No Great Resonances to be Found

Summary:

Resonances with quality factors of 1000 are common, the cavity is the right size and all that can be updated/fixed are the probes and tuning plate.

I verified that the cavity dimensions match the simulations and then spent another good couple of hours playing with the cavity yesterday.  Here is what I learned (pictures taken with my iPhone):

  • Resonances with insertion losses of under 1dB and with Q’s higher then 1000 are common and easy to find.  Here are pictures of two of them:
    • This resonance is the “Best Hope” one which I have shown before and is in the unknown mode (insertion loss 0.5, 1261Q, frequency of 2.44GHz):
    • The resonance below was chosen at random and shows an uncalibrated result, which means the insertion loss measures is really about 0.7dB and the Q about 1000 (f of 2.47GHz).  Again, some unknown mode.

I also wanted to see what a high Q resonance looked like and found one:

A few things to note:

  • The bandwidth (shown as “BW” above) is only 160KHz which would explain the Q of 16000
  • The loss, even after subtracting the amount calibration would adjust for, is 3dB!
    • I think I remember being told once that having a high insertion loss would help Q and I can reproduce that here if I tune the insertion loss even higher,  I can get a Q of 32000.
  • Note how sharp the spike is compared to the first “Best Hope” graph above, both of which have about the same width of 5MHz

The last diagram is what the TE0,1 mode should look like but with a much better insertion loss.

I have brought the cavity home to work on some more.  The first thing I am going to do is bring the probes closer to the simulations by making them rounder and more horizontal.  The probes are twisted slightly which may be the reason for the insertion loss.  I will then fix the tuning plate by putting some copper mesh around it’s outside edges to connect it properly to the cavity.  I will also make sure it is as flat as possible and doesn’t twist when moved in and out.  Kevin pointed out that it may be necessary to investigate another way of tuning and perhaps a movable plate from the large end??  I don’t think it should make any difference, but I will run that through a simulation to see and I will also think about other ramifications, for example, won’t a larger plate flex even more?

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