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BASIC OPERATION > UNDERSTANDING SONAR
32 Fishfinder 250/250C Owner’s Manual
Transducer Coverage
The area covered by the transmitted sound waves is determined by
the cone angle of the transducer and the water depth. Cone angles
may vary among different types of transducers. For example,
a 50kHz frequency may provide a wide 40° cone angle with a
coverage width that is approximately 2/3 of the water depth. As
shown in the diagram on the right, the 40° cone angle (50kHz
frequency) approximately covers the area of a 20-foot diameter
circle at a 30-foot depth. A 200kHz frequency may provide a narrow
10° cone angle with a coverage width that is approximately 2/10 of
the water depth. As shown, the 10° cone angle (200kHz frequency)
approximately covers the area of a 6-foot diameter circle at a 30-foot
depth.
When using the Fishfinder 250/250C with Frequency
set to Dual,
the unit alternately transmits 50kHz and 200kHz signals and
combines the information. This allows the unit to cover a large
area and still retain good bottom resolution. The unit uses the
narrow cone angle (200kHz) to display detailed bottom information
(keeping dead zones to a minimum) and the wide cone angle
(50kHz) for the large coverage area.
200kHz - 10˚ cone angle
6-foot coverage at 30-foot depth
50kHz - 40
˚ cone angle
20-foot coverage at 30-foot depth
Transducer coverage
Fish returns from the wide and narrow beams appear differently on
the display. The wide beam tends to show longer fish returns than
the narrow beam, as shown below.
Wide cone angle Narrow cone angle