In a patch antenna, most of the propagation is above the ground plane and can. It is also desirable to have an estimate of the input impedance of the antenna.
Hi,
I am writing this post to elaborate on how can we determine the input impedance of patch antenna using 'de-embed' option in HFSS. It will be helpful for those who do not know about it or find other help resources insufficient to do that.
The figure below shows a sample patch antenna that was simulated in HFSS. S11 for this design was -18.53 dB at 4.84 GHz. One of the most important part of antenna design is to measure its input impedance so that feed line may be accurately designed to cancel the reactive part and to match with the resistive part. We want to know Zin for this design.
One of the mistake that users normally do is to use de-embed option in HFSS as shown in figure below. But this is wrong. The reason is that de-embed option either 'adds' or 'deletes' a section of transmission line of 'uniform characteristic impedance' to reduce the simulation effort and time. We cannot de-embed the waveport all the way over two or more transmission lines that have 'different' characteristic impedances ( Z1 is not equal to Z2). Mathematically, de-embedding option only calculates the change in phase (beta) in S-parameters based on lengthening or shortening of the transmission line. Just imagine multiplying the previous entries of S-parameters with exp (-j*beta*L), where L is the distance by which we move the wave port. But if the line is also lossy, as is the case for all practical designs, it also calculates the change in magnitude (alpha) of S parameters.
The right way to calculate the Zin is to just use 50 ohm transmission line (Z1 in our case) to connect it with the patch antenna and to de-mbed the waveport at the junction of these two. The resulting plots for Z11 in HFSS will show us the correct input impedance of patch antenna.
![Impedance Impedance](http://www.microwavetools.com/wp-content/uploads/2015/02/Patch_Impedance.png)
As an example, the real and imaginary parts of Zin (Z11) have been shown in the figure below.
Rin = 307 ohm at 4.81GHz whereas the reactive impedance is .0079 ohm at 4.81GHz. Once the user have this data, feed network can be accurately designed to give excellent return loss at a given frequency.
Any questions or comments from the readers will be appreciated.