Section 7 - Electromagnetic Compatibility (EMC)

7.6 Operating Practices and EMC

The likelihood of encountering EMC problems will also be governed by your choices of output power level and transmission mode. The greater your output power the more likely it is that problems will occur.

Constant level transmissions such as FM and some data modes such as PSK offer a relatively low risk of EMC problems. Those whose level changes frequently are most likely to cause problems. SSB, where the level changes continuously with voice amplitude, and to a lesser degree AM, pose the greatest risk of interference problems. CW poses some risks since the carrier is being rapidly switched on and off. Good circuit design will allow the transmitter to make smooth changes from on to off thus minimising potential EMC problems.

7.7-7.8 Immunity and Use of Filters

Immunity is the ability of a device to withstand interference and continue to function correctly. It is now a legal requirement for manufacturers of electrical equipment to ensure that basic immunity standards are met. The introduction of the "CE mark" and the accompanying EEC directives have done much to focus manufacturers minds on EMC issues and we can now be assured of a reasonable level of immunity in all new pieces of equipment.

But what is a "reasonable level of immunity"? In the average house, the equipment will be subjected to many radio signals, from cell phones, commercial radio transmitters etc, but none of these is likely to be anywhere near as strong as those from an amateur radio station next door. As we discussed earlier, the size of a radio wave drops rapidly as distance increases, so although the broadcast transmitters are much much more powerful than our station, by the time the radio signal reaches our neighbour's house it is much smaller. It should not therefore be surprising to find that a certain piece of equipment lack sufficient immunity to cope with our transmissions.

Immunity is normally a designed in attribute, but we can boost a piece of equipment's immunity by fitting readily available filters to it. Ferrite rings or clamp on beads can be fitted to mains power and other leads close to where they enter the equipment. For direct pickup by the aerial, or pickup on the aerial down-lead, a plug-in filter can be fitted to the antenna connector of the affected device.

Filters are readily available from most radio dealers. At the Foundation licence level, we should only use commercially manufactured items, and should not fit any filters that require internal modification of the affected equipment. As we progress to the higher licence classes, our knowledge of EMC will grow and we will be better able to construct, use and assess home made filters.

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