The gain of an antenna is closely related to its directivity. Because directivity is based solely on the shape of the radiation pattern lobes, it does not take into account power losses that may occur in the actual antenna system.

Every Digital Antenna marine antenna is tuned for minimum standing wave ratio (SWR) which approaches 1:1, reducing the system loss significantly. Additionally, a ferrite choke bead is used to contain RF power in the antenna system. By using a choke bead, all RF power is directed into the radiating element and prevented from leaking back to the transmission cable.

This technique increases usable RF power and minimizes cable interference with other electronic equipment. Also, Digital Antenna uses its exclusive marine grade low loss UV stable RG8X cable to reduce transmission line loss in all Digital Antenna marine antennas.

Gain in an antenna is not the creation of power, but rather a simple refocusing of energy from all directions to a specific direction. Gain is measured in decibels (dB). Vertical antennas are in fact quite directional, except in the horizontal or azimuth plane.

The figure to the left shows the radiation pattern of three different length radiating antenna elements. Note that the quarter wavelength antenna has the highest angle of radiation; however, it also has the lowest gain of the three cases. The 5/8 wavelength antenna has both the lowest angle of radiation and the highest gain, which gives superior performance at sea level.

The vertical antenna works well only when placed over a good ground system. The success or failure of an antenna system depends on whether or not it has a good RF ground. Poor grounds cause antennas to operate at less than best efficiency. In fact, it is possible to lose between 50 and 90 percent of the RF power by heating the space under the radiation lobe, instead of transmitting into the air.
  

The advanced ground system internal to all Digital Antenna marine antennas allows 4.5dB through 10dB gain antennas to have a greater range in any sea conditions. The figure below shows the effect that a ground system has on a vertical antenna. 

Note that by increasing the ground system efficiency, useful RF power is actually directed closer to the water surface where it is needed. Most well designed 5/8 wavelength 6dB antennas will transmit the same distance in the sky; however, the ground efficiency determines the actual communication range at sea level.