Reg, G4FGQ wrote:
"The transmitter is OFF when I do it manually and I tune for maximum
noise in the receiver."

As Reg said, reciprocity rules.

You can also satisfactotily tune an antenna to resonance by adjusting it
for maximum output. When I was in Tierra del Fuego we had Land Rovers
and boats equipped rith RCA single-sideband HF radios.

The Rovers had whip antennas atop fiberglass boxes containing loading
coils and motor-driven tap-changing switches. The coil base-loaded the
whip.

I routinely tuned the whips by switching my multimeter to the a-c range
(it used germanium rectifiers) and tuning for a maximum meter
indication. I chose meter-probe positions for a convenient scale
indication.

The above was accomplished with the transmitter ON and fed with a test
tone driving it to put out some power. The whip was mostly
non-directional so it made little difference where the r-f sensing
antenna was placed. The tuning procedure worked fine even with the
multimeter in the near-zone laying on the bonnet (hood) of the Rover.

The capacitor tuning a high-Q loop could also be tuned for maximum
output because that is your goal. The sampling antenna could be located
almost anywhere but probably not in a null of the loop.

Best regards, Richard Harrison, KB5WZI