Re: Coax Probes into Waveguide


Christopher Bartram <cbartram@...>
 

Hello Andy

My experience in that area is not too extensive, but I have been down a similar road to you, while designing my dual mode filter.

I had a good look at things that had previously been published, both professionally and in the amateur radio literature, and found very little to help. I suspect that a lot of information is available in company archives somewhere, but hasn't been published for commercial reasons. Certainly, I wasn't able to find any 'cookbook' design data from easily available sources

My approach was to use EM simulation to understand the whole of the filter. That allowed me to develop the loosely coupled transitions from 2.2mm coax to waveguide which were needed, without cutting metal. While the length of the probes, and their location along the guide were fixed, their penetration into the guide was a variable, allowing good RL to be obtained when the filters were set-up on my aged microwave net-an. I'll send the detail of the matching arrangement if you'd find it useful, but I don't think it would work without a lot of modification in your filter.

Once I finally retire, and have some time of my own, I'll write-up some of the microwave design work which I've done :-) My filter, of which I've made some 35 samples, was built from brass, managed 30MHz -3dB BW at 10368 with an insertion-loss of about 1.5dB at CF. There were no spurious responses below 10.37GHz, and the first spurious response above was at about 13.5GHz. And it didn't use irises!

Incidentally, I've used one of my prototypes as a dual resonator in a 'proof of principle' prototype of what I hope will be a very high performance VCO for 10GHz. The two cavities are effectively in series, so the phase slope of the resonator is extremely steep, resulting in a very high effective loaded Q. A low 1/f noise microwave amplifier is proving to be an 'interesting' exercise. More on that anon.

Vy 73

Chris
GW4DGU

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