John asked -

Why dont Wellbrook or MFJ incorporate noise cancellers into their RX
magnetic loops?
It would make sense.

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Because it wouldn't make sense.

The noise-collecting wire MUST be spaced an appreciable distance from the
loop. Otherwise the noise picked up by both antennas would either be in
phase, or would appear to be coming from the same direction.

More economical to put the noise cancelling arrangements in a separate box
as may be purchased commercially.

If there is only ONE dominent noise source the magloop does a very good job
all by itself. Its pair of 180-degrees apart nulls are extremely sharp
whereas its wanted signal pick-up lobes are very broad. The solitary magloop
fails only when the noise and wanted signal come from very nearly the same
directions. The extra noise cancelling wire would be seldom needed.

If there are TWO noise sources, to simultaneously cancel BOTH of them out is
well nigh impossible. It would require a well-distanced (1/2-wavelength)
noise-pick-up wire to distort the overall radiation pattern such that the
two magloop-nulls are no longer at 180 degrees to each other.

For perfection you would need TWO well-spaced noise-cancelling wires, each
with its own phase and amplitude controls, all of which would interact with
each other and with the rotation of the magloop.

By the time you had twiddled the phasing and amplitude knobs to your
satisfaction you would be half way into the next sun-spot cycle.

Back to the drawing board ? ;o)
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Reg, G4FGQ