Read a Review
of radar reflectors a few months ago (Practical Sailor, I believe). They included the radar-reflective flag in their tests. Bottom line was that the flag returns a radar signal when the signal comes perpendicular to the flag, i.e., when it can bounce off a flat surface straight back to its source, but when the angle changes, the flag's reflectivity drops drastically, way down to be the worst perfomer they tested. The best overall performer was the good old hexagonal reflector, hung in the "raincatcher" position.
