Tuning Plate Added – Still Crappy Q

I got the tuning plate attached and got good results at the university, although, as usual, as expected.

The tuning plate fit this cavity very well, with a fairly uniform and small gap between it and the cavity walls.

The measured results are at least a bit like the expected results and here is a side by side comparison at one location of the tuning plate:

That was the best match I could get across the tuning spectrum and it is close.  Notice the little “hill” just before the “Marker 1” location, labelled “A”?  That seems to match the little hill just before the the TE0,1 mode in the simulations.  There is also the wider bandwidth resonance immediately after (“B”) and although “Marker Mountain” is not as clear as the measurements, that can probably be explained by the lack of measurement points, only 1500 across the entire spectrum from 2.3Ghz to 2.5Ghz, of the network analyzer.  If we were to zoom into the important points of “Marker Mountain”, it would probably resolve a few details.

The unfortunate part is it looks like the resonance only dips to  -13dB, meaning the cavity isn’t very well matched and the insertion loss is unacceptably high at 2dB+!  I will zoom in, recalibrate, and retake measurements this week, but from previous experience, I am guessing the results won’t change much.

However, while I was moving up and down the tuning range, I did find this:

Again, notice the marker mountain on the far left and marker 1 shows the location of interest.  The match is very good at -38dB and the insertion loss also stellar at 0.6db!  Unfortunately,  Q still only looks to be in the hundreds because the 3dB points look to be about 6 to 7 Mhz apart.  From the graph, which has 20Mhz per division, the 3dB points look to be about a fifth or so of that width.  Q is then just 2446Mhz divided by 6Mhz or roughly 407 (unitless), ironically not much of an improvement over the plastic cavity!!  To get a Q in the thousands, the width between the 3dB points has to be 2Mhz or less!  Besides a low Q, it is hard to tell if that resonance is even the TE0,1 mode we want because it doesn’t closely match the simulated results.

Again, I will take a closer look this week but I’m not holding my breath.

Playing with the tuning plate also showed something else – that “Marker Mountain” is pretty much stationary no matter where the tuning plate is.  Tuning does change the mountain’s shape, but it doesn’t move it in frequency.  Moving the tuning plate mostly affects the higher order resonances, like the TE0,1 mode we are looking for, but it isn’t clear if they are moving frequencies or just appearing and disappearing.  Click on the graphic below to see a looping animation that gives you and idea of how the s-parameters change as the tuning plate moves up (making the cavity longer).

Here’s a shot from the documentary footage: