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Cable connectors: A weak link if done improperly
4/20/2020

Many of the failures of electronics on boats are due to connections between cables that are made improperly during installation.  If you’re planning to do some wiring or re-wiring, marine electronics dealer John Barry says to consider frequency response, and be sure to match impedance, use environmentally rated connectors, tie carefully, never mount under tension, don’t over-tighten wire ties and test all connections. And, importantly, he says to always follow manufacturers’ guidelines.

By John Barry


Marine electronics installers use a wide variety of connectors to accomplish the job.  Connectors used in the marine environment range from simple to complex and overlap other industry connectors in their construction and usage.  We share the PL259, the UHF connector used for marine VHF, with many other radio services, like HAM radio or CB radio (Citizens Band).  We also share the RJ45 Ethernet connector with the computer industry.  We all use connectors for all types of connections, but there is more to getting the connection right than just plugging it in.

Of course, connectors, like all technology, is a topic that has evolved and is evolving as we stand on the shoulders of giants.  Connectors have properties that make one connector OK and another not OK!  The most obvious property or specification associated with a connector is its frequency response. A PL259 connector will pass up to 300 MHz, so a signal that is below 300 MHz will pass through the connector with minimal signal loss.  If we connect a 900 MHz cell phone using a PL259 connector there is huge loss, to the extent that it will not work.  TNC (Threaded Neill-Concelman) connectors have a much higher frequency response, and so we often see TNC connectors on cell phones.  Always use a connector that is rated for at least the frequency you are using.
 
Be sure to match impedance

There are considerations other than frequency response when selecting a connector.  For coaxial connectors there is an impedance rating. [In simple terms impedance is the amount of resistance that a component offers to current flow in a circuit at a specific frequency—Editor]

This is because they are used in antenna transmission lines.  Antenna transmission lines are simply the coaxial cable that connects the antenna and the radio.  The impedance rating of the connector is always matched to the impedance of the transmission line.  So if we use 50 ohm coax, as is typical on a VHF antenna, we must use 50 ohm connectors.  If we use 75 ohm coax, as is typical on satellite TV, we must use a 75 ohm connector. When troubleshooting signal loss, always check for an impedance mismatch.

Other properties of connectors such as vibration resistance or waterproofness are greatly dependent on physical construction.  NMEA 2000® uses Device-net connectors which are rated IP67.  The IP rating relates to waterproofness and more, and depending on the environment various IP ratings may be required.  Consumer-grade connectors should not be used in harsh environments or for safety-critical connections.  Installing equipment in dry locations, properly sealing penetrations and chemical preservatives all help with this.  A very thin coating of silicone dielectric grease applied to connectors is a good idea.  

Shielding

One more characteristic of note is shielding. Most coax connectors are metal and maintain the shielding of a wire.  Some connectors are plastic and do not continue the outer shield through a connection.  For instance, we recommend the use of shielded Cat 5e Ethernet wire in marine applications.  Shielded Ethernet connections are common on boats.  If plastic connectors and couplers are used for terminating and connecting these wires, the shielding is not continuous and noise problems are much more likely. 


About the author

John Barry owns and operates Technical Marine Support, Inc., in Pleasant Prairie, WI. He’s a Certified Marine Electronics Technician and a principal instructor of NMEA technical courses.

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