Start by getting the reception equal in the two antennas.
The gain on the noise antenna should be set so that the main antenna
with the noise antenna disconnected (or unpowered, if active) is the
same S meter reading as the noise antenna with the main antenna
disconnected (or unpowered, if active).
You have to shade that to compensate for the phase control increasing
the gain at either extreme setting, but at least you start in the
right ballpark.
That the loop is itself directional introduces a complication. What
you want equal is the signal you're trying to eliminate, in both
antennas.
It can happen that the signal is already nulled in the loop, in which
case the loop can't help in nulling the signal in the other antenna.
Either reorient the loop or listen with the loop alone, in this case.
--
Ron Hardin
On the internet, nobody knows you're a jerk.