Factors Affecting Maximum Range.3-2; Radar Horizon.3-2 - Furuno 1751 Operator's Manual

3. APPLICATION
Factors Affecting Maximum Range
Factors Affecting Maximum Range
It is nearly impossible to State that a radar has a maximum range. The maximum range a radar
will "see'* is dependent on many factors, not just the range marked on the screen. Not only
does the sensitivity of the 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 target, and
atmospheric 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 about the same range to 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 reflected from the target.
Just as you can see a low-to-the-water speedboat only up relatively close to your boat, the
radar can see 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 it is capable of
seeing other targets. However a possible negative effect 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 increased to a greater distance from the boat.
Thus it is not at all uncommon to see a 3000 foot high mountain 50 miles away (provided the
radar has a 50 nautical mile detection capability), while at the same time being only able to
see a small power boat 3 or 4 miles away. (See Figure 3-2.)
The distance to the horizon from the scanner, under normal conditions, is calculated by the
following formula.
Rmax = 2.2x(V^ -h
VJi2)
Where Rmax: Radar horizon (mile), hl: Antenna height (meters), h2: Target height
(meters)
For example, to find the distance to the horizon in Figure 3-2, if the antenna height is 8
meters (26 feet) and the target height is 15.2 meters (50 feet) the maximum range is (when
the cliff begins to appear on the radar),
Rmax = 2.2 x (V8 + V15.2) = 14.8 miles.