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491 Echo Sounder Operations Manual Page 17
Manually
overriding
automatic mode
You can always manually change any of the echo sounder’s
settings: the gain, clutter, noise limiter, shift, and range.
Changing the appearance of the picture (colors, gain, clutter,
and the noise limiter) is described in Chapter 3.
Setting the range and shift is described in Chapter 6.
Changing the frequency of echoes
The 491 uses two transducer frequencies: 50 kHz and 200
kHz. The advantages and disadvantages of each frequency are
shown below.
You can choose from four settings for the transducer
frequency:
• 50 kHz displayed on a full screen
• 200 kHz displayed on a full screen
• 50 kHz on the left half of the screen, and 200 on the right
• 200 kHz on the left half of the screen, and 50 on the right
There are two reasons why you might want to operate with
your preferred frequency on the right side of the screen:
• the zoom function magnifies the frequency on the right
side
Table 2: 50 kHz vs. 200 kHz
50 kHz 200 kHz
Longer wavelength goes deeper,
but with somewhat lower resolu-
tion.
Shorter wavelength provides a
sharper picture, but can’t go as
deep.
Less resolution is better at finding
big fish, but may miss the bait
fish.
Finer resolution is better at find-
ing small fish (baitfish) and subtle
bottom details.
Wider beam is good for seeing
fish not directly beneath the boat.
Narrower beam is very focused.
Reduces the influence of a rough
sea.