We can create a directional antenna either by putting reflectors behind the driven elements
or putting directors in front of the driven elements . ATV aerial is an example of
a directional antenna; however, a TV aerial is just receiving, whereas we need to transmit
and receive. The more elements we add, the higher the forward gain but the narrower
the beamwidth (the bandwidth of the antenna also reduces). Doubling antenna
aperture doubles the gain (+3 dB). However, doubling the aperture of the antenna doubles
its size, which can create wind loading problems on a mast. A 24-element directional
antenna will give a 15 dB gain with a 25° beamwidth but can really only be used
at microwave frequencies. Eight element antennas are quite often used at high-band
VHF and four-element antennas at low-band VHF.
We can increase the aperture of an antenna by coupling it with other antennas either
stacked vertically or bayed horizontally (see Figure 13.3). Every time we double the
number of antennas we double the gain. However, when we stack two antennas, we
halve the vertical beamwidth; when we bay two antennas, we halve the horizontal
beamwidth. At some stage, the coupling losses involved in combining multiple antennas
exceeds the gain achieved. We are also adding cost (mast occupancy and wind
loading) and complexity.
If we change the wavelength distance between the antennas, we can create nulls on
either side of the forward beam (see Figure 13.4). We can use this to reduce interference
to other users and to reduce the interference that the base station sees in the receive path. 301
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