Basic Antenna Principles for Mobile Communications

4.2.4 Antenna Systems
Special applications which cannot be realised by using a single antanna are very often achieved via
antenna combinations. The combination is made up of several single antennas and a distribution
system (power splitter and connecting cable). Very often a combination is designed in order to achieve
a higher gain. Many different antennas are also used to achieve a wide range of horizontal radiation
characteristics by varying the number of antennas, the azimuth direction, the spacing, the phase and
the power ratio. Figure 15 shows 3 simple examples.
A quasi-omnidirectional pattern can also be produced. The required number of antennas increases with
the diameter of the tower. For examples 8 Panels are required at 900MHz for a mast with a diameter
of approximately 1.5m. The omnidirectional radiation is not continuous but a result of the one or two
optimas per panel mounting diameter.
The calculation of such radiation patterns is achieved via vector addition of the amplitude and phase of
each antenna. The amplitude of each pattern can be read from the data sheet but the phase is only
known by the antenna manufacturer. However the phase is the most important factor for the calculation
because a rough estimate using only the amplitude can lead to completely incorrect results.
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