1. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 1 Chelmsford Amateur Radio Society Advanced Course (7) EMC Part-2 - The Cures

2. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-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 Earth Do 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 Well Especially the Transmitter Good Quality Coax and Connectors

3. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 3 Choice of Antenna Type Dipole / 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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 4 Choice of Antenna Type Long Wire System Poor EMC Antenna System Unbalanced Antenna System. Strong RF fields near the house. Poor earth system. Near TV antenna. Station RF Earth Strong RF Field

5. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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 Type Better EMC - Long Wire System Station Kept as far as possible Good Spacing Earth ATU 7MHz Trap Earth Prevent access to antenna at ground level

6. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 6 Excellent EMC Antenna System Antenna balanced system. Well located. Well Earthed. Away from house. Needs a lot of garden Choice of Antenna Type Best EMC - Balanced System Station Balanced Twin Feeder Balanced Antenna as far as possible from house Coax underground Earth Rod Balun Earth Rod

7. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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 at 1/  2 (0.707) of the voltage V in V out C R V in V out R C RC = Resistor Capacitor

8. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 8 Response Graph High Pass Filter Frequency V out Corner of cut-off frequency 0.707 V in V out = V in

9. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 9 Response Graph Low Pass Filter Frequency V out Corner of cut-off frequency 0.707 V in

10. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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 LLV in V out LC High Pass Filter V in V out C C L LC Low Pass Filter

11. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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. V out V in L C V out V in LC Band Pass LC Filters

12. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 12 Response Graph Band Stop / Notch Filter Frequency 0 Pass Band Stop Band Pass Band fcfc Pass Band Loss 10 20 Loss (dB)

13. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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 V in V out L LLCC C V in V out

14. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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. V out V in L C V out V in L C

15. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 15 Mains Filter High Voltage Capacitors * Mains To Equipment Some Mains Filters/Suppressors will have three VDR’s across L-N, L-E, N-E Voltage Dependent Resistor *Do not use ordinary Capacitors

16. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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 L1L2 L1, L2: 4 turns of 20SWG 6mm Dia x 6mm Long

18. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 19 Ferrite Beads Capacitor is 1nF low inductance disk ceramic type Audio & Non RF Circuits (A) has a capacitor across the base-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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 20 Mobile Installations Considerations Installations Care should be taken about the installation of radio equipment in vehicles. Any EMC issue can have SERIOUS safety 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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 19: v1.3, 2-Oct-2007 (7) EMC-2: The Cures 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