# UK Amateur Radio ## Foundation Licence 2002
## EMC Part A
### Basics of electromagnetic compatibility ElectroMagnetic Compatibility or EMC is the ability of one piece of electronic equipment to not interfere with the operation of another.
### How Interference travels Interference travels (propagates) by 1) Radiation as from an antenna. 2) Induction, as between two wires in close proximity. This is similar to transformer action. 3) By conduction, it simply travels along the wires like any other electricity. The interference can be picked up through the electrical wiring in the house, the TV Antenna down lead that is in the back of your TV, Telephone wiring and in particular at VHF/UHF by direct pickup by the internal circuits of the affected equipment itself. Good separation of the source and the recipient or victim equipment can reduce the problem - and also - preventing the radio frequency (r.f.) signals getting into the wiring in the first place. Note. It is not only transmitters that can cause interference. Practically anything electrically or electronically operated can, and possibly will, cause interference to other electronic equipment and radio equipment. For instance when an electric drill is used it is needed to drill a hole - it is not supposed to make an electrical noise which is heard on a near by radio - but sometimes it happens.
### Immunity of radio receiving equipment If a piece of electronic or radio equipment can operate properly in the close proximity to RF signals it is said that the equipment has "immunity" to interference.
### Interference by a transmitting system With regard to Amateur Radio the operation of your Transceiver must not cause any "interference" to any other piece of electronic equipment. By the very nature of a transmitter it is giving out radio emissions else the purpose of the transmitter would not enable you to communicate with others. It is these radio emission that must not cause the interference to any radio or other electronic equipment.
For any transmitting system, only the antenna must radiate the signal. There must be no signal radiating from any other part of the transmitting equipment. The field strength around a transmitting antenna reduces with distance from it, therefore, the greater the separation between the transmitting antenna and the surrounding equipment, the better from an EMC point of view. It is also good practice to keep transmitter output power levels to the minimum necessary to maintain good communication.
### Station design for EMC ##### Less likely to cause interference problems FM CW PSK31 Certain classes of emission are more emc friendly than others. Frequency Modulation (FM) is the least likely to cause interference because there are no amplitude variations on the constant amplitude carrier wave. Therefore, there will be no envelope detection in the receiving equipment. Morse transmissions (CW) and the new PSK31 HF data mode are fairly EMC friendly because relatively low power is possible to maintain contact over long distances. More likely to cause interference problems AM and SSB Amplitude Modulation (AM) and Single Sideband (SSB) are the most likely types of transmission to cause interference because of the syllabic nature of the radiated signal and the fact that receiving and audio equipment, under certain conditions, is able to detect (extract the audio modulating signal) from the modulated carrier wave. As it is likely that SSB will be your most used mode of communication and it is essential that you understand that you can easily cause problems to others. ### What can you do regarding aerials All antennas should be correctly adjusted for the frequency on which they are operating for the best VSWR (Voltage Standing Wave Ratio). Possibly the best antennas - from an emc point of view - are centre-fed dipoles using a BALUN at their centre feed point when fed with coaxial cable. Long wire antennas have the potential to cause interference and should be correctly tuned with the aid of an antenna tuning unit (ATU). Most HF transceivers have a built-in ATU these days - although it may come as an optional extra. EMC problems for VHF/UHF can be minimized by good location of aerials as far away from houses as possible and also to locate them as high as possible. ### Operating power Operate with as low a power as possible and keep transmitting antennas away from neighbours receiving antennas and Hi-Fi equipment to avoid trouble. ### Think of the neighbours It is more than possible that your neighbour's receiving equipment lacks immunity from strong nearby signals. In such cases it is necessary to gain their confidence and carry out a few tests to determine the real source of the problem. Never admit causing interference - although it might seem the case to the neighbour - the problem is more likely to be due to lack of immunity to strong RF fields of the TV and Hi-Fi equipment. Always refer to it as an EMC problem - Never interference. Ask your neighbour to keep a log and see if it ties up with your transmissions which will be properly recorded in your log book!! Such log book on both side of the EMC problem will be useful when checking for correlation is helpful during investigation of EMC problems. ### Other systems can be affected Also security alarms, lights and similar devices can lack immunity and be triggered by an amateur transmitter.
### The Radiocommunication Agency Radiocommunication Agency leaflet RA 179 gives information as to how someone should report interference and RA 234 EMC and the Radio Amateur (Rev. 5, March 2000) is a useful source of more information. You should be aware of these publication.
### What can be done to reduce the likelihood of EMC problems There are many methods and devices available for reducing the level of unwanted signals leaving a transmitting installation. Low Pass filters may be fitted to transmitter antennas and there are special filters for mains leads. Also, ferrite rings are available. These are used by passing a few turns of the leads or cables through them. These devices work by increasing the impedance (A.C. resistance) of the lead to reduce the flow of unwanted signals at higher frequencies. All devices should only be fitted as recommended by the supplier. Do not attempt to fit any device of which you are uncertain. The danger of fire or electrocution is only too real.

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