The problem was summarised by the late Fred Wise in Planning Medium Frequency Broadcast Radio Services, published in IBA Technical Review No. 7, July 1976:
``The strength of a radio signal is expressed in terms of its field strength, and the most straightforward interpretation of the term field strength in the context of the mf band is the ratio of emf induced in a short vertical wire to the effective height of that wire. It is normally expressed in mV/m, or alternatively in dB relative to 1 µV/m.
``In principle, field strength could be determined by measuring the emf induced in a vertical rod. Such a measurement would give directly the magnitude of the electric component of a radio wave, but, because of instrumentation difficulties, an alternative procedure is normally adopted. Thus, in practice, the aerial used is almost invariably a shielded loop which gives an output proportional to the magnitude of the magnetic component of the wave.
``In the case of a plane wave, and in a situation where no discontinuities in the ground constants exist, the electric component E and the magnetic component H are related by the equation
E = 120.pi.H
where E = volts/metres; and H = amps/metre.
``Normally, therefore, although a loop aerial measures the magnetic component of the field, calibration can be directly in terms of the electric component. Should there be any discontinuities in the ground constants, or any reflecting objects in the vicinity such as steel-framed buildings, this relationship no longer applies and the loop aerial can then only be used to indicate the strength of the magnetic component.''
It ought to be possible to design a simple but reasonably accurate field strength meter for 73 kHz. The relationship between field strength and the output of a shielded loop should be predictable, and it ought to be possible to construct a TRF receiver of known gain using good-quality audio op-amps.
Please remember that this method only holds good in the far-field region. The near field will cause errors, so it will be necessary to take readings many kilometres away from the antenna.