Chelmsford Amateur Radio Society EMC 7 ElectroMagnetic Compatibility

1
Chelmsford Amateur Radio Society EMC (7)
ElectroMagnetic Compatibility
2
EMC Regulations

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

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

• 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.

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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.

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.

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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.

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.

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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.

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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.

15
AM or SSB

• Similar patterning on screen,
• Possibly in time with speech,
• Distorted voice like sounds, can be intelligible.
  

16
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.

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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
• PCBs
• 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