Garmin GPSMAP 238 Sounder - Manual (Page 122)

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Appendix I
Transducer Coverage
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 between 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 sidebar, 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 GPSMAP 188/188C/238 in ‘Dual’ frequency mode, the unit transmits both 50kHz and
200kHz signals at the same time. The ‘Dual’ frequency capability of the GPSMAP 188/188C/238 allows
you to have a large coverage area and still retain good bottom resolution. When in ‘Dual’ frequency
mode 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.
Fish returns from the wide and narrow beams appears differently on the display. The wide beam tends
to show longer fi sh returns than the narrow beam.
Wide Cone Angle Narrow Cone Angle
200kHz - 10˚ Cone Angle
72" coverage at 30'
50kHz - 40˚ Cone Angle
20' coverage at 30'
188188C238 Manual Part 2.indd 112 6/30/2003, 3:32:56 PM