chelmsford amateur radio society advanced licence course christopher chapman g0ipu slide set 19:...

Chelmsford Amateur Radio SocietyAdvanced Licence Course

Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007(7) EMC-2: The Cures

1

Chelmsford Amateur Radio Chelmsford Amateur Radio Society Society

Advanced CourseAdvanced Course(7) EMC (7) EMC

Part-2 - The CuresPart-2 - The Cures



2

Balanced Antenna

ATU Filter:-LPF for HF or

BPF for VHF/UHF

Morse Key Use Key Click Filter

Filter Mains

13.8v

PSU

Good RF EarthDo Not Use Mains Earth

Do Not Join Mains & RF Earths

VSWR Meter

Do Not Over-Modulate Transmitter

Restrict Mic Audio Bandwidth

Coax or Balanced Feeder

Balun (if Coax Feed)

Good EMC Station Layout

• Screen All Units WellEspecially the Transmitter

Good Quality Coax and Connectors



3

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

Station

Bond Coax Outer to RF Earth

Coax Underground

Balun

Coax drops at 90°to antenna

Antenna as high as possible and far from House



4

Choice of Antenna Type

Long Wire SystemPoor EMC Antenna System

• Unbalanced Antenna System.

• Strong RF fields nearthe house.

• Poor earth system.

• Near TV antenna.

Station

RF Earth

Strong RF Field

Strong RF Field



5

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 TypeBetter EMC - Long Wire System

Station

Kept as far as possible

Good Spacing

EarthEarth

ATU

7MHz Trap

Earth

Prevent access to antenna at ground level



6

Excellent EMC Antenna System

• Antenna balanced system.

• Well located.

• Well Earthed.

• Away from house.

• Needs a lot of garden

Choice of Antenna TypeBest EMC - Balanced System

Station

Balanced Twin Feeder

Balanced Antenna as far as possible from house

Coax undergroundEarth Rod

Balun

Earth Rod



7

Resistor Capacitor Filters

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

Vin Vout

C

R

Vin Vout

R

C

RC = Resistor Capacitor



8

Response GraphHigh Pass Filter

Frequency

Vout

Corner of cut-off frequency0.707 Vin

Vout = Vin



9

Response GraphLow Pass Filter

Frequency

Vout

Corner of cut-off frequency0.707 Vin



10

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

C

L LVin Vout

LC High Pass Filter

Vin VoutC C

L

LC Low Pass Filter



11

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.

VoutVin

L C

VoutVin L C

Band Pass LC Filters



12

Response GraphBand Stop / Notch Filter

Frequency

0

Pass Band

Stop Band

Pass Band

fc

Pass Band Loss

10

20

Loss (dB)



13

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

LL

LC

C

Vin Vout

L

LLC C

CVin Vout



14

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.

VoutVinL

C

VoutVinL

C



15

Mains Filter

High Voltage Capacitors *

MainsTo Equipment

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

Voltage Dependent Resistor

*Do not use ordinary Capacitors



16

Ferrite Ring Filters

• Using two Ferrite Rings will increase inductance.

• Common Mode (Interference)• Differential Mode (Mains Current)

Approx 8 to 15 turns of cable

Wind only two-thirds of ferrite

Mains or Coax Cable

One or more ferrite rings

Wanted antiphase currents

Unwanted in-phase currents

blocked by choke

Common Mode(RF Interference)

Differential Mode(Mains)



17

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.

High Pass Filter & Braid Breaker

R1 1M5

C1 4p7

C2 4p7

L1 L2

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



18

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.

FM Broadcast Receiver

Coax cut to /4 of the interfering signal

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

Open circuit on end



19

Ferrite Beads

Capacitor is 1nF low inductance disk ceramic type

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.



20

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.

12v 30A Filter

Out of Field of vision “Hands Free” Mic

Secure Antenna Mounts Easy See / Access for Radios

Easy Access Remote PTT

30A Main Fuse

Secure wiring at all times

Radio Fuses



21

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).



22

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.



23

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

chelmsford amateur radio society advanced licence course christopher chapman g0ipu slide set 19:...

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