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if you are in doubt about the amount of gain required, select an antenna that is slightly larger than you think is actually needed. The performance of all components deteriorates slightly during the years they are exposed to rain, sun, wind, and corrosion. Consequently, to insure quality reception for a longer period, choose an antenna with a little more gain than is necessary. (Note: Excessive gain may cause overloading. Therefore, choosing the antenna with the highest gain might create more reception problems than it cures.)

Sensitivity classifications are used to indicate the type of reception area for which the antenna is designed. Certain ranges of gain are best suited for certain types of areas. The appropriate range of gain for a specific area is what a manufacturer is indicating when labeling an antenna. Outer Suburban or Near Fringe Table 1-1 is a chart of typical sensitivity classifications. It indicates in miles, how close an antenna must be to the transmitter to produce the best signal level. The classifications are somewhat general because reception conditions at various sites within a particular type of area may be different.

DIRECTIVITY

Direcitivity is the ability of an antenna to intercept signals from only one direction and reject those from other directions. Directivity indicates the antenna's ability to intercept signals arriving at its front and reject signals coming from the sides and rear. Generally, the more highly directive an antenna, the better it can reject signals from the sides and rear.

The front-to-back ratio of an antenna can be helpful when attempting to determine is directivity. Front-to-back ratio is expressed in decibels (dB) and can be found in the literature accompanying a new antenna. This ratio indicates an antenna's ability to reject signals coming from the rear (rear rejection).For example, an antenna with a front4o-back ratio of 25 dB will receive about 18 times more signal strength from the front than from the back. In most cases, an acceptably accurate estimate of an antenna's directivity can be made by comparing its specified front-to-back ratio with the relative sensitivity classifications listed in Table 1-1.

The sensitivity classifications in Table 1-1 are intended only as general guidelines for evaluating directivity. Other factors, such as the antenna's beamwidth, can affect its directivity. Beamwidth is related to an antenna's overall gain and indicates how wide or narrow the antenna's reception area is. For example, if two antennas have the same front-to-back ratio, the one with the highest overall gain will have the narrowest beamwidth and consequently, will be the most directive. An antenna with a relatively narrow beamwidth generally is best suited for areas where interference from sides is a prob!em. An antenna with a broad beamwidth is best suited for areas where a broad beam is needed to capture the signals. from widely separated stations, and where interference is minimal. Beamwidth information, if included by the manufacturer, is usually displayed by use of polar plots like the one in Figure 1-6. Selecting the most suitable antenna becomes easy with experience, but selecting quality materials should be the first decision to make before starting any installation. High-performance color reception usually requires highly sensitive equipment. In the long run, it pays to use the finest equipment available, from the antenna down to the receiver end of the transmission line. The slight additional cost should be considered an investment in longer system life and optimum performance. You can get an Idea as to how big an antenna you need by entering your zip code at http://www.antennaweb.org

You can enter your address at the web page ANTENNAWEB.ORG and they will tell you most of the channels you will be able to receive over the air with an antenna in your location.

They also teel the direction the stations will be transmitted from and about how large an antenna you will need. They will rate antennas by color chart. Match the color to the CM antennas

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