Radar Reflectors

C

CAVU

New member
Another post with a question about radar reflectors got me to thinking about this subject. Maybe a more in-depth discussion is in order. Many C-Dories don't have radar including me. If you don't have an active system to see others how well do the passive reflectors work so that others can better see you? I bought one of those funny looking three lobed reflectors but I have no idea if it really helps. My gut feeling is on the roof of a C-Dory it is so low to the water that you may disappear in the ground clutter? Any one with practical experience in picking these up on their radar?
 
Ken,

Believe me, they really work! I had one of the Davis Instruments models on my sailboat, where it was mounted about ten feet off the deck, or thirteen feet over the waterline. I was in the Chesapeake heading south and had a Coast Guard friend of mine, who was XO on the Red Birch, a CG Buoy Tender, scan me with their radar. He said that I showed up with a return like that of a large tug! Other times, I radioed other boats and asked what kind of return they were receiving, and it was always very positive.

I'm sure that the height of the reflector over the water and the height of the radar are variables, but I'm not sure how much. Also, how to mount the reflectors, which are typically foil covered cardboard? (They are durable, though, and I only went through two in the eighteen years that I had Spirit.)

There are a few models out there that look like cylindrical fenders, which are the basic unit previously described, but are enclosed in plastic or fiberglass. I'm sure that any of them would be a big improvement over the basic C-Dory.

By the way, we take delivery in the Chesapeake next Thursday!

Nick
Valkyrie
 
Due to their design as corner cube reflectors that send signal back in the same direction from which it came, the reflectors made from a 12.5" circle provide a radar return signal that is approximately equivalent to 12 sq. meters of flat material. That's approx 129 sq. ft. or roughly equal to a 11.3 ft x 11.3 ft chunk of metal perpendicular to the line of site of the radar.

As far as the height goes....
The earth's curvature is only about 8"/mile - hence the reflector doesn't need to be very far up to provide many miles of visibility.

Roger on the SeaDNA
 
dogon dory":38wz7nbr said:
Roger,
I'm not sure where you got your information but you may have taken something out of context. The perfect radar reflector is a flat sheet perpendicular to the radar waves.
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Agreed. But since one doesn't know in advance from where the signal is coming that's not doable in practice. Hence the corner cube design.

dogon dory":38wz7nbr said:
The typical measure of radar signature of a target is to compare it to what size steel sphere would produce the same signature. I think the 12sp.meter equivalent that you refer to is a sphere with 12 sq. meters of surface area, not that size flat sheet.
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It's actually the cross sectional area of a sphere as opposed to the surface area so we were both off on that one ... see
1995 Radar Reflector Test

dogon dory":38wz7nbr said:
The reason for the design of intersecting planar material in radar reflectors and buoys is not to magnify the signal in any way, it is simply to ensure that there is a flat surface to reflect a signal coming from any direction.
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I didn't say it amplified in any way. What I was indicating is that the corner cubes created by the insersecting planes reflect signal back to the source MUCH better than a slightly non-perpendicular plane will. For a 2-dimension explantion of how this works see - this worked example from a University of Texas class in optics. The advantage of the corner cube is that it reflects back to the source even when the planes within are off axis.
 
This is an interesting thread! I was looking at Radar Towers rather than arches and wondered if placing a vertical sheet of metal perpendicular to the shaft of the tower would suffice as a reflector? Since most towers are either Aluminum or Stainless it seems that it would be pretty easy to put an angle on the upright and end up with a mount for a small plate to act as a reflector.

Am I way off on my thinking?
 
gljjr":evrkl7cg said:
This is an interesting thread! I was looking at Radar Towers rather than arches and wondered if placing a vertical sheet of metal perpendicular to the shaft of the tower would suffice as a reflector? Since most towers are either Aluminum or Stainless it seems that it would be pretty easy to put an angle on the upright and end up with a mount for a small plate to act as a reflector.

Am I way off on my thinking?
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I think you'd be much better off just buying (or making) something similar to the Davis reflector. The issue with a sheet of metal is that it's only useful when perpendicular to the source of the radar and that doesn't happen too often...
 
