Determine power within an totally enclosed PEC chamber

[Aspire-Design-Ltd]

Hello Everyone,

I have a totally enclosed PEC chamber with a lumped port source. I have an absorber in the chamber which is effective at different frequencies.

How can I plot the power absorbed vs frequency for the whole chamber due to the absorber? The power into the chamber would be port{1}.P_inc I think.

I limit the simulation by the number of steps because the total energy in the chamber only decays at certain frequencies depending on the absorber position.

Any thoughts greatly appreciated.

MPC.

[Aspire-Design-Ltd]

Hi All,

If I run a simulation for 5000 steps with the different absorbers in the chamber I can see the final energy recording at the end of the simulation decreases. But I can not work out how to determine which frequencies are being attenuated.

1: Final Energy -9.48dB, No Absorber
2: Final Energy -13.89dB, one absorber at position a
3: Final Energy -13.98dB, addition absorber at position b
4: Final Energy -17.23dB, additional absorber at position c
5: Final Energy -20.86dB, additional absorber at position d
6: Final Energy -24.57dB, additional absorber at position e

So the simulations with fixed conditions and limited by timesteps show that energy is being absorbed by the absorbers but I have no way of knowing the frequency response.

Using probes does not help as different positions in the chamber have different frequency responses so the effect of the absorber is masked by the resonances in the chamber. I think I need to be able to do a frequency analysis on the chambers total energy time domain response but I am not sure how the achieve this.

Any thoughts anyone?

Thanks, MPC

[VolkerMuehlhaus]

I think I need to be able to do a frequency analysis on the chambers total energy time domain response but I am not sure how the achieve this.

I would calculate incident power minus reflected power in frequency domain, and always use enough timesteps to ensure convergence to a reasonable limit like -40dB or -50dB.

Halting simulation that is not properly converged, like you did, is difficult (impossible?) to evaluate, the ripple in frequency domain data is just too large when residual energy is large.

[Aspire-Design-Ltd]

Thanks Volker for your thoughts.

I accept the normal method would be to wait for the energy in the chamber to fall to -40dB and do a S parameter response for the antenna but the problem with this method is the antenna has a response and so you do not see the inherent response of just the chamber with the absorber, you see the Chamber + Absorber + Antenna response. Also as the chamber is enclosed in PEC the decay in energy would mean waiting a very long time for the energy to decay because at some frequencies the energy will not be decaying due to the resonance of the chamber.

What I was hoping to do was determine just the frequency response of the chamber and absorber.

I have little experience but I do not think OpenEMS in its current form can do this. I think the only way to do this is to have a time domain recording of the total energy in the chamber as the gaussian pulse decays and then do a DFT on this to see to effect of the absorber on the frequency response of the chamber.

I am not sure of your meaning about the ripple in the frequency domain. Provided the simulation is allowed to run for long enough the resolution in the frequency domain will improve and the points where the absorption is maximum should become resolvable. I think 🤔.

MPC

[VolkerMuehlhaus]

I am not sure of your meaning about the ripple in the frequency domain. Provided the simulation is allowed to run for long enough the resolution in the frequency domain will improve

Yes, this is what I meant. Some years ago I had simulated a (very) high Q cavity (for oscillator) using FDTD and that took ~ 1 million time steps to converge.

That’s said, I see your point regarding the feed/antenna response being included in results.

[thliebig]

I am not sure how helpful or related it is, but I recently pulled a PR adding a rectangular resonant cavity example
Maybe you have a look and decide it is of any help or inspiration.

[Aspire-Design-Ltd]

Thanks Thorsten for the pointer... I will review with a view to trying this out..

May I ask, what effort would it take to adapt openEMS to output a time domain trace for the total energy in a dump box and provide the DFT of the result. I was thinking about having a look but I was not sure where to start. Any pointers?

Not having much experience of DFT I was also not sure how you separate the original gaussian driving pulse from the response of the chamber to the pulse.

[thliebig]

Ehm no such a signal was not on any plan. The current energy convergence metric is also only an estimate (for speed reasons).
But you could just use a normal dump box, record the fields on time domain and then do all the calculations in python. But such a box can get very big very fast... I think you need to think more about how would you do this in real life with real measurement equipment, then you might be able to reproduce it in the same way.