New Probes Tested

I rebuilt the probes using a proper metal bending tool ($100 from Princess Auto) and got to within 1mm accuracy.

(The probe on the far right was my first attempt and you can see it is a bit out-of-round.  The 3mm diameter wire is stiff and difficult to work by hand.  Luckily, 3mm wire is readily available at the local Metal Supermarket and relatively inexpensive.)

After mounting the probes in the cavity, they are between 28mm and 29mm in diameter:

I say “between” because of two things – I drilled the threaded holes a little too far from the probe entrances (the white dielectric) which leads to a slight twisting (they should be straight up and down) and widens them a bit.

However, the new probes did not help the measured S-parameters get any closer to the simulations.

Measured

Expected (simulated)

The resonances below the TE0,1 mode have quite distinct shapes.  The most uniquely shaped and easiest to find resonance is the one that shows up at roughly 2.285Ghz (reference the expected results above).  (By resonance, I mean any place where the S1,2 (blue line) rises and the S1,1 (red line) drops)  The 2.285Ghz resonance is a large broadband resonance followed by two sharp resonances at 2.32Ghz and 2.34Ghz and then, between 2.33Ghz and 2.42Ghz, a long dropping S1,2 curve.  That series of resonances and the dropping S1,2 curve, I arbitrarily call “Marker Mountain”, also shows up in the measured data.

The problem is that simulated resonances above 2.42Ghz are not matching the simulated curves.  Simulated data shows another wide steep S1,2 (blue) line, a couple more resonances and then the TE0,1 mode.  The measured data above “Marker Mountain” does have a bunch of resonances, but none of them are as strong as they should be nor in the right places.

The only other difference between the actual cavity and simulated cavity was the tuning plate, which was easy to delete in the simulation and rerun – Here is what resulted (compare it to the measured above!)

Simulated Results without a Tuning Plate

Closer!

Now the simulated and measure S-parameters are starting to look at least a bit more similar and more importantly, I think I can fix the problem!

A few asides:

  • Without the tuning plate, none of the four or five resonances above “Marker Mountain” are TE0,1 modes (I checked the field configurations for each and nada!)
  • I originally thought that removing the tuning plate would not change the S-parameter graph, except to move the resonances lower in frequency, but it would seem that is not the case.

My next task is to:

  • Mount the tuning plate and find the TE0,1 mode!

From last week:

  • Find a way to built mm accurate probes out of copper.  The good news is they are small and should be fairly inexpensive to get made.
  • Add the rest of the status updates to the blog. 82 posts now going back to 2007!
  • I will also run a monster simulation with the probe at 28mm, across a large frequency range that will probably take a day or so (likely running into hundreds of sample frequencies) .  With the resulting S-parameter chart, I can then compare it against the measured S-parameter chart and quickly find the TE0,1 mode.