The work described in this report was carried out at the request of the BBC World Service and the EBU, who represent the interests of broadcasters at the ITU and CEPT/SE24 Group. These Groups are concerned with defining acceptable limits to the stray magnetic fields generated by Wireless Power Transfer (WPT) devices. In essence, the WPT transmitter uses a coil to generate an alternating magnetic field, whilst the WPT receiver uses a second coil to convert the magnetic field into electricity. In other words, the transmitter and receiver together act as a transformer.

BBC R&D provided an earlier report, ‘WPT: Plain Carrier Interference to AM Reception’, for CEPT/SE24. This dealt with acceptable interference levels and concluded that the existing protection requirements laid down by the ITU for the protection of one AM broadcast transmission from another remain appropriate for WPT. (With AM interferers, the carrier component causes the most audible effect.) An important concession was that the strict limits on interference levels can be relaxed if the interferer is kept within 50 Hz of the victim transmission’s carrier frequency.

More recently, interest has centred on Qi (‘Chee’) devices. The low-power specification, for delivery powers of up to 5 W, was first published in 2009 by the Wireless Power Consortium, and devices conforming to this have become very popular for charging mobile phones in particular. However, the BBC is concerned that the devices could cause serious interference to AM reception. This report looks at the workings of these Qi devices in some detail, and goes on to develop a model to predict the harmonic content of their stray magnetic fields. It then compares the predictions with reality. Finally, by combining these results with those of the earlier report, it establishes the minimum separation needed between the Qi device and an AM receiver if reception is to be unimpaired.

The conclusions are as follows:

The model developed is based on a +5 V supply being switched across a tuned circuit of which the inductor forms the coupling element to the receiver (the item being charged). The emission levels predicted by the model agree well with practical measurements made on a range of devices — at least when the devices are unloaded.

When the devices are loaded, emission levels can be up to 20 dB lower, depending on the construction of the devices and factors such as the physical placement of the load. However, it is likely that the devices will be left switched on but unloaded, and so the emission levels predicted by the model should be used when assessing the acceptability of the system.

The frequency of an emission cannot be guaranteed to be stable — indeed the system relies on adjustment of the frequency to control the power transfer. Hence the concession mentioned in the earlier report is not available.

As theory predicts, and is found in practice, the field-strength of an emission is proportional to the inverse cube of the distance from the source.

Assuming that the maximum acceptable interference level is –43 dBμA/m, as determined during earlier work, and that the strength of the emission is 31 dBμA/m at a distance of 1 metre, interference is a potential problem at distances below 17 metres.

This publication is part of the Distribution Core Technologies section