When a broadcast engineer is putting a station together, he spends a lot of time choosing an antenna, transmission line, and transmitter. Usually, there are three initial factors that influence his decision. The first is his authorized ERP. The second is HAAT, antenna Height Above Average Terrain. The third is a particular transmitter he may have in mind. The question remains, how does he select the remainder of the system? By tabulating the items we have to consider, we can get a picture of what steps should be taken in the decision-making process. These items are:
The ERP and antenna height are usually known. In selecting the antenna, the station engineer has to consider the gain required to make the ERP with the transmitter power available at tower top. He must also ask the question, does this antenna, with its associated null structure, properly cover the service area? In selecting an antenna it is always better to go with a lower gain and more transmitter power for better coverage. Otherwise, a high gain antenna might guarantee the ERP at great distances, but less signal and more nulls at closer distances. When considering the transmission line, the engineer has to be concerned with two things. He has to insure that the insertion loss of the line is not so great that he cannot deliver enough power to tower top and secondly, that the line can handle the required power. The following table lists some common line sizes, their average power rating at 40 degrees C. ambient and their attenuation values per 100 feet.
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