Urgent: how to simulate 50 ohms feedline in HFSS?

[kae_jolie]

I am relatively new to HFSS and need to simulate a 50 ohm feedline for a monopole slot antenna. The antenna is from the literature and it specifies a 50 ohm feedline, and I am not sure how that is simulated in HFSS. The feedline width and length is 2mmx16mm and the substrate thickness is 2mm with relative permittivity of 4.4. I have entered these paramaters in this characteristic impedance calculator at
https://chemandy.com/calculators/microstrip_transmission_line_calculator_IPC_2141.htm

but it's coming up with 70 ohms instead. So I am not sure where 50 ohms come from.

Also, I learned that transmission feedline length should be lambda/4, but how does that relate exactly to a feedline with characteristic impedance of 50 ohms.

If you would like to check out the antenna I am trying to simulate, it's at
**broken link removed**
and it's the first one: Printed Rectangular-Slot Disc Monopole Antenna

I would really appreciate your straightforward reply to my query as I have researched the forum already and there is a hit and miss discussion around this topic.

Thank you.

P.S. This is really an urgent request.

 

[kae_jolie]

Guys...please help....this should be a fairly simple question for the experts out there...


Thanks.

 

[roxxon]

Without having tried to simulate that antenna in particular, I think you're right: a microstrip line, width 2mm on 2mm FR4 (eps_r=4.4) will have a characteristic impedance of about 70 Ohms, not 50. In my experience, on 1.6mm FR4 a 50 Ohm line is somewhat more than 3mm wide. So for 2mm thick substrate it has to be even wider, I think about 3.7mm. Nevertheless, that antenna with the 2mm-wide line could in total be matched onto 50 Ohms.

Also, a transmission line does certainly not have to be lambda/4 long, that only goes for impedance transformers. In that case, however, the 70 Ohm would make sense, if the foot point of the circular dish would show about 100 Ohms, then the impedance transformer of length lambda/4 would transform it to Z=Z_line^2/Z_footpoint=50Ohms.

Anyway, I would try simulating everything in HFSS, it should really not be a hard task. Remember to put the radiation boundary at least lambda/4... lambda/3 away from the antenna and you'll be fine.

 

[kae_jolie]

Thanks roxxon. You're making things clearer...

where do you come up with this equation Z=Z_line^2/Z_footpoint for impedance transformer, and how do we do measure the impedance of the foot point of the circular dish?

Also, what's the difference between assigning 50 ohms to a lumped port or wave port terminal and having a transmission line with characteristic impedance of 50 ohms? Can we do both or have to pick one for impedance matching?


When I simulate the antenna I don't get the same as much S11 back as the paper states. I am attaching my hfss simulation file for your review.

Thanks for prompt reply.

 

[roxxon]

Well, first of all: that was the normal relation for impedance transformers: Z_T=sqrt(Z_L*Z_A) with T denoting the characteristic impedance of the transformer line, L the line impedance which you want to obtain in the end (e.g. 50Ohms) and Z_A the impedance of the antenna at its footpoint. As I mentioned, then the transformer length has to be lambda/4 (with lambda being the effective wavelength on the substrate). Google impedance transformer or quarter-wave transformer to get more information.

Then, let me first state my opinion based on my experience: UWB antennas are difficult to simulate (with HFSS, I do not have experience with other tools) because the solving frequency is far away from the full bandwidth which you want results of. Therefore, the result can be almost arbitrarily wrong. The paper shows very good agreement with measurement, maybe they were lucky.

That being said, there are a few basic mistakes in your model: first, the way you applied the waveport is wrong. The width of the waveport has to be about 3 times the width of the problem domain (i.e. the width of your microstrip line) and about 5 times the height of the substrate. The way you applied it (with just the height of the substrate and the width of the line) is how a lumped port would be modelled -- that however can not be applied on radiating boundary. So, I think the waveport is the way to go in your case, but it has to be larger.

Also, all outermost surfaces will become Perfect E in HFSS, unless defined otherwise. So, the radiating boundary will stay radiating, but the ground surface, where you did not apply anything else will become Perfect E -- extending your ground plane all the way under your antenna. Clearly you do not want that. Also, as a rule of thumb, the radiation boundary has to be about lambda/4 (or in my experience rather lambda/3) away from the antenna structure at all places (except for the waveport, of course). There again, you see why UWB is difficult to simulate, because either the radiation boundary makes your model very electrically large for the highest frequency or the boundary is to near for the low frequency.
So, in my opinion you need to make your air container (at least somewhat) larger and away from the ground plane and then apply the radiation boundary all around.

The third problem, at least by looking at your problem, is that the line does not really connect to the disc, because that is built by several segments. So, I would always let the line extend into the disc and then use the "unite button" to make one single area of both. But that might not be a problem for you, depending on the settings for the resolution of the disc.

So, that is all I can tell you, I haven't actually tried to simulate your model, but I think that would be the way to go. Also, please, if you're gonna be writing a publication, make sure to include all information necessary to reproduce your results. The publication of your antenna is for example missing information on the thickness of the copper plating, which in my experience can have a big impact on the solutions.

 

[kae_jolie]

thanks a lot roxxon. I will try your suggestions...

BTW...I used some waveport size calculator on emtalk.com to come up with the waveport size in my simulation.

Added after 51 minutes:

roxxon,
you didn't answer my question on how I can determine the input impedance of the antenna?or the impedance at the foot point of circular dish.

Thanks.

Added after 21 minutes:

roxxon,
I simulated the antenna with the bigger waveport, and lo and behold, I am getting higher return loss (-3dB) instead of <-10dB. Basically any waveport size larger than 2x2mm yields poor S11, in addition I get the HFSS message of poor convergence in computing port dispersion....Any thoughts?

 

[roxxon]

As for the waveport size: I'm confused about what the EM-Talk calculator gives for 2x2mm, e.g. for 2mm on 1.6mm substrate it would give 3x2mm... and for 1x1.6mm it gives 20x4mm... I don't know what that thing does... but in my opinion there is something bogus there. Have a look at the port tutorial I attached, there the succestion is even larger than mine, which I got from another tutorial, which however I am not able to retrieve at the moment.
If you already know that you want to match your antenna to 50 Ohms in the end, I suggest using a lumped port (width and height of the microstrip line) and applying 50 Ohms as port impedance (just make sure you make the radiation boundary so large that it does not tough the lumped port, but you want it to be larger anyway).

The antenna footpoint impedance can not be determined straightforwardingly (I think). I suggest looking at the smith chart and doing some simulations. Remember that you actually read Z/Z0 from the smith chart and take the microstrip line to have the 70 Ohms you got from the microstrip calculator. Or, approximately, cut the line away by as far as the ground plane reaches and apply a lumped port from the ground up to the rest of the line and the disc. Apply e.g. 100 Ohms as port impedance and do a simulation. Then you'll see, that in fact the impedance at this point - neglecting all the effects from the line which was cut away etc - shows a footpoint impedance in that range, at least in some parts of your frequency range.

 

[roxxon]

I got it now: the em-talk waveport calculator assumes a 50Ohm microstrip line. Clearly yours is not 50Ohm, therefore the calculator can not be used.

 

[shadowpanther4real]

hi,

I want to know how to make a 50 ohm impedance and quarter wavelength transformer in HFSS. I am making a simple rectangular patch antenna. please help me in this problem.

thanks.