- … a vessel’s VHF communications aerial mast or antenna is undoubtedly a key component in an invisible ‘lifeline’ that has the potential to save lives at sea.
- Giving attention to the detail of your boat’s anatomy is the key to trouble-free boating …
- … ‘preventative maintenance’ should be essential watchwords for anyone venturing offshore.
Compared to the more glamorous parts of a vessel’s anatomy, some items on its inventory can be viewed as ‘lesser subjects’ or ‘poor cousins’, until the day they break or malfunction ‒ usually when you need them the most! Here we look at the subject of marine antennas and masts ‒ key components in an invisible ‘lifeline’ that has the ability to save lives at sea. We also not only dispel a few myths and shed light on a few essential facts, but give guidance on fitting the right type to your boat.
Even the best marine radios in the world will not perform to their optimum if connected to the wrong antenna. Much like the tyres of a sports car are the only contact point with the ground, providing the necessary grip and traction when racing around a track, a radio’s only contact point with the outside world is its antenna. If the race car tyres are of low quality or worn, the car will not perform to its best capability ‒ similarly, a radio needs a well-matched load (antenna) to allow the efficient conversion of electrical energy into radio waves.
The antenna also needs to be well designed so that the radiated waves travel in the required direction ‒ there is no point directing radio waves up into space when all of your contacts are at or around sea level! A poor-quality antenna will not provide the required good match and may leave you with questions about your installation and whether your radio equipment is working correctly. That is why we suggest you use a good-quality antenna from a reliable manufacturer.
Here are just some of the factors you should consider before you purchase an antenna:
The higher your antenna is above the water, the greater the distance you’ll achieve. VHF radio waves travel pretty much in straight lines, and your communication range will often be referred to as ‘line of sight’. In a sense, your antenna has to be able to ‘see’ the other antenna, or put another way, there must be a clear unobstructed path between antennas. Because of the curvature of the earth, as the distance between two antennas increases, they eventually fall below the horizon and can no longer communicate with each other. In most cases, communicating by way of VHF is limited to about 35‒50 miles, depending upon antenna height. Sailing boats have a distinct advantage here as you can mount an antenna at the top of the mast and reach great distances.
When it comes to antennas, size matters ‒ the longer the antenna, the higher the gain. Antenna gain refers to the effective increase in signal strength achieved by the manipulation of the transmitted (or received) signal pattern ‒ much like a TV antenna on the roof of your house that is concentrated in a particular direction and thus is said to have high gain, but in that direction only. Swing the antenna away from the signal source and your TV loses picture quality. For marine purposes, you can’t use a TV-type directional antenna as you wish to communicate the full 360 degrees, or omnidirectionally, so instead of concentrating the signal in a particular direction, a marine antenna with gain will compress or ‘squash’ the horizontal signal. Generally speaking, the higher the antenna gain, the flatter to the horizon the signal will get. But hold on, isn’t that what you want, as aiming a signal at the horizon will give you good range? Remember, however, that your boat will be pitching and rolling on the waves, so your very flat compressed signal will then also be pitching and rolling, particularly if your antenna is mounted at the masthead where the motion of your vessel will be most pronounced. This is not conducive to achieving good-quality reliable communications as every time the vessel rocks, you will lose signal.
It is therefore vital that antenna gain is carefully selected, and typically we would not suggest more than 0dBd for the masthead or 3dBd elsewhere. This introduces another problem as many low-quality antennas [SB2] are not totally truthful about their gain claims, or do not quantify gain figures with performance graphs. You also have to be very careful when comparing gain as there are a few different figures used by different suppliers: dB, dBi, dBd and dB marine are all used but only dBi and dBd can be quantified as both refer to performance relative to a control antenna, albeit a theoretical antenna (in space) for the dBi figures. To compare dBi and dBd you can use the following formula: dBi gain = dBd gain + 2.1; thus a 0dBd antenna is 0 + 2.1 = 2.1dBi gain.
Because antennas work due to the principles of physics and radio wavelengths, you will generally find marine antennas in four common lengths:
i) shortened antennas: up to 50cm = -3dBd
ii) half-wave (dipole) antennas: approx. 1m = 0dBd
iii) collinear gain: approx. 2.5m = 3dBd
iv) collinear high gain: approx. 5m+ = 5‒6 dBd
You can thus compare like-for-like gain antennas by checking the antenna length in the brochure ‒ if antenna A quotes 3dB but is 1m long and antenna B quotes 0dBd but is also 1m long, then you know that both have similar 0dBd gain. We would be more tempted to go with antenna B as they are not trying to artificially inflate performance figures for sales purposes.
Mounting the aerial correctly
How you attach your antenna to your boat depends on the features of your boat. The three most common mounting methods are:
- Rail mounting: Available in a variety of sizes and includes ratchets to allow easy lowering
- Mast mounting: For sailing boats
- Surface mounting: You can use a flange mount if you have a truly vertical or horizontal surface to mount to. In most cases you will use a ratchet mount that can adjust for the slope of the mounting surface. You can easily lower the antenna with this mount also.
You should follow these general guidelines for mounting:
- Mount as high as possible to take advantage of line of sight (in the case of motor boats, this will likely be atop the cabin roof, on a purpose-made antenna frame or, in the case of a RIB, on an arch mast)
- Mount away from large metal objects
- Mount away from other antennas
- Mount at least 3ft away from your marine radio
- Use quality coaxial cable and connectors:
You can lose considerable signal strength with poor cable and connections. The longer the cable, the more signal loss there will be. Cable lengths of 10 to 20 ft are not of much concern, but a sailing boat using small-diameter cable running 100ft long can lose 80% of its signal strength. As the cable length to your antenna increases, so should the diameter of the cable being used ‒ RG58 is OK for short lengths and RG213/UR67 would be more suitable for longer runs.
- Combination antennas
Whenever possible, you will get the best results using an individual antenna for each purpose. You can get a VHF antenna that is also your AM/FM antenna, but it won’t work quite as well as the single-purpose antenna. It’s also sound practice to carry a spare VHF aerial in case you suffer a breakage at sea. You can even buy emergency ‘jury rig’-style aerial kits, but be warned that trying to use a VHF with a damaged mast can cause damage to the radio set itself.
In summary, a vessel’s VHF communications aerial mast or antenna is undoubtedly a key component in an invisible ‘lifeline’ that has the potential to save lives at sea. Giving attention to the detail of your boat’s anatomy is the key to trouble-free boating, and ‘preventative maintenance’ should be essential watchwords for anyone venturing offshore. After every run, take the time to check the base of the aerial mast for any signs of weakness, along with its connections and wiring. Look after this item and it will look after you. One day you’ll be grateful you did.
Our thanks to antennaPRO for their expert contributions to this feature.