Furuno WR2120, Руководство по установке

Страница: 11 / 73

5
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.
 Azimuth direction:
Antenna side lobes show both P0N
(unmodulated pulse emission for radars)
and Q0N (frequency modulation within
each pulse) covered area.
 Range direction:
Pulse compression (Range) side lobes
show Q0N only. It improves sensitivity of
long pulse but increase blind range.
However this blind area can recover
using un- modulated pulse.
Pulse compression
：
Pulse compression is useful for weather radar to increase the average transmitted power by
transmitting a longer pulse but without reducing the range resolution of the radar.
Side lobe suppression is helping to eliminate the problem. For a radar pulse signal after
compression, the narrower pulse main lobe is always accompanied by higher side lobes. So, all
pulse compression radars suffer from range side lobes which cause energy from strong
reflections to leak into adjacent range cells. High suppression of side lobes is not required in
some other non-meteorological radar, but is important for meteorological radar because weather
phenomena can have significant reflectivity gradients, and ground clutter echo can be much
larger than medium rain. Furthermore, the side lobes of strong signal will falsely be recognized as
an existence of small target. Therefore, range side lobes must be suppressed by a large amount
to prevent contamination in adjacent range cells.
Antenna
Side lobe
Range
Side lobe
Figure 1.5:
Antenna pattern in a polar-coordinate graph
Figure 1.6: High level echo with side lobes
Front to Back
Side lobe
Main lobe
Beam width