Spaceflight 101's New Horizons Spacecraft Overview is full of interesting information and shows this RF Comm System Block Diagram. The thin (red) lines to the dish antenna area are coaxial cables and the thick (blue) lines are waveguides. Switching is done at the waveguide level.
Like Voyager, the secondary mirror reflects some signals to a pair of coaxial couplers at the secondary focus, while allows other signals to pass through to the primary focus to another pair of coaxial couplers. Each pair has an RHC and LHC label, but I don't know what those mean.
However, there is a third pair of coaxial cables that appear to end on the surface of the secondary reflector and would therefore not be in focus.
What is the function and nature of this third RHC/LCH pair?