Technical talk

Two factors are specified by antenna manufacturers: VSWR and gain. VSWR stands for voltage standing wave ratio, and is also referred to as standing wave ratio (SWR). VSWR is a function of the reflection coefficient, which describes the power reflected from the cable and antenna. If the combination of connections, cable and antenna were perfect, the VSWR would be 1:1. In reality, it is more likely to be between 1.1:1 to 1.5:1 (as for the antennas tested). AnTennA gAin: This is a relative measure of an antenna’s ability to direct or concentrate radio frequency energy in a particular direction or pattern. The measurement is typically measured in dBi (decibels) relative to an isotropic radiator. An isotropic radiator radiates in all directions in a spherical pattern: an antenna which performs like this would have no gain written as 0dBi. A 3dBi antenna is more focused and concentrates the power into a narrower beam width in the horizontal plane. A higher gain, say 6dBi, would concentrate it so narrowly it could miss the receiving antenna when the boat heels. (See PBO Oct 2012*, ‘8 antennas tested’ for a fuller description). *Available from the Copy Service, Tel: 01202 440832