The transmitter of an amateur radio station has the potential to expose the domestic environment to much more powerful radio signals than would be present in the average domestic situation. Any piece of metal, including wires, exposed to these signals will act a bit like an antenna, and pick up some of these signals conveying them into susceptible pieces of equipment in a manner almost impossible to fully predict.
In order to manage EMC effectively we must consider the three main ways in which interference is caused;
1. Direct pickup (Pickup through the antenna).
The radio waves are too strong for the affected equipment to handle. TV and VCR's are often affected in this way by VHF and UHF transmissions. These will effectively overload the RF stages of the tuner and cause an apparent reduction in the strength of the wanted signal. The effect will be seen on a television as full or partial loss of picture and sound.
2. Indirect pickup through interconnecting leads
The interconnecting leads act like an antenna, picking up the radio signals and feeding them into the affected equipment. Commonly seen examples include interference to televisions by HF transmissions where the signal is being picked up by the braid of the television's coaxial antenna feeder, and interference to telephones where HF transmissions are picked up by the telephone wires, particularly common where overhead wiring is in use, and the electronic circuitry of the telephone acts as a detector causing the transmitter operator's voice to be heard over the telephones of many surrounding properties. Television interference of this mode is often seen as loss of colour, a "herring-bone" pattern on the picture, and interference to sound, often in the form of a loud buzz.
3. Indirect pickup through power and earth wiring.
Radio signals may be conducted through the domestic wiring system. In modern housing it is not uncommon for the earth conductors in many houses to be linked before being actually connected to earth. Thus a radio signal may easily pass from one property to another via the house wiring. Radio signals may leak from the transmitter along power supply leads, or from a poorly screened ATU. It may also be fed back into the earth system by poor siting of the antenna, poor RF earthing arrangements, or it may even be picked up by a water pipe that is bonded to the domestic electricity earth.
As
we have seen there are many ways in which a radio signal may enter a piece of
electrical equipment causing interference. By considering these we may take the
appropriate action to minimise the risks of EMC problems. The following pages
will deal with how this may be achieved.
In all cases of interference we should stop transmitting and not resume until the problem has been eliminated.
