# Section 7 - Electromagnetic Compatibility (EMC)

## 7.9 RF Earthing

This section should be read in conjunction with the sections on safety. If you are in any doubt about your ability to correctly assess the implications of any issues discussed in this section you should seek the advise of a qualified electrician.

### Safety Earth

All equipment which is not double insulated must be earthed for safety reasons. On no account must any such earth conductor be removed or disconnected. Mains electricity can kill! Most of the equipment we use in the amateur station is not double insulated and therefore will require a connection to the mains earth. This is provided by the earth conductor of the mains plug. If you use an extension lead for any reason, check that it's earth connection is secure before use.

### RF Earth

STOP: Some properties in the UK use an earthing system where the earth conductor is bonded to the neutral at the supply station, and not bonded to an earth electrode at the property. This system is called Protective Multiple Earthing or PME. Under certain fault conditions a high potential may exist between the mains earth of a PME supplied property and any earth electrode fitted at that property. Because of the safety risks associated with such a fault condition, properties using the PME system MUST NOT install supplementary earth electrodes at the properties. Before installing any supplementary RF earth, you should check that your property is not connected to a PME type supply. Your electricity supplier should be able to provide you with this information. If you are in any doubt, consult a qualified electrician. Remember electricity kills!

All currents flow in a loop. Where a single conductor is used for the feed to an antenna, such as the centre pin of a coaxial cable, the return loop is via earth. Obviously this means that if the transmitter is connected to the mains, then RF currents will flow in the mains earth of the house wiring and may potentially flow to other appliances in the house or even adjoining properties. This may give rise to EMC problems.

RF current flowing in the house's earth wiring can be avoided if we provide a separate RF earth. This is achieved by putting an earth rod into the ground as close as possible to the point where the feeder enters the house. This earth rod is connected directly to the transmitter with very heavy gauge wire. Other devices connected to the transmitter, such as the ATU will also have a heavy gauge connection to this RF earth. Note that this RF earth is in addition to the mains earth and NOT a substitute for it.

RF current flow in the mains earth is prevented (or substantially reduced) by fitting a ferrite ring filter to the transmitter's power lead. The mains lead is wound five or six times thorough a set of five ferrite rings to provide sufficient attenuation.

Another benefit of a good RF earth will be noticed when receiving. By using the filter to remove RF from the mains supply, we remove one source of interference to our receiver, so weak signals can be heard more easily. In order to benefit from this it may be necessary to improve the RF earth beyond the single earth rod mentioned previously. Detailed study of RF earth is beyond the scope of Foundation Licence.

.

Back | Index | Next