Jim and others,

I also find that external effects affect the depth of null you get - in my
field strength measuring experiments, I found some locations where there
was almost no null - This seemed to occur near overhead wires and power
lines, which were presumably acting as parasitic antennas, inducing

signals

in the loop.

I am aware that coverage surveys of LF and MF AM transmitters do not conduct
field strength measurements in locations near power lines, as they otherwise
get false readings.  For amateur LF receiving, much of the QRM is associated
with power lines and connected appliances, so consideration of conduction,
induction and radiation is needed to analyse scenarios.

In my view the specific location of a loop receiving antenna, suitably
separated from mains wiring, has far more impact on weak signal performance
than whether it is unbalanced, partly balanced, floating, or well-balanced.
Near fields reduce rapidly with distance (and different rate for E and H
component), while radiated (far) fields are basically inverse with distance,
so the differential distance with respect to the DX wanted signal is
negligible within the choice of options for siting a loop receiving antenna.
The LF far field is also vertically polarised.

The side-on null of a small loop receiving antenna is only effective in
reducing QRM for "point source" emissions.  The actual near field situation
is that mains wires are the launching mechanism, so the mode of coupling can
be modelled on a "line source".  A deep null is of little use in minimising
coupling with a "line source".

The size of the loop is also a compromise between being sufficiently small
to maintain useful separation from local QRM sources (induction coupling),
while being sufficiently large so that QRN (external fields) dominate
thermal noise (internal noise).  A square shape with 2 metres each side
appears to be about right for the LF bands of interest to amateurs.

73, Bob ZL2CA