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6. INTERPRETING THE DISPLAY
6-4
6.5 False Echo by Sidelobe
An ultrasonic wave is emitted only in the direction set by the TILT lever but, in practice,
there are some emissions outside the main beam that are called "sidelobes." Energy
of the sidelobe is fairly weak but when the sonar is used in comparatively shallow wa-
ter with a hard and rocky bottom, strong target signals are detected by the sidelobe.
These are represented on the screen as a false echo as shown below. To weaken the
sidelobe echoes, set [VER BEAMWIDTH] to [WIDE], on the [SONAR] menu.
Sidelobe echo
6.6 Noise and Interference
If the fishing ground is crowded with many fishing boats, the sonar is subject to inter-
ference from ultrasonic equipment such as an echo sounder, sonar, etc. on board oth-
er boats as well as those on board own ship. For instance, interference from the sonar
operated on board other boats will appear as a ring as shown in (A). This interference
can be suppressed by properly changing the TX cycle. Electrical equipment on own
ship can also cause interference to the sonar as shown in (B). The noise from some
marine life appears on the screen as in (C). Those types of noise can be suppressed
with the interference rejector.
Noise and interference
Sidelobe
Mainlobe
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.
(A) Interference from
other sonar
(B) Electrical
Interference
(C) Marine life noise