EMC (7) Electro Magnetic Compatibility

Karl Davies

East Kent Radio Society

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EMC Regulations

All electronic equipment is capable of radiating and absorbing radio frequency (RF) energy.

The principle behind Electro Magnetic Compatibility is that equipments should limit radiation to below a specified level and also be able to withstand a certain level of incident RF radiation.

The levels are given in the EMC regulations. You need to be aware that they exist, but you will not

need to know them for the exam.

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Domestic Environments

Transmitters in domestic environments may give rise toRF fields stronger than the specified limits.

Special measures may have to be taken to limit interference.

Intermediate Licence allows 50 Watts - and this is likely to cause interference in a domestic environment.

You may have to reduce power if you can not cure the interference.

This may be enforced by the Secretary of State / Ofcom

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New Equipment

New equipment, manufactured since 1996, should meet the EMC directive immunity requirements and the incidence of problems with new equipment is usually lower.

Older equipment and poor installation practices in new equipment do exist and problems do occur.

A TV downlead does have a service life: It is subject to water ingress. PVC Breakdown due to UV Poor quality leads have very little copper braiding with

poor screening properties

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Station Layout

Correct interconnection of the transmitter, microphone, power supply, SWR meter and band or low pass filters, using appropriate cables, minimises EMC problems.

Always have a good layout to keep RF and audio or power leads apart.

Use: Good quality screened cable with

correctly fitted plugs and sockets Filters (on the antenna side of the SWR

meter) Correct earthing.

Station Layout

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Filters

Filters can be fitted in the leads from the power supply to the transmitter to help minimise RF energy entering the mains wiring.

• Ferrite ring filters on the 12V power leads and the mains lead.– Home made power supplies should fit filters by design.

• Any other equipment in the shack, especially if interconnected to the transmitter, such as: – TNCs, Voice keyer, CW keyer, Other audio Interfaces – Computer sound cards, TV cards, Digital camera leads.

• Should be similarly filtered on:– signal leads, power leads, mains leads.

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RF Earth - What is one ?

A good RF earth will comprise an earth rod driven into the ground plus a number of radials so as to form an earthed mat under the antenna.

The ends of the radials may also have earth rods. The radials will be just under the grass, out of sight and

away from damage but still serving the purpose. It must be close to the transmitter and ideally at the

point where the feeder enters the building.

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RF Earth - What is one ?

A Heavy Duty Earth Wire (thick copper braiding) should run directly to the back of the transmitter.

If the feeder terminates in a socket on the wall with a fly lead to the transmitter, an extra earth lead should be connected from the termination of the RF earth lead at the wall socket, to the transmitter.

Do not just rely on the braid of the fly lead.

Mains Safety earths and RF earths are two separate requirements.

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Spurious & Harmonic Emissions

A general coverage receiver is the most effective tool available in the average shack.

Scan methodically through the bands, stopping on odd signals and switching off the suspect item whilst listening to the signal on the receiver.

Calculation of harmonics are covered in transmitters. The wavemeter is deaf in this regard, but can work.

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Siting Antennas

Siting a transmitting antenna close to mains wiring, TV or radio aerials, and downleads, is a potential problem

Especially the use of a loft or indoor transmitting antenna.

The field close to the transmitting antenna is very strong.

Far higher than any equipment can reasonably be expected to withstand

Signals induced in any wiring, mains, TV, audio, telephone or alarm system, will be considerable and must considered.

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Indoor or Loft Transmitting Antennas

Indoor or loft transmitting antennas have two disadvantages: Closeness of coupling and any TV antenna sharing the

loft. Attenuation of the wanted signal as it passes through

the roofing materials. Being an indoor or loft transmitting antennas make it

more susceptible to causing / receiving interference.

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Forms of Interference caused by Amateur Radio

The forms of interference caused by amateur radio transmissions. Patterning on the TV screen, Loss of colour, voice on TV sound, Sounds on / loss of sound on Radio, Sounds on Telephone, Sounds on audio systems.

Interference to digital televisions is different. The picture may freeze, become jerky or disappear.

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Good Analogue Picture

A good example of an analogue TV picture.

Received with no interference.

BBC2 Sports Program

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FM Transmission

Wavy, herringbone patterning on TV

Possible loss of colour

No effect on sound But on severe cases

may cause distorted or loss of sound.

Herringbone patterning on TV

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AM or SSB

Similar patterning on screen,

Possibly in time with speech,

Distorted voice like sounds, can be intelligible.

AM / SSB Patterning on screen

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CW - Morse code

Possibility of flickering picture or wavy effects,

Possibility of clicks or distortion on sound.

Morse code - possibility of clicks or distortion on sound

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Digital TV

Digital TV is affected quite differently.

There is no visual evidence of what is happening other than the picture becoming jerky, forming blocks as if it is a jig-saw,

Freezing or disappearing.

These effects are the same as if there is a weak signal.

The neighbour is more likely to call the service engineer believing a fault on the TV.

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Other Sources

Other sources and effects on a TV are: Electric Motors Spots on TV screen Possibility of buzz (burr) on sound Thermostats 2-10 seconds of spots and lines on screen Possibly quite intense and noise like screwing up paper

on sound Vehicle ignition spots/lines on screen, Clicks on sound in time with engine speed.

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Direct (internal) pickup

Direct pickup in affected devices tends to be independent of the transmitted frequency.

Direct (internal) pickup and conducted pickup e.g. by: loudspeaker leads Internal wiring PCB’s etc

And occurs over a wide range of frequencies.

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Masthead and Downlead

Masthead and Downlead TV amplifiers are broadband and so they amplify a wide range of frequencies, including amateur frequencies.

This can overload the TV input. If the masthead amplifier is

needed, a filter at the TV input will help as will a band specific amplifier.

Avoid using Masthead and Downlead amplifiers in conjunction with a video unit.

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Ferrite Rings

Ferrite rings are effective for R.F. signals on mains and audio leads and for R.F. signals picked up by the screen of coaxial feeders and downleads.

If the signal is picked up by an antenna on the inner conductor, then ferrite rings are ineffective, a filter tuned to the appropriate frequency is needed.

In-line mains suppression filters are effective for interference conducted along the mains wires.

Transmitting into a dummy load is a good test for any unwanted R.F. being conducted out of the transmitter along its power supply leads and into the mains

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Conducting Tests & Neighbours

Conduct tests in co-operation with the complainant in instances of interference.

Remember that a logbook can be of considerable assistance in confirming sources of interference to neighbours.

The Complainant should also keep a log of the instances of interference.

Ofcom can require Logbooks to be kept over a period of time to assist Interference Investigations

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Help from Ofcom & RSGB

Advice is available from the RSGB EMC Committee http://www.rsgb.org/emc/

Local Ofcom officers will help with cases of interference. RSGB have a wide range of EMC information leaflets on their EMC

website. The RA Archive on the Ofcom Website also has:

RA-234: EMC and the Radio Amateur RA-179: Television and Radio InterferenceRA-272: Problems Thermostats can cause to Television & Radio

ReceptionRA-323: Guidelines for Improving Television and Radio ReceptionRA-415: Guidelines for Improving Digital Television and Radio

Reception

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