jimh
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A boater asked for advice on antenna length to enable communication with shore at a distance of 30-miles using VHF Marine Band radio. This question is interesting on several levels.

The first element of the question that provokes comment is the incorrect assumption that the length of an antenna used by one station in a VHF radio communication path will be influential on the range of communication to a second station. The length of an antenna plays little to no part in determining the range of communication to another station. For VHF propagation paths, reliable communication with another station is generally possible if the radio horizons of the two stations overlap. The radio horizon is proportional to the height of the antenna, not the length of the antenna.

Second, the range at which two particular stations can communicate reliably with VHF radios is determined by the sum of their radio horizons. If you wanted to be certain to be able to reach any station at a particular distance, you would have to assume the other station's radio horizon to be very short, and your station's radio horizon would have to reach all the way to the other station. On the other hand, if the other station has a very long radio horizon, you can communicate with that station even though your station's radio horizon is much more modest.

To illustrate, let's assume a typical recreational boat uses an antenna mounted 6-feet above the sea. (Here the distance is to be measured to the center of the antenna's radiating element.) The radio horizon is not the usually cited "line of sight" or optical horizon but is longer. A general assumption is made that refraction of radio waves in the atmosphere will cause a bending of the wave travel toward the earth. This refraction occurs at a greater rate than with light waves, which are also refracted, because radio waves are much longer wavelength. The result is a radio wave has a distance to its radio horizon that is greater than an optical wave to its optical horizon. Thus radio communication is not "line of sight" or to the optical horizon, but farther, to the radio horizon. (For a detailed explanation and derivation, see my article on calculating the radio horizon.)

The radio horizon can be estimated by the relationship d = (2h)^0.5, where d is the radio horizon in miles and h is antenna height above sea in feet. For an antenna at 6-feet above the sea, the radio horizon will be approximately 3.5-miles.

If a boat with an antenna at 6-feet elevation wants to communication with another station that is 30-miles away, the other station must have a radio horizon of 30 - 3.5 = 26.5-miles. To have such a long radio horizon we can calculated the height of antenna required from h = (d^2)/2. Solving for h for d=26.5 gives an antenna height of 350-feet. Conversely, if a boat wants to be able to communicate with other stations with antennas 6-feet high at a range of 30-miles, the first boat must have an antenna 350-feet high. Clearly this is impractical, as even the largest boats in the world are limited in their vertical draft to much less, usually to the vertical clearance of bridges under which they must transit.

The propagation of radio waves at VHF is not always exactly or precisely limited to the distance of the radio horizon, and in some cases more refraction can occur, causing the radio horizon distance to increase. Other propagation effects can also occur. Under certain conditions VHF radio waves will travel farther than the calculated radio horizon, and unusually long communication paths can occur. These longer paths are not reliable. To have reliable communication, the estimated range using the overlapped radio horizon method should be a good predictor.

Investigation into radio wave propagation at VHF frequencies has shown that antenna height has an even greater influence on range than would be predicted simply by the radio horizon analysis. It has been observed that doubling the antenna height results in a four-times improvement in signal strength. (See "Effect of Antenna Height on Signal Strength.") For this reason, the best way to improve communication range with VHF radios is to increase the height of the antenna.

jimh
Posts: 6463
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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