Any return from a reflector on a kayak is likely to be lost in the sea clutter. The efficiency of a reflector, like all radar targets is improved with height, a difficult achievement on a kayak. On calm water a target may be visible at close range (1/4 mile as suggested)but if there is any sea running the radar operator will use the clutter control to suppress the congestion of returns from nearby waves. This congestion usually spreads out to and sometimes beyond 1/4 mile and is caused by the angle of incidence between the radar aerial (scanner) and the face of the wave, the higher the aerial the more sea clutter generally speaking. Therefore it is unlikely that a radar reflector will be of much use to the paddler except on calm water. It also follows that the larger the vessel, the less likely it is to receive a useful echo from your relector. All this leads me to believe that the best course of action in fog is to head for the nearest beach!