Furuno 1830 - Indirect Echoes and Radar Picture Comparison

range marked on the screen. Not only does Ihe sensitivity of Ihe receiver
and power of the transmitter but also the height above the water of both the
scanner and target, the size, shape and composition of the targei, and
atcnospheric conditions contribute to increase or decrease the maximum
detectable range.
Radar Horizon
Radar is by its very nature essentially a "line-of-sight" phenomenon. That
means that you have just ahout the same range to the horizon with a radar
as you do with your own eyes, However under normal atmospheric
conditions, the radar horizon is 6% greater than the optical horizon.
Therefore, if the target does not rise above the horizon the radar beam
cannot be refiecied from the target-
Just as you can see a low-to-the-water speedboat only up relatively close to
your boat, the radar can ses a target high off the water farther than it can
see an object which is close to the water. Further, the higher the antenna
is mounted over the water the farther il is capable of seeing other targets.
However a possible negative effeet with mounting the antenna too high off
the water is that due to the finite vertical beamwidth of the scanner, the
amount of sea dutter due to reflections from nearby waves is inereased to a
greater distance from the boat.
Thus it is not at all xincommon to see a 3000 foot high mountain 50 miles
away (provided the radar has a SOnm detection capability), while at the same
time being only able to see a small power boat 3 or 4 miles away. (Se«
Fig.38.)
The distance to the horizon from the scanner, under normal conditions, is
calcuiaLed by the foliowlng formula.
Rmax =5 2.2 x ( <1 hl + Ih2)
Where Rmax: Radar horizon (mile)
hl: Antenna height (meters)
h2: Target height (meters)
For example, to find the distance to the horizon in Fig.38, if the antenna
height is 8 meters (26 feet) and the target height is 15.2 meters (50 feet)
the maximum ranga is (when the eliff begins to appear on the radar),
Rmax = 2.2 (/S* +Jl5.2) = 14.8 miles.
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