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.
The connector was originally called a UHF connector, and still is in some circles, which is odd because it was designed for use at frequencies up to 300 MHz, the VHF band, and not the UHF band which only starts at 300 MHz. Mr Quackenbush probably had nothing to do with the misnaming of his creation.
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 all about its non-constant impedance but at marine frequencies, around 150 MHz, this doesn’t matter a jot.
You can find fitting instructions on the Salty John website, under ‘articles and links’.
The PL259 needs to fit cable diameters from 5mm up to 10.4mm. (The various cable types are discussed elsewhere on the blog). Although you can buy 5mm PL259s it’s more usual to use a standard connector with an adapter insert to suit the appropriate size cable. I like using an adapter because it grips the coaxial braid firmly and that means you don’t need to solder the braid to the connector body. You still need to solder the centre conductor to the centre pin, of course, but that’s easy.
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 deck you have make 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 all called PL363 barrel connectors and you have to specify the length.
The PL363 comes with a pair of nuts to secure it through the bulkhead – be that a wall 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.
You could consider fitting a PL259 to your AIS antenna lead and then use an SO239/BNC adapter to connect to the AIS unit – this makes sense because the PL259 is easier to fit than the BNC female connector and, should you lose your masthead VHF antenna, you can simply stick the AIS antenna lead, sans adapter, into the back of your VHF radio and you’re back in business radio-wise.
Another connector you might encounter on boats is the N connector – named for that serial 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. 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.
I think I’ve rambled on quite enough for one day – I hope some will have found the foregoing illuminating, and I know that many will have slipped into a coma after the first paragraph or two.
