I see you are a fellow Juno user and may not be on the Web.
If you can get on, there's lots of pictures & data about feed horns..
A feed horn looks like metal container with one end open.
It can be round or square, straight or flared (to the open end).
There is normally a small wire probe inside that is connected to
an RF connector on the side.
The feed horn (as most often used) is a small antenna that is
placed at the focal point of a reflecting Dish. RF energy enters
the open end of the feed horn an is picked up by the probe.
That energy is then passed on to the connector, LNA & etc..
The dimensions of the feedhorn depend on what frequency
it's designed for and the shape of the Dish. It's main
gain patten (or beam width) must cover the surface of
the dish, with a minimum amount of side-lobes..
73, Rich <>
Richard Tyndall NJ1A Woburn, MA. USA
RTyndall@juno.com or firstname.lastname@example.org
ARGUS Observatory FN42jl