I have been running a converted Inverto Black Ultra for three years now without any problems.
The interface consists of two parts made in a lathe and held together with a tight shrink-fit.
(a tube for the circular waveguide and a plate for the flange, a thin line shows where they meet)
Normally you would have to remove a lot of metal in the lathe if the part is both long and with
a large diameter somewhere but this is made with a shrink fit between a tube going into the
old feedhorn and a plate to make up the C120 flange. You can also optimise each part before assembly.
There is metal to metal contact in the bottom of the new circular waveguide. It can be improved further
by conducting paint (with lots of silver in it) but this one works without it. The tiny locking screws
press the interface down onto the other side. The polarisation angle is adjustable so it can be made to be
the same as other LNB's, easier to swap LNB's for tests if they are close to the sweet spot at the start.
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