|
Radar Reflector Study provided by West Marine, for additional information dial (800)BOATING Other Trihedral-based Reflectors
The Firdell Blipper is not an octahedral reflector, but it still uses the basic trihedral corner reflectors. Rather than eight corner reflectors oriented around a sphere as in the case of the octahedral reflector, the Firdell uses ten trihedral corner reflectors oriented approximately 36° to each other, and optimized for angles close to the horizontal. The theory is that by avoiding the regular geometry of the octahedron, the deep nulls can be avoided. Its a good theory, but the problem is that in order to fit the corner reflectors into a package of reasonable size, the individual reflectors must be made fairly small, with a radius of only 4" in the case of the popular 210-5. Since the performance of a trihedral reflector is proportional to the fourth power of its size, this is a serious loss. For example, a circular 6.25" trihedral element (such as the Davis Echomaster) will have an RCS 2.5 times (4 dB) greater than a 4" trihedral element such as the Firdell. We tested two models of the Firdell Blipper, the popular 210-5, which measures 20" in height and 8½" in diameter, and weighs 3¾ pounds, and the slightly (10%) larger 210-7. Both are designed to be mounted vertically, either on the forward side of a sailboat mast, suspended vertically using a small halyard, or mounted vertically on a flat surface. It should be noted that mounting the Firdell (or any reflector) on the front of a mast will shadow it from the rear, making it ineffective over an angle that can be 90° or more.
The cylindrical Mobri reflector is another variation on the trihedral theme, but in this case they are stacked in either a 2" or a 4" diameter cylinder. With the radar beam exactly at right angles, they act as a series of dihedral reflectors, but even small heel angles cause it to operate in a deep null with little reflection. The series of end plates that would form the third side of each trihedral are too small to be effective, even in X-band, and are operating too close to edge-on at small heel angles. The smaller 2" diameter unit suffers an additional problem in that the 1" radius of each dihedral reflector is less than a wavelength even at X-band. Both Mobri reflectors have provision for hanging the cylinder from top and bottom, or can be strapped to a wire or spar. The manufacturer suggests mounting one on each cap shroud above the spreaders, which would provide a reflection at two narrow angles of heel, rather than just one.
The High Gain Rotation is an 8" diameter plastic sphere with a gimbaled quadrahedral reflector inside. Unable to determine how the gimbaling was accomplished, we cut open the plastic shell after testing. It has two intersecting aluminum plates which are embedded in a combination float/ballast base, which in turn floats on what appeared to be water. This allows the reflector to remain vertical through 360° of pitch and roll. It does not allow the attitude of the reflector surface to be known while testing, however, since it is completely enclosed in the sphere and it is free to rotate. In addition, the effect of rapid boat movement is difficult to predict, since the period of oscillation of the reflector will be in and out of phase with the motion of the boat. Our test gave little insight into the workings of the High Gain Rotation, except that its performance at 0° and 20° of heel was very similar, which would be expected.
The Cyclops 1 is the smallest of the three Cyclops models, and has trihedral reflectors facing fore and aft and biconic reflectors facing athwartship. It designed to be masthead mounted, and has a provision for attaching masthead lights above it. It is a sealed plastic dome, with pointed ends fore and aft, and measures 13"L x 10.5"W x 7"H.
The Lensref is a Luneburg lens device, and is the one significantly different reflector that was tested. The Lensref is an 8" diameter sphere with layers of plastic (frequently likened to the layers of an onion) which vary in their index of refraction. By focusing the radar energy to a reflective band around the "equator" of the lens, and then back along the same path to the source of the energy, a claimed 360° reflection is achieved. It has a 10 mm bolt at its top and bottom which can be mounted in an optional mast bracket, or bolted to the top or bottom of a vertical surface. The angle of heel is limited, however, by the width of the metalized band that provides the actual reflection. Beyond about 18 degrees of heel the focused beam misses the metalized band completely, and the reflector quits working. Providing a wider reflective band would increase the range of heel angles, but at the expense of overall performance since more of the "front" surface (towards the radar beam) would be covered. Mounting the Lensref on a gimbal would significantly enhance its performance under sail.
|
