Research and analysis

EMFs: exposure from lightweight aviation transponders

This report (HPA-RPD-031) includes an investigation of exposure to electromagnetic fields from proposed lightweight aviation transponders.

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An investigation was carried out to determine exposure to electromagnetic fields from proposed lightweight aviation transponders operating in Mode S and used near the body. The assessment has been made for peak radiated powers of 30 watts (W) and 80 W with a duty factor of 0.55%, in accordance with advice from the Civil Aviation Authority.

Numerical models of a generic transponder have been developed with typical antennas and simulations carried out to investigate the electric and magnetic fields. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) public exposure reference levels were exceeded within 3.6 cm and 5.4 cm for 30 W and 80 W transponders respectively. For occupational exposure, the corresponding distances were 3.0 and 3.9 cm respectively.

A scientific literature review was performed and indicated that the localised specific energy absorption rate in the head and body should be within the 2 W kg-1 public exposure basic restrictions for 30 W transponders with similar size and shape to mobile phones. The 10 W kg-1 occupational exposure basic restriction seems unlikely to be exceeded by transponders.

Lightweight transponders will have similar output powers to other devices used near the body for which testing is carried out by manufacturers against the ICNIRP guideline values. The technical standards which are evolving to support this testing may contain procedures suitable for testing aviation transponders, and such testing by manufacturers would seem appropriate, particularly for 80 W transponders.