strip is not grounded...
The antenna should have a resonance near 400MHz and the feeding line has been studied to have a characteristic impedance of 50 Ohm.
How can I know the input impedance of the antenna?
I've measured the input impedance Z11 at the wave port. How can I extract the real input impedance of the antenna? I know approximately it's value by means of an article about this kind of antenna and I want to know the antenna impedance so I can realize an adequate matching network.
Problem...I've measured S11 at the port for varying lengths of the feeding strip and I've found that it varies in an inacceptable way. What's wrong?
I've also have read something oh topics about using lumped ports but I've tried with bad results.
Could anyone help me please?
Thanks in adv.
PS.I've attached an image containing the model I've simulated.

 

[jallem]

input impedance of antenna and z11

can you post the .hfss file?

 

[ifelix]

characteristic impedance hfss how to measure

This is the .hfss file about my model.
Thanks in adv.

Added after 13 minutes:

I had an error message when trying to upload .hfss file so I've added a .pdf extension.Remove it to have .hfss file. Thanks.

 

[adel_48]

measure antenna impedance

you can measure the antenna impedance using the value of s11, simply by determining the input impedance from s11, normalizing, placing the normalized impedance on smith chart then moving toward the load by a length equal to that of the feeding line. the point you will reach is the normalized input impedance of the antenna.

you can do this algebraicaly on matlab using the formula
zin = z0 * (zl-z0*tan(beta * L))/(z0-zl * tan(beta * L))

(but note here that L should be negative since you are moving toward the load.

Hope this might help.

best regards,
Adel

 

[jallem]

how to measure antenna impedance

I think you have to model the strip as starting from
the rectangule 3 of the cell. Every time you change
the distance the strip disconnect from the cell
(form a gap). I made the
changes to the model but I will post it tomorrow.
You will see what I am talking abut.

 

[madengr]

antenna impedance s11

Be careful since the S-parameters from waveports are generalized, that means they are normalized to the wave port impedance which may not be 50 ohms. There is an option to re-normalize to 50 ohms. Lumped ports are 50 ohms however they are not de-embedded

 

[ifelix]

Sorry but there're some things not clear...
I'm not sure of one thing:can I make the operation Adel told me also if the terminal part of the feeding strip is not grounded?I tell this because that formula is valid when we speak about tx lines(so a microstrip grounded is similar to a tx line) but if I've a strip not grounded can I still define that strip as a tx line? I think this isn't right...correct me if I'm failing please...
Instead, in the case this would be right, can I take the value of Z11 that HFSS gives me and utilize it in the formula of Adel to extract the value of ZL? I think it's obvious I've to renormalize the wave port to 50 Ohm...
Thanks.

 

[madengr]

The integration line on the port should start on the ground plane and end on the strip. You have it along the whole port.

Also, analyze the structure with just a short at the end of the transmission line instead of an antenna. Verify your result of Z11, it should just be a small inductance.