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

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

2. 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

3. 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

4. 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.

5. 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.

6. 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

7. 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

8. Response GraphHigh Pass Filter Vout Vout = Vin Frequency 0.707 Vin Corner of cut-off frequency

9. Response GraphLow Pass Filter Vout Frequency 0.707 Vin Corner of cut-off frequency

10. 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

11. 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.

12. Response GraphBand Stop / Notch Filter Stop Band Pass Band Loss 0 Frequency Loss (dB) 10 Pass Band Pass Band 20 fc

13. 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

14. 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.

15. 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

16. 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

17. 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.

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. 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

19. 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

20. 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.

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