By Tom Burden
Radar Reflectors (more accurately called Radar Target Enhancers, or RTEs) reflect radar energy from other vessels’ radars so that your boat shows up as a larger and more consistent “target.” If you operate your boat in areas with shipping traffic or where fog and low visibility are common, the ability to be seen by radar-equipped ships can make the difference between being seen and being sunk.
RTEs work by reflecting radar energy directly back to the radar antenna so that your boat appears to be a larger target. The best analogy we can think of is the reflective “dots” on many highways that make it so much easier to see where the lanes are. These light reflectors use small triangular-shaped prisms that bounce the light around and reflect it precisely back at its source.
The effectiveness of an RTE is disproportionately related to its size. Assume that you have three theoretical reflectors of the same design, but of different sizes. Look at how rapidly the RCS (Radar Cross Section) increases with size. The RCS of a given reflector goes up by the fourth power of the radius, resulting in this dramatic increase in effectiveness. For example: a reflector of twice the size of a similar but smaller model has a RCS that is 16-times larger. Moral: buy the largest RTE you can, based on your budget, windage, weight, space available, and aesthetic judgment.
|Radius||Peak Radar Cross Section|
There are two main types of passive reflector technologies in common use: the octahedral corner reflector and the Luneberg lens. Without boring you (too late!), octahedral reflectors like the Davis Echomasters, the Blipper and the Echomax have characteristically strong peak reflections separated by zones or areas of very little reflection.
In other words, slightly different orientations of the RTE cause dramatically different RCS. This causes the size of the radar target to vary as the RTE or the boat yaws and pitches in a seaway. Your boat appears and disappears, mimicking “surface clutter” or wave interference to a ship using radar, especially if you are oriented in a consistently “bad” direction relative to the other vessel. Consider also that a ship’s radar antenna is mounted high above the surface, so you may be at a null angle even in calm water.
The Echomax, and other multi element reflectors, attempt to smooth out these peaks and nulls by using a number of corner reflectors. This approach results in a somewhat ragged, but more uniform response over azimuth and altitude.
Luneberg lenses focus the radar energy to a reflective band or hemisphere inside the element, which is reflected back on a reciprocal bearing back to the source. Imagine a fun house mirror where you can stand in a wide variety of locations and still see your reflection. This produces a reflective pattern with a very uniform range of operation with limited “nulls” or blind spots.
A test conducted in March 2007 by defense research firm QinetiQ for the British Marine Accident Investigation Branch (MAIB) agency (which is available here) showed that the Echomax and West Marine Tri-Lens reflectors performed consistently well, but not as effectively as an electronically amplified reflector called the Sea-Me. This active reflector amplifies the radar signals it receives and sends back an enhanced pulse, drawing 150mA of power in standby mode and 350mA when transmitting.
The results showed that the Echomax “shows good peak and average RCS performance compared to its competitors but its stated performance level falls to around 0.m2 above an elevation angle of 10°,” so it was not as effective off-angle or when heeled. The Tri-Lens reflector did a better job, “the Large Tri-Lens performs especially well at larger angles of heel and elevation...” The smaller Standard Tri-Lens (We call it the Medium Tri-Lens, Model 2343069) “shows average performance. The peak RCS was quite low at 4m2, but as the elevation angle increased the relative performance of this reflector increased. Above 15° it outperformed the Blipper and Echomax in terms of average.”
QinetiQ’s recommendations, based on both field and laboratory testing:
Download their full Radar Reflectors Report
West Marine tested radar reflectors in 1995, lead by Chuck Hawley along with experts Stan Honey, Dick Honey and Jim Corenman. Although 1995 was a long time ago, the tests are still relevant and interesting, because just about all of the products and technology are unchanged.
Download the 1995 Radar Reflectors Test
In crowded harbors, AIS tranceivers are another new collision-avoidance technology. These VHF radio-based identification devices are especially useful in heavily-trafficked inshore or near shore areas like the US Eastern Seaboard or Western Europe. In areas like those, we think you’re better off with AIS. In remote open ocean situations we think that knowing that somebody else is in the neighborhood might be very handy, so the Active-X or the Sea-Me might be preferred.