HFSS - 50 Ohm Coaxial line terminated with 50 Ohm impedance boundary. |S11| too large

I'm modelling a 50 Ohm coaxial cable in HFSS that is excited with a waveport and terminated with a 50 Ohm impedance boundary.

The Z0 of the wave port is 50 Ohms (refined through tweaking dimensions of the teflon and pin size around standard SMA 50 Ohm cable designs).

The problem is that S11 mag in dB (at 4 GHz) is only -3.26 dB.. I'd expect that to be much lower given the 50 Ohm impedance boundary.

This result doesn't change when changing the following:

• Changing the length of the cable (S11 mag doesn't change, but phase does, as expected)
• Changing port characteristic impedance - Zpi, Zpv, Zvi, Zwave
• Changing PEC to copper
• Using curvilinear coordinates for curved surfaces
• Deembedding (same on and off)
• Normalizing port to 50 Ohms (no change, as expected due to Z0 being very close to 50 already)
• Mesh density (smallest tried is Lambda refinement of 0.25)

Any ideas?
Ian

I've realized that the "impedance" boundary is designed for sheet impedances, defined as ohm/square.. So I tried with a "radiation" boundary and got -70dB S11 mag.. But I feel like that's cheating, however it does demonstrate what I hoped to demonstrate

Why is your center conductor hollow?

Where is your energyzing port?

I'm not sure that the impedance boundary is designed to be used like that.

You might want to specify your own resistive material for your termination. Make it out of carbon and specify the bulk conductivity to simulate the 50Ω of the termination. You may have to run empirical tests to get the conductivity correct.

HFSS doesn't have discrete parts, of you could just run 10 parallel 500Ω across the termination. I think that designing your own termination would be better.

Yes, the 50 Ohm impedance boundary would be 50 Ohms/square.

Whenever I need a high-quality modal termination, I just use another wave port, and make sure it isn't excited.