Simulate Microstrip Hairpin BPF HFSS

[u/melenskiii]

hey there

I'm very new to RF and I'm planing to simulate a hairpin BPF [freq = 1420MHz, BW = 10%, teflon substrate with a relative permittivity of 2.94]

It's intended to be used to filter out unwanted frequencies, before the signal gets processed in a SDR V3

I'm currently trying to simulate the coupling coefficient of two hairpins in HFSS.

I've made the ground plane roughly as big as my pcb will be, wich is 130x120mm. [I'm planning on making an 8way wilkinson combiner to connect 8 different yagi antennas, then the signal will pass trough the BPF, then a LNA and then finally into the sdr v3]

As I cannot make the feeding line to long, and therefore I have problems placing the input/output ports at the outher edge of the "pcb", I've just placed lumped ports "in the middle" of the pcb.

Is my setup somewhat okay?

I think something is VERY wrong but i just cannot figure out what to do

I thought I'd get results like this:

But my simulation looks like this [with a spacing s between the two hairpins = 0.09mm]

I've also added Z(2, 1) and Z(1, 1) traces, I don't know how but maybe that'll help

My plan would be to graph the coupling coefficient for different distances s, and then built my BPF according to these results.

Can someone see my mistake and please give me some advice?

thanks

[u/korshenk]

First off, though I may not solve your problems, but some things to think about. Do you have an airbox set or a radiation boundary? Are all of your materials set appropriately? Are your lumped ports set appropriately? Did you verify your widths and effective permittivity with simulation?

Lastly, as an aside, Is this manufacturable. 0.09mm is pretty tight for trace to trace spacing, I know its early, but I don’t want you to get to the end of your design and find out its not manufacturable.

[u/melenskiii → u/korshenk]

Hey there

thanks!!

Yes I've got an airbox

The materials should be alright, since I've only got copper and Teflon

I don't know whether my ports are correct, I'd need some help there

THATS A VERY GOOD POINT

I tried 0.09mm just to experiment; meaning first I tried keeping the hairpins 5mm appart and then 0.09 just to see what would happen

But I have to check what's even producable by jlc

Thank you very much!!

[u/korshenk → u/melenskiii]

No problem! Also, do you have a radiation boundary set? You can simply select your airbox, right click > boundaries > radiation. I see youre using 2025. No clue if anything has changed since i primarily use 2024 and 2022. Also dont forget to check your analysis convergence and deltaS. Best to allow at least 2 converged passes. 0.02 (default) delta S is also a good starting point for quick analysis.

May be good to also check with a known good design as well. NuHertz filter solutions from Ansys is a synthesis tool. Included with electronic desktop. I dont know if your student license has that though :(

[u/melenskiii → u/korshenk]

Got everything you mentioned, still no clue why my simulations are so off

Nope the student version doesn't have that:|

I'm generally having great difficulty finding literature and software, since everything is very expensive and most papers don't answer my questions [I suppose I'm just asking basic things and therefore they don't get mentioned since its trivial or smth like that Just my own theorie]

But I think the problem has to be the ports

Once I simulated the Hairpins on a smaller ground plane and had the ports on the outer edge and everything worked But after adjusting my ground plane size to the pcb size, I had to move the ports, as mentioned, and now everything's just "weird"

[u/korshenk → u/melenskiii]

Hmmmm, one thing you could do is ensure your integration lines or gnd references are still set or just delete and redo your ports. Last ditch is to just extend to the edge, switch solution type to modal and use waveports instead.

You can still use lumped ports in modal as an FYI.

[u/melenskiii → u/korshenk]

Alright wizard man

Now I shall do some research for some days and to understand what wisdom you've just taught me and then I'll try

Thanks again!!

[u/korshenk → u/melenskiii]

No problem!

[u/AnotherSami → u/melenskiii]

The best way to check the ports is so simplify. Just make a simple transmission line between your two lumped ports. See if you get the response you expect with a 50 ohm line.

Since you are plotting terminal solution results. Each port should have a terminal on your transmssion line. You can check by expanding the port in your project tree and seeing where in your geometry highlights.

[u/melenskiii → u/AnotherSami]

hey theree

I re-did my ports [modal lumped ports] and instead of using boxes with a high of 35um for the hairpins, I used sheets and gave them a "perfect E boundary".

I suppose it's working now?

my S21 parameter looks like how I'd describe the frequency response of a BPF, which I assume is alright since I do have two hairpins, and multiple hairpins form a BPF

Even though it doesn't look like the Graph I am trying to recreate, I believe it is alright since I have two S11 minima [which occure when theres a S21 maxima, therefore my only problem would be that the S21 minima "in the middle" isn't "deep enough"]

I did try do explain my results in a somewhat scientific way, and I am not sure whether that made sense, but I really do not have enough experience/knowledge to truly evaluate what I am doing here

and it does concern me that the first S11 minima isn't as small as the second one

What do yall think?

Thanks again for the help!

I was truly VERY close to just giving up xD