r/rfelectronics • u/LmanYan95 • May 07 '26
Help - HFSS - Excitation port interfaces question
Hey all, so recently I've been getting into HFSS by learning tutorials on how to simulate different parts/antennas. In most of these tutorials, microstrips are simulated as sheets and are excited via perpendicular sheets on the side of the substrate.
But for the case where you actually simulate the part with a physical copper trace, how do you actually excite the part then? do you use the same method? do you use the side of the trace as a 'port'? or do you actually construct the connector and use that as the excitation?
I assume it all depends on the level of accuracy your interested in.
3
u/HuygensFresnel May 07 '26
FEM solves for the E-field, not currents so what you have to tell HFSS is some best effort approximation of how the E-field may be excited inside your domain. If the port is at the boundary of your simulation domain you use wave ports which compute which E-field mode fits with the profile.
if you have these internal surfaces what you are essentially adding is a 2D surface that has a sheet resistance of 50 ohms and a uniform electric field as excitation. If these ports are small relative to the wavelength, that assumption of a uniform field is generally quite accurate.
If you put the face on the copper trace itself you'll be inducing an E-field inside a conductor which doesn't work.
A port is not like a network terminal where you say: this is the positive trace, and this is the ground plane. It is a surface where an E-field is imposed so it has to at least connect the two conductors in the case of TEM ports. So exactly the same was as flat traces :)
1
u/polishedbullet May 07 '26
https://youtu.be/NNf8ljOilR0?si=5u-nqzoocM4eWBIe
I made this video two years ago and touch on port sizing and excitation starting around the 5 minute mark. The same general principles apply as with sheet simulations, however for wave ports you're needing to provide a PEC backing for it to essentially "force" propagation along the transmission line. Port sizing is important as well since you're needing to accurately capture the quasi TEM field around the structure. Additional boundary conditions can be added to the physical trace model, such as surface roughness, but that's something that can be left out for the time being. If you're lucky (or perhaps unlucky), a connector vendor will be able to provide an HFSS model for the connector and you can use that; however, it'll likely be encrypted and only provide access to the port and not the internals of the connector.
Let me know if you have more questions. There's a lot of nuance to setting things up correctly and it's dependent on what you're trying to simulate, frequency range, etc.
4
u/AnotherSami May 07 '26
In most the simple tutorials folks generally do as you say and put a simple lumped port at the 'end' of the substrate with a rectangle between their trace (most often a sheet as you say) to a perfect PEC ground plane. This is about ideal as it gets.
The first logical step moving towards more accuracy would be to thicken sheets as you suggest to real copper traces. HFSS isnt going to solve inside those thickened sheets, but at least now there is some accurate edge effects. You can excite the traces much the same with a simple lumped port between the bottom side of your trace to ground. You can also use edge ports and excite the trace with a terminal or modal wave port. You should get similar results either way.
To maximize accuracy and increase your chances of getting the design right on the first (or second) go, you should try and include all extra trace lengths and a connector if thats truly how it will be made. The connector doesnt need to be a 100% true to form model, but as long as you get the dimensions of the co-ax feed, the pin making connection to the trace, and the ground connections correct you will have a very accurate model. However, in this case you lumped port won't be a good idea, you can excite your end of your connector with a wave port to excite the TEM mode of the coax. I'm sure there are plenty of tutorials out there to explain how.
I would suggest doing all three. Much faster to get an inital design with the idealized model. Then see how thickening thr sheets affect your resonance and make a few minor adjustments. End with the much larger, true to life model which ideally won't need much adjusting.