The range and effectiveness of your yacht’s radio communication depends on the dimensions, position and quality of your VHF antenna. Sam Fortescue explains
When Guglielmo Marconi began sending radio messages between coastal radio stations and ships at sea in 1897, there were plenty who predicted the technology would have very limited scope. Yet, just four years later, Marconi succeeded in transmitting a radio signal across the Atlantic Ocean, from Cornwall to Newfoundland, an experiment that required an extraordinary 500ft-tall antenna held up in a gale by an enormous kite.
Today, a better understanding of the physics behind radio waves, along with huge leaps in technology, mean that the equipment now needed for yacht comms has been successfully miniaturised without sacrificing too much performance. A quality radio set with a good antenna setup can send and receive VHF signals over line of sight distances of 50 miles, even further in favourable conditions.
And the antenna is still absolutely key, as Ian Lockyer of radio set manufacturer Icom UK explains: ‘Even the best marine radios in the world will not perform to their optimum if connected to the wrong antenna. Much as the tyres of a sports car are the only contact point with the ground, providing the necessary grip and traction, a radio’s only contact point with the outside world is its antenna. It needs a well-matched antenna to allow the efficient conversion of electrical energy into radio waves.’
Just as worn tyres will undermine a car’s performance, an unsuitable VHF antenna or poor quality cabling will hamstring your radio. Poor positioning of your antenna or interference from other nearby antennas can also reduce range and clarity dramatically.
It’s also wise to have a redundant VHF antenna setup if you’re planning to head offshore for racing or blue-water cruising. Indeed, many events, such as the ARC, require it. You can, for example, run a spare cable up the companionway if the mast unit is damaged and mount a spare antenna on the pushpit.
Amplifying the signal
An antenna’s job is to convert electromagnetic radiation (radio waves) into electric current and back again, depending on whether it is transmitting or receiving. Physics ensures that the electrons in the antenna do this in a predictable way, which can be amplified by your equipment to make the signal stronger – that is, to increase the volume you hear via a speaker, or achieve longer range in transmission.
In a perfect world, the antenna would be made of copper or some other ultra-conductive metal. But at the masthead, strength and corrosion resistance are all important, so stainless steel generally rules here. That said, leading antenna manufacturers such as Glomex, Shakespeare and Amphenol Procom do all make marine aerials with a copper antenna sheathed in protective fibreglass. The problem is the extra windage.
Antenna length vs wavelength
The dimensions of the antenna are key. Operating at 156-162MHz, maritime VHF bands have a wavelength between 1.72m and 1.92m (the distance between peaks). There is a relationship between this wavelength and the antenna length, which means that VHF aerials range from about 25cm up to 5-6m. In marine, they are available as a 50cm shortened, a 1m dipole (half wave),
a 2.5m collinear gain or 5m-plus collinear gain unit. As sailors mount their antenna at the masthead, the 100cm option is the most common.
The longer the antenna, the higher the so-called ‘gain’ – a measure of the increase in the signal strength. With no gain, the signal would resemble an apple – being broadcast evenly in all directions. The higher the gain, the more it is squashed, like a flat peach – until it becomes a narrow disc. So higher gain means better range, but a narrower beam horizontal width.
‘On a sailboat, you would probably have 3dBi antenna, because the signal that comes out is like a big fat doughnut, or a lightbulb,’ says Dave Manasseh of antenna-maker Shakespeare.
‘So, as the boat rolls or pitches, you’ll still be able to see it. On a 6dBi antenna the signal is more like a row of torches, where the beam shoots up into the sky and down to the sea as you roll. If you’re going to use a higher gain antenna, you need to put it on a stable platform.’
VHF antenna height
Because radio signals are limited to a line-of-sight range, height is vital. An antenna positioned 2m above the waves will have a horizon of five miles, whereas one at 15m could manage ten miles. Assuming the receiving station has its antenna at a similar height, the range is doubled. Sea waves can obstruct the signal, which has a greater impact at lower heights.
You can have the best antenna in the world, perfectly installed at the masthead, but if the cabling that connects it to your VHF radio set is of poor quality, you’ll never get a good signal out of it. In the car analogy, it would be as if the fuel line to the engine were too narrow and you couldn’t use its maximum power.
