I Think I Know What To Fix!


Summary – From simulations, it looks like I can fix three things on the cavity, increase the probe length from 3mm to 15mm, move the probe backward to 280mm and get rid of the gap around the tuning plate.

For the test runs  below, I varied four variables

  • the probe loop diameter
  • the length between the probe mount and the probe loop (how far the loop stuck into the cavity)
  • the distance of both the measurement and power probe from the tuning plate
  • the gap around the tuning plate

Below is probably the most indecipherable graph ever but it shows all the test runs:


  • The best result seems to be a loop 28mm in diameter 15mm in length and 280mm from the short with no gap around the tuning plate.
  • The three variables that had the most dramatic affect, did so in two different ways:
    • Probe diameter and probe location affected the insertion loss dramatically but not the frequency, e.g.compare (E), (G) & (J) where the loop went from 20mm to 28mm and the insertion loss went from 1.5dB to 0.34dB.
    • Probe length seemed to affect the resonant frequency, starting at a high frequency of 2.4417Ghz with a probe length of 3mm, swinging as low as 2.435Ghz @ 20mm and then returning to 2.4414Ghz for a probe length of 30mm.
  • The current tuning plate has about a 1 to 2mm gap around the edge in order to avoid the tuning plate scratching the inside of the cavity.  However, as a previous status report showed, having a completely aluminum cavity has little affect on the TE0,1 mode and a few scratches from the tuning plate will have negligible effect.
    • The simulation results above suggest the tuning plate gap at 2mm, has a slight affect on both the insertion loss and the resonant frequency.  For example, for test run (G) without a gap and (H) with a 2mm gap, the frequency moved 3Mhz and the loss got better (?).
  • One of the surprises during this round of simulation was the 9dB loss for test (B).  This shows that not only is 292.5mm a bad match, but it is also more susceptible to other factors.  At 280mm, the probe length could vary from 3mm to 20mm in length and have little affect on the insertion loss, wheres as at 292.5mm, going from 3mm to 15mm, the loss went from 0.6dB to 9dB!!  The susceptibility to other factors at that probe location is likely the problem with the current cavity.

For the coming next week:

  • It is clear that having the probe at 280mm is much better then 292.2mm and I want to test, via simulation, if I can use the same probe location, but bend the probe backward such that the loop is in the right location (15mm above and 280mm from the short).
  • Once done the bent probe simulation, I will then decide if I need to drill a new hole or not.  I will then build a new probe and remove the gap around the tuning plate.

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