"Al" wrote
The shielded three-foot diameter loop I built is for receive only. The
primary loop is six turns paralleled by a variable capacitor, and the
secondary (link loop) is one turn (same diameter) fed to a preamp or
directly to the receiver. The band of operation of the loop is about
200kHz
to 500kHz (NDB chaser).

One question that is not answered in rjeloop3 is what should be the
distance
from the output (link) loop to the main loop? The antenna's I built used
flat cable for the wiring and one of the turns was the link so the
distance
was 0.050 inches. In other articles I have read that the Q, gain, and S/N
ratio can be varied by varying the distance between the two loops.
Approximately what is the distance? Are we talking a small fraction of an
inch or 2, 3, or perhaps six inches apart? I would just like a feeler
distance to work with. If the distance is to be over one inch, I'm
considering building the primary loop in one hoola-hoop form and the
secondary loop in a second hoola-hoop then varying the distance between
the
hoops. Any thoughts on this or is this a waste of time and effort?

=====================================
Hi Al,
To have a significant effect on operating Q it is necessary to shift the
coupling loop away from the main loop by more than about 1/10 of loop
diameter. This is mechanically inconvenient and anyway I've forgotten how to
calculate the coupling coefficient between the two coils. A more convenient
way of varying coupling between main loop and receiver is to place a smaller
loop inside the main loop in the same plane. Just like a Magloop
transmitting loop. Additional variation can be obtained by rotating the
coupling loop relative to the other. It is easier to calculate.

The two loops constitute a transformer. It makes little difference
wherabouts the small loop is located inside the large loop. So it can be
located near the bottom of the main loop near the tuning capacitor. It is
better to have no direct connection between the two loops. It may upset the
natural balance between the main loop and its surroundings and ground. One
side of the coupling loop is grounded at the receiver end of the line.

If the line is an appreciable fraction of a wavelength (very unlikely) then
use an impedance Zo in the same ballpark as the receiver input Z. It hardly
matters whether it's coax or very loosely twisted pair.

The transformer has an effective turns ratio of -

N = (N1*D1) / (N2*D2).

N1 = main loop turns.
D1 = main loop diameter.

N2 = coupling loop turns.
D2 = coupling loop diameter.

Which may be very interesting but will not be of the slightest use unless
the receiver input resistance is known.

RJELOOP3 calculates the parallel L and C resonant resistance of the main
loop. Call this R1. To match this to the receiver input resistance R2, make
N = SquareRoot(R1/R2).

When impedance-matched to the main loop the receiver input resistance damps
the intrinsic Q of the loop to exactly half of its unloaded value. The
received signal is then maximised.

Prefer to use a larger diameter for the coupling loop rather than a larger
number of turns.

To maximise operating Q and selectivity match the loop to 1/2 or 1/3 times
the receiver's actual input resistance. There will be a few dB loss in
signal strength.

None of the foregoing will have any effect on signal to noise ratio except
when the signal bandwidth is appreciably smaller than the bandwith accepted
by the loop. But it is best to manage channel bandwidth in the IF amplifier.

The natural Q of a good receiving loop at LF and MF is already high enough
to spoil the audio quality of broadcast stations.
---
Regards, Reg, G4FGQ