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  VHF Marine Antenna Gain and Pattern

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Author Topic:   VHF Marine Antenna Gain and Pattern
jimh posted 10-11-2008 11:36 AM ET (US)   Profile for jimh   Send Email to jimh  
The figures for antenna gain cited by most VHF Marine Band antenna manufacturers seem quite fanciful to me. All figures of gain have to be in reference to another antenna. Since gain in an antenna comes from concentration of the radiation in a favored direction at the expense of non-favored directions, it is generally considered that an antenna which radiates equally in all directions has the lowest possible gain. Such an antenna is called an isotropic radiator. In actual practice it is difficult to physically produce an isotropic radiator, since almost any embodiment of an antenna has some variation in its radiation pattern. So an isotropic antenna is only a theoretical reference. It is also generally assumed that the isotropic radiator operates in free space, that is, there is no other conducting surface anywhere near it.

A practical reference antenna is the half-wave dipole. A half-wave dipole antenna is easily constructed and can be used as the reference for measurement of another antenna's gain. It can be shown that a half-wave dipole has a gain over an isotropic antenna of about 2-dB in free space.

Cf.: http://en.wikipedia.org/wiki/Antenna_gain

In the real world, antennas are not operating in free space. They operate near the surface of the earth, which is electrically conductive. When an antenna is operated above a conducting surface, other electrical effects occur which affect the gain. In general, an antenna which is operated above a conductive surface will have increased gain because of the influence on the pattern of the antenna from the conducting surface. If nothing else, the earth cuts off some of the space in which the antenna radiation can occur, so it is reasonable that more radiation is concentrated into the space where it can occur. Thus operation above the earth creates antenna gain by itself.

We have come to the point now where just about any practical antenna operated above the earth will have gain referenced to an isotropic radiator in free space. There will generally be about 2-dB of gain from the physical nature of the antenna itself causing some concentration of radiation pattern, and additional gain from operation above a conductive surface. The conductivity of the earth also varies with frequency. At lower radio frequencies, such as the AM broadcast band, the earth is quite a good conductor. Since marine antennas are operated above the water and at VHF frequencies, we have a different situation. We might also consider the difference between freshwater (generally a poor conductor) and saltwater (a good conductor).

In recreational marine electronics, we often see antennas with a rating of 3-dB gain. Assuming that this claim is in reference to an isotropic radiator in free space, then the so-called 3-dB marine antenna is really about the minimum realizable practical antenna. In this sense one ought to call it a 0-dB gain antenna. These ratings may be in reference to operation above a perfectly conducting surface.

When antennas are made longer than a half-wave length, they can also improve their gain by concentration of more radiation into one direction. Again, all antenna gain comes by concentration of radiation into a main lobe at the expense of radiation in other directions. The gain is realized at the remote end of the circuit only when the main lobe of the antenna is directed at that receiving point. As an antenna's gain increases, the concentration of radiation becomes more narrow, and aiming of the antenna with its main lobe toward the receiving point become more critical.

Measurement of antenna gain is difficult. The usual technique is to construct an antenna test range. A test range consists of a transmitting site and a distant receiver site. The reference antenna is set up as a transmitting antenna, and a known amount of power is applied. The level produced at the distant receiver site is noted. Then the antenna under test is placed at the transmitter site. The transmit power is adjusted to produce the same receive level at the distant receive site, and the power is noted. By comparison of the two power levels, the gain of the antenna relative to the reference antenna can be noted. One problem with this technique is the construction of a valid reference antenna with known gain characteristics.

Antenna test ranges also provide for the measurement of the antenna's radiation pattern. The antenna under test is often rotated or moved while the remote receive level is noted. In this way the pattern can be deduced.

The antenna test range site itself must be carefully controlled. The orientation of the transmit and receive antennas with respect to the ground can influence the results. Reflections from the ground can cause nulls or reinforcements in the antenna radiation which will affect the measurement of gain. For marine antennas the test site ought to be over water, both freshwater and saltwater, if the real effects are to be observed.

Another influence on an antenna's gain is its efficiency. In theoretical comparisons, all antennas are assumed to work equally efficiently. In the real world there are practical considerations which affect the antenna's efficiency. Losses due to electrical resistance in the conductors of the antenna are an influence, as are losses from imperfect dielectric materials. Poorly constructed antennas with high ohmic and dielectric loss will not work as efficiently as better constructed antennas, and the lossy antenna will not display as much gain, even if of the same size and type of antenna, as a similar antenna constructed of better materials.

Gain is so difficult to measure accurately that at least one respected communications journal (QST) has had a long-standing policy of refusing to publish any advertised gain figures for antennas. In recreational marine literature, gain figures are freely quoted, but there is almost no controlled testing of any antennas to verify their performance. Manufacturers appear to be free to cite any gain figures they choose.

For a small boat there are really only two likely antenna choices, the so-called 3-dB and 6-dB marine antennas. Again these are probably more properly called the 0-dB or reference gain and 3-dB gain antennas. Antennas with more than 3-dB of real gain are generally physically too large for use on small boats.

Beam Tilt

When a vertical antenna is operated over the earth it is generally assumed that the orientation will be straight up. However, a vertical radiator can be tilted from straight up, and this often occurs in a small boat when the boat pitches or rolls. Illustrations are often seen in which the radiation pattern of the antenna is shown to exactly follow this physical change in orientation, but these illustrations are somewhat misleading. Physical tilting of a vertical radiator does not result in an exactly corresponding tilt in its main beam. The influence of the conductive surface near the antenna affects this. At lower frequencies and above a conducting earth, vertical antennas can be tilted slightly without much change in their radiation pattern. At VHF frequencies and above freshwater, there is probably a potential for more beam tilting to occur than at lower frequencies above a conducting earth. I don't know if this has been carefully studied and reported. It would be interesting to find any scientific literature on beam tilt of VHF monopoles above freshwater.

When a boat rolls or pitches it is probably reasonable to assume there is some alteration of the radiation pattern of its vertical antenna. We have all observed the influence of such motion on reception of distant stations on our VHF Marine Band radios. In this regard, the lower gain antenna with a broader pattern may be more attractive in some situations than a higher gain antennas with narrower pattern. On a boat, however, the antenna tilting is also usually accompanied with a decrease in antenna height. This may also be influential in creating a signal loss.

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