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How to select the correct VHF antenna for your boat

 

General considerations

The VHF antenna is the most important part of your VHF installation, as using a good VHF radio and a low quality VHF antenna will definitely compromise the performance your communication system. A little money spent on a good antenna will do more than a lot spent on a better radio.

 

To better your ability to communicate with shore station and other boats you need to understand how a VHF antenna work and how to select the best antenna for your purposes. Since there are many factors that influence the selection of a proper antenna for a specific application, we will try to help you in your decision making.

 

Gain

Gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction. As a receiving antenna, the figure describes how well the antenna converts radio waves arriving from a specified direction into electrical power. When no direction is specified, "gain" is understood to refer to the peak value of the gain.

 

Gain is stated in decibels (dB). Generally, the higher the gain, the greater the communicating range. On the other hand the higher the gain the more compressed the beam width becomes. A narrow beam can actually fade in and fade out as the boat rolls and pitches. Small, lightweight boats, which roll excessively in heavy seas, normally do not use high gain antennas. Sailboats should use a 3dB antenna mounted at the top of the mast whenever possible, or a ground plane antenna.

 Radiation Patterns

Range

The range in nautical miles - that is, how far your radio can 'see' to the horizon - is equal to 2.08 times the square root of the height of your antenna in meters. 

 

Range (nautical miles) = 2.08 x  (metres)

 

Keep in mind that the vessel to which you are transmitting also has his antenna above the water, so you must add the calculations for both vessels to find the range between them.

To maximize the range is very important to install the antenna as high as possible.

 

Several other factors including the type of antenna affect range, such as the sensitivity of the receiving radio, background electromagnetic noise, land masses, weather, are a few others. 

 

Cable

Coaxial cables are vital to the performance of your antenna and radio system. The cable selected will affect the system’s cost, coverage and reliability.

One thing to note is the loss of signal through the coaxial cable which leads from the radio to the antenna.  A powerboat's cable is often only a few meters, so the loss is negligible.  However, on a sailboat, the antenna cable is much longer.  To make up for this, a heavier cable, with less loss is required. 
RG-58 cable is sufficient for cable runs up to 20 meters, while for runs over 20 meters the RG-213 is preferable (or RG-8).

 

SCOUT marine antennas are equipped with low-loss high quality coaxial cables made in Italy, specifically designed for marine use.

 

Antenna installation

The higher the antenna is mounted the better the antenna's range and performance. Install the antenna as high as possible and avoid positions where it is shielded by the boat's structure or by persons.

 

If you install more than one antenna always mount them far away from each other.

 

Use the mount that best support the antenna, don't use plastic mounts with long antennas made with stainless steel ferrule.

 

Original SCOUT mounts and cables will help you in making the right installation.

 

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