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Old 03-02-2008, 13:11   #31
JCF
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J,

Here is a link to an "Ocean Navigator" letter in which the author has measured (in a radar anechoic chamber) a 12-inch diameter plastic bag full of crumpled aluminum: Aluminum foil as an emergency radar reflector | Articles |. His measurement results were not encouraging. Of course, this is not exactly the same thing as you are suggesting, but it does have some similarities.
Hello Paul,

I read the article and as already noted, it is not exactly the same since the author has moved away from the "linear" principle, to the round principle. Round is best for trihedral planforms. His results would have been more favorable, and definitely marginally, oppossed to a total failure, for the planform and size, if he would have given the medium a different form.

The small "balls, would create a lot of nulls and would provide lots of nooks and crannies for the wave to explore infinitely. Not a real good set-up and quite honestly, he stuck to the hypotheses to test, but he could have picked a better one.

His "bag" experiment would have produced even better results if he would have poured aluminum paint in a balloon, blew it up and ensured uniform coverage inside. Put ten of those lightweight babies up on the spreaders and extraterrestrial life can see you.

You test my memory once again, but here goes. He tested at 9.4 Ghz, and that wave length is something like 32 millimeters long or 1.25 inches long. If he would have cut strips "chaff" the length of the wave, 1.25", and half as wide as the wavelength, 5/8", he would have been succesful at documenting useful data, albeit it may be only marginally useful on a practical level. Still, I believe the results would have been more favorable.

Man, that is enough physics for a lifetime and definitely enough for today. Okay, I give in and accept your disagreement in its entirety as acceptable to me.

Good day.

J
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Old 03-02-2008, 13:18   #32
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Radar waves will disburse upon their journey and the curvature of the earth will limit their distance to objects on the horizon. I think the rule of thumb for wave existense is 1300 miles for every yard above water level. [...]

At 20 feet of height the top of the mast may encounter a wavelength from 8666 miles away if conditions are optimal, which they never are, and those kinds of ranges are generated only by military early detection systems, but do you see the point of the height "linear" and area? [...]

Either way, there is no substitute for a watch at all times and the false sense of security from radar reflectors should not be entertained. Do we agree on that?
J
J,

First, I emphatically agree with you on watchkeeping and not relying on reflectors! However, the original poster is planning a singlehanded passage... By the way, I'm not arguing that he shouldn't do this singlehanded (and I'd like to do the same some day), but that he should take what reasonable steps he can to improve his chances.

I'm having trouble with your range numbers. The only radar that I am aware of which can work over 1000's of miles is High Frequency "over the horizon" radar -- definitely not the type we are discussing! The effective range for typical microwave radar is only slightly beyond visual line of sight. From Bowditch: d = 1.22 * sqrt(h), where d = distance to radar horizon in miles and h = height of radar in feet. So, for a radar at 50 ft, and a reflector at 20 ft, the maximum range is 1.22 * sqrt(50) + 1.22 * sqrt(20), or about 14 nautical miles. Inside this range there are "Fresnel zones", or areas where the return is greatly diminished, and these depend on radar and reflector height. I've seen articles where theis Fresnel Zone information is used to calculate an "optimum" height for your reflector.

All this theory is fun (if you like that sort of thing, and I do), but fine-tuning my reflector is actually pretty far down on my list of sailing priorities.

By the way, I am pretty certain that an aluminum mast will completely hide anything you stuff inside of it. The radar beam will just bounce off the outside surface of the mast. I did a little searching, and I've not yet found any authoritative data on the crumpled aluminum foil reflectors, but I have found several completely conflicting first, second, and third-hand reports about them. Some say they worked better than a commercial reflector, others said they were fairly useless. I do know that in the real world there are many confounding factors, making it difficult to perform a valid comparison.

Back to AIS -- I believe that this is now the best way to alert yourself to large ships in the vicinity. It is mandated that all large commercial ships carry AIS, and you are not depending on the ship having it's radar on and monitored. AIS won't help with the smaller vessels, but these are also the vessels that are less likely to be running and watching their radar. Once you get out of coastal waters, it's the large ships that you are likely to cross paths with anyway.
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Old 03-02-2008, 17:03   #33
JCF
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J,

First...
...it's the large ships that you are likely to cross paths with anyway.
Hello Paul...

You snuck one more in there huh?

