Experiments with helix element diameter
I did some sun noise tests recently on a 0.33 f/D prime focus 1.2m dish at 10GHz, using two different diameter wires for the helix. The 10GHz element of the feed is a 22mm diameter water pipe squeezed in a vice to fit inside a WG16 (WR90) flange (to connect to my DU3BC preamp).
The feed is shown below:
Sun noise measurements:
helix element not fitted = 8.5dB
1mm diameter helix wire = 8.0dB
2.75mm diameter helix wire = 7.0dB
These measurements seem to confirm that the reduction in performance on receive is caused by blockage from the helix, and that thinner wire is better.
The 1mm wire helix was no problem to match to 50ohms using the quarter turn strip per the G3RUH design, and seemed to have a good axial ratio.