# Cavity Resonators by two waveguide coaxial adapters

1. Jan 17, 2010

### lukacsmw

One of my professors made an X-band cavity resonator by putting together, end to end, two X281A coaxial waveguide adapters (n-type) with cut center pins. It works pretty well - but I need some more information. Does anyone know where can I find some more information on this setup?

Specifically, I'm trying to determine the Q factor of such a resonator.

2. Jan 18, 2010

### Mike_In_Plano

Wow, pretty cool. So he decreased the coupling by cutting back the stubs and making the waveguide into a resonate cavity. I could see how that would work, but I'm not sure how you would find the Q for it.

I suggest you try rfcafe.com

- Mike

3. Jan 18, 2010

### f95toli

I seriously doubt there is an easy way to calculate Q. It is only when the geometry happens to be very simple (like a TM001 mode in a cylinder) that you can calculate Q with reasonable accuracy, and then only if you know the surface resistance at that frequency of the material (which depends on how well polished it is etc). And of course this will only give you the unloaded Q, in order to get the loaded Q you would need a complete model for the whole system.

Why not simply use a VNA and measure the Q?

4. Jan 18, 2010

### waht

Waveguide adapters are pretty expensive, I'm not sure why he would just cut the stub for a demo. The calculation for Q of a rectangular cavity and any relevant info can be found in Pozar if I remember correctly. Once you know the Q, you can do some perturbation to approximate how a short stub affects the Q. Generally, the smaller the stub, the higher the Q.

5. Jan 18, 2010

### Mike_In_Plano

I think it depends on whether his losses are dominated by the coupling to the stubs or the waveguide structure. I suspect that problem breaks down cleanly if the stub mismatch is low. Again, the fellows at rfcafe.com have a solid background in this type of problem.

6. Jan 18, 2010

### f95toli

Yes it can (pp 280, I always have a copy of Pozar on my desk). But again, in order to get a reasonably accurate value you need to know not only the dimensions but also Rs; which can be quite hard to find (you can of course just measure it experimentally for a given resonator, and then use THAT value to calculate Q for other dimensions which is what I normally do, I've learned from experience never to trust calculated Q values for high-Q resonators).

The best you can hope for is -in my experience- is to get an order-of-magnitude estimate; although then you might as well use some experience and say that the Q for a system like this will probably be of the order of a few thousand (I have a similar setup somewhere in the lab that I use for testing).
But even that estimate assumes that the system is undercoupled (which is by no means obvious in this case).

So again, why not simply measure Q? Labs where you can find these kinds of adapters normally have at least one VNA...