Usable Q(!) and Tuning Plate Tilt

Results are rolling in now, hot and fast.  I played with the cavity again today and got better results, even, maybe ones we can actually power up the cavity with.  The first result is from the “best hope” resonance that I mentioned in the last part of the previous email, which I zoomed in on today and got this:

A few things are notable:

  • The insertion loss is pretty high (1.9dB) when our simulations predict 0.7.  A cavity with a loss of 3dB or greater, is useless (3dB is a log scale which means each step looses exponentially more power).
  • The Q is the best I have measured yet at nearly 3000 (subtract markers 3 from 2 which resulted in 2467.140Mhz divided by 0.838)
  • It isn’t clear from trying to match the simulation s-parameter curve, if this is the TE0,1 mode.  However, if the insertion loss can be brought into the sub zero range (see below) and the Q is still high, then this is the resonance to use!

As mentioned at the beginning of the previous email, I then tuned and zoomed in on the location which looked to have the best match with the simulated location of the TE0,1 mode.  Here are the results (got some reflection in the picture):

An analysis:

  • Wow – an insertion loss of only 0.5dB! Excellent! (WAIT – this is not right – look at the graph, S1,1 goes above zero!!! Check out the next blog for an explanation)
    • However, that insertion loss was finicky and could only be created by pushing the tuning rod toward the back as the diagram shows (note the arrow).

      I think the reason is the tuning plate rests at a slight angle and when straightened, the insertion loss dropped by about one dB.  The Q didn’t change at all though.
  • The Q isn’t as high as the “Best Hope” but was still very decent at 1500
  • I also figured out how to get the network Analyzer to automatically calculate Q, as you can see above.  It should be noted that it says “loss: -10.718 dB” however, it is taking that result from the wrong s-parameter as the loss is 0.57dB
  • I had a bit of trouble with calibration which produced some funky results like this (note the jagged lines):

    I’m not sure what happened, but you can tell it’s a calibration problem because all the other graphs were smooth and the S1,1 actually protrudes momentarily above zero which is nigh impossible.

If we do end up with a resonance that we can use, I have been thinking about how to power it.  The problem is this:

A high Q means the energy will only be accepted by that mode over a small frequency range, usually under a megahertz.  However, the magnetron from a microwave is built for power not pin-point frequency control and wanders as it heats up or there are changes in input power.  Worse, the better the Q, the narrower the frequency range over which it will accept energy.  If we are going for a Q of 50,000, it will have a bandwidth of 0.05Mhz or 50 Khz!

Possible solutions:

  • The one thing that might save us is that a microwave transmits over a wide range of frequencies, usually 30Mhz or more, which means if we can center it on the resonant frequency, some of the energy will excite the resonance.
  • Most of the energy will be reflected, not only because it is outside the frequency bandwidth of the resonance, but also because once resonance has reached steady state, all the energy will start to be reflected.  For much longer runs, once steady state is reach, it should be possible to turn the magnetron off periodically in order to just pump in enough energy that only a bit of it is reflected.   An analogy is like pushing a kid on a swing, when getting them up and going, you need to push hard, and the fastest way is to run behind them and push from one side to the other.  After they are swinging high enough, you can just use a single hand to keep the resonance going.

I will be back up at the university tomorrow to test if changing the angle of the tuning plate at the 3000Q resonance makes any difference to the insertion loss.  I also plan on getting the new water cooled magnetron up and running too!

My attempts to avoid reflection when taking pictures of the results, means I spend most of my time like this: