electromagnetic interference (emi) - nmea ibex presentation emi.pdf · electromagnetic interference...

IBEX 2011Speakers:

David Gratton- Martek-Palm Beach, FLJohnny Lindstrom- Westport Shipyard, WA

Electromagnetic Interference (EMI)

Property of the NMEA. Shall not be copied or re-distributed.

Seminar Overview

• EMI Troubleshooting• EMI Prevention with Proper Cabling & Terminations• Lightning Protection• Corrosion• Grounding


Electromagnetic Interference

• Unwanted Periodic Signal/Energy– “One person’s signal is another person’s EMI.”

• Interrupts, Obstructs, Degrades, or Limits Equipment Performance


Electromagnetic Interference

• Radiated Emissions – RF Energy That Reaches Susceptible Equipment via Broadcast

• Conducted Emissions – RF Energy That Reaches Susceptible Equipment via Common Connections


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 RadiatorsProperty 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• Wiring Property of the NMEA. Shall not


EMI Recipients

• Electronic Compasses• AV Systems• Multifunction Displays• Just about any Electronic Device


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


Avoiding EMI Problems• Layout and Space Planning

– Identify Potential EMI Radiation Sources– Identify Potential EMI Conducted Sources– Avoid Potential Hot Spots

VHFRadio

Auto-pilot

F

RF TransmissionLine

Rudder FeedbackSignal

Parallel cable runs are not desirable


Mitigation Is Application Dependent

Internal HighFrequency

SignalDC PowerSupplyLeads

Signal OutputStage

AC/RFOutput

RF Outputto Antenna(AC Signal

Wave Form)Property of the NMEA. Shall not


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”.


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


Ferrite Geometry


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 FlexibilityFoil 100% poor poorBraid 85-95 % good good


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


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


Signal Loss Calculations

Total Loss = Cable Loss + Connector Losses

(Depends on Type)Cable Loss = Loss in dB per 100 ft. x Cable Length / 100

Connector Loss = Number of Connectors x 0.5 dB


Connector Selection

Connector Type

Maximum Frequency

Impedance Permitted Uses

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


Cable Bend Radius

Cable Type Bend Radius (inches)

RG58U 2.0RG8X 2.4RG8U 4.5RG213 5.0LMR240 0.75LMR400 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

Nominal O.D.

3/16" 1/4" 13/32" 13/32" 1/4" 13/32”

Conductor (AWG)

20 16 13 13 15 9

Impedance(Ohms)

50 50 52 50 50 50

Impedance Match within 2 Ohms


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


Lightning Strikes- GOOD LUCK!

• 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.

Refer to ABYC TE-4 — Lightning Protection for more information

Lightning Protection:Two schools of thought

• 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

Refer to ABYC TE-4 — Lightning Protection for more information


Corrosion

• 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

Refer to ABYC E-11 — AC & DC Electrical Systems on Boats for more information

Other Corrosion Sources

• 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 BondAge Old Debate!

• 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

Refer to ABYC E-2 — Cathodic Protection for more information


DC Common Grounding System

• Shorts Stray Potentials to Ground

• May Provide Cathodic Protection

• Usually Main Connection between Vessel and Earth Ground



Grounding System Interconnect

• Objective: Single Point with No Current Flow• Practical: Few Interconnections with No Common Currents


Vessel Grounding Systems

System Type When Required

DC Ground or Negative Reference All Vessels with DC Systems

AC Neutral Reference Vessels with AC Shore Power, Generator, or Inverter Installed

AC Grounding (Safety) Safety

RF Ground Performance Vessels with Electronics Equipment Installed

Single Side Band (SSB) Ground

Performance Vessels with SSB Transceiver Installed

DC Grounding Reference

Lightning Ground Safety



electromagnetic interference (emi) - nmea ibex presentation emi.pdf · electromagnetic interference...

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