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Old 23-04-2008, 02:08   #46
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Excellent stuff Paul Elliot. You must have put in a lot of effort to get this together. I think that's the clearest presentation I have ever seen on this subject. You should try and get it published more widely. Well done Sir.

Also, from what you your graphs demonstrate, it seems to me that there is strong argument for installing two radar reflectors, one at a height of say 4m, and another at say 8m?

Maybe this would seem a bit over the top for your average yachty, but the technical argument shown here is very convincing, and would have some real benefit.
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Old 23-04-2008, 03:44   #47
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Excellent stuff Paul Elliot. You must have put in a lot of effort to get this together. I think that's the clearest presentation I have ever seen on this subject. You should try and get it published more widely. Well done Sir...
... the technical argument shown here is very convincing, and would have some real benefit.
EXCELLENT presentation Paul!
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Old 23-04-2008, 10:04   #48
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Thanks guys! I caution you that this is largely a theoretical excercise, with some pretty simplified assumptions. I was mainly interested in exploring the concept of Fresnel / Multipath nulls, which I had seen (somewhere?) in a magazine article about radar reflectors. I did find other references to this issue, and even Bowditch mentions destructive interference in the radar section.

There are (as always) additional confounding factors that I haven't figured in; the ones that come to mind are the variations in sea-surface reflectivity, and different path-losses for the direct and reflected signals. These factors will reduce the depth of the nulls, so make the problem not quite as bad as it would be in a theoretically ideal world.

Fortunately for me, I think a theoretically ideal world might be a little boring!

Also, I have completely ignored the effect of all the other metal bits on your boat. The larger pieces will act as reflectors too -- far from ideal reflectors, but at close range (where the nulls really become dense) these other reflectons will fill in the return and compensate somewhat.

During my limited research I found references to using vertically stacked reflectors to mitigate the effects of these Fresnel nulls as Panthablue (and others) have suggested. I have also gained an appreciation for the reflectors such as the "Firdell Blipper" and even (gasp) the Mobri, both of which have vertically-stacked reflectors. Unfortunately these have not tested very well as basic reflectors (the Mobri is close to invisible), but the stacking concept does have merit.

The active transponders (Sea-Me, one or two others) are attractive, but do have their issues -- not so much the performance, but practical ones of mounting, power consumption, and I assume having to turn them off when you are using your own radar (?). Also, the null effect ought to apply to a transponder signal as well. Since they are usually mounted up high, I wonder if this might be an issue. At a distance, the transponder signal will be much stronger than that of the reflector, so perhaps any dead-zones will become extremely narrow? Still, if you can manage the installation issues the transponder should be much more effective than a passive reflector.

I'm not sure how much I am willing to put into the radar reflector on my actual boat. For the type of sailing I've been doing, my AIS receiver is probably the more useful tool for detecting ships, and it fits my "active avoidance" philosophy. If I were singlehanding, or sailed in different waters, the reflector might be more beneficial.

Of course, I've still got to consider the fishing boats in the fog. Interestingly, since they will carry their radar at a lower elevation than a big tanker, the null problem will be less.

Here is a website by a sailor who has *really* put some effort into reflector analysis: The Radar Reflector Site He also seems to have the background that makes his opinion worth considering. Still, he is mostly analyzing reflector performance in isolation, with little accounting for other factors. But that is where you need to start -- first the reflector has to reflect!

On a cold and rainy day,
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Old 23-04-2008, 13:46   #49
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Fortunately for me, I think a theoretically ideal world might be a little boring!

Here is a website by a sailor who has *really* put some effort into reflector analysis: The Radar Reflector Site He also seems to have the background that makes his opinion worth considering.
And for those wondering about his qualifcations ...


Quote:
Ah, found it. The article is from Sept 2006 issue of Sail Magazine (pages 96 to 100) the author was Phil Gallman. From the by line of the aritcle, may I present his bona fides:
Phil Gallman has a PhD in engineering and applied science and spent 27 years working as a defense contractor in radar and communications systems. He is the author of Radar Reflectors for Cruising Sailboats.
He also has a website.
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Old 23-04-2008, 14:00   #50
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To add REAL LIFE to this Very informative thread, which has led to an epiphany ...

About a year ago, coming back from Santa Cruz Island to Channel Islands Harbor (on a POWER boat) we were in the middle of the Traffic Lanes (crossing at 90 degrees to traffic) in medium fog and keeping a VERY sharp eye out visually and using the radar. NOTHING ... all clear - we could "see" the islands and the two closer oil platforms, couple of boats 5 or 6 miles off. And then BOING!!!! VISUAL about 1/4 mile behind us -a HUGE freak'n cargo ship. We couldn't figure out why we didn't (and couldn't for about 3 minutes) see it.

Since we were on a power boat, and the radar was unencumbered by any mast or other obstruction, we were stumped. Maybe these inherent nulls were the problem??
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Old 23-04-2008, 14:13   #51
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I don't know if you have all seen this report: -

http://www.ybw.com/pbo/pdfs/radar_reflectors.pdf

I have also heard that the German made Rasmus 1 reflector is better than the See-Me unit.
Adaero,
could you give more info on the plus points for the Rasmus? I am planning to install an active radar transponder soon and know of only these two currently on the market.

Thanks
Roger
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Old 24-04-2008, 15:54   #52
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Wow.

I'm blown away. Thanks for the in depth response Paul!

Sounds like two radar reflectors at different heights are a good idea, to stagger the nulls.

Thanks!

Zach
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Old 01-05-2008, 16:04   #53
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Adaero,
could you give more info on the plus points for the Rasmus? I am planning to install an active radar transponder soon and know of only these two currently on the market.

