Naturally, if you mount the antenna higher by locating it on a spreader
or mast, you can gain additional range.
On a small boat at sea, however, an
antenna located too high will be rocking so much that much of the advantage provided by elevation is wasted.
For most small craft, pole-mount-ing an antenna at a height of nine
to 12 feet is considered appropriate.
of radar is 0.25 miles or
0.125 miles. But often the
last 50 yards or so are
filled with so much noise
that you’ll see a solid
blob on-screen; in some
other cases the area within 50 or 100 yards of
the antenna is intentionally filtered out with “bang
suppression,” which merely leaves a cleaner-looking
blob or nothing at all at the
center of your radar screen.
So while the pulse length
and height considerations are limiting factors, often they aren’t the practical limitation to minimum radar range.
There’s one major exception to mini-mum-range limitations, which is provided
by the cutting-edge technology found in
broadband radar. Broadband doesn’t emit
strong microwave pulses, like traditional
radar. Instead it emits a tiny fraction of the
power and measures the frequency change
between the emitted waves and those
bounced back to the antenna.
There’s little excess electrical noise
created, and bang suppression is completely unnecessary. As a result, it’s possible to
see objects that are just a few feet away
Radar also has a minimum range,
which is a bit more complex to determine
as there are several variables: the pulse
length and processing of the microwave
signal, a geometric element that arises from
the shadowed region that lies below the
beam pulse, and the intentional squelching
of excess close-proximity electrical noise.
The vertical width of a typical radar
beam is about +/- 15 degrees from horizontal. That beam first strikes the water at
distance of its height in feet, divided by tan
( 15 degrees). For a 30-foot antenna, this
is 30/0.268, which is 112 feet from the
antenna. With a 12-foot antenna, this distance is reduced to 44 feet. So on a typical
small craft, even one with a high-mounted
antenna, this is not a huge limitation.
Radar resolution is dependent on many factors,
including bearing and range.
The next limitation to add into the
mix is electrical limitation, which is 164
yards for each microsecond of pulse length.
Most radars switch to shorter pulse lengths
at lower ranges, with something in the
order of 0.12 microseconds being typical
for ranges less than a mile. This translates
to 0.12 x 164, or about 20 yards from
the antenna — but signal processing usually doubles this electronic limitation. Thus
the lowest range scale on many makes
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