I have loosely assembled a copy of Peter’s G3PHO’s plumbing fittings implementation of a W2IMU dual-mode horn. At the moment, it is stuck together with insulation tape. To this I have added my own experimental design of a waveguide crook. Behind the horn is a 135 deg bend, followed by a 180 deg sweep bend up to the main bit of waveguide. Then there is a wide 90 deg sweep into the box. So it is a bit like a shepherd’s crook, but bendier.
This has probably all been done before for an off-set dish, so it is probably not a “G3YJR Crook” at all, it which case, here it is again!
I’ve used 22mm copper pipe as circular waveguide. I’m not using any rectangular waveguide. Rectangular waveguide seems popular and there are probably very good reasons for using it, but as a newcomer to microwaves, I don’t know them.
There is a twist to the polarisation along the waveguide. I haven’t measured this yet, but it looks like about 45deg at the coax feed point. My SMA-coax transition accounts for this. The twist seems stable. I’ve mounted a weather-protective plastic box behind the dish. The SMA feed transition sits inside the box along with the electrickery.
The idea is to move the pre-amp/transverter from the end of the off-set arm into the box behind the dish in order to minimise the weight on the off-set arm, whilst keeping the losses low by using rigid waveguide. My coax transition has two SMA feeds at right-angles in order to allow for switching polarisation.
With the dish in the terrestrial position, looking at the horizon, the waveguide slopes gently down to the horn so that any condensation should dribble down & escape from the horn.
Feeding the crook/horn from the DB6NT shows the same monitor output as I got using my ex-LNB horn with my home-brew SMA-to-rectangular transition: 5 “units” on the moving-coil meter scale. This looks encouraging. I need a way to measure the insertion loss.
I added a bracket under the box so as to not distort the dish with the weight of the box (not in this picture). I bought the box from Toolstation.
I tested the horn straight into the SMA transition using GB3FNY as a source. And then with the crook in between.
The results seemed comparable with the old LNB horn, so that seems reasonable. One surprise was that when I removed the back-short (the 22mm plumbing cap) from the SMA transition and left the waveguide open, I couldn’t see any difference in the signal strength.
Using the vertical polarisation SMA feed resulted in about a 20dB drop in signal (as predicted by Bryan G8DDK).