For all leisure boat applications I advocate using 1m, 3dbi gain whip type antennas such as the Metz so let’s look at the various ways you can mount these antennas.
Height above sea level trumps most other factors when it comes to getting range from your VHF antenna so the top of the mast on a sail boat is the optimum location.
To mount a 1m whip antenna at the mast head you can use a simple L bracket through which the threaded socket at the base of the antenna is secured and which screws onto the face of the mast.
This is a perfectly satisfactory arrangement but in some cases you may need to get the antenna away from the usual masthead clutter of wind instruments and navigation lights so you’d use an offset bracket which will get the antenna around 200mm away from the mast.
If you are mounting two antennas, perhaps AIS and VHF radio, you can usually get better than a half metre separation by using a standoff bracket for each antenna on opposite sides of the mast – this should be good enough to prevent interference between the two. You can help to reduce masthead clutter by fitting a Windex type wind indicator over the whip antenna.
On power boats, narrowboats and RIBs you don’t have the masthead mounting option and even on a yacht you may choose to place an antenna at deck level.
For mounting an antenna on a standard boat rail of 22mm, 25mm or 32mm (or imperial equivalents) you can use a rail clamp with an L bracket attached to it and mount the antenna just as you would for masthead fitting.
However, there is also a wide range of antenna mounts that use a 1” threaded boss to attach the antenna and for this you need the appropriate type of antenna or use an adapter. In this range there are rail mounts as well as deck mounts and also ratchet type lay-down brackets – very useful if you need to go under low bridges or want to fit a boat cover.
By the way, ratchet brackets can be set so that the antenna leans back at a jaunty angle to give a racy look but don’t be tempted to do this – antennas should be vertical so that they radiate their signal towards the horizon.
You can get improved range by mounting your antenna on an extension pole -proprietary stainless steel, aluminium or fibreglass poles with male and female 1” threads at either end or you can use a threaded adapter on the top of a plain pole.
There’s more than one way to mount an antenna! Have a look at the various options at http://saltyjohn.co.uk