Electro-Magnetic Compatibility (EMC)

 

Introduction

 

Electro-Magnetic Compatibility (usually abbreviated as EMC) is the avoidance of interference between various pieces of electronic equipment.

 

Causes of Interference

 

Radio transmitters can cause interference to nearby electronic and radio equipment.

 

Radio receivers can also suffer interference from local sources.

 

There are various ways that nearby radio transmissions can result in interference: -

 

·        Getting into mains electrical house wiring

·        TV antenna down leads

·        Telephone Wiring

·        Direct pick of the transmitted signal by the equipment itself (particularly at v.h.f. and u.h.f.)

 

çExample of Picture with

Interference from

Nearby Transmitter

Same Picture

 with No Interferenceè

 

Methods to Prevent / Minimise Interference

 

EMC problems can be minimised by: -

 

·        Siting antennas as far away from houses as possible

·        Siting antennas as high as possible

·        Using balance antennas at h.f.

·        Using Horizontal Dipoles at h.f.

·        Not using end-fed long wires at h.f.

·        Reducing Power

·        Using types of transmissions less likely to cause interference

 

Information on avoiding interference problems by the correct choice and siting of antennas, and suitable operating procedures is available from several sources including the R.S.G.B. and local amateur radio clubs.

 

Note that just because your television picture etc. is not getting interference, it is possible that other people’s TVs might be getting interference.

 

The more power that a station runs, the more likely it is to cause interference.

 

Some types of transmissions are more likely to cause interference than others.

 

Types of Transmissions and the likelihood of causing interference
Less likely	More likely
f.m.	s.s.b. *
c.w (Morse)
PSK31 (data mode)

     

(* Most amateurs can run s.s.b. without interference problems).

 

Dealing with EMC Problems

 

The ability of any piece of electronic or radio equipment to function correctly in the presence of strong r.f. signals is known as immunity.

 

Fitting suitable external chokes and filters in mains and TV antenna leads can increase the immunity of most types of equipment.  To be most effective, filters should be fitted as close as possible to the affected piece of equipment.

 

Anything fitted to the mains wiring must be properly made for the purpose.  Homemade items (apart form ferrite rings) are potentially dangerous.

 

Information about the purchase, making and fitting of chokes and filters is available from several sources including the R.S.G.B. and local amateur radio clubs.

 

EMC problems have the potential for causing neighbour disputes.  To make progress, you need to be diplomatic and try to resolve the problem.  Information on dealing with EMC problems is available from several sources including the R.S.G.B. and local amateur radio clubs.  If all else fails, then the local office of Ofcom can be brought in, either by your neighbours or by the amateur.  It is much better to try the various methods listed above and seek help from your local amateur radio club.  Don’t just carry on transmitting without tackling the EMC problem – to do so will only upset your neighbours even more! 

r.f. Earth

 

In an h.f. amateur radio station, an r.f. earth connection is used to provide a path to ground to minimise r.f. currents entering the mains earth systems and causing interference to other electronic equipment.

 

An r.f. earth is NOT the same as the electrical safety earthing that is part of the mains plug.

 

(Some of the pictures on this page are reproduced by courtesy of John Owen G4VWL)