The gain and vertical radiation pattern of typical antennas used in the VHF Marine Band radio service are examined in some detail. The effects of the gain on the vertical radiation pattern are investigated. The effects of tilting the radiator are considered in terms of operation of small boats.
The most common VHF Marine Band radio antenna configuration on small recreational vessels is a vertical mono-pole radiator. These antennas consist of a single vertical radiator. They have an omnidirectional horizontal radiation pattern, and radiate equally well in all azimuthal directions. The vertical radiation pattern of these antenna typically is optimized for radiation toward the horizon. As the antenna is made physically and electrically larger, it is possible to achieve antenna gain by concentrating more of the radiation into a main lobe directed at the horizon at the expense of radiation in other directions. Because of this, the antenna is said to have gain in its main lobe as compared to an antenna that does not concentrate its radiation in one particular direction or, in this case, one vertical angle.
When describing antenna gain, the increase in radiation in the main lobe is compared to the radiation that would occur from a reference antenna in that same direction. There are only two common reference antennas: a theoretical antenna, which radiates equally well in all directions, both horizontal and vertical, called an isotropic antenna; and a theoretical half-wavelength dipole. The half-wavelength (abbreviated 0.5 λ) dipole has a pattern which concentrates radiation into two broad lobes, and therefore it can be described as having gain over an isotropic antenna. In theory, a half-wavelength dipole will show a power gain of 1.64 compared to an isotropic.
Comparison of antenna gain is generally measured using decibels, a logarithmic comparator. The relationship is:
dB = 10 LOG (P2/P1),
where P1 and P2 are the two power levels being compared. Thus a half-wavelength dipole antenna with a power gain of 1.64 is said to have a decibel gain compared to an isotropic antenna of:
dB = 10 LOG (1.64)
dB = 10 (0.215)
dB = 2.15
At the frequencies used in the VHF Marine Band, 156 to 162-MHz, building a practical antenna that is a half-wavelength dipole antenna is not difficult; the length required is only three feet. One can then say that such a practical half-wavelength antenna has gain compared to an isotropic antenna. By selecting the reference antenna it is possible to manipulate the amount of gain that can be claimed to have occurred in practical antennas. If the gain is referenced to an isotropic antenna, the gain will be stated as being higher (by 2.15 dB) than if the gain is compared to an antenna like a half-wavelength dipole.
In theoretical analysis of gain, an antenna is assumed to be operated in free space, an infinite environment in which there are no other electrical conductors present. In all practical applications, antennas are operated on or close to the surface of the earth. Because the earth is conductive, its presence influences the radiation pattern of an antenna. In some cases the influence is beneficial, and the radiation pattern is changed in a useful way. This results in an increase in the gain of the antenna in its main lobe. When stating the gain of a practical antenna, the amount of gain claimed can be further increased if the reference antenna is assumed to be in free space while the practical antenna is operated on earth. The amount of additional gain varies with the assumptions made about the antenna's position relative to the earth and the conductivity of the earth that is assumed. If the antenna's position relative to the earth is optimized, and if the earth is assumed to be a perfectly conducting surface, even more gain can be claimed for an antenna compared to other antennas that operate in free space.
Claims of gain in decibels are usually distinguished or noted to indicate the reference antenna. If the reference is to an isotropic radiator, the term dBi is used. If the gain is in reference to a dipole, the term dBd is used.
It is possible to produce more antenna gain by building larger antennas. Unfortunately, if a single radiator is used, as the radiator is made longer the pattern begins to develop many minor lobes. Because of this, larger antennas usually are constructed using arrays of half-wavelength radiator elements. When the elements of the array lie on a line, the array is called a collinear array. A collinear array vertically stacks radiators one above the other, interposing some means to align the current flow in each element so as to be favorable for creating the desired pattern. As the number of elements in the array increases, it become more difficult to obtain gain in this manner. Gain in antenna arrays comes principally from the flow of mutually induced currents, and as the array elements become more distant from each other, this effect decreases. When an array is initially doubled in size, the gain may also double. But this trend cannot be maintained. Each time the array is doubled in size it become more difficult to achieve more gain. In practical collinear arrays, four elements is about the limit of useful expansion. If further gain is needed, other techniques are employed, such as assembly of collinear arrays into broadside or end-fire arrays. But such broadside or end-fire arrays have directivity in their azimuthal pattern, and they are generally not useful for maritime applications.
