Microstrip Power Absorption

Hey everyone,

I have designed a 50 ohm copper microstrip line in HFSS with these parameters : F = 10GHz , Er = 4.4, t = 17um, W = 2mm, H = 1mm, l = 20mm
I have terminated each end of the line with 50ohm excitation lumped Port. I have also used a plane wave excitation propagating in -z direction toward the microstrip line as shown in the picture attached.

I would like to know how I can measure the power absorption on the terminating loads, the power loss in substrate and power loss in the copper microstrip line.

Does anyone know how to get these power calculation in HFSS simulation tool ?

Please help, I need an urgent answer

Hi Arash_S,

You can do this with scattering parameters and the fields calculator.

The power terminated in the ports is just the real component of the Poynting vector (normal to the port surface) integrated over the surface.

The power lost in the substrate can be determined from the square of S21. The power lost in the copper should be very close to 0 at 10 GHz.

Hi PlanerMetamaterials,

Thank you for your reply. I have several other questions that I think you might be able to help me with.

The reason for my simulation is to plot a graph of total power absorption of the terminating loads over frequency range of 1 - 20 GHz and then divide it by the power generated by the plane wave to the get the power absorption of the microstrip line. I will then carry out the same procedure with different angle of propagation (Plane Wave) to get the average power absorption of the microstrip as if it was placed inside a reverberation chamber.

1) For terminating the microstrip with 50ohm at each end, I am using the excitation lumped port rather than the lumped port provided under the Boundaries option. what is the difference between these two and which one I should use for my situation?

2) In my design, I am using a plane wave excitation. What I have found is that we can only alter the magnitude of the EM plane wave by right clicking on the Field overlay and choosing the Edit sources option. Is this the correct way ? if not how can I get the plane wave magnitude ? Please see the attached Picture

3) For the power absorption of the terminating loads I did exactly what you said in your comment but the value that I calculate does not seems right. Here is a picture of what I have calculated using the field calculator :

Thank you very much for helping me

That all sounds correct.

You may want to try using a wave port and having it excite (receive) two modes. The second mode will be the surface-wave mode supported by the MS, which could be excited by the plane wave. Why doesn't the integrated power look correct?

If we use the wave port then it needs to be applied to the surface of the boundary therefore we can't have the radiation boundary wavelength/4 away from the PCB. I don't know if this does effect the simulation output or not.

Could you please explain me how having two modes would help ?

Regarding the integrated power on the loads, I have tried to do some quick calculation to see if the absorbed power by the loads are correct or not but I don't know if I'm doing it correct. I used the simulation to plot the poynting vector inside the radiation boundary (W/m^2) total Field. Converted the units to W/mm^2 and then multiplied it by the microstrip area 40mm^2.
I guess the integrated power is also in W/m^2 so I have converted that also to W/mm^2 and then compared the total incident power and the total power absorbed by the loads.
Would this be a right way of checking if the integrated power is correct or not ?

Could you also have a look at my project file to make sure everything is set correctly please.

Thanks for all the support

The wavelength/4 rule for radiation boundaries only applies to free space modes, not guided modes.

You'd be including power propagating in both modes, instead of just one.

I don't think so. I don't think the exact area of the microstrip should really affect the coupling much.

Sure

No problem.

Than you for all the help.

Have a quick question regarding designing 50 ohm microstrip line. I have used some equations to calculate the width and height of my microstrip line for 50 ohm. When I used HFSS to make sure the line is matched, the S-parameters indicates that the microstrip line is not 50 ohms.
On thing that I noticed is that when I reduce the physical length of the microstrip, the impedance of microstrip comes closer to 50 ohm.

Does the physical length affect the impedance ?

One more thing, what is the Electrical length or how we can calculate it because the Circuit Design tool on HFSS has a calculator that request this parameter.

Thank You

As far as I can tell your setup is valid.

If the length of the microstrip affects return loss, then the microstrip is not matched at the other end -- because yes, length affects input impedance.

Electrical length is the length of the line in wavelengths.

Another advantage to using waveports is they automatically match themselves for you -- so you'd be able to tell what the actual characteristic impedance is.