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Thread: radar reflectors and rules

  1. #11
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    There is a commercially-sold passive radar reflector that is up there at 10m2. It's called th Tri-Lens, Luneberg lens reflector. The smaller model is nowhere near 10m2, but the larger model, which weighs 13 pounds, is.

    Before looking at it, though, you might read this:

    http://www.tri-lens.com/practical_sailor.txt

    ...here's an advertisement for it at Defender... cha-chinnngggg.. $500

    http://www.defender.com/product.jsp?path=-1|118|107602|275622&id=303311

    West Marine sells the little model, which is about 2.5 m2, for $150



    http://www.westmarine.com/webapp/wcs...3&classNum=109

    By all accounts the Mobri sucks, has all sorts of valleys in its response and only works even halfway well at very narrow angles of heel. Still and all, they're easy to get into the rigging, they don't have a lot of windage, and I've found an ebay seller that makes a knockoff for $15 instead of $70. That's cheap enough that you can put 3-4 of them around the boat, at assorted points, slung at different angles and hope that at least one of them will be "right" at any given time.

    That's been my hope and prayer...three Mobri knockoffs in the rigging and an Echomaster on a stern mount about 10 feet off the water.
    Last edited by AlanH; 10-19-2007 at 03:20 PM.
    1968 Selmer Series 9 B-flat and A clarinets
    1962 Buesher "Aristocrat" tenor saxophone
    Piper One Design 24, Hull #35; "Alpha"

  2. #12
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    Are radar reflectors really not cumulative? I've been thinking about this, and it seems to me that adding radar reflectors *must* be cumulative in some way, perhaps just not in the way we think.

    Take the following hypothetical case. Take a steel battleship. Glue Davis reflectors all over it so it is completely covered. Am I to believe the RCS is now only the same as a single reflector? I don't think so....

    Or, pile up a bunch of Davis reflectors into a mound a mile high and a mile wide. Is the RCS that of a single reflector?

    - Mark

  3. #13
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    Question Mobri or mast??

    The Davis chaffed through its halyard in 2002. I replaced it with a Mobri but since the advent of AIS I have asked every ship about my radar signature and have always been told that it was excellent with and without the Mobri. something tells me that most of the credit goes to the big wide and tall aluminum mast. Am I correct in this assumption? Or flirting with death.
    Lou

  4. #14
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    Lou, I think you are definitely flirting with death, but so are we all, regardless of radar reflectors. Back to the real question though. I believe that an elliptical aluminum mast has a radar signature that is pretty good, particularly in larger sizes when in a certain orientation to the radar signal. However in other orientations it is near nothing. Look at the radar returns of the various reflectors that are designed to be optimal. There are still areas where the returns are very weak. The Pardey's with the wrinkled aluminum foil is another example. I am sure that in some instances they have a great reflector but in others they don't. In all the independent testing I am aware of only a very few commercially available reflectors seem to pass muster. Some of these are heavy and expensive or don't work well at more than a little heel. I will carry what the committee deems acceptable but I will not depend on it to guarantee I will be seen by another radar (whether there is an observer at the screen or not).

    Mark is on the right track though about the cumulative effect though I think a better analogy is a small light bulb. A thousand of them would be seen better than one but still not a far or distinct as a focused fresnel type lens makes one. Al

  5. #15
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    Another study done in England, http://www.ybw.com/pbo/pdfs/radar_reflectors.pdf
    Very interesting conclusions and recommendations at the end. Al

  6. #16
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    Default Radar reflectors

    There's an interesting article in Cruising World this month about the boat that was sunk by a sperm whale at the same time some of us were returning in'06. The part that caught my attention was the writer's contention that the boat's kevlar main was a great radar reflector. Makes sense but I hadn't heard this claim before. If true, it would seem to satisfy the race requirement.

    Bill Merrick

  7. #17
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    I find it pretty difficult to believe that kevlar can reflect radar. Anyone know of any other references to support this?

    The other consideration is that even if kevlar *were* reflective, that still doesn't make it an effective safety device. Radar reflectors are designed to reflect a signal directly back at the transmitter. A flat sheet of steel can be invisible to radar if you angle it such that the radar signal reflects off in a direction away from the transmitter. That's why stealth aircraft have all those flat surfaces. So even if a sail were reflective, the odds of it reflecting a signal back to the radar transmitter would be slim.

    -Mark

  8. #18
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    As I recall, the sails were carbon fiber. Even so, carbon fiber shouldn't be a particularly good reflector, since I believe that it is quite lossy. Some forms of carbon are used in dissipative elements for radio-frequency anechoic rooms, although I admit that I don't know if CF has the same properties.

    Even if the sails were reflective at radar frequencies, the orientation of the sails would have to be correct to send back a signal. I wouldn't trust that this would be the case.

    So, how to explain the report of a strong radar return? I don't know, but it sounds iffy to me.

    - Paul
    - S/V VALIS

  9. #19
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    Quote Originally Posted by Alchera View Post
    Are radar reflectors really not cumulative? I've been thinking about this, and it seems to me that adding radar reflectors *must* be cumulative in some way, perhaps just not in the way we think.

    Take the following hypothetical case. Take a steel battleship. Glue Davis reflectors all over it so it is completely covered. Am I to believe the RCS is now only the same as a single reflector? I don't think so....

    Or, pile up a bunch of Davis reflectors into a mound a mile high and a mile wide. Is the RCS that of a single reflector?

    - Mark
    I would be careful about assuming the additive nature of separate radar reflectors. If the reflected signals are in phase they would add together, e.g. 1+1=2. However, if one radar reflector is 1/2 wavelength further away than the other it will be completely out of phase, so the reflected signal would be 1-1=0, or no measurable radar reflection. The 1/2 wavelength of 2.45 GHz microwave radiation is about 2 1/2 inches. You cannot control the relative position of the reflector on a boat to that accuracy. The constructive and destructive interference also occurs with light. That's where the antireflective coating on your camera lens comes from. I could go into this ad nauseum, but the bottom line is that you can not assume that individual reflectors are additive. If my analysis is incorrect, please let me know.

  10. #20
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    Quote Originally Posted by Chesapeake View Post
    I would be careful about assuming the additive nature of separate radar reflectors. If the reflected signals are in phase they would add together, e.g. 1+1=2. However, if one radar reflector is 1/2 wavelength further away than the other it will be completely out of phase, so the reflected signal would be 1-1=0, or no measurable radar reflection. The 1/2 wavelength of 2.45 GHz microwave radiation is about 2 1/2 inches. You cannot control the relative position of the reflector on a boat to that accuracy. The constructive and destructive interference also occurs with light. That's where the antireflective coating on your camera lens comes from. I could go into this ad nauseum, but the bottom line is that you can not assume that individual reflectors are additive. If my analysis is incorrect, please let me know.
    While this may be true in a carefully controlled laboratory environment using perfectly coherent radar pulses and perfectly designed and arranged radar reflectors, I don't think this effect is significant in the real world. Radar pulses are not perfectly coherent frequencies, and reflections distort and modify the waveform and polarization, even from a radar reflector. If the destructive interference effect were a serious factor, then it would be the same problem with large targets even without radar reflectors, since you have many surfaces all reflecting back to the transmitter in that case as well. But we all know that large radar targets in general have larger returns than small radar targets.

    - Mark

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