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Despite the advent of the internet and streamed services, there is still an interest in traditional broadcasting. With that interest comes a need to measure the associated electromagnetic field strengths, both of the wanted transmissions and of atmospheric noise that could be interfering with them. For instance, at the time of writing, the Â鶹ԼÅÄ intends to transmit a trial DRM service on 3,955 kHz. It is also concerned about interference from wireless power transfer (WPT) devices to AM transmissions in the LF and MF bands. An accurately calibrated and sensitive antenna is needed for such work.

This White Paper looks at the characterisation of magnetic loop antennas — those sensitive to the magnetic component of an electromagnetic field. Generally, such antennas are intended for use below 30 MHz. Unfortunately, generation of a travelling electromagnetic wave at these frequencies — the ideal test signal — is impracticable within any reasonable enclosed space; the wavelength at 200 kHz is 1,500 m, for instance. To make matters worse, these antennas are often too large to be tested satisfactorily in a substitute such as a GTEM- or PTEM-cell.

An alternative to these traditional methods involves the use of a ‘small coil’ placed at the centre of the loop antenna. This is described here, along with the work needed to eliminate possible sources of error. The result is a reliable plot of antenna factor (the field strength divided by the antenna output) versus frequency. By combining the antenna factor with the noise measured at the output of the antenna, it is then possible to calculate the equivalent noise field — an essential parameter in deciding whether an antenna is capable of measuring the expected levels of environmental noise.

There is also a short discussion on the use of magnetic loop antennas and possible shortcomings such as spurious sensitivity to electric fields.