John,

 

if you would use a large loop laid out flat on the ground, the resulting polarisation would probably suppress most of the vertically polarized far-field interference. In addition, you could use a gradiometer pair, eg. 2 turns with 50 m diameter inside a single reversed turn with 70 m, acting as a series-connected active shield. The receive-"antenna" sensitivity would ultimately be limited only by the thermal noise of the ground conductance, coupled by eddy currents. Under these conditions, the optimum loop diameter is similar to the depth of the (magnetic dipole) source in the ground.

 

73 de Markus, DF6NM

 

Jim,

Do you think this approach would be worth trying at the surface station of a 
cave radio link?

Regards,
John G3PAI