Resources for Designing Cavity Resonators  - Page 1
Forum section: electronics / rf-microwave

author: WideBandwidth
author_rank: Contributor
timestamp: June 12, 2021, 11:54:35 pm
content: Hello,Taking a look at the HP 8640B sig gen and its cavity resonator main oscillator got me interested in cavity design. Does anyone have any good textbooks or other resources about either the EM cavity theory or practical design? I was taking a look at Pozar's book, but there's fewer than 10 pages on the subject. I was hoping for a little more! I suppose some of my main questions include how best to couple a (coaxial, for example) signal into a resonator and ideal methods for tuning.If anyone has experience with designing tunable oscillators with cavity resonators, I'd be highly interested to hear about it too.Thanks

author: mawyatt
author_rank: Super Contributor
timestamp: June 13, 2021, 01:11:50 am
content: Zverev's book, "Handbook of Filter Synthesis" has some discussion of Helical Cavity filters in Section 9. Never designed a Cavity filter but used YIG devices in tunable oscillators, these are very interesting devices that operate based upon EM field theory.Best

author: mawyatt
author_rank: Super Contributor
timestamp: June 13, 2021, 02:59:11 pm
content: Quote from: evb149 on June 13, 2021, 07:03:31 amAwhile back (can't remember details but maybe Discovery) there was a fascinating video on MIT Radiation Labs, discussing the setup to work with the British supplied Magnitron and the name chosen to fool the Nazi into thinking it was for nuclear work, not radar.Best,

author: xmo
author_rank: Regular Contributor
timestamp: June 13, 2021, 10:53:20 pm
content: Another reference:https://web.mit.edu/22.09/ClassHandouts/Charged%20Particle%20Accel/CHAP12.PDF

author: WideBandwidth
author_rank: Contributor
timestamp: June 14, 2021, 03:41:46 am
content: All,This is great! Thank you all for the suggested readings, they will keep me busy for a while.And @evb149, thanks for the information on coupling/tuning methods.I suppose one follow-up question: is it feasible to prototype a cavity with a plastic shell lined with copper tape, or would I encounter high loss and other badness? I'm currently mainly interested in the UHF range where skin depths in copper are around 3um. I imagine tape thickness somewhere around 50um would make loss a nonissue, but perhaps I'm not thinking of something else.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 14, 2021, 10:09:17 am
content: Lining inside a plastic enclosure sounds awful. Wrapping the outside though, that has some promise. Use EMI tape, the kind that makes contact through the adhesive side, or if nothing else, solder over the seams.The plastic being on the inside, has a minor effect on resonant frequency. Choose plastics with low losses, i.e. avoid PVC and PET, most hydrocarbons are okay.Can also solder together sheets of copper clad PCB, again doing up the seams fully. Can't exactly get a soldering iron down inside a shape, you'll have to do it from the outside which means the inside copper is kinda just dead space. Might rather use single-sided, actually? Maybe glue up the seams for strength, then solder foil around the corners? Oh, or solder foil around the edges of double-sided, then solder it up the rest of the way.Tim

author: arlo_g
author_rank: Contributor
timestamp: June 14, 2021, 12:07:19 pm
content: You might want to have a look at the cavity filter pages on Scotty's Spectrum Analyzer site:https://scottyspectrumanalyzer.us/https://scottyspectrumanalyzer.us/cavity.htmlScotty Sprowls came up with a design for a high Q ~1GHz coaxial cavity filter that many people have been able to reproduce. For the spectrum analyzer application, multiple filter stages were needed.There are many articles around about "pipe cap" cavity filters for microwave ham radio use. Most of these pipe cap filters operate as lowish Q coaxial resonators with the "tuning screw" being the resonant structure. Higher Q could be achieved from pipe caps by operating them at resonance of the cavity, with a small screw only perturbing the resonant frequency, but I don't think that is done very often, possibly because higher modes of the cavity are closer in than are higher modes of coaxial resonators.

author: cdev
author_rank: Super Contributor
timestamp: June 16, 2021, 04:06:08 am
content: Here is what I would do, I'd get an idea of the rough size, throw it onnthe nanoVNA and tune it that way. Every time I saw a big metal garbage can or something, Id make the tuning screw an make two ports, then throw it on the VNA and see if it could b e tuned to act as a high Q filter at some useful frequency, then mark on the outside what it is, fairly soon you would have made some useful resonators out of junk and could just pick one out of your junk box when you needed one. You could also use it for cooking foodW1GHZ has some web pages on making them cheaply out of plumbing fixtures.