Antenna Gain

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Antenna Gain

Postby jimh » Thu Nov 09, 2017 9:07 am

Antenna gain is generally stated in comparison to an isotropic antenna, which is a theoretical antenna that radiates equally well in all directions. All real-world practical antennas tend to have some sort of directional pattern, even the simplest of antennas, such as the half-wave antenna. A half-wave antenna has a broad directional pattern and the pattern name is often used to describe a half-wave antenna: a dipole antenna.

Engineers have calculated the gain of a half-wave dipole compared to an isotropic antenna. By gain they mean the intensity of the half-wave antenna in its main lobe compared to the intensity that an isotropic antenna has in all directions. The dipole in its main lobe has a gain of about 2-dB in this comparison. Why does a simple half-wave dipole antenna have gain? Because it has some directivity. A dipole does not radiate uniformly in all directions. It has two main lobes of radiation, and two deep nulls with almost no radiation. The directivity is the source of the gain. By producing a stronger signal in one direction at the expense of a weaker signal in another direction, the intensity of the signal in the stronger lobe will be higher than the intensity produced by the theoretical isotropic radiator that creates a signal of uniform strength in all directions. You can figure that all marine antenna gain claims are made in reference to an isotropic, so, in essence, they are all getting a free 2-dB of gain because they are making a comparison to a theoretical antenna.

The theoretical antenna is also assumed to operate in free space, that is a vacuum with no other conducting surfaces anywhere. Such an environment does not exist in the real word. Practical antennas all operate above a ground. The ground is conductive. Exactly how conductive the ground is considered to be will affect the antenna gain.

If we assume that an antenna is to be operated above a perfectly conductive ground plane that extends (a very long way) in all directions, we will find that this alters the antenna characteristics. The presence of the ground affects the antenna impedance. The ground also tends to reflect radiation from the antenna above much like a mirror would reflect light. In theory an antenna operating above a perfectly conducting ground plane will have a mirror image of itself reflected from the ground plane, and the antenna will behave like an array of these two antennas, one the actual antenna and the other the mirror image in the ground plane. (By the way, this is how a quarter-wave whip antenna works; it needs a ground plane.)

If we let the ground plane be perfectly conductive, we can calculate the gain of the antenna to be about 3-dB higher than the same antenna would have if it were in free space away from all other conductors. This is how we get to the figure of gain for most marine antennas. A typical marine half-wave antenna has 2-dB of gain compared to an isotropic. It is assumed to operate above a perfectly conductive ground plane, so it gets 3-dB of gain from the ground. Now we can say we have an antenna that in its main lobe has about 5-dB of gain compared to an isotropic antenna in free space. There is nothing exaggerated or misleading about this to engineers.

I suspect that all VHF Marine Band antenna manufacturers are including a 3-dB gain boost to their claims of antenna gain by assuming their antenna is going to be operating above a perfectly conductive ground plane that extends a long way in all directions. So they all get another free 3-dB of gain.

In the real world, particularly if operating above saltwater, the conductivity of the saltwater will tend to enhance antenna gain. Saltwater is relatively conductive--more conductive than a lot of dry dirt and cement. If operating in freshwater that has very low impurities, there is very little conductivity and little enhancement of gain. These effects apply to all marine antennas--they're laws of Physics and they don't discriminate.

If a manufacturer says their antenna has a gain of 5-dBi, they are basically saying it has little or no gain--just as a practical half-wave antenna ought to not have any gain. That lowercase "i" at the end of the "dB" means "compared to an isotropic radiator." To get more gain than we can obtain from the most basic half-wave dipole radiator, we need to use other methods. One method is to make an array of radiating elements and control their spacing and antenna current in a precise manner to tend to create a single strong main lobe. This takes us into much too complex antenna theory to describe here.

Also, remember that all antenna gain is occurring only in the antenna's main lobe. The main lobe of the antenna must be aimed at the other station in a circuit in order for the gain to be useful. If the antenna main lobe is pointing away from the other station, there is no realization of gain, and, in fact, there may be a significant decrease if the other station is in a null in the pattern of the antenna. The pattern nulls can be very deep, perhaps only 1/100-th of the strength of the main lobe. If there is a main lobe there must be nulls somewhere else, otherwise there would be no gain.

There is nothing about practical antennas that makes them intrinsically work right. Every real-world antenna has to be carefully designed, manufactured, and tested. Some work well and some not so well, and that is because some are well designed, well manufactured, and well tested. Some others are not. The diameter of the conductors, the arrangement of the conductors, the matching of the antenna impedance to the transmission line impedance are all variables in antenna design and construction, and they will effect the actual performance of the antenna.