This article Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz used a 600mm a side square loop which was originally designed for field strength measurements on the 40m in an effort to understand and document BPL (PLC) emissions.
As part of validation of the antenna, a free space NEC model with external excitation was developed. This article publishes a graphic summary of the antenna characteristic. The model antenna is loaded with 50+j0Ω and includes 10m of RG58A/U which was used for the BPL related measurements with FSM.
The antenna can be considered a small loop at frequencies where the perimeter is less than λ/10, which in this case is 12.5MHz. At these frequencies, the current magnitude and phase around the loop is approximately uniform.
Antenna Factor vs Frequency
Though the Antenna Factor becomes quite large by the lower end of the AM MW BC band, the antenna is still quite usable within the service area of a transmitter because the Field Strength is typically quite high, >70dBµV/m.
The antenna has been used to good effect with a tinySA Ultra spectrum analyser (see the referenced article).
Gain vs Frequency
