Question:? Are we talking about resonant or non-resonant coupling coils?? There is considerable difference in how these should be handled.? I have used a resonant ferrite loop to couple a resonant spiral loop to the receiver and it worked well for medium wave frequencies, and the two antennas can be phased to produce peaks or nulls.? Otherwise I have always used low-impedance, shielded, non-resonant loops for medium wave.
Several years ago I started building loops for shortwave, again resonant, spiral primary circuits.? I soon found low-impedance coupling coils to be quite critical regarding size (inductance).? Since these loops covered a range of several MHz, I have always suspected that their field impedance has varied with frequency.? I have had to tap the low-impedance coupling coils using a rotary switch to match the impedance to the low impedance of the transmission line (twisted pair) and receiver.? Just one turn difference on a small coil can make a huge difference in Q.? A mis-sized coupling coil can actually kill the input from the primary circuit.
My point is, there may be something to the 1/5 rule, which I have also heard about, but there appears to exist an impedance matching issue, which I have failed to fully master.
Bob C.
Dexter