I meant to respond to your question earlier, but didn't.
I'm not sure of the specifications of the radio switches, but even if they are
quite horrible, they should be able to overcome a 60dB path loss. Your cable
(in the coaxial mode) will have an attenuation of about 10dB at 300MHz. So your
couplers can stand a 25dB loss at each end - that is, the coupling can be very
There are many good ways to do it, but what I would do is to use the coupling
loop, similar to that used in many SWR bridges, as proposed by Ian GM3SEK. I
would get a small die-cast box (~3"x1"x1") and put a connector on each end.
Then join the inners together. Then have another connector on the side of the
box which goes to the RF switch TX/RX. I would connect onto that a stiff wire
loop from the inner of the side connector, an inch or so long, and parallel
with the main line, and then going to a 47R resistor back to the outer of the
connector. (Or just using the resistor leads, cutting the earthy end short at
the resistor body.) This will then be a directional coupler (although not a
very good one), so the resistors should be at the ends connected to the feeder.
The length of transmission line in the box will not be 50 or 75 Ohms, but is
such a small length on 160m that it doesn't matter. The coupling loop is quite
short at 300MHz, so terminating it in 47R gives the switch transmitter some
sort of load - which probably doesn't matter but just might.
You can put the boxes wherever they are convenient in the feedline - as you
said you are using the differential mode - so common mode choking is
irrelevant. DC injection points however may matter - depending on their
circuitry they may greatly attenuate 300MHz.
I (and I strongly suspect others) will be very interested to hear how you get
on. The idea of doing the same thing appeals here, although it will have to
wait until the spring. We are promised -35C here tonight with the wind chill,
so antenna weather has long gone!
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