Page 33 - Antennas for Base Stations in Wireless Communications

6 Chapter One

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Figure 1.5 Rectangular plot of radiation pattern

half-power beamwidth is sometimes referred to as the 3-dB beamwidth.
Both horizontal and vertical beamwidths are usually considered.

1.1.3.2 Sidelobes and Nulls No antenna is able to radiate all the energy
in one preferred direction. Some energy is inevitably radiated in other
directions with lower levels than the main beam. These smaller peaks
are referred to as sidelobes, commonly specified in dB down from the
main lobe.
In an antenna radiation pattern, a null is a zone in which the effective
radiated power is at a minimum. A null often has a narrow directivity
angle compared to that of the main beam. Thus, the null is useful for
several purposes, such as suppressing interfering signals in a given
direction.
Comparing the front-to-back ratio of directional antennas is often
useful. This is the ratio of the maximum directivity of an antenna to its
directivity in the opposite direction. For example, when the radiation
pattern is plotted on a relative dB scale, the front-to-back ratio is the
difference in dB between the level of the maximum radiation in the
forward direction and the level of radiation at 180°. This number is
meaningless for an omnidirectional antenna, but it gives one an idea of
the amount of power directed forward on a very directional antenna.

1.1.4 Polarization of the Antenna
The polarization of an antenna is the orientation of the electric field
(E-plane) of the radio wave with respect to the Earth’s surface and is