Seiwa SWRx series - Manual (Page 48)

50 User Manual
Side lobe
Antenna
Main
Beam
Fig. A.1.2 - Antenna Pattern
A.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))
h1 h2
Line of sight
Radar Radio Wave
Earth
Fig. A.2 - Radar wave
A.2.1 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 fibreglass,
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
3
1
HU
Fig. A.2.1 - Targets difficult to display on screen
A.2.2 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 superstruc-
ture 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.