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2.1 The Antenna Installation
2.1.1 Radiation Diagram
The bridge units are supplied with a choice of two different antennas.
The type of antenna used depends on the type of link (point-to-point or point-to-
multipoint) and the required range. All antennas are directional by nature; they
cannot be made to radiate uniformly in all directions. Therefore, antennas are
designed with controlled properties to guide available RF energy in the desired
direction. This directivity multiplied by antenna efficiency is gain. Gain is
expressed in decibels relative to a hypothetical isotropic source that radiates
uniformly over a spherical surface. Figure 1.1 displays the radiation pattern of an
antenna with the gain referenced to an equivalent isotropic source. The gain of
the isotropic source is the unity or zero decibels. Depending on the directive
behavior of an antenna and the site requirements, the type of antenna is chosen.
This diagram displays the radiation power versus the direction. Figure 2.1
displays the radiation diagram of a Stub Loaded Helix (SLH) antenna.
Figure 2.1 The radiation diagram of a Stub Loaded Helix (SLH) antenna
The more directive an antenna is, the more gain an antenna has in one specific
direction. Installing an omni directional antenna for a point-to-point link is
inefficient, unless the range between the two antennas is short. In such a case
the choice for an omni-directional antenna is a purely economical.
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