Coaxial cable used in VHF setups has a conductive core surrounded by an insulator, with a metallic shield around that and an outer plastic sheath to protect it. The insulator keeps the core and the shield at a constant distance from each other, to maximise signal quality. And the shield is there to intercept electromagnetic interference and conduct it away from the sensitive core.
Just like electrical wiring, coaxial cable comes in different thicknesses and different qualities. Most of us will have the standard RG58 cable aboard. It’s entry-level stuff, with a copper core and a braided metal shield. It is flexible and relatively cheap, and good for shorter cable runs. For longer runs, you would benefit from using a more conductive cable, such as RG213/UR67, which has twice the diameter.
RG8X is another commonly used cable which sits between the two in terms of performance. But the sky is the limit – you can spend money on silver plating, quadruple shields, and more. ‘Coaxial cable has losses within it, so you want the best quality conductor for your antenna,’ explains Manasseh. ‘A solid copper element coated in silver has even fewer losses, but it only makes a difference at the extremities and costs twice the price.’
Ian Lockyer at Icom UK says that it depends on the length of the cable run. ‘As a rule of thumb, RG58 is good for up to 10m, and if you are using RG213, that is good for up to 30m. Any longer than that, and you need to go for Heliax.’
Good quality connectors are also important, and bear in mind that the more connectors there are in the system, the more points of potential failure and resistance there will be. Gold connectors offer the best conductivity, but are often subject to poor installation.
Many sailors will have Shakespeare’s V-Tronix Hawk fitted, combining a wind indicator and a 1m antenna. This comes with 20m of RG58 cable, and its base connector means
that no other type of cable fits here.
‘Our standard Hawk whip flex comes with 20m of cable,’ says Manasseh. ‘It’s an acceptable amount of loss, but we advise you cut it to a minimum. Some people will keep the cable and coil it up behind the radio – never do that. Others leave a short 50cm pigtail of the RG58 to connect to the Hawk, then use an adapter to wire in a better quality cable down the mast and into the radio set.’
Masthead real-estate is pretty limited, and it’s tough to run new cables down the mast and through the deck. So most people prefer to use a single antenna for all their VHF requirements, including FM radio (88-108MHz) and AIS (161.975MHz and 162.025MHz). The same antenna and cable can certainly do this job, but you’ll need a signal splitter to separate the three elements and send them to the right devices.
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Where you have an AIS receiver only, you can fit a simple splitter for around £50. It is designed to prioritise the VHF and prevent your broadcast from being directed into the sensitive electronics of the FM and AIS gear.
It gets a little more complicated when you have an AIS transceiver that is also broadcasting at up to 12W itself. In this case, you need an active splitter which will switch between the two and isolate both. Some AIS units have these already built in, but you can buy external ones costing £200-£600. If you’re looking at something with a much lower price tag than that, beware! It probably only works with AIS receivers.
There is a downside to splitters, however. The electronics reduce the receive signal strength a little, chipping away at the gain of your antenna. This is especially true if you buy a cheap one. On the other hand, Vesper Marine claims its SP-160 splitter has the opposite effect because it contains a low-noise amplifier that actually boosts the incoming AIS signal (its 12dB figure provides no context, however). This could prove valuable with the proliferation of low-power MOB and SART beacons that use AIS.
The VHF spectrum is finite and there’s huge pressure on its use. Some busy marine areas inshore are so saturated with AIS data that broadcasting and reception becomes haphazard. Work is, though, now under way to broaden use of the marine VHF spectrum and harmonise it with satellite and terrestrial-based transceivers. It’s hoped that this so-called VHF Data Exchange System (VDES) will achieve 32 times better bandwidth.
VDES is still in early testing, so there are no products for leisure sailors that are capable of tapping into it. But many in the industry believe that will change in the next decade. Real-time weather observations, two-way comms and ice forecasts all become possible under VDES. And it will all be integrated into one unit. It’s a process that is already under way in products like Vesper Marine’s Cortex. Fortunately, the VHF component will function perfectly well with an existing antenna.
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