24GHz Horn Optimisation


Chris Bartram G4DGU
 

Hello Neil

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 went through exactly that problem in the late 2000s when I was developing a feed for my then 2.4m dish. The basic design was conceived on paper and optimised with the aid of a simulation tool to which I used have access.

My solution FWIW was to find a couple of measurement quality directional couplers going cheap at a Roundtable. Not cross couplers, but broadside coupled designs: cross couplers are OK, but it's more difficult to get good directivity the 20dB level. From memory the coupling losses of mine are 20 and 30dB. ISTR they were made by Flann. There is design info. available I was also able to locate a couple of 3.5mm - coax adaptors, a couple of WG loads and courtesy of the late G3GNR, a tunable WG20 Gunn source. The difficult thing was to find a good 3.5mm coax termination. I did, but my lips are sealed as to the source. All I will say is that it was a rep. who like me has since retired!

Incidentally, there is  range of affordable 25GHz SMA connectors made by Emerson.

It's always worth having the ability to look at things out-of-band. It can show a lot of potential problems.

My own 24 GHz system uses WG20 and a Rellcom TR relay. I did use a coax system initially, but it didn't work as well as the current WG system.

Hope that's useful.

Chris G4DGU