Most of this is a copy/paste from an article by John Schofield, with a few comments of my own (in blue) thrown in for good measure. Hope it helps a little.
VHF radio range is line of sight, so height is a very important factor in achieving maximum range. On a yacht the most suitable antenna location is, therefore, at the top of the mast. Two boats, each with a masthead antenna 60’ above sea level can theoretically communicate at about 20 miles. (since this is not practical on a V20 the distance will be less) (A transmitter at a coast station, perhaps 1000 feet above sea level, could communicate with these same boats at closer to 50 miles.
For those who wish to do the maths themselves the formula is: D1 + D2 (total distance between stations in nm) is equal to 1.23(0H1 +0H2), where H is height of the antennas in feet.
Accepting that the VHF antenna must be positioned as high as possible on the boat we can turn to other factors that will effect radio performance:
It is a simple fact that the potential performance of a marine VHF radio is limited by the quality of the antenna and its installation. A badly designed antenna fitted with undersized cable and imperfect connections will make
the performance of even the most exquisite and expensive radio unacceptable. (I underlined this sentence because it's the most important one in the article)
It is important to select the right antenna and install it in such a way that it maximises the performance of the radio to which it is attached.
Antenna performance, or
gain, is expressed in decibels – (expressed as the symbol dB).
A 9dB antenna increases the signal power eightfold, a 6dB antenna fourfold and a 3dB antenna, the best selection for masthead applications, doubles signal power.
But why choose a 3dB antenna for the masthead when you could get all that extra power from a 9dB antenna?
Well, the increased signal strength of a high gain antenna is achieved by concentrating the signal radiating from it into a narrow disc. (remember, all antennas radiate their signal from the tip of the antenna, NOT along the entire length) The signal radiates at right angles to the antenna with very little radiation upwards or downwards. When the boat heels and the antenna rocks backwards and forwards the concentrated disc-like
radiation pattern points the signal at the sky, or at the sea, instead of at the horizon. A 3dB gain antenna has a radiation pattern that looks more like a fat doughnut than a disc, with a significant portion of the signal radiating upwards and downwards. No matter how much the boat heels, such an antenna will still have some portion of its signal pointing at the horizon and, because VHF transmissions are line-of-sight, this is vital to performance. Hence the universally accepted use of a 3dB gain antenna for boating applications.
Another measure of antenna performance is the SWR – Standing Wave Ratio. (You may see this referred to as VSWR, Voltage Standing Wave Ratio, but it is the same thing). (Very important point, since your SWR's will effect your transmission range immensely)
This is, in very simple terms, a measure of the amount of the transmission power that is lost in the antenna system. If the antenna system were perfect and the entire signal power leaving the radio was transmitted by the antenna (and its cables and connections) the SWR would be 1:1. Sadly this is not achievable, but getting as close as possible is a worthy aim. (It is the goal of every good installation ) A SWR of 2.0:1 over
the whole system represents a ½ dB loss in signal strength and is barely acceptable. To achieve or, preferably, better this performance the antenna itself should have the best possible SWR. A top quality antenna such as the Metz Manta-6 has a SWR of less than 1.2:1. Others will have a SWR of 1.5:1 or higher. (This is a masthead antenna and you can ignore it's recommendation here, but the stated goal of using a top quality antenna with the lowest SWR is correct.)
Other criteria for selecting a marine antenna, particularly one for masthead mounting, would be:
•
Resistance to UV degradation. Clearly all stainless steel construction wins hands down. (But in our applications it's impractable due to the closeness of the antenna to people. Whip antennas are called that for a reason. Use a fiberglass or carbon fiber encased antenna)
•
Resistance to bird strikes. Again, a stainless steel whip is less vulnerable than a rigid plastic one.(Debateable)
• Ability to be removed when the mast is taken down. This is when the antenna is particularly vulnerable and those with factory crimped connections cannot be removed without removing all the cable with
them. The connection at the antenna should be an SO239 socket which takes the standard PL259 connector.
•
Low weight.
•
Sturdy mounting bracket.
Once the antenna has been selected we can turn to the other components of the antenna system; the cable and connectors.
Cable:
Marine VHF applications require 50 ohm coax. TV cable is 75 ohm and is not suitable. (Repeat after me, TV cable is 75 ohm and IS NOT suitable)
Suitable cables include RG-58 (smallest), RG-8X, RG-8U and RG213. RG-213 is the same size as RG-8U but with completely waterproof and ultra-violet resistant insulation. RG-213 is more difficult to work with when it
comes to making connections and is very expensive, so probably best left to superyachts. RG58 is only suitable for interconnections between equipment and for very short runs. (But since all we need is a very short run from the normal antenna mounting area on the side of the boat to the radio, RG-58 is perfect for our applications. Ignore the rest unless you want to be working with 1/2" thick cable)
Coax must have copper braid, preferably tinned copper braid. (That is, the metal webbing just under the outside insulation and surrounding the center insulating core) Braid is described by percentage figures – 98% is the best, 96% is good. It is important that the dielectric, the insulation surrounding the center core of the cable, is solid polyethylene. Foam core dielectrics can suck moisture into the cable via the connections and cause the core and braid to corrode. A good UV resistant cover is essential for longevity.
One of the biggest causes of failure in an antenna system is faulty connections. These allow water into the coaxial cable causing corrosion of the braid and center conductor. So, high quality connectors are required.
The cable terminal is the PL-259 plug. This plug fits RG8U cable and, with suitable adaptors, RG8X and RG58. The PL259 connector mates with an SO-239 socket fitted to the antenna, (unless the antenna comes with the cable already connected internally to it) and to the back or the radio.
Connections should be (MUST BE) soldered. Many manufacturers use pressure crimped connections but these are subject to corrosion and need special tools to create the connection. (If you look at the PL-259 connector, you'll notice that there are (usually) three holes drilled into the sides of the connector, spaced evenly around the barrel of the connector. When you peel back the braid, you should twist it into strands and feed it through these holes, then apply solder to the braid and connector at the holes, filling them with solder and making a solid permanent connection of the cable braid to the connector, ensuring that all holes are completely sealed with solder to help prevent water from getting into the holes. This is very important as most of your line loss will occur at the connectors. The better the mechanical joint, the less the signal loss and the lower your SWR's) There are a ton of different ways to actually attach the 259 to the cable. Do a search on the internet and you'll find several hundred, all claiming to be the best way. My advice is that you'll never get in trouble if you use the Amphenol method. Oh, and use silver plated connectors. If you use the nickel plated ones you'll have to file or sand off the plating before you can solder the wire to them. And try to find ones that say "Made in the USA". 
There was not many that were talking over me!
