Coax 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.
Coaxial cable. 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.

Storm tactics: Heaving-to

When the wind pipes up and the going gets tough heaving-to is a great tactic that lets you stop the world and get off for a while. Or, as Bernard Moitessier says in his classic The Long Way: “….when you no longer know what to do: come about without touching the sheets, put the helm alee, stretch out in the cockpit, eyes closed, and then see things as they are….”.

You can heave-to to have lunch or to weather a storm or, of course, to lie in the cockpit and contemplate. How you do it depends to a large extent on your boat and you should practice the manoeuvre so that you can do it when you need to. For most it’s a matter of tightening up to close hauled and then tacking without releasing the jib sheet. Once the jib is aback, let out the main a little and lash the helm alee. Each boat will behave somewhat differently but the principle of setting the helm and main to drive the boat against the backed jib remains – it’s a matter of finding the right balance for your boat and the prevailing conditions.

In storm conditions you’d be down to storm jib and fully reefed main or trysail but you can heave-to with a fuller sail plan if you just want to stop for lunch or to carry out some task which is best done with the boat still.

Hove-to, the boat should lie about 40º or 50º off the wind and forereach slowly. You are underway so need to act accordingly regarding collision avoidance.

A good skill to acquire is heaving-to.

The anchor snubber

The snubber is a length of nylon or polyester three-strand line that takes the anchor load from the chain to a deck cleat or Samson post, absorbing the shocks and leaving the chain hanging in a loose bight, resting lightly and relatively noiselessly in the bow roller.

The snubber is attached to the chain by a chain hook of some sort – there are a range of proprietary variations available – or a rolling hitch. After a few months, we dispensed with our clunky chain hook in favour of the rolling hitch – we found this more positive than the chain hook and more deck and toe friendly. The rolling hitch is particularly suited to this purpose, it doesn’t tighten under load and so won’t jam and become difficult to undo.

The snubber for a 35 to 40-foot cruising boat would be typically 12mm diameter and at least 12m long. If you choose a line that’s too heavy you won’t get enough of the beneficial stretch into the system, which is why old halyards and sheets aren’t really suitable for this purpose, they tend to be low stretch. The snubber is attached to the chain and a strong point on deck and then the chain is run out until the snubber comes up taught, then a few more feet to give a nice healthy loop of chain and you’re set. If the snubber chafes through the chain retakes the load.

A snubber is also useful in anchorages where the swell comes from a different direction to the wind, curving around a headland, perhaps. The boat, lying to the wind, may take the swell on the beam and roll uncomfortably. In this case, lead the snubber line all the way aft to a cleat or sheet-winch on the side away from the swell. Then, as you let out more anchor chain, the boat will turn her head toward the swell as the anchor lead point moves aft. This bridle arrangement can mean a good night’s sleep in an otherwise impossibly rolly anchorage.

An essential thing is the snubber.

Night sailing

When the sun has melted into the horizon like a knob of butter on a hotplate you flick on the navigation lights and prepare for a night at sea.

Night sailing is at times magical, at other times intimidating. Deep water with plenty of sea room, no traffic, a gentle breeze and a big moon are the ingredients for a pleasant night passage.

We enjoyed just such an untroubled passage between Dominican Republic and Puerto Rico on our sloop Adriana. It had been hot and windless during the day but as night fell the breeze came back and the lights of Mayaguez twinkled on the horizon. I set the jib, slacked the mainsheet and cut the engine. Adriana leaned her shoulder into the sea and came alive. The sky grew into deeper shades of night, beset with a million jewels, as we cut a swathe through the boisterous sea.

Other night passages have been less idyllic: Battling to windward along the north coast of Hispaniola under a grim moonless sky, hugging the rocky coastline to stay within the umbra provided by the land, lightning blooming on the horizon – this wasn’t the most relaxing of night watches.

Dawn creeps up with the promise of delight or of dire warning – radiant sunburst or red tinged clouds. Another day at sea begins. What will it bring?

Forestay tension on catamarans

Forestay tension is achieved by tensioning the backstay. On most boats the amount of backstay tension you need to apply is less than the resultant forestay tension because the forestay makes a more acute angle with the mast than does the backstay.

However, on many catamarans the backstay makes a more acute angle than the forestay, as can be seen on this Prout sail plan. This means that for every kilogram of tension needed in the forestay you have to crank on more backstay tension. So, be careful.

A Loos tension gauge is a handy tool for setting up standing rigging.

 

The Metz VHF antenna

To survive in the marine environment your VHF antenna needs to be strongly built of appropriate materials – look for stainless steel components, including both the body and the whip. How the antenna is built internally isn’t so obvious – you’ll need to rely on reputation and a good warranty.

A marine VHF whip antenna depends for its performance on proper design and build-quality. That tin-can at the bottom of the antenna contains the DC shunted coil that must be precisely tuned to the proper resonance. Getting this bit of the design and build right is the difference between an antenna that performs well and one that doesn’t.

The US built Metz antenna is based on a heavy gauge stainless steel shell which encloses the coil wound around a substantial form. The coil assembly is sealed in a solid epoxy compound. This build method allows prolonged transmission without danger of coil distortion as the antenna heats up. It carries a life-time warranty.

The Metz antenna is used by professionals such as UK Search and Rescue organisations, professional fishermen, tour boats and the Clipper Round the World fleet.

Lower quality antennas have fibreglass or mild steel bodies (see picture) enclosing light gauge coils and inadequate forms all sealed in a waxy substance. This flimsy internal construction leads to distortion of the coil as the antenna heats up when transmitting, which changes the antenna characteristics, leading to poor performance.

You don’t need to pay through the nose for top quality construction: the Metz Manta, at £59.95 including UK postage, is no more expensive than ordinary antennas and lower in price than some other stainless bodied antennas. Check it out at the Salty John on-line shop.