Reg Edwards wrote:
One of the most serious sources of error will be pick-up on the long
line between the small loop and the receiver. With a coax line there
will be a greater signal pick up on the coax braid than there is in
the loop. They are both located in the same field.

So best to use very low impedance balanced pair line such as 50 ohms
perhaps with a screening braid. A good choke balun or a 1-to-1 wound
transformer would be advisable between the line and receiver input.


Agreed. Because of the inherent balance of the whole loop I have use a
"Voltage Balun", see http://www.vk1od.net/bpl/loop.jpg . I have made
observations of the received signal level when close to aerial telephone
lines carrying ADSL and the pickup level seems the same no matter which
side of the loop is nearest the aerial line.

Also, depending on frequency, length and impedances, there may be
standing waves on the line which could make a mess of your
calculations.

I have made the assumption that the line is adequately terminated in 50
ohms (the attenuator, and there should be now standing waves. Doesn't
that seem reasonable?


A change in line length is a good way to check for errors of this
sort.

Fortunately, field strength measurements are seldom needed to great
accuracy. Strength is usually required only to be less than or greater
than some specified value and there is an ample margin for error.

Personally, I think a tuned loop, in the fashion of a magloop, is a
better bet. With its small coupling loop the main loop can be
completely isolated from the line and the line can be ordinary coax
which matches a 50-ohm receiver.

A tuned loop is far more sensitive than the untuned variety. But its
operating frequency range is somwhat restricted.


Noted.

I have encouraged another ham friend to design an active loop with an AF
good enough to get the system noise floor below -10dBuV at 7MHz. That
is another alternative, and it has issues I know.

I am also considering trying to measure the performance of a portable
short dipole such as a buddipole ( http://www.buddipole.com/ ) for the
purposes of measurement down to ambient noise and a little lower.

Field strength measurements are essentially power level measurements
and, ideally, the pick-up loop should be impedance matched to the
receiver. Result : no reflections.

But if the rx terminates the line, does it matter whether the
"generator" impedance is matched? (I am not trying to bait anyone here,
but Reg, I think I understand the standing wave issue you are raising,
but my reasoning is that if the rx terminates the line sufficiently
well, then standing wave ratio will be small and the error contribution
negligible.)

(I think the lights have gone out on the other side of the big pond.)

Owen