This way I could use separate left hand/right hand circular feedhorns for receive and transmit.
The down side is of course, that this is a somewhat cumbersome mechanical contraption.
The good thing about this setup is that you can separately optimize the TX feedhorn for max gain and RX feedhorn for min noise.
The RX feedhorn + low noise preamp could also be put together into a thermal enclosure for cooling, as radio-astronomers routinely do on mm waves. Usually, with a cooled preamp, the biggest problem is the input RF cable: it must be low loss RF-wise but must also have low thermal conductivity - that is a mutually exclusive requirement. By putting the feedhorn in the cold box, this problem is eliminated.
This is the mechanical layout:
green=fixed part
red=moving part
grey=ball bearing
blue=motor
yellow=end switches
This is the geometrical layout:
Axes of the feedhorns meet at the rotation axis!
yellow='slow' switches
green='stop' switches
red=elastic fingers that engage the switches
First, I used a single speed schematic like this:
Note that at the end switch, the motor is short-circuited for braking. Even so, the moving part did bang into the endstops with too much force, because I used a relatively high speed, to be able to hear my own echoes. (Switch time less than one second).
Because of that, I have added two more switches, that reduce the speed just before the final position:
The "TX gate" output (+12V when switched to TX) goes to the twt supply where it allows the beam to be switched on only after the switching to the TX horn has been completed.
Here are two photos of this thing:
and here is a MPEG movie (2.6MB) of the thing in operation - it is the old, full-speed only version, you can hear the bangs!