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Chelmsford Amateur Radio Society EMC 7 ElectroMagnetic Compatibility

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Title: 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.

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
  • 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
  • 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.

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

14
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.

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
  • PCBs
  • 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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