Furuno 1730 Operator's Manual page 8

Setting the GAIN Control
The GAIN Control is used to adjust the sensitivity of the receiver, and thus the strength of
echoes as they appear on the screen. It is adjusted so that a speckled noise background is
just visible on the CRT.
To become acquainted with the way the GAIN control \vorks, try rotating it between its
fully counterclockwise and clockwise position as you observe the display. To properly set
the gain, one of the higher ranges (16nm or 24nm) should be used--the speckled noise
background is more apparent on these ranges. As you slowly turn the GAIN control
clockwise you should be able to see the speckled background appear when the position of
the control is between 2 and 3 o'clock. If you set up for too little gain, weak echoes may
be missed. If you turn the
GAIN
control too far clockwise, yielding too much speckled
noise background, strong targets may be missed because of the poor contrast between
desired echoes and the background noise on the display. Fig.2-3 illustrates examples of
gain settings which are too high, proper, and too low.
(A) too high
Fig.2-3 Setting the
GAIN
Control
In certain circiinistanccs it may be useful to slightly reduce the gain to improve range
resolution; clear iij) the picture; or reduce dutter caused by rain or snow.
Range resolution is a mcasiire of the capability of a radar to display as separate pips the
echoes receivcd from two targets which are on the same bearing, and are close together
radially. With reditclion in the gain setting, the echoes may be made to appear as
separate pips on the display screen.
When sailing or eruising in crowded regions a slight reduction in gain often helps to clear
up the picture. d'his should be done carefully, otherwise weak targets may be missed.
Echoes from ships inside a squall or storm may be obscured if the gain is at its normal
setting, since the dutter may have in asked, but not completely, echoes from the targets.
In all cases, the gain should be returned to its original position after any temporary
reduction is no longer required.
Adjusting the A/C SEA Control
Echoes from waves can be troublesome, covering the central part of the display with
random signals known as "sea dutter." The higher the waves, and the higher the scanner
above the water, the further the dutter will extend. Sea dutter appear s on the screen as a
large number of small echoes which might affeet radar performance (see Fig.2-4 A). dlie
action of the A/C SEA is to reduce the amplification of echoes at short ranges (where
dutter is the greatest) and progressively inerease amplification as the range inereases, so
that amplification will be normal at those ranges where sea dutter is not experienced.
The control is only effeetive up to a maximum of about 4 miles.
The proper setting of the A/C SEA is such that the dutter is broken up into small dots,
and small targets become distinguishable. If the control is not sufficiently advanced,
other targets will be hidden in the dutter, while if it is set too high, sea dutter and targets
will both disappear from the screen. As a general rule of thumb, turn the control
clockwise until dutter has disappeared to leeward, but a little is still visible windward.
Fig.2-4 illustrates how to adjust the A/c sea control.
(A) Sea Clutter,
A/C SEA
control "OFF" (B) A/C SEA properly adjusted
Fig.2-4 Adjusting the A/C SEA
A common mistake is too over adjust the control so that all the dutter is removed. By
rotating the control fully clockwise you will see how dangerous this may be; a dark zone is
created near the center of the screen. This dark zone can be dangerous (targets may be
missed), espedally if the gain has not been properly adjusted. Always leave a little clutter
visible on the screen, this ensures weak echoes will not be suppressed. If no clutter is
visible on the screen, leave the control in the fully counterclockwise position.
The GAIN is normally set to the point where there is a trace of noise speckles showing on
the screen on the 16 or 24 mile range, and then tlie A/C SEA is adjusted on the 0.25 mile
range scale so that close-in targets in a harbor situation are clearly seen. This equalizes
the GAIN and A/C SEA characteristics for all ranges, short and long.
In moderate conditions on the open sea, where there are no definite targets to be seen on
the shorter ranges, you should still adjust the GAIN on the 16 or 24 mile range for some
noise speckles on the CRT, and then go down to the 0.5 or 1 mile range to adjust the A/C
SEA until a bit of sea clutter is observed dose to the boat.
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