A marine radar radio
pulse is transmitted from a special antenna
that focuses the RF (radio frequency) energy into a very narrow beam.
Consider it the same as a beam of light from a narrow beam spotlight.
If this pulsed RF beam intercepts a flat surface that is vertically & horizontally at 90deg to the beam,then a strong reflection is sent back toward the transmitting/receiving antenna
,just as would happen if the light beam hit a perfectly vert/horiz mirror.
The real world is not flat 90deg surfaces(except for a slab sided big ship
The pulsed RF beam mostly will hit rounded or slanted surfaces. Some of the energy will be reflected back to the T/R antenna,but much of the energy will be reflected off vertically & horizontally & never arrive at the T/R antenna.
A proper radar reflector has specific dimension that "fit" the wavelength/frequency of marine radar transmitters-ie it is a tuned receiving antenna & is much better at "grabbing" radar transmissions than a random sized reflective surface.
Secondly,a radar reflector is built of cones with 90deg angles to each other & is supposed to be mounted such that the focal point of these cones is horiz. to the water
. As shown in the attd.,the cone captures the RF beams,concentrates them into a stronger beam & reflects this concentrated beam back to the T/R antenna.
The same thing is being done in the new LED spotlight reflectors.
When a reflected pulse is received back at the radar antenna,a voltmeter circuit measures the strength of the pulse & commands the computer in the display to print a pixel whose brightness/size/color is relative to the strength of that echo.(in simple terms)
GRP is not a good electrical
conductor,so it makes a poor RF reflector.
I daresay wet sails
Rounded or non vertical shapes,regardless of material,are poor RF reflectors.
Add to this that both vessels are "rocking & rolling" not in sync.
Luckily,your radar is spitting out & waiting for returns from "several hundreds" of pulses per second. The antenna,turning @ 24-48 RPM
is basically "stopped" while several thousand pulses happen at each bearing.
I live in fog
country & you would not believe the difference that a $50 radar reflector makes in any vessel's visibility,clear or foggy,night or day.
That even includes dorys & other small craft.
Don't leave home without it IMHO.
Should be a requirement under safety
Radar Basics - corner reflectors