Page 168 - Antennas for Base Stations in Wireless Communications

Advanced Antennas for Radio Base Stations 141

the difference in the resulting shape of the covered area for the two tilt
methods (ignoring surrounding cells).
A combination of mechanical and electrical beamtilt is sometimes
an attractive solution. By applying a fixed mechanical beamtilt, the
requirements on the electrical tilt range offered by the antenna may be
reduced and the hardware implementation becomes less complex. One
application of combined mechanical and electrical tilt is for reducing
backlobe interference for antennas that have a distinct backlobe that
behaves similarly to the forward main beam when the element excita-
tions are varied. This can be achieved by mechanically tilting the main
beam, either upward or downward, and then offsetting the mechani-
cal tilt by electrically tilting the beam in the opposite direction. The
combined result is that the main beam gets the desired beamtilt angle
(the difference between the mechanical and the electrical tilt angles)
in the forward direction whereas the backlobe is tilted away from the
horizontal plane, as shown in Figure 4.7.

α m
Antenna normal

α e Horizon
α − α m
e
α + α m
e
Back lobe Main beam

Figure 4.7 Main beam and backlobe directions for an antenna with simultaneous
mechanical uptilt (a m ) and electrical downtilt (a e )