Foundation Course

EMC

Overview of EMC

EMC = Electromagnetic Compatibility

(The ability of radio transceivers and domestic appliances to co-exist without interfering with each other).

Interference caused to Domestic appliances

• Interference to domestic appliances may manifest by clicking, buzzing, muffled or clear speech or even in the case of a TV, patterning on the picture.

• Amateur Radio generates far more RF power than normal domestic appliances and unless care is taken with the installation a great deal of interference can be caused.

• Although the likelihood of causing interference increases with power, it should be remembered that even a few watts may be sufficient to cause interference.

Interference caused to Domestic Appliances

There are many ways that neighbouring houses as well as your own might pick up amateur transmissions. Examples include

• Telephone wiring• Mains wiring• Audio loudspeaker wiring • TV aerial/downlead.• At VHF/UHF by direct pickup by the internal circuits of

the affected equipment itself

If left unchecked this has the ability to cause neighbourhood disputes.

Interference caused to Radio receivers

• Interference can be caused to a receiver by many domestic electrical appliances.

• The ultimate cause is often a switch or a motor within the appliance.

• Other domestic appliances which may cause interference are plasma televisions, solar panels, inferior power supplies.

Station Design for EMC

EMC problems can be minimized by:-

• Siting antennas as far away from houses as possible, as high as possible.

• At HF use of (horizontal) dipoles (with a balun when fed by coaxial cable).

• At HF avoiding the use of end-fed wires. If these are to be used ensure proper matching with the transmitter with an ATU (Antenna Tuning Unit).

• Reducing your power where possible. The higher the power the greater the likelihood of interference.

The impact of mode on interference

• AM and SSB are the modes most likely to cause interference, it is even possible for domestic appliances to decode the transmitted audio and for this to be heard in place of or together with the domestic audio.

• FM is a far less likely mode to cause interference.

• Data modes such as PSK31 or morse code (CW) are also less likely to cause interference owing to the lower powers normally used/required.

RF Earth Connection• A good RF Earth connection will help reduce interference from HF

transmissions.

• This works by earthing stray RF currents and reduces the likelihood of them entering the mains earth system and interfering with other electronic equipment.

• Ideally this should consist of the shortest possible length of thick copper braiding, attached to a copper earth stake in the ground.

EMC Precautions/possible solutions

• Low pass/ band pass filters may be fitted to transmitters to suppress harmonics where TV Interference is being experienced.

• Dedicated RF Earths should be used as opposed to mains safety earths.

• Use a balanced antenna such as a dipole and/or fit a balun to the antenna feeder/cable.

• Inline filters/ferrite rings may be fitted to domestic equipment such as TV, HIFI, PC etc.. If used these should be as close as possible to the affected equipment.

• Advice can be sought from the RSGB EMC Committee or Ofcom.

Revision Questions

Which of these could be fitted to the mains lead of a television to reduce interference from a nearby transmitter?

A [ ] Dummy LoadB [ ] Ferrite RingC [ ] SWR MeterD [ ] PL259 Plug

Answer : B) A Ferrite Ring.

Which of the following would INCREASE the risk of EMC problems?:

A [ ] Using SSB rather than FM.B [ ] Operating with reduced RF output.C [ ] Using a horizontal balanced antenna.D [ ] Moving the transmitting antenna away from nearby houses.

Answer : A) Using SSB rather than FM.

Thurrock Training Team