Antenna systems for 800 MHz

The assignment of the 800 MHz frequencies has brought into sharper focus the long standing requirement for omnidirectional coverage from base station antennas that are not individually mounted at the top of a dedicated tower. The cellular and trunking systems with their large number of channels at any one site must, with present state of the art transmitter combiners, use more than one transmit antenna. Since each tower has only one top; the requirement to place several antennas there becomes evident. Figures are presented illustrating the variation in pattern and gain caused by this multiplicity of antennas mounted in such candelabra arrangements of two, three and five antennas. The spacing of the antennas varies from less than two wavelengths to greater than seven wavelengths in these tower top clusters. In the past, existing large broadcast towers have been utilized as antenna mounting locations: 800 MHz systems will be no different. The top of these towers are generally not available for land mobile antennas as they are occupied by TV or FM broadcast stations. As a result, side mounting becomes absolutely necessary. The tower may be many wavelengths in width at the antenna mounting elevation. The patterns resulting from side mounting antennas on these large structures as well as smaller structures are shown.