I'm very grateful to Bob ZL2CA for his important observations and
Firstly, a couple of points of clarification:
1) My loop is orientated in the vertical plane. It is a
delta-loop, having a 65 m perimeter, fed at the top with balanced
feeder. The line of the loop runs east-west, and therefore has
deep nulls to the north and south - very handy for attenuating
the noise sidebands from the Loran TX, located to the south of me
(in France). Further details can be found at:
2) The vertical antenna is located at one end of the loop.
The absolute gain of your short vertical will be higher than your
loop, so whatever figure of mutual coupling is found to apply, on
receive the loop will be affected more by the tuning situation of the
vertical than the other way round. With smaller frame loops for
receiving the impact of the vertical for reception on the loop is even
more pronounced (due to the much higher absolute gain of the vertical).
I had not appreciated this important point before now. I guess
it's one of those 'key facts' that every LFer needs to learn.
I find that on receive, unless the LF vertical transmitting antenna is
detuned, it receives local QRM from a fairly wide area and re-radiates
it to any LF receiving antenna on my property.
Again, thanks for the tip, Bob. I'll try and detect that effect
with my own set-up.
Regards to all,