Furuno FAR-1513-BB Operator's Manual page 98

2. RADAR OBSERVATION
Sidelobe echoes
Every time the radar pulse is transmitted, some radiation escapes on each side of the
beam, called "sidelobes". If a target exists where it can be detected by the side lobes
as well as the main lobe, the side echoes may be represented on both sides of the true
echo at the same range. Side lobes show usually only on short ranges and from strong
targets. They can be reduced through careful reduction of the gain or proper adjust-
ment of the A/C SEA control.
Sidelobe
Virtual image
A relatively large target close to your ship may be represented at two positions on the
screen. One of them is the true echo directly reflected by the target and the other is a
false echo which is caused by the mirror effect of a large object on or close to your
ship as shown in the figure below. If your ship comes close to a large metal bridge, for
example, such a false echo may temporarily be seen on the screen.
Mirror image
of target ship
2-4
Mainlobe (beam)
Sidelobe
Antenna
Target ship
Route for direct reflection
Your ship
Bridge
Route for
indirect reflection
True target
False echoes by sidelobes
True
echo
False
echo