Use of High Gain Omnidirectional Antennas

Despite our best efforts to discourage the use of high gain omnidirectional antennas in point-to-multipoint systems, our sales for these types of antennas have increased in recent years, and as expected, so have the complaints from the field, with most of the problems being due to improper installation practices. Rural radio systems are often 'stretched' to the limit where every dB of antenna gain becomes important. In a point-to-multipoint system, the ideal is to maximize the gain of the central station antenna so that the cost of the outstation antenna systems (antennas, transmission lines, antenna support structures) can be minimized. When considering a 13 dBi omnidirectional antenna at the central station, the following points should be kept in mind: 1. In order to increase the gain of an omnidirectional antenna from 10 dBi to 13 dBi, the half-power elevation beamwidth must be decreased by half. The elevation beamwidth (3 dB points) of a 13 dBi omnidirectional antenna is only 4 degrees (+ 2 degrees). 2. The nulls between the main lobe and the first sidelobes in the elevation pattern for a collinear array fall at approximately twice the 3 dB beamwidth i.e. + 4 degrees for a 13 dBi omnidirectional antenna.