Electromagnetic Interference (EMI) NMEA Conference 2014
Seminar Overview • EMI Troubleshooting • EMI Prevention with Proper Cabling & Terminations • Lightning Protection • Corrosion • Grounding Property of the NMEA. Shall not be copied or re-distributed.
Electromagnetic Interference • Unwanted Periodic Signal/Energy – “One person’s signal is another person’s EMI.” • Interrupts, Obstructs, Degrades, or Limits Equipment Performance Property of the NMEA. Shall not be copied or re-distributed.
Electromagnetic Interference • Radiated Emissions – RF Energy That Reaches Susceptible Equipment via Broadcast • Conducted Emissions – RF Energy That Reaches Susceptible Equipment via Common Connections Property of the NMEA. Shall not be copied or re-distributed.
EMI Propagation • Radiated Emissions – – Signals/Energy that Reaches Susceptible Equipment via Broadcast – Radiated Power Decreases by Distance Squared • Conducted Emissions – – Signals/Energy that Reaches Susceptible Equipment via Common Connections • Combined Modes – – Signals/Energy that Propagates via Cable Connections that then Become Signal Radiators Property of the NMEA. Shall not be copied or re-distributed.
EMI Sources • AC Units • Alternators • Battery Chargers • Blower fans • Engines • Generators • Inverters • Propeller Shafts • Radars Property of the NMEA. Shall not • Wiring be copied or re-distributed.
EMI Recipients • Electronic Compasses • AV Systems • Multifunction Displays • Just about any Electronic Device Property of the NMEA. Shall not be copied or re-distributed.
Identifying EMI Sources • Trial-and-Error Process of Elimination – Turn off All Equipment Except for Affected Device – Turn on a Device and Check for Symptoms – Repeat Until Interference Symptoms Return • Additional Testing May Be Required to Determine If Interference Is Radiated or Conducted • Interference May Be Radiated from Cables Connected to Interference Source Property of the NMEA. Shall not be copied or re-distributed.
Avoiding EMI Problems • Layout and Space Planning – Identify Potential EMI Radiation Sources – Identify Potential EMI Conducted Sources – Avoid Potential Hot Spots Rudder Feedback Signal Auto- Parallel cable runs pilot are not desirable RF Transmission Line VHF Radio F Property of the NMEA. Shall not be copied or re-distributed.
Mitigation Is Application Dependent Internal High Frequency DC Power Signal Supply Leads Signal Output Stage RF Output AC/RF to Antenna Output (AC Signal Wave Form) Property of the NMEA. Shall not be copied or re-distributed.
Mitigation for Unintended Signals • Objective: Block the Signal – Prevent Signal Transmission on Cables – Prevent Signal Transmission through Enclosure • Shielded Enclosure with Attached Ground • Input and Output Cables – Active Filters- usually BandPass or Notch – Ferrites – Best Practice is to attempt to solve problems within the “offender” rather than the “offended”. Property of the NMEA. Shall not be copied or re-distributed.
Ferrites as an EMI Suppressor • Ferrites Composed of Ferrous Oxide and One or More Powdered Metals • Composite Material Resists Imposed EMI Fields by Suppressing Electron Movement • Variable Sensitivity to Frequency – Lower Frequencies Pass without Significant Loss – Above Resonant Frequency Signal Becomes Coupled to Ferrite, Causing a High Impedance • Increasing Turns Increases Effectiveness Property of the NMEA. Shall not be copied or re-distributed.
Ferrite Geometry Property of the NMEA. Shall not be copied or re-distributed.
Mitigation for Signal Cables • Objective: Preserve Signal on Cable; Prevent Radiation to Other Cables and Devices • Balanced Signals Cancel Radiation • Shielding Captures Radiation and Diverts Signal to Ground (not applicable to coax). • Find it with troubleshooting Type Coverage Conductivity Flexibility Foil 100% poor poor Braid 85-95 % good good Property of the NMEA. Shall not be copied or re-distributed.
Eliminating Interference: Power cables • Shielded Cables – Connect One End of Shield to RF Ground • Grounding – Connect Case to RF Ground – Better to Connect Source’s Case – Less Desirable to Connect Affected Device’s Case • Filters – Install in Power Leads of offender first • Ferrites – Effective for Conducted and Radiated Noise Property of the NMEA. Shall not be copied or re-distributed.
Other Mitigation • Relocate Cable Runs • Relocate Equipment Displays • Relocate Antennas – Consider Antenna Radiation Patterns • Consider Cable Lengths – Avoid Multiples of ¼ Wave Length with power & data cables – This is not an issue with coax cables Property of the NMEA. Shall not be copied or re-distributed.
