Chapter1 MDP-621/641/640
General Information Operation manual
Radar sight distance (NM) = 2.22 ( antenna height (m) + target height (m))
h1
h2
Line of sight
Radar Radio
Earth
Targets difficult to display on screen
Figure 1. 7 Radar wave propagation
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 is 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
3
1
Figure 1. 8 Targets difficult to display on screen
Shadow zones of radar
Radar waves propagate in a straight line like light. 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 scan, this will also create
a shadow zone. No targets will be recognized behind it and it could create a dangerous situation.
False echoes
Sometimes radar will display targets on screen that do not exist in the real world. You should be aware
of how and why this happens.
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