Instruction Manual

RA51/52/53/54/55 INSTRUCTION MANUAL – 05 3

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))

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.

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.

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.

A. Ghost echoes

Sometimes one large object very near your boat will appear as two different targets onscreen. One

is the actual radar echo. The other is a ghost echo generated by a re-reflection of the original

signal. It comes back to your own boat, bounces back to the target, and then is picked up by the

Line of sight

Radar Radio

Wave

Earth

Apparent coastline

Actual(invisible)

coastline

Invisible

Visible