Ed, I've added our correspondence here to maybe get additional advice! :) Ed: This is awesome to see. It’s great mind food. The first question I have is whether the cavity resonance is really shifting from the gas within, or whether it’s simply being crushed by the lack of air pressure. If you were to put it into a 55 gallon plastic drum with a V band clamp and simultaneously pump both down, and my theory is correct, the pressure change shouldn’t cause frequency change. You wouldn’t be able to take all of the atmosphere out, but even a 2% drop ought to be doable ought to be easily observable. Letting air back into the drum would then show the pressure shift. Or you could even pressurize the drum slightly and see the shift increase. My rationale is that 100 microstrains while seemingly tiny is not completely insignificant with the precision you require. Corby: Thanks for the thoughts! It is not caused by pressure difference as the cavity is entirely within the vacuum chamber. Vacuum inside and outside so there is no pressure difference to affect the measurements. My main problem now is that before the varactor repair the cavity was around 800kHz high at 22 degrees C. With that case as the temperature of the cavity increased to the target of 50 degrees C the cavity frequency dropped to the proper value. With the varactor repair at 22 degrees C I am way to low, almost 900kHz, so as the cavity heats it heads way from resonance! :( So I'm going to try a different varactor and add a tiny chip of alumina to see if reducing the cavity volume would increase the frequency by the proper amount. Probably am looking at several or many tries to see if this works. Ed" Using a series capacitor that has higher Q than the varactor would reduce its capacitance, and tuning range. You’d need an inductor that’s open circuit to the cavity frequency, which is easy. What was the varactor you’re using, and in what package style? Corby: Attached is a PIX of the varactor "loop". The electrodes on each side of the gap are flattened slightly. Originally the varactor was a tiny black epoxy blob on a tiny ceramic base. No way to determine its part number. :( In the PIX you can see a tiny wire from the blob to the other side electrode. Also there was a tiny rectangle of alumina epoxied across the gap on the opposite side to mechanically anchor the loop ends. I removed the shorted varactor and also the alumina rectangle. I installed an SOD882 cased SMV1405 (2.67pf @ volts and .63pf @ 30V.) that I had attached two tiny wires to. Next plan is to install an SC-79 cased diode that is about half the capacitance and to also add back an alumina stiffener. Other thought is to wind a tiny wire around part of the loop to add volume??? Then I could unwind wire till I get to the spot I need. Ed: Before worrying about any of that, perhaps rotating the coupling loop so it's interacting less with the fields might be good. I don't think changing the loop size without knowing whether it's close to resonant frequency If the thread screws in to stop against cavity wall, add a metal washer as a shim that's only part of a thread thick. Ultimately a -210 ceramic pill or a -240 ceramic disk package varactor would be good in a newly fabricated stud mount with a tempered spring steel hook to clamp it in compression (silver plated of course). I get it you want to make it operate, but it would suck the tear one lead of a beam lead diode. Or maybe you could harvest a connector pin of the right diameter from a D-sub connector or something to serve as a socket to hold the diode. Less coupling is a good thing ... if you shorten the loop there's no way add back length chopped out. Corby: I think maybe adding that shim might be the way to go. After the next varactor is installed and tested that might be the way to go!