10A. EMC

Definitions

• Electromagnetic Compatibility (EMC) is the ability of equipment to function satisfactorily in its electromagnetic environment without introducing intolerable disturbance to anything in that environment
• Noise limiter would comprise of two back to back diodes connected across the headphone circuit
• Field strength is measured in Volts (or µV) induced into each metre of receiving wire

Diagrams

• Motor with suppressor

  

• Simple noise limiter circuit

  

Circuit diagrams

Formula

• Field strength = 7 x (square root of the ERP) where Field strength is in V/m, square root of ERP is Effective Radiated Power

Notes

Example resolution

• Electric motor
  • the carbon brushes sparking on the commutator and radiating interference from the wiring
  • solution:
    • each motor was manufactured with a suppressor fitted
    • this damped the spark and greatly reduced its annoying radiation
• Headphone circuit
  • solution:
    • spike resulting from a distant spark would have a much greater amplitude and ‘switch on’ the diodes
    • this would chop off the top of the spike making it less objectionable to the listener
• Radio transmitters
  • intentionally radiates an electromagnetic wave
  • Modern domestic radios, TVs and Hi-Fi’s are not always as efficient at ‘ignoring’ local electromagnetic transmissions as were their valve predecessors
• Television
  • quite possible to manufacture TV sets that are immune to strong electromagnetic fields
  • most TV’s do not have to operate in the presence of a strong electromagnetic field
  • to keep their production casts to a minimum they omit any components not absolutely necessary under normal operating conditions

Precaution with Radio

Amateur radio station

• transmitting aerial should be in the clear
• as far possible from anyone’s TV (and aerial) or Hi-Fl equipment
• efforts should be made to minimize the coupling between the transmitting stem and the TV coax by not having the two running parallel
• would be unwise to install a vertical amateur aerial anywhere near where the TV coax runs up the wall to the roof

How can an Amateur transmission get into a TV or Hi-Fi?

• via aerial coaxial feeder cable
• via mains cable
• directly into TV/Hi-Fl chassis
• via loud speaker leads (Hi-Fl)

End Fed Aerials

• Steps should be taken to avoid creating strong electromagnetic fields within the house in order to prevent it being picked up by the house electrical wiring
• At certain frequencies the internal portion of the end fed wire could create a very strong electromagnetic field within the house
• most TV’s and Hi-Fl’s have wooden or plastic cases and therefore have no inherent screening
• solution:
  • best not to use an ‘end fed aerial’ unless the aerial tuning unit can be fitted just inside the outside wall
  • treat plastic to give it screening qualities, but it is costly

Earth

• A good RF earth must have a low impedance
• Ideally the earth cable should be:
  • short:
    • care must be taken that it is not equal to a quarter wavelength, or an odd number of quarter waves at the frequencies to be used.
    • transmitter that is ‘earthed’ via this length will be at a very high impedance point
  • straight
  • stout

Warning: NEVER in any circumstances should the mains Earth be used as a RF ground. Not only will it not work effectively, it can be dangerous in the event of an earth fault.

Field strength

• strength of a field (E) reduces as you move away from transmitting aerial
• reduce the transmitted power to reduce field strength
• it is good practice to use only enough power necessary for adequate communication
• quite easy for several fairly low power stations to operate around the same frequency
• just one amateur using unnecessarily high power renders a whole block of frequencies unusable for everybody else

Home equipments

TV

• TV sets have two time-base oscillators:
  • (horizontal) line timebase
  • (vertical) frame timebase
• a 625 line system the line oscillator runs at 15.625 kHz
• shape of the waveform is like a ‘sawtooth’
• waveform is very rich in harmonics
• can heard throughout the Amateur Bands up to at least 30MHz
• pitch of the buzz will vary in sympathy with changes in the TV picture
• TV time-base radiation can often be reduced by fitting the usual filters in the:
  • TV aerial lead
  • TV mains lead

Computers

• Tiny circuits are either on or off, 1 or 0
• switch continuously and rapidly resulting in many nasty squares
• 5 Volt square waves
• rich in harmonics
• create ‘hash’ throughout the Amateur Bands right up to at least 200 MHz
• all small computers are, unfortunately, built in untreated plastic boxes
• unscreened ribbon cable acts, unwittingly, as an aerial and ensures that the hash is spread over a wider neighborhood

Older equipments

• Worn thermostat contacts
• fluorescent strip lights

Power measurement in your log book

• present license now requires the log entry to include the output ‘power’
• Most power meters are calibrated directly in Watts rather than dB

Decibel	Watt
9dB	6W
10dB	10W
14dB	25W
15dB	32W
16dB	40W
20dB	100W
22dB	160W
26dB	400W
30dB	1000W