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rate targets at a distance. The critical factors are the antenna's beam
width and side lobe level. Typically, a radar antenna will radiate a
tightly focused beam from the front of the array. The longer the an-
tenna array is, the narrower the beam width. Additionally, it will also
emit smaller amounts of energy to each side. A lower side lobe level
lessens the effect of a false echo.
Side lobe
The beam radiating the strongest radio signal from the antenna is
called the "main lobe". Those beams radiated in other directions are
referred to as "side lobes". The side lobe level indicates the difference in
level (signal strength) between the largest side lobe and the main lobe.
Characteristics of Radar Wave
Radio waves travel out from the antenna, bending slightly along the
earth's surface. The amount they bend depends on atmospheric condi-
tions. The sight distance of a radar generally is about 6% longer than
the optical sight distance and is calculated using this equation:
Where h is the height of the scanner, the distance to the radar horizon
in nautical miles (Nm)
= 1.22 h(feet) or 2.21 h(meters)
Targets difficult to display on screen
The intensity of the reflected radio signal from a target depends on the
distance, height, size of the target, the target's material and shape
along with the radar’s transmitter power output and antenna size.
Targets made of fiberglass, wood, or other low-reflectance materials or
those that have a small incident angle are difficult to display on a
screen. Sandy beaches and sandy or muddy shallows can be difficult to
pick up. A coastline can actually be closer to your boat than it appears
on the screen, because there's not much to reflect a signal back to you.
Radar Shadow Zones
Radar waves propagate in a straight line. A high outcropping of land or
a large ship will create a shadow zone behind it and prevent you from
seeing targets on the other side. Radar is a line-of-sight view of targets
and obstructions.
More importantly, if a mast or some part of the boat's superstructure is
in the path of the antenna's sweep, this will also create a shadow zone.
No targets will be recognized behind it and it could create a dangerous
situation.