In many applications, vertical antennas are operated on or very close to a reflecting conductive surface, and in these cases the analysis of the gain involves consideration of an image antenna which is created by the effect of the conducting surface which is called a ground plane. The image antenna is considered as a reflection of the actual antenna, and it contributes to the calculation of the pattern and gain. Antennas designed for installation on cars usually anticipate the presence of this ground plane in the form of the metal body of the car roof or trunk. Vertical antennas intended for operation on fiberglass boats must be designed for operation in the absence of any sort of ground plane. Vertical antennas which require a ground plane can be designed with additional elements or conductors incorporated into them which simulate a ground plane. A simulated ground plane is called a counterpoise. In some cases certain element lengths in an array are increased to 0.625λ, which increases the gain slightly compared to a 0.5λ element. The 0.625λ (often called a "five-eights" wavelength) vertical radiator requires a ground plane or counterpoise to work properly, so it cannot be used in simple antennas for marine use on fiberglass boats which cannot provide a conductive ground plane. A 0.625λ element can be used in arrays where another element or portion of the antenna can function as its counterpoise.
Manufacturers of antennas specifically marketed to recreational VHF Marine Band radio users seem to have taken full advantage of the freedom to claim antenna gain by choosing a suitable reference antenna. Indeed, in general there is no reference antenna description provided at all from the manufacturers, and figures for gain are just offered without explanation. Since the gain figures are quite generous, it is reasonable to make the inference that the gain has been measured against the most favorable reference antenna, an isotropic antenna in free space, while the actual antenna is operated in a real environment above a conducting earth (or sea water). By doing this, marine antenna manufacturers have given their antennas additional gain. They also seem fond of rounding up to the next higher whole number. This practice seem uniform across all marine antenna manufacturers marketing to recreational boats. All over estimate the gain of their antennas by 3-dB as compared to a reference half-wavelength dipole.
The typical gain figures reported by marine antenna manufacturers are summarized in the table below. The physical length refers to the typical size of practical antennas that can show the associated gain at the VHF Marine Band frequencies.
|LENGTH||Relative GAIN in dB|
|8-foot||2 x 0.5 λ||3||5.1||"6"|
|16-foot||4 x 0.5 λ||6||8.1||"9"|
Because all of the typical vertical mono-pole antennas used on small recreational boats have an omnidirectional horizontal pattern, the difference among the antennas will be found in their vertical radiation patterns. We now look more closely at the vertical radiation pattern of three actual vertical antennas which have gain. These are not patterns of any actual common VHF Marine Band antenna marketed to recreational mariners, in part because the manufacturers of those antennas have not published any such information. However, the patterns are from actual antennas operating at the same frequencies (156 to 162-MHz). They are typical of real-world results when building vertical antennas with gain. The precise details of the pattern measurement are not known, but they are assumed to have been made with the antenna elevated above the earth by several wavelength. This should remove most effects of the earth, but some asymmetry can be seen in the patterns which is likely an influence of the earth.
A common VHF Marine Band radio antenna has a radiator that is approximately three-feet long, or a half-wavelength. These antennas are sometimes fabricated in longer assemblies, sometimes in fiberglass tubes as long as eight feet in which the radiator is located in the upper half of the assembly. A common configuration for these antennas is as an exposed metal whip antenna with a base matching coil or as a similar radiator assembled into a four-foot long fiberglass tube. In marine terms, this antenna is generally called a "3-dB gain" antenna. The typical vertical radiation pattern is shown below. (This pattern is from an actual measurement of a practical half-wavelength antenna which has a claimed gain of 0-dBd. The radial scale is 10dB per division relative to the main lobe.)
Another common VHF Marine Band radio antenna has a radiator that employs two half-wavelength sections in a collinear arrangement and is approximately eight-feet long. These antennas are typically fabricated in assemblies of fiberglass tubes. In marine terms, this antenna is generally called a "6-dB gain" antenna. The typical vertical radiation pattern is shown below. (This pattern is from an actual measurement of a practical antenna which has a claimed gain of only 3-dBd. The radial scale is 10dB per division relative to the main lobe.)
VHF Marine Band radio antennas are often made into even larger collinear arrays of radiators. These antennas are typically fabricated in assemblies of very large fiberglass tubes. In marine terms, these antenna are generally called a "9-dB gain" antenna. A typical vertical radiation pattern is shown below. (This pattern is from an actual measurement of a practical antenna which has a claimed gain of only 6-dBd. The radial scale is 10dB per division relative to the main lobe.)