Thanks
Roger

Here is the site in german: rasmus Marine e. K.

search for radar reflektor

Regards

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Old 04-05-2008, 13:18   #54
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Here is the site in german: rasmus Marine e. K.

search for radar reflektor

Regards

Alan
Hi Alan,

Thanks. I had already downloaded a radar reflector test done by a German yachting magazin. Both the Sea-me and Rasmus received full marks, the Rasmus apparently returning a better signal over all angles. That said, the Rasmus needed a second test with a different piece of equipment to perform well, seems the first product was a "Monday product"?!

Both the Sea-Me and Rasmus have / can have an audible alarm. Can this be switched off after being activated?

Regards
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Old 04-05-2008, 14:21   #55
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I am of the opinion that if your radar can't find a boat with a 200 lb.+ block of iron in it along with all the assorted other metal objects like the stove, mast, boom, anchor, assorted junk in the lockers, it will be unable to find a little round or other "radar reflector". I have been looking at boats via radar for years and when they pass, I make a mental note of whether or not they have a radar reflector. I am unable to discern any difference in the radar blip with and without. The only vessels that return a larger target are vessels with a larger surface area such as ships, barges and land masses.
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Old 04-05-2008, 15:58   #56
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Great info! We were traveling in heavy fog from Nantucket to Martha's Vineyard last summer and it was spooky. Got a Davis via Defender along w a military surplus collapsable [$5] So 4 meters is best....right?
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Old 04-05-2008, 16:06   #57
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Several years ago, I read an article about a radar reflector that was being used by the military, manufactured here in San Diego. I went out to the plant to find out more. The result was that I built a mockup of a masthead, topped by a tri-lens, and capped with an Aquasignal masthead running light unit. The company was able to convince WEST Marine to try the units. Several lab tests by independent magazines demonstrated the value of the reflectors. Rozendahl Associates, the designer/builder, gave me one as a thankyou gift. The tests are available for those who have the capacity to understand them. Many folks in San Diego now sport these units. I leave it to you folks to sort things out.

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Old 04-05-2008, 21:19   #58
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Roy,
I can't help but wonder if those trilens reflectors are in reality 3 corner reflectors ala Davis types mounted in 3 different configurations....

Steve B.
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Old 05-05-2008, 13:04   #59
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Think of it as three onions placed tightly together on the table. Each layer of the onion reflects radar. The "blind" spots are very small, and given the movement of a boat on the sea, virtually negligible. They showed me their test facility, the innards of the unit, and any data I asked for. I believed it, and so did the military procurors. West Marine, and some other reliable folks, tested the product to their satisfaction and now promote it. Look at the web site to see the graphs of the radar cross sections of the tri-lens and other competitors. Especially the "radar reflective flag"....

I guess, what I'm saying is, does it truly matter how expensive a product is when compared to the overall investment, and the incomparable cost of the crew? We're not talking thousands of bucks here. It's like having an anchor that you bet your life on (and those others who may be along for the ride). I tend to go a bit overboard, having way too many halon and foam extinguishers, AIS, and other costly stuff. But it is reassuring when the dark closes in and you don't know if that cargo ship is being actively helmed or the "watch" is otherwise engaged. And short of beaming over to their bridge, I can't think of anything else I can do to ensure my boat and crew's safety. Well, maybe at the next boat show, I'll find something that wakes them up and gets them out of my harm's way.
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Old 05-05-2008, 13:54   #60
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I am of the opinion that if your radar can't find a boat with a 200 lb.+ block of iron in it along with all the assorted other metal objects like the stove, mast, boom, anchor, assorted junk in the lockers, it will be unable to find a little round or other "radar reflector". I have been looking at boats via radar for years and when they pass, I make a mental note of whether or not they have a radar reflector. I am unable to discern any difference in the radar blip with and without. The only vessels that return a larger target are vessels with a larger surface area such as ships, barges and land masses.
I do not wish to throw stones, but this does rather show a lack of understanding of radar.

The 200lb mass is below the waterline, thus to all normal radars, invisible.

A larger surface area does not necessarily mean a larger return. If it is a flat area, then there will be one angle that provides a reasonable return, but the best returns are those provided by 90 degree corners. Most merchant ships have plenty of these. New warships do not, and are thus becoming more and more stealthy.

Recent trials discovered that the metal flag locker on one ship (lots of 90 degree bends) nearly doubled the return on one ship.
I have also been staring at radar screens for more years than I care to recall, and can assure you that 90 degree angles will definitely provide a decent return above that which would have been there without the reflector. That is why the old fashioned octahedron has been such a long lived item. However, radar return is dependent on size as well as numbers of reflectors, for normal reflectors, and thus for a decent return you need a large reflector with lots of 90 degree corners. This increase is measureable, and for a decent radar is significant. But is well below the threshold of most yacht radars.
So what do you do.

The "see-me" will pulse a radar return that will make a captain think he has a supertanker coming towards him, but this only works on one of the frequency bands used by merchant ships, and this is normally the one in use as they close the harbours. When they are further out, they normally run on the other radar, so the see-me is useless (admitedly a number of the better ones use both radars all the time, and these are the ships that also keep a better look out - but the worst lookouts probably are also only running on a single radar (if that)). Furthermore, the see-me also uses precious power.

The latest technology is the luneberg lens - a technology developed to enhance a towed aerial target for warships to test their abilities. For multihulls, this is definetly worth consideration, because the resultant return is focussed and magnified. But (there is always a but) you do need at least 3 lens for all round coverage, and the angle of heel of a lot of monohulls would be outside the limits of the lens capabilities.
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