Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Waveguide impedance matching with quarter wave transformers in Ansys HFSS

Not open for further replies.


Newbie level 4
May 22, 2022
Reaction score
Trophy points
Activity points
I'm currently working on a project which consists in matching impedances in a transmission line composed of rectangular waveguides.
I'm working at 2.45E9 Hz in TE10 mode.
There is one WR430 to coax 430WCAN on each end of the line with a 50 ohms port. Then there is a waveguide that I had to determine that is used as a quarter lambda transformer between the WR430 and the rest of the line. In the middle of the line, there is a 10cm x 4.5cm rectangular waveguide filled with epoxy.
I have also added small sections to both the WR430 and the Epoxy-filled waveguide so that their lengths are odd multiples of 1/4lambda to avoid complex loads.
So I have calculated everything I needed (lambda, k, Beta, ...) and managed to determine that, in order to do the matching, I'd need the quarter lambda transformers to be 9.8cm x 4.9cm rectangular waveguide filled with air, which has a characteristic impedance of around 482.06 ohms (the characteristic impedance that I calculated was 483.05 ohms). With all of that, I simulate and I find VSWRs around 1.68, which isn't terrible(about -12dB S-parameters) but is still much worse than what I expected, considering I redid the calculations and found an impedance almost exactly equal to the WR430's characteristic impedance.
So, that's why I come here, to see if anyone knows what the problem might be. I've already checked if the imaginary part in the characteristic impedance of the Epoxy-filled wg was responsible for it and came to the conclusion that it barely changed anything(at least not enough for the VSWRs to be that far from the expected ones).
I've also checked the Betas in HFSS's solution data and found that the Epoxy filled wg's was quite different from the one I had calculated, as is its characteristic impedance for some reason and so I changed its length and obtained the results mentioned before.
Something weird about that is that I determined that for the characteristic impedance of the quarter wave transformers to even matter, the length of the Epoxy-filled wg would have to be an odd multiple of 1/4lambda, and 46.09cm is an even multiple of that lambda g (34* 1.53cm), yet it yields better results than the closest odd multiple of lambda g (35*1.53cm) so yeah I'm not sure why that is.


  • quarterdaedit.PNG
    16.9 KB · Views: 101

Hello, thanks a lot for answering. Could you clarify what you mean by simplified circuit?

Could you clarify what you mean by simplified circuit?
Since you're working with a single mode at a single frequency, you should be able to reasonably well-model your various waveguides sections as transmission lines, with the appropriate characteristic impedances and phase delays. For example, I would interpret your setup as being modelled by the following circuit in ADS:


I obtained these rough values from using your values and some rules of thumb. Could you fill in the actual expected data (i.e., line impedances and electrical lengths)? What are the resulting S-parameters? Using my circuit as-is, I get about -20dB S11.



    Points: 2
    Helpful Answer Positive Rating
Okay, that's kind of what I had in mind but I wasn't sure.
I'm assuming the WGImpedance parameters represent the characteristic impedance? If so, the TE10 mode impedance I calculated/checked on HFSS were 455Ohms for the WR430s and 170Ohms for the epoxy-filled waveguide. I don't see any other difference between your simplified circuit and my design. I don't think I have access to ADS, so I'll make a frequency sweep on HFSS and post the results here.
Thanks a lot for your time

Not open for further replies.

Part and Inventory Search

Welcome to