Author Topic: Resources for Designing Cavity Resonators  (Read 923 times)

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Offline WideBandwidth

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Resources for Designing Cavity Resonators
« on: June 12, 2021, 11:54:35 pm »
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
 

Online mawyatt

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Re: Resources for Designing Cavity Resonators
« Reply #1 on: June 13, 2021, 01:11:50 am »
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 
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Offline evb149

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Re: Resources for Designing Cavity Resonators
« Reply #2 on: June 13, 2021, 07:03:31 am »
The mit radlab series has some practical information on that, and is free online, as I recall.
Obviously it isn't modern, but some things never change, and if anything it's more from the
peak times of cavity r&d than things made in subsequent decades.

You could see if microwave journal, microwaves101 web site have relevant articles.

BSTJ probably has some, too, and it's also free online last I knew.

 

Offline evb149

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Re: Resources for Designing Cavity Resonators
« Reply #3 on: June 13, 2021, 07:33:34 am »
To couple coax to cavity often involves one of two approaches to actually launch the signal.

1: know what mode in the guide you're trying to excite and therefore also what modes exist in the physical spatial design of the guide, so you'll know where the E and H field maxima can be located geometrically and what their polarization is.

2: An E field probe can be just a finger like essentially a monopole antenna aligned with producing the mode of the E field you're trying to excite. Basically ground the braid of the coax to the guide's wall and extend the innner conductor into the guide straight or bent in an "L" shape after it clears the guide wall by a small amount so the section in free space away from the wall is aligned with producing the E field

3: An H field probe is similar except it is a loop like a "J" where the top of the J is conceptually axial with the coaxial inner conductor and the end tip of the J is connected well electrically to the inner wall of the guide, and the braid is electrically connected to the guide as before.  So you're forming a small loop antenna where the inner conductor forms a small coil that is electrically returned to the guide wall more or less opposite where the braid is attached.  The inductance couples the H field aligned with the mode polarization you want to excite.

In either case you can adjust:
A: alter the length of the probe feed conductor inside the guide from "tiny" to "small".
B: rotate the physical angle of the loop / monopole a bit relative to the guide field mode.
C: Move the feed point closer to or away from the line of the guide where the H/E field maxima occurs.
D: Move the feed point closer to or away from the end of the guide where presumably your reflecting wall end cap is.

...so that the degree of coupling is adjusted and the coupling impedance can be changed.

Oh you might also check these, they may have something useful; I'll see about checking my paper copies in a while but in the mean time:

Measurements at centimeter wavelenghts
https://archive.org/details/measurementatcen0000unse

Microwave spectroscopy (has something about probes / launchers / couplers etc. as I recall)
https://archive.org/details/dli.ernet.285394/285394-Microwave%20Spectroscopy

Very high frequency techniques (?):
https://archive.org/details/in.ernet.dli.2015.5539/2015.5539.Very-High-Frequency-Techniques-Vol-ii?q=%22ultra-high%22+frequency+techniques

Terman's radio engineering handbook (probably not a lot of content but maybe something useful)

ARRL handbook (probably not a lot of content)
 
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Offline evb149

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Offline evb149

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Re: Resources for Designing Cavity Resonators
« Reply #5 on: June 13, 2021, 08:33:10 am »
Reference data for radio engineers 4th edition paper copy has some useful information in chapter 21
"waveguides and resonators".  I assume other editions may have similar.
https://archive.org/details/referencedatafor00west
https://archive.org/details/referencedatafor00itt

And as cited in excerpt for discussion / reference uhf techniques has some few pages on it
 
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Online mawyatt

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Re: Resources for Designing Cavity Resonators
« Reply #7 on: June 13, 2021, 02:59:11 pm »
The mit radlab series has some practical information on that, and is free online, as I recall.
Obviously it isn't modern, but some things never change, and if anything it's more from the
peak times of cavity r&d than things made in subsequent decades.

You could see if microwave journal, microwaves101 web site have relevant articles.

BSTJ probably has some, too, and it's also free online last I knew.

Awhile 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,
Research is like a treasure hunt, you don't know where to look or what you'll find!
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Offline xmo

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Offline WideBandwidth

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Re: Resources for Designing Cavity Resonators
« Reply #9 on: June 14, 2021, 03:41:46 am »
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
 

Offline evb149

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Re: Resources for Designing Cavity Resonators
« Reply #10 on: June 14, 2021, 07:09:21 am »
It depends on how you do it.

If the tape is very long / wide so you end up with mostly a single piece in there, that might not be so bad.

The issues are:
1: Surface roughness -- as you say with a 3um skin depth anything that isn't polished flat to within a small fraction of that creates signal disturbance and added resistance no matter if there is a thick solid copper plane just below it.

2: Seams.  Since the skin depth is small a solder joint along an edge does produce a "long" region where the signal transitions from the surface of the Cu to the surface of the soldered seam, over the top of the solder, down to the Cu layer where the other end of the joint is.  Not the end of the world, but lossy.

"pro" RF cavities are not only say brass or copper or what not inside, but they're then electroplated with a polished layer of something like ultra high conductivity silver lining on the inside then that's possibly coated with something to protect the silver from oxidation.  Just the difference from pure polished copper to pure polished silver is enough to matter for the Q in not a few cases.

But it depends on your application and quality and stability requirements.  There are enough amateur radio articles talking about making cavities out of pipe or metal cans / bins.  And at minimum some people make VHF cavities others up into
high UHF or cm wavelengths.  Lower frequency is more forgiving dimensionally and in materials.

But just get any old paint can, or maybe tin can attach a probe, hook up your VNA and you'll probably see a strong resonance at some relatively predictable dimension just like an old grid dip meter operated somewhre roughly near a lumped tuned circuit.  So you absolutely can experiment with this with simple materials. 
You just might not achieve more than a fraction of the stability / Q which is possible with more careful materials and designs.

You can also experiment with coaxial line resonators or patch / plate resonators and so forth and get relatively high Q.

Dielectric loaded cavities are another possibility where you can fill it with low loss wax or polystyrene or silica or some such thing that has low loss @ F vs its Dk and maybe thereby tune a smaller (easier to make) resonator down to resonate at some significantly lower frequency.

Reentrant designs can be useful / interesting, too, or making a cylindrical one ywith a tuning piston and maybe a quarter-wave choke flange for the tuning piston if you're so inclined.

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
 

Offline T3sl4co1l

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Re: Resources for Designing Cavity Resonators
« Reply #11 on: June 14, 2021, 10:09:17 am »
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
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline arlo_g

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Re: Resources for Designing Cavity Resonators
« Reply #12 on: June 14, 2021, 12:07:19 pm »
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.html

Scotty 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.
 

Offline cdev

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Re: Resources for Designing Cavity Resonators
« Reply #13 on: June 16, 2021, 04:06:08 am »
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 food

W1GHZ has some web pages on making them cheaply out of plumbing fixtures.
"What the large print giveth, the small print taketh away."
 


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