Radio Range Increase

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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Location: Michigan, Lower Peninsula
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Radio Range Increase

Postby jimh » Fri Jul 21, 2017 11:42 am

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
Contact:

Re: Radio Range Increase

Postby jimh » Sat Jul 22, 2017 12:20 pm

Regarding any influence of antenna length on range of communication, there is some possible influence, but I would not consider it to be primary. In the case of vertical monopole antennas, it is possible that a longer antenna may have some antenna gain. Additional gain in the antenna can improve the radio range if the path loss is such that the received signals are at the margin of being copied. However, the notion that longer antennas will always improve range is not always obtained in practice. There are several limits.

In order for an antenna to have gain, the pattern of antenna radiation must concentrate power into one major lobe and substantially reduce radiation in all other directions. To produce a concentration of radiated power in one lobe typically requires an array of antenna elements. Since the typical marine antenna is a vertical monopole, the arrangement of the antenna elements must be collinear (in a line). Collinear vertical antenna arrays can produce gain, but they typically do so by compressing the vertical pattern of radiation into a narrow beam directed toward the horizon. In theory, significant gain can be obtained by a vertical colinear array, but in actual practice the gain is not always obtained with ease. That a particular VHF Marine Band antenna is longer than another antenna does not guarantee that it will produce more gain. As in any implementation of theory into practice, the actual design, implementation, and construction of the antenna has great influence on how much gain will be produced--if any. Some published test results have shown that VHF Marine Band antennas of three-foot length produce stronger signals than antennas of eight-foot length.

If we assume that a longer antenna will produce some gain, a second consideration will be to know how uniform the radiation pattern will be and if there is a single, clean, well-defined main lobe. It is very common in antenna arrays of multiple elements that their radiation pattern will have a number of minor lobes and many deep nulls. The existence of minor lobes and deep nulls in the antenna radiation pattern can cause problems in use on boats, because the typical small boat at sea is not a stable platform. The vertical orientation of the antenna often is constantly changing at the boat works in the waves. If the antenna pattern has many lobes and deep nulls between those lobes, there is no guarantee that the main lobe of the antenna will always be pointing in the optimum direction, at the distant horizon. In actual practice, obtaining some gain in an antenna array is generally possible, but obtaining a clean, well-defined radiation pattern free from deep nulls and minor lobes is much more difficult and requires careful design and construction.

If a longer antenna does provide gain and does have a clean and well-defined major lobe free of deep nulls, there is still the problem that the major lobe from such an antenna is, by nature, going to be a narrower beam than the major lobe of a well-designed shorter antenna will less gain. The motion of the boat is seas will tend to direct a narrower beam from an antenna to directions away from the horizon. The result will be that the longer antenna may actually produce less gain at the horizon compared to a shorter antenna with a broader main lobe.

The longer antenna must overcome these three problems: it must actually create some gain, the main lobe must be clean, and the main lobe must remain directed at the horizon. None of these conditions are intrinsic in just having a longer antenna.