Signal Loss Calculations Total Loss = Cable Loss + Connector Losses Cable Loss = Loss in dB per 100 ft. (Depends on Type) x Cable Length / 100 Connector Loss = Number of Connectors x 0.5 dB Property of the NMEA. Shall not be copied or re-distributed.
Connector Selection Connector Maximum Impedance Permitted Uses Type Frequency UHF (PL-259) 300 MHz 50-Ohm VHF , SSB, DGPS, Stereo BNC 4.0 GHz 50-Ohm VHF , SSB, DGPS, Cell, GPS TNC 2.5 GHz 50-Ohm VHF , SSB, DGPS, Cell, GPS, MINI-M N 11.0 GHz 50-Ohm VHF , SSB, DGPS, Cell, GPS F 2.0 GHz 75-Ohm TV , GPS Mini UHF 2.5 GHz 50-Ohm Cell SMA 12.0 GHz 50-Ohm SAT Phone SMB 4.0 GHz 50-Ohm FME 200 MHz 50-Ohm VHF , SSB Property of the NMEA. Shall not be copied or re-distributed.
Cable Bend Radius Cable Type Bend Radius (inches) RG58U 2.0 RG8X 2.4 RG8U 4.5 RG213 5.0 LMR240 0.75 LMR400 5.0 Better to route coax cables using gentle S-curves (green) than tight right-angle turns (yellow) when possible.
Coax Cables • Attenuation – Transmission Losses (dB per ft.) – Connector Losses (dB) – Impedance Mismatch (Avoidable) 50 Ω - 75 Ω – Also affected by installation • Cable Length – Minimum Length Necessary (Most systems have no “tuned” length) • Equipment Connections – Match Application and Cable Type • Extensions –beware of the allowed loss
Cable Selection RG58U RG8X RG8U RG213 LMR240 LMR400 3 / 16 " 1 / 4 " 13 / 32 " 13 / 32 " 1 / 4 " 13 / 32 ” Nominal O.D. Conductor 20 16 13 13 15 9 (AWG) Impedance 50 50 52 50 50 50 (Ohms) Impedance Match within ± 2 Ohms Property of the NMEA. Shall not be copied or re-distributed.
Shielding Connections of Power • Shields Connected to RF Ground System • Effectiveness Dependent on Low Impedance to Ground • Best Ground Path for High Frequency EMI signals is via Copper Foil – # 8 AWG Stranded Copper Wire Acceptable Property of the NMEA. Shall not be copied or re-distributed.
Lightning Strikes- GOOD LUCK! Refer to ABYC TE-4 — Lightning Protection for more information • Direct Strike – Direct Hit on a Part of the Vessel, Such as an Antenna or Mast. • Conductive Strike – Strike on a Utility Line and Conducted Aboard Through the AC Power Cord • Inductive Strike – Nearby Strike Causing a Large Magnetic Field, Which in Turn Induces a Voltage in the Vessel’s Wiring • There is no sure way to prevent damage from any type of strike.
Lightning Protection: Two schools of thought Refer to ABYC TE-4 — Lightning Protection for more information • Conductive and Inductive – Commercially Available Surge Protection Products • Direct Strikes – Air Terminals Connected to Properly Sized Conductors, with Relatively Straight Paths to Ground – Towers Grounded Port and Starboard Property of the NMEA. Shall not be copied or re-distributed.
Corrosion Refer to ABYC E-11 — AC & DC Electrical Systems on Boats for more information • Electrolysis – Chemical and/or Electrochemical Change Due to Electric Current • Galvanic Corrosion – Corrosion from Electric Current Flow between Connected but Dissimilar Metals in the Same Electrolyte
Other Corrosion Sources Refer to ABYC E-11 — AC & DC Electrical Systems on Boats for more information • Stray Current Corrosion – Electric Current Flow Caused by an Outside Source. (In a Marina). • Velocity Corrosion – Electric Current Flow Caused by Strong Water Currents • Selective Corrosion – Electric Current Flow Caused by Areas with Different Composition within the Same Alloy • Oxygen Starvation – Electric Current Flow Caused by Trapped Electrolytes Making Adjacent Surfaces More Positive to the Remaining Metal Surface
To Bond or Not to Bond Age Old Debate! Refer to ABYC E-2 — Cathodic Protection for more information • Decision Is Vessel Dependent • Bonding Is Fairly Common in the Industry • Aluminum Is Less Noble Than Almost Everything Else • Always Isolate From Bonding System And/or Bond by Itself to a Sacrificial Anode Selected Especially for Aluminum Property of the NMEA. Shall not be copied or re-distributed.
DC Common Grounding System Refer to ABYC E-11 — AC & DC Electrical Systems on Boats for more information • Shorts Stray Potentials to Ground • May Provide Cathodic Protection • Usually Main Connection between Vessel and Earth Ground Property of the NMEA. Shall not be copied or re-distributed.
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