Radar Reflectors

When I modify or add a stainless steel fitting to the mast of my sailboat, it frequently weighs more than I had expected. I just shrug my shoulders and mutter: "It is a radar reflector . . ."

I have some questions for this forum: How can those foil-covered cardboard reflectors generate 10 square meters of radar reflection? Is there a good website to coach me on radar reflector design? Are there guidelines or rules-of-thumb for reflector design? To avoid clutter and minimize windage, can a reflector be dropped down inside an aluminum mast and still be effective? Of all the gadgets and fittings up a typical mast, is there any that are begging to be modified to incorporate radar reflection? The spreaders? The backstay crane? The mast light?

 

The radar reflectors are passive devices. They send back to the radar some of the signal it generates. All objects reflect the signal. However, a radar reflector is designed to do it more efficiently. The simplest shape, three intersecting circles, presents the equivalent to a flat circle perpendicular to the direccion of the signal. A mast, even if is has a square section, only presents a flat surface in two positions. With the movement of a boat this would rarely happen. Any metal parts will increase the footprint on the radar screen. A reflector inside a metal mast won't work. Only the shape of the outside metallic surface matters. A flag with metal fibers will help, but the signal will be erratic.

 

Often, a radar reflector is no more than three square sheets of metal all tack welded together like the corner of a box. Each of these plates are measured to be on some harmonic of the signal it is intended to reflect.

You can see an example of this style of reflector at your local airport along side of the runway. (If the facility has an active radar control) Usually they're painted international orange and white.

Other types of reflectors can be a simple dipole arangement inside the focal point of a parabolic dish aimed at the radar transciever. Some are passive and some are battery powered to amplify any entering signal, sometimes superimposing a digital marker onto the returning wave.

Probably more than anyone wanted to know.

D

 

Dave, you mentioned that the dimensions are somehow related to the frequency of the wave to be reflected. Are there guidelines for this?

I am guessing that the intersecting planes only act to present some surface area no matter which way the reflector is turned. Or do they somehow amplify the signal? Would 3 vertical intersecting planes perform better than 2? Would 8 perform better than 3? Would a device that resembled a folding paper bell hung at Christmas perform the same or better?

If the mast spreader tips were vertical planes (like the stabilizers on the tips of the wings of some commercial jets) they would present some surface area when viewed from abeam. Would this surface reflect as well as the same area with perpendicular, intersecting shapes?

 

If you really want to be seen -and not have all that windage etc. get one of these..

http://www.sea-me.co.uk/

Cheers!


Paul