How can the output voltage of a multiple-turn receiving loop antenna be increased?
Why A small receiving loop works by magnetic induction: the voltage induced is proportional to the number of turns times the area enclosed times the rate of change of the magnetic field (V = 2π f N A B / λ-related constant). So doubling the turns or doubling the enclosed area roughly doubles the output. That is why direction-finding loops are wound with many turns on a fairly large form.
Watch out Using high impedance wire for the coupling loop does nothing useful; the coupling loop just transfers energy to the feed line, and adding resistance only adds loss. Permeability of a shield is not a voltage-boosting factor either, since the shield is there to reject the electric field component.
Loop voltage goes up with N times A: more turns, more area, more signal.