Chelmsford Amateur Radio Society Advanced Course 7 EMC

Chelmsford Amateur Radio Society Advanced Course (7) EMC Part-2 - The Cures Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 1

Good EMC Station Layout Balanced Antenna Morse Key Use Key Click Filter Balun (if Coax Feed) Good Quality Coax and Connectors Coax or Balanced Feeder ATU Filter: LPF for HF or BPF for VHF/UHF Do Not Over-Modulate Transmitter VSWR Meter Good RF Earth Do Not Use Mains Earth Do Not Join Mains & RF Earths Filter Mains 13. 8 v • Screen All Units Well Especially the Transmitter Chelmsford Amateur Radio Society Advanced Licence Course Restrict Mic Audio Bandwidth Christopher Chapman G 0 IPU PSU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 2

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 Antenna as high as possible and far from House Balun Coax drops at 90°to antenna Station Coax Underground • Balun used. Bond Coax Outer to RF Earth • Balanced Antenna System Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 3

Choice of Antenna Type Long Wire System Poor EMC Antenna System • Unbalanced Antenna System. Strong RF Field • Strong RF fields near the house. Station • Poor earth system. Strong RF Field • Near TV antenna. RF Earth Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 4

Choice of Antenna Type Better EMC - Long Wire System Better EMC Antenna System • Unbalanced Antenna System. • Coax fed to remote ATU. • Earths at each end of Coax. • 7 MHz trap keeps higher frequencies (HF) away from house. 7 MHz Trap Kept as far as possible Good Spacing Prevent access to antenna at ground level Station ATU Earth • LF still too close though. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 5

Choice of Antenna Type Best EMC - Balanced System Excellent EMC Antenna System • Antenna balanced system. • Well located. Balanced Antenna as far as possible from house • Well Earthed. Balanced Twin Feeder • Away from house. Balun • Needs a lot of garden Coax underground Earth Rod Chelmsford Amateur Radio Society Advanced Licence Course Station Christopher Chapman G 0 IPU Earth Rod Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 6

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 • Passes low frequencies • Blocks high frequencies (attenuates) • 50% Power Cut-off point is at 1/ 2 (0. 707) of the voltage Chelmsford Amateur Radio Society Advanced Licence Course C R Vin Vout R Vin Christopher Chapman G 0 IPU C Vout Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 7

Response Graph High Pass Filter Vout = Vin 0. 707 Vin Corner of cut-off frequency Frequency Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 8

Response Graph Low Pass Filter Vout 0. 707 Vin Corner of cut-off frequency Frequency Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 9

LC Cascade (pi) Filters • Inductors are used as they vary in reactance with frequency (unlike resistors). Vin • There are three frequency sensitive elements. C L Vout C Vout LC High Pass Filter • They roll off three times faster than RC Filters • Little attenuation at pass band L L Vin C LC Low Pass Filter Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 10

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. Chelmsford Amateur Radio Society Advanced Licence Course L C Vin Christopher Chapman G 0 IPU Vout L C Vout Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 11

Response Graph Band Stop / Notch Filter Stop Band 0 Pass Band Loss (d. B) 10 20 Pass Band Frequency fc Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 12

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 L Vin C Chelmsford Amateur Radio Society Advanced Licence Course L Christopher Chapman G 0 IPU Vout L C Vout C L Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 13

Band Stop / Notch Filters Series • Low Impedance at resonance. • Stops a given band of frequencies at resonance. • Passing others. L Vin C Parallel • High Impedance at resonance. • Blocks the unwanted signal. • Passing others outside of resonance. Notch Filter • When response is sharp they are notch Vin filters, removing a particular frequency. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Vout C L Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures Vout 14

Mains Filter High Voltage Capacitors * To Equipment Mains Voltage Dependent Resistor *Do not use ordinary Capacitors • Some Mains Filters/Suppressors will have three VDR’s across L-N, L-E, N-E Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 15

Ferrite Ring Filters • Using two Ferrite Rings will increase inductance. • Common Mode (Interference) • Differential Mode (Mains Current) Common Mode (RF Interference) Unwanted in-phase currents blocked by choke One or more ferrite rings Approx 8 to 15 turns of cable Mains or Coax Cable Wind only two-thirds of ferrite Differential Mode (Mains) Wanted antiphase currents Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 16

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 C 1 4 p 7 L 1 L 2 C 2 4 p 7 R 1 1 M 5 L 1, L 2: 4 turns of 20 SWG 6 mm Dia x 6 mm Long Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 17

Coaxial Trap/Stub Filters Coaxial Trap/Stub Filter • Open circuit on end Coax cut to /4 of the interfering signal 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. taking in account the velocity of propagation in coax (0. 66) FM Broadcast Receiver Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 18

Ferrite Beads 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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Capacitor is 1 n. F low inductance disk ceramic type Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 19

Mobile Installations Considerations Installations Out of Field of vision • Care should be taken “Hands Free” Mic about the installation of radio equipment in vehicles. • Any EMC issue can have SERIOUS safety implications. Secure wiring at all times 30 A Main Fuse 12 v 30 A Filter • Fit filters to any power supply where possible to radio. • Keep car cable looms, Engine Management Units and RF cables separate at all times. Chelmsford Amateur Radio Society Advanced Licence Course Radio Fuses Easy Access Remote PTT Secure Antenna Mounts Easy See / Access for Radios Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 20

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). Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 21

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. Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 22

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 G 0 IPU Slide Set 19: v 1. 3, 2 -Oct-2007 (7) EMC-2: The Cures 23