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Sidelobe echoes (false echoes)
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 en-
ergy 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 rep-
resented on the display as a false echo as shown below.
Sidelobe
Noise and interference
When the fishing ground is crowded with many fishing boats, the sonar is subject to
interference from ultrasonic equipment (echo sounder or sonar) on other boats as well
as those on own ship.
For instance, interference from the sonar operated on other boats will show itself on
the display as in (A) in the figure below. This interference can be suppressed by
changing the TX interval, with [TX Interval] on the [H-Scan Setting] menu. Noise from
marine life shows itself on the displays as in (B) in the figure below. This type of noise
can be suppressed with the interference rejector on the [Picture Setting] menu.
School of fish
Bottom
Ring of interference
Mainlobe
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 (yours or others)
2. HORIZONTAL MODE
Mainlobe echo
Sidelobe echo
(B) Interference from marine life
2-43
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