Sailing Today Issue 149 September 2009

RADAR REFLECTORS
Which refl ector
How the tests
Radar
were conducted
Scanner
Direct Path
The test day was hot with a gentle
Reflector
northerly breeze and high atmospheric
pressure at 1028mb. A halyard was rigged
half way along the starboard spreader on
works best?
the HR352, 5.9m above the deck. Gaffer
tape was stuck to the deck to mark the
Surface Reflected Path
10° and 20° angles, a distance of 1.040m
and 2.147m aft respectively. Beginning at
Designers of passive refl ectors do their best to choose a combination of angles of
a range of one nautical mile, we viewed
metal plates or lenses that work together to best refl ect signals.
the boat with no reflector up, then hoisted
each reflector in turn, first vertically, then at
10° and 20° of heel. We also viewed the
created by surface plane intersections. answer. Designers of passive reﬂ ectors, boat without a reflector and checked the
RCS is shown in square metres; the higher therefore, do their best to work out what radar reflective performance of a pressure
the number, the better the reﬂ ector should combination of angles of metal plates or cooker. Progressively we increased the
be. We decided that because this was a lenses work together to best reﬂ ect the range, eliminating products that failed to
practical evaluation, not a theoretical test, signal back in the direction it came, with improve the target on the radar.
we would not compare quoted RCS as little as possible of the signal shooting All tests were done stern-on, or nearly
ﬁ gures, because it could cause confusion, off in the wrong direction. Most designers so, to give as small a ‘paint’ as possible.
particularly as they can be quoted either then put their collection of plates and A problem we had was that the HR352
as a peak value, a mean value or lenses into a housing so they look nice. seems to be quite a good radar reflector
sometimes even as SPL (Speciﬁ ed The casings also help to hold the all on her own. On a normal day this
Performance Level). component parts together. would be quite surprising, because the
Without a radar reﬂ ector, a small boat boat does not have a very wide transom.
will have an RCS that varies radically How active reflectors work We are confident that the high pressure,
with the aspect. It will be small from Active type reﬂ ectors are more correctly combined with very calm water, caused
ahead and astern and peak when beam on. known as Radar Target Enhancers (RTE). the boat on her own (without a reflector
Adding a radar reﬂ ector should even this We reviewed the science behind these in up) to be visible at a greater distance
Radar
out, but the effect will only be noticeable
Min Signal
the Tech Talk section of ST148. In a than would normally be the case in our
when you get to the range where the RCS
Max Signal
nutshell, each device has two antennae – climate. (See the test results later where
of the reﬂ ector – which will be mounted Sea Surface one for receive and the other for transmit this is explained.)
high up – is bigger than that of the boat. – and a power ampliﬁ er. The incoming The tracking boat was a Nelson
A large RCS does not mean that the radar pulse is picked up by the receive 42 fitted with a Raymarine HD digital
‘blob’ made by your boat on a radar will antenna, passed through the ampliﬁ er and radar. The scanner, a 4ft open array,
be bigger, it means it is more likely to be is re-transmitted through the transmit giving a 1.5° beam width, was mounted
there on each sweep and will be more antenna. A 12V DC supply is needed to approximately 4m above sea level. The
visible against background clutter. Finally, power the ampliﬁ er. The RCS of active display, from which all the screen shots
the curvature of the earth ultimately limits reﬂ ectors is much greater than that of were recorded, was the Raymarine E120.
the range at which you can be seen. The passive ones, probably by a factor of 10.
higher your reﬂ ector and the higher the
radar, the greater this is. Beyond about X-band and S-band
eight miles, even with a passive reﬂ ector, Radars operate in one of two frequency
the returns may be so weak that you will bands, X-band and S-band. Radars on
not be detected. This is where the stronger small craft operate in X-band (8-12GHz);
returns of the active reﬂ ector will big ships carry both X-band and S-band
deﬁ nitely win out. (2-4GHz) radars. S-band radars are too
big for small craft, because they work on
Bouncing it back a lower frequency/longer wavelength, so
Passive reﬂ ectors are designed to mirror have much larger antennae. S-band is best
the incoming radar wave and bounce it for long range target tracking and when
back whence it came. In theory, the best the weather is poor, because the lower
passive radar reﬂ ector is a ﬂ at metal plate, frequency is less affected by rain and
aimed square on to the incoming signal. choppy sea conditions. X-band is better in
The problem with that design is that you good weather and at shorter range.
don’t know where the signal is coming in Passive radar reﬂ ectors are optimised for
from, so can’t aim it back. There’s also X-band and work to a lesser degree in S-
the problem of the boat pitching, rolling band. Current active radar reﬂ ectors work
and yawing, so clearly a ﬂ at plate isn’t the only in X-band.
RDELL • ACTIVE-X • SEA-ME • ECHOMAX •
September 09 Sailing Today 53
ST149 Grp Radar Reflect_RTG.indd45 45 16/7/09 14:14:20
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