1 Chelmsford Amateur Radio Society Intermediate Licence Course Christopher Chapman G0IPU Slide Set...

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Chelmsford Amateur Radio SocietyIntermediate Licence Course

Christopher Chapman G0IPU Slide Set 11: v1.2 16-Jun-2007 (7) EMC

Chelmsford Amateur Radio Chelmsford Amateur Radio Society Society

EMC (7)EMC (7) EElectroMMagnetic CCompatibility

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EMC Regulations

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

• The The principle behind ElectroMagnetic 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 LayoutStation 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 Interference

RA-272: Problems Thermostats can cause to Television & Radio Reception

RA-323: Guidelines for Improving Television and Radio Reception

RA-415: Guidelines for Improving Digital Television and Radio Reception

1 Chelmsford Amateur Radio Society Intermediate Licence Course Christopher Chapman G0IPU Slide Set...

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