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EMC (7) E lectro M agnetic C ompatibility Karl Davies

EMC (7) E lectro M agnetic C ompatibility Karl Davies. East Kent Radio Society. EMC Regulations. All electronic equipment is capable of radiating and absorbing radio frequency (RF) energy.

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EMC (7) E lectro M agnetic C ompatibility Karl Davies

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  1. EMC (7)Electro Magnetic CompatibilityKarl Davies East Kent Radio Society

  2. 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.

  3. 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

  4. 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

  5. Station Layout 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.

  6. 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.

  7. 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.

  8. RF Earth - What is one ? • AHeavy 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. BBC2 Sports Program Good Analogue Picture • A good example of an analogue TV picture. • Received with no interference.

  14. Herringbone patterning on TV 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.

  15. AM / SSB Patterning on screen AM or SSB • Similar patterning on screen, • Possibly in time with speech, • Distorted voice like sounds, can be intelligible.

  16. Morse code - possibility of clicks or distortion on sound CW - Morse code • Possibility of flickering picture or wavy effects, • Possibility of clicks or distortion on sound.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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

  22. 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

  23. 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

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