I have an offset dish which has an f/d around 0.6, so I decided to make one of Jeffrey Pawlan WA6KBL’s linear dual-mode feedhorns as published in DUBUS 2016/1.  The design uses a stepped horn and an oval iris with direct WR90/WG16 flange connection.  The horn and round iris were turned from 65mm diameter aluminium bar, and the oval iris was milled from 10mm flat bar.

I used metric M2.5 capscrews to fix the three sections together, and tapped the waveguide mounts M4.

I found a 3/4 inch end mill in one of my dad’s old toolboxes, so I used it, not realising that he’d modified the grind, and that the sides were not parallel.  Took me three wrecked versions of the oval iris to realise what was wrong.  Simple enough to fix, I just milled out the waste in 0.5mm steps.

Although the joints appear to be perfect, I think I’ll need to add a bit of protection to prevent water ingress at the interfaces.

I will probably make a polyethylene cap for the end, although building foam was also recommended as a fill, to prevent any chance of condensation from diurnal pumping

Apart from the issues with that end mill, it was a nice straightforward project.  Next step is to measure the return loss and then make a mount to replace the LNA collar on the dish mount.  Might use a three-rod mount using aluminium tube or stainless steel rod.