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Microwave resonances

Posted: Sat Aug 22, 2015 6:36 pm
by fdavies
One of the things that I love about electronic instruments is that one can often 'see' the invisible. I made a resonant cavity from a 23 oz. Ice Tea can (the largest aluminum can that I could find in the market). I cut out the end and made two small holes on opposite sides about 20% of the way from the open end to the closed end. I put a bulkhead SMA connector with a j shaped loop antenna in each hole and pressed the open end against a copper board. I then connected one SMA to the TX connection of my bladeRF, and the other to the RX connection. Here are two photos:
Here is a summary of my measurements:
dimensions of cavity:
length: 181mm (approx)
diam: 73mm (approx)

Mode: Calculated Measured (GHz)
TE111: 2.54e9 2.551, 2.555
TE112: 2.92e9 2.908, 2.915
TM010: 3.14e9 3.067
TM011: 3.25e9 3.263
TE113: 3.46e9 3.428, 3.431
TM012: 3.55e6 3.543
TE211: 4.08e9 out of range
TM110: 5.09e9 out of range

The three TE modes are not radially symmetric, so each is split into two resonances by asymmetries in the cavity, like the SMA connectors. The three TM modes are radially symmetric, so they do not split.

I think that I could get higher Q's (narrower resonance spikes) if I had a better way of closing off the cavity.

Re: Microwave resonances

Posted: Sun Aug 23, 2015 6:25 am
by bpadalino
These are some awesome measurements!

What software package do you use to make the images that you have superimposed on the graph?

Re: Microwave resonances

Posted: Sun Aug 23, 2015 11:37 am
by fdavies
I used Mathematica. I am usually a open source fan, but I got the $300 individual version of Mathematica for a present, and it can certainly make pretty pictures. It would have taken me a lot more effort to make the lines curve with Octave.