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.

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

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.

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