Furuno CH-37 - Manual (Page 38)

2-22
False echo by sidelobe
An ultrasonic wave is emitted only in the direction set by the
TILT control, however there are some emissions outside the
main beam. These are called sidelobes. The energy of the
sidelobe is fairly weak but when the water is comparatively
shallow and the bottom is rocky and hard, strong signals are
detected by the sidelobe. These are represented on the dis-
play as a false echo as shown below.
Mainlobe
Sidelobe
Mainlobe echo
Sidelobe echo
The bottom echo
detected by sidelobe
appears at a certain
tilt angle when the
sidelobe points ver-
tically. Also, poor
soundome projection
may result in a similar
false echo.
Figure 2-21 Sidelobe echoes