[Ground-station] Dual band feed for Phase 4A

[Michelle Thompson]

We're putting together a design for a dual band feed for Phase 4A on Phase
4 Ground Slack and I wanted to share it here for wider comment.

We've been asked about this a lot since we published the dual band
(5GHz/10GHz) feed for Phase 4B.

The idea for Phase 4A is a helical (for 2.4GHz, LHCP) and an x-band
horn/cylindrical waveguide in the middle.

We executed a crude drawing, reviewed it briefly at San Diego Microwave
Group meeting on Monday evening, and attached is draft calculations for the
helical from a popular online calculator.

It can be 3d printed, which always excites me, but can also be put together
very quickly with waveguide and spacers.

Please pitch in with the 10GHz part if you have dimensions and we'll get
some made and tested.

There are two 10GHz transponders, one narrow band and one wideband for 4A.
They are vertical/horizontal to separate. One can physically twist the
entire feed to go from one to another, but I'm also interested in other
more elegant ways to solve this part.

Comment and critique welcomed and encouraged.

-Michelle W5NYV
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