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EMC or Electromagnetic Compatibility is concerned with avoiding interference between different pieces of electronic equipment.
Interference is the degradation or disruption of a desired radio signal caused by unwanted electromagnetic energy. It can manifest as static, noise, signal loss, or distorted audio.
Transmitters can cause interference. Receivers can suffer from inteference.
The more power a station uses, the more likely it is to cause interference.
Immunity is the ability of a piece of equipment to function correctly in the presence of strong RF signals.
Different transmission modes are more or less likely to cause interference.
AM and SSB are more likely to cause interference.
FM and some HF data modes are least likely (remember "Friendly Modulation").
Occurences of interference
Nearby radio transmissions can cause interference to:
- domestic mains wiring
- TV aerial leads
- Speaker leads
- Telephone wiring
- Internal circuitry
Methods of reducing interference
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Fitting filters (known as chokes) in mains or antenna leads as close to the affected device as possible
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Using balanced antennas at HF
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Using horizontal dipoles
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Avoiding the use of end-fed antenna types
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Siting antennas as far from houses as possible and as high as pissible
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Using a dedicated RF earth to prevent RF current from entering the mains earth system
Dummy loads
A dummy load is a screened resistor attached to a transmitter instead of an aerial. This enables the transmitter to operate without putting out a signal which can be useful when detecting and managing interference.