Roy Lewallen wrote in
:

Owen Duffy wrote:
. . .
Fundamentally, wouldn't you expect S/N to degrade in a linear
receiver system as you decrease the Signal+Off-Air-Noise wrt the
receiver equivalent internal noise?
. . .

I wouldn't expect it to change by any discernible amount at HF, until
the antenna gets very short or very mismatched. The noise figure of
most HF receivers is good enough, and the atmospheric noise high
enough, that it takes a poor antenna indeed before the receiver noise
becomes apparent. This is certainly emphatically true on 80 and 160
meters in Florida.

Agreed Roy.

My graphic at http://www.vk1od.net/bpl/AreYouReady.htm shows the IRU-R
P.372-8 predicted galactic noise levels to be some 35dB above a good
receiver noise floor at 80m, and man made noise and especially
atmospherics are well above that.

Harking back to IMD noise, it is a result of non-linearity in the
receiver (front end usually), and can be improved by reducing the level
of unwanted (ie out of band) signals reaching the electronics. Any form
of front end filtering (like a detuned ATU) that favours out of band
signals relative to inband will only exacerbate IMD noise, not improve
it, so the proposed technique should not improve IMD noise.

As you noted, IMD noise can be an issue with a substandard receiver...
for example I experience it with an Icom IC-R20 on a half wave dipole on
80m. I agree with you that most good receivers should have very low
noise from IMD, so that AI4QJ's proposition without an apparent
explanation.

Owen