Antenna Patterns and Polarizations

A simple dipole antenna produces a signal polarized in the same plane as the antenna. A vertically mounted dipole (or array of collinear dipoles) produces a vertically polarized signal. Likewise, more complicated antennas, such as YAGIs, with all elements in the same plane produce a signal polarized in the plane of the elements. For example, a YAGI antenna can be used to produce a vertically or horizontally polarized signal depending on the mounting orientation of the antenna.

Antennas that produce a simple horizontally or vertically polarized signal can be represented by two antenna patterns: the pattern in the same plane as the elements, which is the pattern of the signal in the plane of polarization, and the pattern in the plane orthogonal to, or 90 degrees from, the plane of the elements (the "cross-polarized" signal). For antennas such as these, the azimuth pattern and the elevation pattern correspond to the co-polarized and cross-polarized patterns depending on the mounting orientation of the antenna. For example, if a YAGI antenna is mounted with the radiating elements oriented vertically it will produce a vertically polarized signal. The azimuth pattern would then correspond to the co-polarized pattern (in the plane of the elements, sometimes called the E-plane, or the plane of the electric field produced by the antenna). The elevation pattern in this example would correspond to the cross-polarized pattern (90 degrees from the antenna elements, the H-plane, or the plane of the magnetic field produced by the antenna). If the YAGI was mounted with the elements oriented horizontally, the azimuth and elevation patterns would be interchanged.

The significance of the relationship between the antenna polarization and pattern relates to the signal available to be received at a given location distant from the antenna by a receiver with a particular polarization. Ideally the receiving antenna is co-polarized with the transmitting antenna, and a cross-polarized receiving antenna offers significant rejection of the transmitted signal. For example, with the vertically polarized YAGI transmitting antenna described above, the azimuth pattern is only meaningful for vertically polarized receive antennas.

Complex antennas, such as many television broadcast antennas, include a significant signal in both the horizontal and vertical polarizations. The azimuth pattern for these antennas is often supplied for both polarizations, and the complexity of the antenna can result in significantly different azimuth patterns for the two polarizations. The azimuth pattern polarization which should be used in field strength calculations depends on the expected polarization of the receive antenna. Typically, television receivers are designed for horizontally polarized reception, so the horizontally polarized azimuth pattern should be used. However, since many receivers are often equipped with "rabbit ears" type antennas (which are usually extended vertically resulting in vertical polarization), the vertically polarized azimuth pattern can be used to compute the coverage area for this receiver configuration. To accomplish this with the TAP system, you would enter two separate directional antenna files, one for each of the polarization azimuth patterns. You would also need to enter the appropriate effective radiated power (ERP) for each configuration when the contours are computed, since the absolute gain value for an antenna is typically different for different polarizations. Note that since most communications services are designed and regulated for a particular polarization, the calculation of two coverage contours for the two polarizations would be a very unconventional approach and would not normally be required.

The discussion so far has only involved the azimuth patterns for the antenna. Further information is necessary (usually available from the antenna manufacturer) to describe the elevation patterns. The elevation pattern information is especially important for high gain antennas, since the gain in the azimuth pattern is typically achieved by reducing radiation above and below the horizontal plane. (Imagine squeezing a balloon flatter, so it bulges in the horizontal plane as the size in the vertical plane is reduced.) With such an antenna the elevation pattern is important, since deep nulls often exist in the elevation pattern. These nulls often appear in the elevation pattern at angles of only a few degrees below (and above) the horizontal plane, so knowledge of the null locations is very important in system design. If the null(s) happen to be at a depression angle that includes a desired coverage area (such as a populated area), the actual received signal will be significantly reduced. The elevation pattern for each polarization should be entered in the appropriate TAP antenna file (the one with the azimuth pattern for the same polarization).

After the azimuth and elevation patterns have been entered for both polarizations, you could compute two coverage contours that describe the service area of transmitting facilities using such an antenna: one for horizontally polarized receive antennas (typical roof-mounted YAGI TV antennas), and on for vertically polarized receive antennas (such as rabbit ears). Obviously, if you are only interested in one of the polarizations (such as horizontal for television), you would only need to compute the coverage contour for that particular polarization.

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