Gary- The plate would only reflect radar waves when is was perpendicular (square) to the radar sender (source). The radar waves are reflected (kind of) like a BB projectile, only bouncing straight back when the reflecting surface is square to the source. Most of the time, your boat's and the plate's angles are at something other than totally perpendicular to the source due to direction of travel and pitching and rolling angles.

The real beauty of the 3-cornered reflector is that the geometry of the reflector will bounce the radar waves back to the source as it moves about on the boat, regardless of the boat's motion or that of the reflector. Has to do with incoming and reflected angles trapped in a perpendicular reflecting corner, in this case in three dimensional form, so you could mount a three cornered radar reflector below the radar tower instead of the plate, providing it will fit.

Joe.
 
I've been wanting to add a radar reflector, but I don't know where to mount it. I have a radome on a 17" tall radar arch on the back of the roof. I will have a dingy attached to the roof under the radar arch. I assume that I'll need to get the reflector up out of the way of the radar. My gps antenna, vhf antenna and anchor light are all mounted to the radar arch. Is there a standard mount that will work on the arch, but get the reflector up out of the way?
Lyle
 
Valkyrie,
Thanks for the info. That was just what I wanted to hear. I have a model of reflector from West Marine called a Tri-Lens reflector. It got a good review from Practical Sailor magazine but I was concerned with its height above the water. On the roof of the C-Dory it is probably only 6' about the surface of the water. I like to fish out of Ucluelet off the west coast of Vancouver Island and I have been caught in the fog a few times. I slow way down and keep a good eye out but I get very nervous coming into the entrance buoy where all the fishing boats tend to concentrate. I have had a few boats come roaring by going much faster than I would deem safe in the fog. I guess they think since they have radar they don't need to slow down.
 
CAVU,

Glad I could help, but my "It works, get it" answer seems to have been overpowered by the scientific discourse that followed my post!

I never cease to be amazed by the level of experience and knowledge found on this site.

Nick
Valkyrie
 
Plan B made a reflector out of cardboard, covered with aluminum foil, a few years ago -- looked like the West Marine "corner cube" twin circle -- except his was twin semi-circles. He mounted it flush atop his radar dome and we tested it with our radar -- BOOM -- a marvelous reflection. For do-it-yourselfers or for those who enjoy experimenting, it might make an interesting afternoon and give a CD an inexpensive, easily mounted, adequate reflector.
 
dogon dory":ias52evn said:
Bess-C":ias52evn said:
I've been wanting to add a radar reflector, but I don't know where to mount it....
Lyle
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One thing that is not intuitively obvious is that you do not have to mount the reflector on the outside of your boat. The very reason that you need a reflector on a fiberglass boat is because the glass does not reflect the radar waves. Due to that phenomenon, you can stick the reflector in a locker down below and it is perfectly functional. You could put it under the spash well, under the sink, in the back of a locker, etc.

As for worrying about height above the surface, if all you are worrying about is being run over, height is somewhat moot. You're only concerned about being visible within a mile or so which is when most small boats will begin to think about taking evasive maneuvers anyway. And if they aren't watching their radar it is really moot. And if it a super tanker and you are counting on them yielding right of way...well... :smileo

rogerbum,
Between the two of us we are probably half right :smile Excellent article that you provided the link for. Very similar to one published by Practical Sailor years ago.

CAVU,
Valkyrie made best comments of all.... THEY WORK, GET ONE... :thup
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Dan,

Good point about the reflector being useful even if it's inside the boat. Height does help though as being visible from farther away means you're more likely to be seen by the captain whose eyes are not continuously scanning the radar (and there are definitely some of those).

Also, I seem to remember that in the discussion about insulating the inside of the cabin, someone had used a foil coated bubble wrap. Something like that is a possible 2-for-one job - get the insulation and increase the reflectivity of the boat.

BTW - I think both or us are individually correct more the 50% percent of the time which (by my math) means that between the two of us we must be right MORE than 100% of the time. :lol:
 
.............and El and Bill's post about Plan B's homemade foil covered reflector mounted atop the radome is what I recall from a thread on the old C-Dog site from a couple years back, which is what started this whole and very interesting discussion in the first place.
 
Thanks Fred, But for those of you who are handy with aluminum foil and want a conversation piece at the dock, Jo and David on Spirit developed this fine reflector. They don't know if it works, but hey, live on the edge. :shock:

Blakely_05_103.sized.jpg


Robbi
 
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