Antenna cable connectors

Here’s a short look at the type of coaxial cable connector you might encounter when installing your radio and AIS antenna systems.

Boats use 50 ohm coaxial cable for their radio and AIS systems – it comes in different sizes but the three you are most likely to encounter are RG58 (nominal size 6mm, but rarely more than 5.5mm), RG8X (nominal size 7mm but usually 6.5mm and the best choice for boats up to super-yacht size) and RG213 (nominally 10mm, actually about 9.5mm – the cable you need for long runs, 30m or more). Coax cable comprises an outer jacket covering a layer of fine mesh or braid, an insulating layer called the dielectric and then the centre conductor. The outer braid and the centre conductor must never meet. Whichever coax size you have you’ll need to join it to the equipment, and to itself, using a variety of connectors:

The PL259 and its female partner the SO239. This connector pair was developed in the late 1930’s by a designer with the fantastic name of E. Clark Quackenbush. He worked for Amphenol at the time and I wouldn’t have mentioned him at all were it not for that magnificent name. Anyway, he designed what was to become the most widely used connector in the amateur radio field.

PL stands for plug and the number, 259, is the inventory number assigned to it by the US military. The socket into which it plugs is given another inventory number, 239, and the prefix SO for socket.

All marine VHF radios have a built-in SO239 antenna socket to accept a PL259. Top quality marine antennas use the same connector, so the antenna cable will have a PL259 at each end, whatever other connectors it has for intermediate joins.

The PL259 is simple, mechanically rugged and relatively easy to fit. That’s why it’s popular on boats. Purist radio techies will tell you about its non-constant impedance but at marine frequencies, around 150 MHz, this doesn’t matter a jot.

Fitting: Most PL259s are the solder type but usually only the centre conductor is actually soldered into the pin, the braid being held mechanically. In some cases the cable is crimped onto the cable, making contact with the braid through the pvc jacket. For this you need an appropriate crimping tool so this type is more popular for industrial application. The leisure sailor is more likely to choose a screw-in-earth type, where the cable is screwed into the threaded inlet of the plug once a section of braid has been exposed, and then the centre conductor is soldered into the pin. Many top quality PL259s are made for the bigger cable size and have an adapter insert to suit the smaller sizes of cable. Using an adapter is convenient because it grips the coaxial braid firmly. I like silver plated connectors because they avoid corrosion and they solder very well – and I’d always have silver or gold plated centre pins for this reason.

You can get PL259s that require no soldering, they are entirely mechanically assembled. The Shakespeare Center-Pin connector is a good example. A little expensive, perhaps, but they are great for solderphobes.

The PL259 is not fully waterproof and the join should be protected with silicone self fusing tape when used outside.

When the cable run on a boat encounters a bulkhead or the deck you have a choice – do you drill a hole and pass the cable through it, continuing the unbroken run, or do you use a bulkhead connector of some sort? I’ll save the debate over the relative merits of deck plugs, deck glands and the various joining methods for another time, but no discussion of the PL259 would be complete without a mention of the barrel connector.

The barrel connector is a double female – you can plug a PL259 into each end and make a mechanically strong connection between two sections of cable. The barrel connector comes in a variety of lengths starting with the small, discontinuously threaded version about 1” long, up to a 12 inch long monster.

The short barrel connector is called a PL258. This shows that the bloke in the spares department in the US military wasn’t on his toes when it came to designating inventory numbers because this is clearly a double socket (SO) and not a plug (PL).

The longer versions are often called PL363 barrel connectors, or bulkhead connectors, or double-females and some other weird designations. You have to specify the length. The PL363 comes with a pair of nuts to secure it through the bulkhead or the deck or a radar arch base. The standard nuts are a bit wimpy but you can buy more substantial ones – the thread is 5/8” 24 tpi.

The BNC connector is a bayonet connector designed for applications where frequent connecting and disconnecting occurs, such as on laboratory oscilloscopes. Despite this it has found its way into applications such as connecting the antenna to an AIS unit, or even for cable to cable connections.

BNC stands for Bayonet Neill Concelman, after its two designers.

Aware that the bayonet design allowed noise to intrude when the cable was subjected to vibration the Neill Concelman partnership came up with a more secure variation, the TNC, for Threaded Neill Concelman.

Both connectors have male and female halves – typically the male bit is attached to the AIS unit and the antenna cable is fitted with the mating female connector. Barrel connectors are also available for cable to cable joins. BNC and TNC connector sets are often chosen as cable to cable connectors when the reliable but chunky PL259/barrel connector/PL259 connection is unworkable.

BNC and TNC connectors are fiddlier to fit to the cable than the good old PL259 but they are high performance connectors, used for frequencies as high as 11 GHz. That’s a gazillion times more critical than the simple 150 MHz of VHF.

Another connector you might encounter on boats is the N connector – named for that prolific connector designer Mr Paul Neill of Bell Labs who designed it in the 1940s. This is another connector set that has high performance, being suitable for frequencies up to 11 GHz. Large commercial VHF antennas often come with an N connector and RG213 cable.

If you have satellite communications on your boat you may encounter the F connector to attach to a remote antenna system and if you want to connect your handheld VHF radio to a fixed antenna you might use an SMA connector, although some manufacturers have their own proprietary antenna connector.

So there you have it, the low down on RF connectors for boats.

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