too high can be a problem for radar, as the boat’s
rolling and pitching is amplified at a masthead,
for example. A compromise is therefore in order.
The NMEA recommends a minimum height of
8 feet abovedeck, and a maximum height above
the waterline of 30 feet. Keep in mind that large
objects in the same horizontal plane as the radar
beam will interfere with its signal and may cause
blind spots or false targets to appear on the radar
display.
On many sailboats, the radar scanner is
mounted high on the leading edge of the
mast, which may cause a blind spot on the
radar screen directly aft of the boat. One way
to minimize this effect is to mount a wooden
block between the back of the radome and the
mast, as wood is less reflective to the radar
beam and will be less likely to create an
obscure target on the radar screen.
On powerboats, the concern is the fore-and-
aft trim angle of the boat. If you have a planing-
hull powerboat, you can fabricate a wedge or
bracket to adjust the antenna’s plane angle to
accommodate the rise of the bow when the boat
is on plane (see illustrations next page). For own-
ers of displacement-hull powerboats, this is not
much of a problem since they don’t experience
as much bow rise underway as planing hulls.
Radar scanners are available in different
power configurations. Typical small-boat units
are available in the 2 kW to 5 kW power range.
Larger craft often use units with power ratings
up to 10 kW. Higher-output units work better in
fog and rain, which is generally when a radar is
most needed, because fog and mist absorb some
of the radiated energy from the antenna, effec-
tively reducing the potential range of the unit.
But radar is a line-of-sight system in both
the Transmit and Receive modes. So more
important than transmitting power to the sys-
tem’s range capability are the relative heights
of the radar antenna on your boat and the tar-
get in the distance; tall is good, short is harder
to see. It’s worthwhile to question whether you
coaxial cable and antennas 185satellite
domeGPSGPS
differential
antennaradomemounting
poleinsulated
standoffssplit backstayAn effective radar mount on a sailboat. It is up out of the
way of crew, elevated enough to provide an acceptable
line-of-sight range, and far enough behind the mast so that
the mast will interfere only minimally with its line of sight.
Some mounts of this type articulate automatically from side
to side, leveling the antenna to compensate for the boat’s
heeling angle. (This photo also shows the split-backstay
SSB antenna installation discussed on page 182.)really need a radar with a 50-mile range,
which the vast majority of boaters never use.
As a boater, I’m more concerned with seeing
targets as they get closer to me so that I can
avoid hitting them! Rather than impressing
people with a 10 kW, 50-mile radar, I’d rather