Fifty years ago I fabricated and sold 1-M diameter, single-turn, resonant-loop antennas for the AM broadcast band.? These were designed to recover a virtually noise-free signal for AM broadcasters with studios in downtown office buildings where elevator noise and other interference was a significant problem.? Coaxial cable lead-ins just did not work.? The solution was simply shielded audio wire with a ferrite balanced-to-unbalanced transformers at the modulation monitor or other equipment end, as the very low impedance feed from the center-grounded loop antenna was well-balanced.? As I recall, such shielded audio cables measured in the 100 to 150-ohm range.
Bruce, KG6OJI
ebrucehunter