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Due to space limitations in the September issue, John Battison's RF Engineering column is shown here in its entirety.
The average ground system is quiet, dependable and retiring, performing its work efficiently, without demand for attention. A properly installed and maintained ground system will usually continue to conduct space current to the transmitter's common return until the system begins to deteriorate. Unfortunately, too many stations try to economize when installing or maintaining ground systems.
Early on, vertical radiators were the most suitable for AM broadcasting. AM operation is generally based on ground wave coverage, although skywave coverage is often desired and implemented intentionally. The single vertical dipole tower around 90 degrees in height has become a very popular AM radiator. When operated over the best achievable low-resistance ground system it is very efficient. The phrase "low-resistance ground system" is the crux of the operation with vertical radiators usually between 90 and 180 degrees high.
The most efficient operation of a vertical radiator requires mounting over a perfectly conducting plane surface. This is rarely achievable even with the use of very low loss conductors in the ground and high conductivity. Dry, sandy or rocky soil is generally the least satisfactory. Antennas mounted over bodies of water are not necessarily as effective: Water levels can vary and affect the effective length of the antennas. Pure fresh water does not conduct as well as saltwater, and antennas mounted over tidal areas can be subject to considerable changes in radiation efficiency as water levels change antenna height above ground. In order to provide a workable technical standard for comparison of proposed operations, and achievable results, the FCC developed basic minimum ground system specifications involving a specified minimum resistance factor for use when planning new stations.
For years the standard ground system has been 120 buried radials each one-quarter wavelength long, spaced 3 degrees apart around each tower base. The ground wires should be 8-gauge copper, sometimes Copperweld is used. Usually the radial array is buried about 8" below the surface of the ground. This has been the FCC's minimum requirement. Any deviation has usually resulted in a condition on the Construction Permit requiring a proof of measured radiation efficiency after construction is completed. In the 1980s the FCC began to adopt and conform to some of the regulations of the CCIR for the North American region.
In recent years, shortage of available land and building restrictions have led to more exotic antenna and ground systems, and above-ground, elevated radials are not uncommon today. Unfortunately, the shortages of copper and large increases in copper prices have led to vandalism and thefts of ground system wire and copper.
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Also in the September Issue
- Trends in Technology: Electronic News Gathering for Radio
- Tower Rules Update
- AM NRSC Measurements
- Field Report: FLIR ONE
- Small Remote Controls