First. I think he should go ahead with the solo circumnavigation. Caution and care, less sleep and a good loud alarm clock will make it ok. Sleep is overrated and underpaid so he better watch his six carefully because life is short and death is really freaking long.

Radar range is a product of radar strength and height above the water. I believe that you have calculated the "minimum" hit on the radar based on the "radar horizon" and height of two objects. In your example, the painted vessel would be reflected at that distance. However, anything beyond that distance would also be painted as long as it is above the radar horizon and there is enough kilowatts. If this were not the case, then a 48 mile radar would never paint a target at that distance. In essence, and knowing both heights, you have calculated distance to the object, but you can get hits before or beyond the target.

The horizon I used is based on infinite distance and in reality, some radars could read beyond the infinite horizon because of diffraction, but that calculation is left out altogether. Anything sticking above the infinite horizon and above the radar horizon, will be painted, given enough power.

I suspect that you may be right about the aluminum mast. I'm not inclined to believe that the foiled mast worked better that the engineered reflectors as those reports you read have indicated. I think they may have worked "differently" and acceptably, but not better, and definitely under certain material constraints. Either way, something is better than nothing.

AIS is the way to go but I would still have another radar for the "masses" just in case, or as a redundant. Well, not really...I'm kind of old school and prefer doing it the "hard" way as my son goes out of his way to advise.

I took his beligerent *ss out one day to three days out from sight of land and told him to take us home without all the electronics. It took him 2 days to figure out the sun goes up in the east and 2 minutes to apologize and ask me to teach him the sextant, chronometer and log.

Didn't give him any water or food for 3 days and I lectured on the importance of learning these things for 60 hrs straight so that when lightning strikes he wont starve or dehydrate trying to figure out why his electronics don't work. The tension with the wifey was thick and lasted a long time, but I made him a better man. Now he keeps paper charts and "backs up" his observations electronically.

J
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Old 04-02-2008, 00:10   #34
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J,

When an interesting topic comes up I am like a pit bull -- sometimes I just won't let go. Fortunately for everyone else, I do have a short attention span. Oh look, a bunny!

Anyway, this is definitely an interesting topic. I'm not sure how much use the typical radar reflector is going to be in practice, but if you're going to put something up it might as well be something that works reasonably well. I see the tiny "Mobri" reflectors on some boats, and while they sure are small and light, they just don't work well and you might as well not bother.

Here is a reflector study that looks decent:
1995 Radar Reflector Test

And this guy's website has some good info (see the Articles and Blogs" page). The author does have a book for sale on the subject, and I don't know if he has any company affiliations. He does like the "TriLens" reflector -- if it weren't for the weight I would probably have one of those onboard now:
The Radar Reflector Site

I do wish I were sailing now, instead of acting like the technology geek I am!
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Old 04-02-2008, 07:05   #35
JCF
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J,

When an interesting topic comes up I am like a pit bull -- sometimes I just won't let go. Fortunately for everyone else, I do have a short attention span. Oh look, a bunny!

Anyway, this is definitely an interesting topic. I'm not sure how much use the typical radar reflector is going to be in practice, but if you're going to put something up it might as well be something that works reasonably well. I see the tiny "Mobri" reflectors on some boats, and while they sure are small and light, they just don't work well and you might as well not bother.

Here is a reflector study that looks decent:
1995 Radar Reflector Test

And this guy's website has some good info (see the Articles and Blogs" page). The author does have a book for sale on the subject, and I don't know if he has any company affiliations. He does like the "TriLens" reflector -- if it weren't for the weight I would probably have one of those onboard now:
The Radar Reflector Site

I do wish I were sailing now, instead of acting like the technology geek I am!
Hello Paul,

Don't worry about it. Sometimes it's good to belabor the issue to death than not have a full undersdtanding.

I agree that if you're going to put up something it should be something that works reasonably well, but in some situations, this being one of them, I would prefer to put up the best. The Mobri's are okay but they have been measured and weighed and found to be wanting. I don't think someone would detect you with that even if you threw it at them and hit their hull.

Those lens-ref are in my mind the way to go. As you have already observed, the weight is an issue. Personally, I don't think that the weight issue can't be overcome with some engineering. Personally, for me anyway, I would put it to use differently dependent on the conditions and circumstances and believe the whole "swinging" situation could be moderately resolved. I don't think they produce those anymore. Geez, what friggin idiot would discontinue something that has a chance? Oh I know, the non passive supplier.

AMEN to sailing!!!

J
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