Owen,
I gather you are interested in measurents only in the 40m band which
makes life easier. The photograph of the loop and line makes it more
clear what you are up to.

Yes, there will be no standing waves on the line if the line Zo is
equal to receiver input impedance. (I didn't make myself clear). If a
balun is used it doesn't matter much what Zo is, provided the balun
has the correct ratio. So it is necessary to know what Zo actually is
just as accurately as the input impedance is known.

With your setup it is impossible to match the loop to the line. But if
it WAS possible (eg., as with a magloop) it would NOT be to prevent
standing waves.

Your calculations take the loop/line mismatch loss into account. Its
only a few dB.

Incidentally, have you considered what effects increasing the number
of turns to 2 or 3 would have? They MIGHT possibly be beneficial. It
needs more calculations. As with just increasing the size of the loop
which almost certainly would be beneficial.

You have set yourself a most interesting and useful task. I wish you
well with it.
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Reg, G4FGQ