Seiwa MDS-18910R - Manual (Page 8)

MDS-1R/8R/9R/10R Installation
2.2 Characteristics of Radar Wave
Radio waves travel out from the antenna while bending slightly along the earth's surface. The
amount they bend depends on atmospheric conditions. The sight distance of a radar generally
is about 6% longer than the optical sight distance and is calculated using this equation:
Radar sight distance (NM) = 2.22 ( antenna height (m) + target height (m))
Line of sight
Radar Radio Wave
h1 h2
Earth
Fig.2-3 Radar wave
Targets difficult to display on screen
The intensity of the reflected radio signal from a target depends on the distance, height, and
size of the target, as well as its 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 catch. Because there's not much to reflect a
signal back to you, a coastline can actually be closer to your boat than it appears on the
screen.
Apparent coastline
Actual(invisible)
coastline
Invisible
Visible
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1
HU
Fig.2-4 Targets difficult to display on screen
Shadow zones of radar
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. 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.
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