Chelmsford amateur radio society advanced course 7 emc part 2 the cures
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Chelmsford Amateur Radio Society Advanced Course (7) EMC Part-2 - The Cures. Balanced Antenna. Morse Key Use Key Click Filter. Balun (if Coax Feed). Restrict Mic Audio Bandwidth. Coax or Balanced Feeder. Do Not Over-Modulate Transmitter. ATU. Filter:- LPF for HF or BPF for VHF/UHF.

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Chelmsford amateur radio society advanced course 7 emc part 2 the cures

Chelmsford Amateur Radio Society Advanced Course(7) EMC Part-2 - The Cures


Good emc station layout

Balanced Antenna

Morse Key Use Key Click Filter

Balun (if Coax Feed)

Restrict Mic Audio Bandwidth

Coax or Balanced Feeder

Do Not

Over-Modulate Transmitter

ATU

Filter:-

LPF for HF or

BPF for VHF/UHF

VSWR

Meter

Good RF Earth

Do Not Use Mains Earth

Do Not Join Mains & RF Earths

Filter Mains

13.8v

PSU

Good EMC Station Layout

  • Screen All Units WellEspecially the Transmitter

Good Quality Coax and Connectors


Choice of antenna type dipole balanced system

Antenna as high as possible and far from House

Balun

Coax drops at 90°to antenna

Station

Coax Underground

Bond Coax Outer to RF Earth

Choice of Antenna TypeDipole / Balanced System

EMC Correct Antenna System

  • Antenna sited as far away and as high as possible.

  • Good Quality coax run under ground where possible.

  • Coax earthed at point of entry to house

  • Balun used.

  • Balanced Antenna System


Choice of antenna type long wire system

Station

Strong RF Field

Strong RF Field

RF Earth

Choice of Antenna TypeLong Wire System

Poor EMC Antenna System

  • Unbalanced Antenna System.

  • Strong RF fields nearthe house.

  • Poor earth system.

  • Near TV antenna.


Choice of antenna type better emc long wire system

Kept as far as possible

7MHz Trap

Good Spacing

Prevent access to antenna at ground level

Station

ATU

Earth

Earth

Earth

Choice of Antenna TypeBetter EMC - Long Wire System

Better EMC Antenna System

  • Unbalanced Antenna System.

  • Coax fed to remote ATU.

  • Earths at each end of Coax.

  • 7MHz trap keeps higher frequencies (HF) away from house.

  • LF still too close though.


Choice of antenna type best emc balanced system

Balanced Antenna as far as possible from house

Balanced Twin Feeder

Station

Balun

Coax underground

Earth Rod

Earth Rod

Choice of Antenna TypeBest EMC - Balanced System

Excellent EMC Antenna System

  • Antenna balanced system.

  • Well located.

  • Well Earthed.

  • Away from house.

  • Needs a lot of garden


Resistor capacitor filters

C

R

Vout

Vin

R

Vin

C

Vout

Resistor Capacitor Filters

RC = Resistor Capacitor

RC High Pass Filter

  • High Pass Filter

  • Passes high frequencies

  • Blocks low frequencies (attenuates)

  • 50% Power Cut-off point is at 1/2 (0.707) of the voltage

    RC Low Pass Filter

  • Low Pass Filter

  • Passes low frequencies

  • Blocks high frequencies (attenuates)

  • 50% Power Cut-off point is at1/2 (0.707) of the voltage


Response graph high pass filter
Response GraphHigh Pass Filter

Vout

Vout = Vin

Frequency

0.707 Vin

Corner of cut-off frequency


Response graph low pass filter
Response GraphLow Pass Filter

Vout

Frequency

0.707 Vin

Corner of cut-off frequency


Lc cascade pi filters

C

Vin

L

L

Vout

LC High Pass Filter

L

C

Vin

C

Vout

LC Low Pass Filter

LC Cascade  (pi) Filters

  • Inductors are used as they vary in reactance with frequency (unlike resistors).

  • There are three frequency sensitive elements.

  • They roll off three times faster than RC Filters

  • Little attenuation at pass band


Band pass lc filters

L

C

Vin

Vout

Vin

L

C

Vout

Band Pass LC Filters

Series

  • High Input Impedance.

  • Blocking frequencies outside the pass band.

  • Little attenuation at pass band.

    Parallel

  • Low Input Impedance.

  • Blocking frequencies outside the pass band.

  • Shorts unwanted signals to earth.

  • Little attenuation at pass band.


