Furuno 9ZWWR2120, Руководство по установке

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S2E-17-0209_IM_WR2120_C
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1.5. Side Lobe
The energy radiated from an antenna forms a field having a definite
radiation pattern
. A radiation
pattern is a way of plotting the radiated energy from an antenna. This energy is measured at
various angles at a constant distance from the antenna. The shape of this pattern depends on the
type of antenna used.
To plot this pattern, two different types of graphs, rectangular-and polar-coordinate graphs are
used. The
polar-coordinated graph
has proved to be of great use in studying radiation patterns.
In the polar-coordinate graph, points are located by projection along a rotating axis (radius) to an
intersection with one of several concentric, equally-spaced circles. The polar-coordinate graph of
the measured radiation is shown in Figure 1.7.
The
main beam
(or
main lobe
) is the region around the direction of maximum radiation (usually
the region that is within 3 dB of the peak of the main beam). The main beam in Figure 1.7 is
northbound.
The
side lobes
are smaller beams that are away from the main beam. These side lobes are
usually radiation in undesired directions which can never be completely eliminated. The side lobe
level (or side lobe ratio) is an important parameter used to characterize radiation patterns. It is the
maximum value of the side lobes away from the main beam and is expressed in Decibels. One
side lobe is called
back lobe
. This is the portion of radiation pattern that is directed opposing the
main beam direction.
The now following graph shows the
rectangular-coordinated graph
for the same source. In the
rectangular-coordinate graph, points are located by projection from a pair of stationary,
perpendicular axes. The horizontal axis on the rectangular-coordinate graph corresponds to the
circles on the polar-coordinate graph. The vertical axis on the rectangular-coordinate graph
corresponds to the rotating axis (radius) on the polar-coordinate graph. The measurement scales
in the graphs can have linear as well as logarithmic steps.
Beam Width:
The angular range of the antenna pattern in which at least half of the maximum power is still
emitted is described as a „Beam With”. Bordering points of this main lobe are therefore the points
at which the field strength has fallen in the room around 3 dB regarding the maximum field
strength. This angle is then described as beam width or aperture angle or half power (- 3 dB) angle
- with notation Θ (also φ). The beam width Θ is exactly the angle between the 2 red marked
directions in the upper pictures. The angle Θ can be determined in the horizontal plane (with
notation Θ
AZ
) as well as in the vertical plane (with notation Θ
EL
).
Main and Side Lobes (Minor Lobes):
The pattern shown in the upper figures has radiation concentrated in several lobes. The radiation
intensity in one lobe is considerably stronger than in the other. The strongest lobe is called
main
lobe
; the others are (minor)
side lobes
. Since the complex radiation patterns associated with
arrays frequently contain several lobes of varying intensity, you should learn to use appropriate
terminology. In general, main lobes are those in which the greatest amount of radiation occurs.
Side or minor lobes are those in which the radiation intensity is least.
Front-to-Back Ratio:
The front-to-back ratio of an antenna is the proportion of energy radiated in the principal direction
of radiation to the energy radiated in the opposite direction. A high front-to-back ratio is desirable
because this means that a minimum amount of energy is radiated in the undesired direction.
Cumulonimbus
early detection
from low
altitudes
Start of
detection with
FURUNO radar
Start of detection with
conventional radar
Conventional
radar
FURUNO
radar
FURUNO radar
Conventional radar
Figure 1.4: FURUNO radar detects cumulonimbus
cloud faster than conventional radar
Figure 1.3: Image of using 3 radars