Furuno CH-37 Operator's Manual page 21

Case 1
Case 2
Fish School a
Bottom
Fish school b
Case 1
Figure 2-4 Fish echo and tilt angle
Points to consider
• Normally, a vertically distributed fish school is a better sonar
target than the bottom, because it reflects the transmitted
pulse back toward the transducer.
• In case 3, both fish schools a and b are presented. Gener-
ally speaking, however, midwater fish schools tend to be
larger than bottom fish schools and they are often displayed
near the bottom on the display.
• It is difficult to detect bottom fish when they are not distrib-
uted vertically.
Tilt angle for surface fish
Sound emitted from the sonar transducer forms an oval-shaped
beam with a width of approximately 13 (for 60 kHz transducer)
in the vertical direction (vertical beam width). The tilt angle is
indicated by the angle between the center line of the beam and
the horizontal plane. Then, if the tilt angle is set to 0 , the cen-
ter line is parallel with the sea surface and one half of the emit-
ted sound goes upward, toward the sea surface.
This causes one half of the emitted sound to be reflected to-
ward the transducer and displayed on the screen as sea sur-
face reflections. When the sea is calm, since the sound is
reflected just like a light hitting a mirror at a narrow incident
angle, it propagates away and the sea surface reflections be-
come negligible.
Fish school a
Bottom
Fish school b
Case 2
Case 3
Fish school a
Fish school b
Case 3
2-5