The Conundrum – Why the Simulation Does Not Match Actual?


  • Simulations suggest rotated probes don’t make any difference
  • Upgraded computer to 16GB of faster memory and faster processor to handle larger simulations
  • Still trying to solve the conundrum – why the simulations don’t match real world results.

At the end of July, it was clear that simulated results were not matching the real world cavity, enough so that running a full-power test would not provide any useful information.  The problem is because of three things:

  • The resonant frequency of the cavity for the TE0,1 mode is 20Mhz higher then in the simulations (no matter the position of the tuning plate).
  • The insertion loss at the TE0,1 mode is 2.2dB versus an expected 0.5dB
  • The Q is 2500 versus an expected Q of 10,000+.

So far, here are all the things I have checked:

  • Size of cavity (length, diameter of narrow and large end) – cavity matches simulation
  • Size of probes (28mm, optimal size) – matches
  • Skin depth for microwaves – the electroplating thickness is more then enough (12.7 micrometers versus a skin depth of 2 micrometers)
  • Orientation of the probe –
    • In the simulation, I rotated the power probe by 5, 10, 15 and 25 degrees to see if it affected the resonant frequency, Q or insertion loss – it does not.  Below are the four examples which show the TE0,1 mode from 5 to 25 degrees (hold your mouse over to see the file name):
  • Tuning Plate at large end – I also ran a series of simulations with the tuning plate at the large end, but it didn’t lead to any conclusive results.  It did however, result in an interesting resonance in the center of the cavity:
  • Rebuilt the cavity model from scratch with better probes and a finer mesh for important parts.
    • Part of the problem with the old cavity model was that I had imported the probe from a previous model and the probe parameters (like diameter, height, etc.) could not be independently controlled (the power probe was a mirror of the measurement probe).  I also wanted to rebuild the model from scratch in the CST 2010 environment to double check all the variables and use any updated static data, for example, the conductivity of copper is now more accurate in CST 2010 because it differentiates between annealed and hard-drawn. (we use the later)
    • The two pictures below are good examples of the differences between v1 and v2 of the model:
      • The diagram below is from the first generation model which shows the sharp edges where the the probe loop connects and has a much rougher meshing.
      • The second diagram below is from v2.0 of the cavity model and has a finer mesh and probes that look much closer to the actual probe shape. The model still only takes about 350K+ tetrahedrals, but the meshing is now finer around important sections.

One thing I plan to check with the new version two is how the orientation and flatness of the tuning plate affects results.  My end goal is to see if I can duplicate what I see on the bench with a simulation. If I can, then I should be able to fix the problem.

I am currently running a simulation where I check to see how sensitive the TE0,1 mode is to the position of the probes from large end of the cavity.   That is one thing I haven’t check yet and from my preliminary results, it may be the cause of the problem.

In order to run higher accuracy simulations with more tetrahedrals (>500K), I upgraded my machine to 16GB of memory.  The CPU is also slightly faster now at 3.7Ghz instead of 3.2Ghz (stock) and the memory also runs at 1333Mhz versus 800Mhz ($350 in upgrades).  The upgrade also allows me to drop in the “Bulldozer” eight core AMD processor when it comes out in the fall.