"Bob Bob" wrote
I would also
like to get an idea how "critical" it is to make sure ones antenna
truly
is horizontal (eg not an inverted V or quad loop) if noise is the
greatest concern.

One would assume you also get a similar affect of "less horizontal
noise" from the actual noise source for the same reason. eg power
lines
radiate well upwards but not so well in groundwave.
========================================
Bob,

The angle of the 'horizontal' dipole relative to the horizontal,
whether it is an inverted-V or not, makes negligible difference to the
amount of noise it collects. It is non-critical in this respect.

The incoming, mainly distant noise comes in from all directions and
angles and is randomly polarised. Except, that is, for locally
generated noise, which is mainly a vertically polarised ground-wave
and from low angles to which the horizontal dipole is quite
insensitive.

Noise radiated from nearby elevated power lines is probably randomly
polarised and is collected in similar proportions by both horizontal
and vertical antennas. When a power line is half-mile or more away I
would guess that the received noise reverts to vertically polarised
groundwaves which at HF are rapidly attenuated. The horizontal waves
are even more rapidly attenuated.

Noise can be studied only from its statistical probability
distributions versus direction, angle, frequency, receiver bandwidth
and time. The opinions and anecdotes of individuals matter only to the
inviduals concerned and their locations on the Earth's surface.
----
Reg.