[Elecraft] Elecraft K3 RX Antenna OUT termination

[Michael Carter]

Not disagreeing with the general advice Dick, W1REJ offered, but inspection of the schematic of the KXV3B module (and I recall that the KXV3 and /3A versions are similarly configured) reveals that if RX ANT is not selected on the front panel, the RX ANT IN and RX ANT OUT ports are not connected together save for any capacitive coupling across the relay poles.  Moreover, these ports are not coupled to other circuits internally if RX ANT is not selected.

I have not had occasion to do this, but my understanding is that the RX ANT IN and RX ANT OUT ports can be connected to the output and input, respectively, of a low noise preamp or bandpass filter.  The old Elecraft PR6 and PR6-10 external preamps were designed for just this purpose to allow use of the transmit antenna routed through the external preamp on a band-selected basis for receiving only using the RX ANT per-band selection.

If RX ANT IN is connected to the outside world, and RX ANT is selected, the RX ANT OUT port is connected on receive only to the transmit antenna port.  Could extraneous noise be coupled from the transmit antenna into the receiver front end?  Sure, but unless the stray coupling in the RF board and/or KXV3 module is pretty strong between the RX ANT OUT traces and RX ANT IN traces, or across the relay poles where those signals are routed, the impact of the unintended transmit antenna noise is minimal.  Terminating the RX ANT OUT port in 50 ohms may not attenuate that internal coupling from whatever does exist.  Perhaps someone has already measured the coupling/attenuation between the two RX ANT ports?

73,
Mike, K8CN