Furuno CSH-5L/CSH-8L manual False Echo by Sidelobe, Noise and Interference, Sidelobe echo

6. INTERPRETING THE DISPLAY

6.5False 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 water 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.

	Mainlobe echo
Sidelobe	Sidelobe echo
	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.
	Sidelobe echo

6.6Noise and Interference

If the fishing ground is crowded with many fishing boats, the sonar is subject to interference from ultrasonic equipment such as an echo sounder, sonar, etc. on board other 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.

(A) Interference from	(B) Electrical	(C) Marine life noise
other sonar	interference

Noise and interference

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