Re: Testing return loss of 24 GHz antennas and transitions



Amongst the things in my junkbox  are some nice Flann broadwall WG20 directional couplers and once in the WG domain I can measure return loss to a reasonable accuracy (Directivity  is 35dBmin)

Even with my best commercial coax transitions, the measurement uncertainly becomes  interesting.

 I have a 26GHz 50 ohm calibrated load  which helps  but without a proper coaxial S Parameter test set and Scalar  analyser,  I struggle.

The only mechanism I have left but not yet explored at 24GHz, is wobbulation.

A non-directional coupler  (signal sampler) and detector with a resolution of 0.1dB is all that needed. Absolute accuracy is unimportant

 You either change the sample point along the transmission line ( difficult in coax) or sweep over a suitable frequency range

 The resultant  amplitude vs frequency ( or distance) response will  vary,   FFT analysis will give you DTF / TDR*  information / antenna return loss information. You can do 'qualitative'  FFT anaylsis in your head :)  You wont be able to  determine accurate DTF  info this way but you can optimise antennas.  The technique does not lend itself well to very short transmission lines

 To calibrate this, ideally  you need a good know 50 Ohm load. With that I can probably help by 'matching' my good reference to a secondary load with a similar response.

 I believe I could achieve a secondary and traceable measurement system with a directivity of at least 25dB.  Not great but adequate.

On the subject of silver components, if you need silver, i have plenty and can cast into large ish  rough shapes suitable for your machining  skills, (Delft Clay casting method)

 I have about 300g of silver 'in stock' for casting, so there is scope for experimentation.

 There are few component that would benefit greatly from being made of silver  but if you find a need, give me a call.

 I can cast gold too of course but that is really not very cost effective. I don't have much surplus 18k + gold either :)

Good luck..  This is a field that few amateurs master with reasonable accuracy but it is possible  at reasonable cost, with effort, even at 24GHz.


* Note that the terms DTF (Distance to fault) and TDR (Time domain Reflectometry) are often interchangeable but there are are differences ,  associated with the way the data is measured. Measurement  parameters are  important  for these techniques and it is easy to  get things wrong and 'contaminate' results with errors  that are not obvious.

On 19/03/2021 20:09, Neil Smith G4DBN wrote:
I'm just embarking on machining some coax-input feedhorns and waveguide transitions to use with these Wavelab units on 24 GHz, but I don't have anything I can use to check the return loss of the transitions or horns. I can make up a 20 dB cross-coupler in waveguide, but while I can sort-of use that with my spectrum analyser as detector, assuming my best 50 ohm load looks vaguely resistive at that frequency, I can't think how to check a coax-fed feedhorn.

Without access to a 24GHz-capable VNA and the right connectors, obviously.

It is just a question of modelling it to get close and then trimming for maximum smoke on TX and use cold sky versus moon/sun noise to verify that it is working right?      Is there some magic bit of testgear I can make or obtain to get the thing tuned spot on and remove any uncertainty?

I'm thinking of using a 2.92 mm connector with a hermetic seal feedthru and a solid silver probe, because why not. They seem to be cheaper than 3.5mm or high-end SMAs. That much Sterling silver is about 10p.

It would be SO much easier with a waveguide launch PA/LNA and w/g relay, wouldn't it...

Neil G4DBN

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