Justin Gill wrote:
"Is anyone aware of any source of information / theory on H Field
antennas, such as Chelton Loop for HF?'

Search on H-field antenna. Then click on "Standard H-field NRSC antenna
-Chris Scott and Associates. The LP-S series stanard H-field Antenna is
specifically designed for emission measurement of AM broadcast stations
using a spectrum analyzer or other calibrated receiver.

Best regards, Richard Harrison, KB5WZI