Response graph band stop notch filter
Response GraphBand Stop / Notch Filter

Stop Band

Pass Band Loss

0

Frequency

Loss (dB)

10

Pass Band

Pass Band

20

fc


Cascade bandpass filters

L

L

C

Vin

Vout

C

L

L

C

Vin

Vout

C

L

C

L

Cascade Bandpass Filters

Three LC Stages:

High Input Impedance

  • The input / output impedance is governed by the type of LC circuit used.

    Low Input Impedance


Band stop notch filters

L

Vin

Vout

C

C

L

Vin

Vout

Band Stop / Notch Filters

Series

  • Low Impedance at resonance.

  • Stops a given band of frequencies at resonance.

  • Passing others.

    Parallel

  • High Impedance at resonance.

  • Blocks the unwanted signal.

  • Passing others outside of resonance.

    Notch Filter

  • When response is sharp they are notch filters, removing a particular frequency.


Mains filter
Mains Filter

High Voltage Capacitors *

To Equipment

Mains

Voltage Dependent Resistor

*Do not use ordinary Capacitors

  • Some Mains Filters/Suppressors will havethree VDR’s across L-N, L-E, N-E


Ferrite ring filters

One or more ferrite rings

Approx 8 to 15 turns of cable

Mains or Coax Cable

Wind only two-thirds of ferrite

Ferrite Ring Filters

  • Using two Ferrite Rings will increase inductance.

  • Common Mode (Interference)

  • Differential Mode (Mains Current)

Common Mode

(RF Interference)

Differential Mode

(Mains)

Unwanted in-phase currents blocked by choke

Wanted antiphase currents


Braid braking filters

High Pass Filter & Braid Breaker

C1 4p7

L1

L2

C2 4p7

R1 1M5

L1, L2: 4 turns of 20SWG 6mm Dia x 6mm Long

Braid Braking Filters

  • Signals picked up by the aerial are in Differential Mode in the coax

  • Signals picked up by the braid will be Common Mode and will be attenuated.


Coaxial trap stub filters
Coaxial Trap/Stub Filters

Coaxial Trap/Stub Filter

  • Comprises a /4 of coax, resonant on the frequency to be reduced connected in parallel (Tee’d on to) the main feeder and left open circuit.

Open circuit on end

Coax cut to /4 of the interfering signal

taking in account the velocity of propagation in coax (0.66)

FM Broadcast Receiver


Ferrite beads
Ferrite Beads

Audio & Non RF Circuits

(A) has a capacitor across thebase-emitter.

Base-emitter is forward biased.

Behaves like a diode.

Rectifies RF to a Voltage.

Upsetting operation of transistor.

Or adds modulation to wanted signal

(B) has a ferrite bead acting as a RF choke.

(C) has a ferrite bead in the track added (if there is space)

(D) has both methods.

Capacitor is 1nF low inductance disk ceramic type


Mobile installations considerations

Out of Field of vision “Hands Free” Mic

Secure wiring at all times

30A Main Fuse

Radio Fuses

12v 30A Filter

Easy Access Remote PTT

Secure Antenna Mounts

Easy See / Access for Radios

Mobile InstallationsConsiderations

Installations

  • Care should be taken about the installation of radio equipment in vehicles.

  • Any EMC issue can have SERIOUSsafety implications.

  • Fit filters to any power supply where possible to radio.

  • Keep car cable looms, Engine Management Units and RF cables separate at all times.


Good emc house keeping
Good EMC House Keeping

  • The key thing to remember is:

  • Keep your station in good working order.

    • Do checks from time to time on the station equipment.

      • This means “the emitted frequency of the apparatus comprised in the Radio Equipment is stable and free from Unwanted Emissions as the state of technical development of amateur radio apparatus reasonably permits”

      • And that “the bandwidth occupied by the emission is such that not more than 1% of the mean power of the transmission falls outside the nominal modulated carrier bandwidth”

  • You will need to show this using a wavemeter or the station receiver.

  • Always treat any reported EMC issue.

    • If you can help a neighbour without invalidating any of their warranties (try not to modify their equipment).

    • Use a quality filter (not a birds nest construction).


Social issues
Social Issues

  • Be diplomatic - even if you can’t cure the EMC problem.

    • You may have to modify your operating practices.

    • Only use as much power necessary to make the contact - This will reduce the EMC potential

  • Remember that the station log may be of considerable assistance in confirming sources of interference to neighbours.

    • The Complainant should also keep a log, of the instances of interference.

    • If Ofcom becomes involved they may well expect both parties to keep a log.

  • Conduct tests in co-operation with the complainant in instances of interference.


Advice
Advice

  • Advice is available from the RSGB EMC Committee

    http://www.rsgb.org.uk/emc

  • Local Ofcom officers can also help in cases of interference.

THE END


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