In most marine applications the vertical direction to the other station will be at the horizon or at very low angles above the horizon (as would be the case communicating with a shore station using a very high antenna over a short distance). Radiation at other angles is of little use for most marine communication. Comparing the three radiation patterns shown above, they all display their maximum radiation toward the horizon. However, as the gain of the antenna increases, the width of the main lobe toward the horizon narrows. With a unity gain (or so-called "3" dB gain marine antenna), the width of the main lobe is broad, and it appear that gain does not decrease by -3 dB until about 30° off axis. With a 3 dBd gain antenna (or so-called "6" dB gain marine antenna) the width of the main lobe is noticeably narrower, and it appears that gain decreases by -3 dB when just over 10° off axis. With a 6 dBd gain antenna (or a so called "9" dB gain marine antenna) the main lobe is very narrow, and gain drops off rapidly. When 10° off axis, gain will be down -9 dB.
If we compare the relative gain of our three antennas when 10° off axis, the results are interesting:
|LENGTH||GAIN in dBd|
|Physical||Electrical||On Axis||10° Off Axis|
|8-foot||2 x 0.5 λ||3||Unity|
|16-foot||4 x 0.5 λ||6||-3|
We now repeat this investigation at 20° off axis:
|LENGTH||GAIN in dBd|
|Physical||Electrical||On Axis||20° Off Axis|
|8-foot||2 x 0.5 λ||3||-2.5|
|16-foot||4 x 0.5 λ||6||-10|
The surprising result is that the lowest "gain" antenna provides the best signal when 20° off axis.
When we look at the vertical pattern of an antenna, we should not assume that if the radiator element is tilted with repect to the earth that the vertical radiation pattern will also be precisely tilted at that same angle. While this would be the case in a free space environment, in the real world the effect of the earth may provide some influence. At some point I hope to either carry out some experiments on this behavior myself, or to discover results of others who have investigated it. But at this moment, I don't have any good data to predict or show exactly how a marine vertical antenna will behave when tilted from its normal vertical position.
Tilting a vertical antenna from a true vertical orientation will affect the pattern of radiation from the antenna. The effects are hard to determine with precision, as the height above the sea and the presence of other conducting elements will affect the outcome, but, in a general sense, one can say that the antenna will no longer function as it was intended or designed, including loss of gain and severe distortion of the radiation patter. For this reason the mechanical tilting of vertical antennas should be avoided.
The effect of the ground, or in our marine environment, the sea, can be considered in two ways: the distance of the antenna above the sea, and the conductivity of the water. It seems reasonable to assume that any influence of the sea would be reduced as the antenna was moved farther from it, and thus the higher an antenna is mounted the less likely the sea will alter the vertical radiation pattern when the antenna is tilted. The conductivity of the sea may also have an influence. In this regard, sea water is much more conductive than fresh water. It seems likely that antennas operating above fresh water may not have their vertical radiation pattern influenced by the water when tilted. A tall antenna above fresh water is most likely to behave like an antenna in free space, that is, its vertical pattern will be affected by tilting. An antenna low above saltwater may show some influences on the its pattern when tilted, but there may be some moderation of the effect due to the proximity to a conductive earth (or sea). Again, this is just speculation and needs to be confirmed either with modeling or experimentation.
Tilting a vertical antenna which is mounted above the point of rotation decreases its height. We can illustrate this with an example:
If an antenna (i.e., the center of the radiator) is mounted 10-feet above the roll axis of a boat (and we assume the roll axis of the hull is at the waterline), and the boat undergoes a 20° roll, the height of the antenna above the waterline will decrease to 10cos(20), or 9.4 feet. While this may seem like an insignificant reduction, any lowering of the antenna reduces its range for line-of-sight propagation.
Increasing antenna gain is beneficial, but it comes with the price of a narrower main lobe. Movement of the antenna on a small boat may cause a change in the orientation of the vertical radiator, which in turn may cause a change in the vertical radiation pattern. Particularly on longer antennas with higher gain, a narrow main lobe may prove to be undesireable in a small boat. Considering that a pitch or roll motion of 20° is not particularly unusual in a small boat operating in rough seas, in those conditions the broader main lobe of a unity gain antenna may prove to be more effective than the narrower main lobe of higher gain antennas. Even with movements as small as 10-degrees, the gain of a longer antenna may be lost when compared with a smaller antenna having a broader pattern.
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Author: James W. Hebert
This article first appeared January 19, 2009.