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    Re: Lights etc.
    From: George Huxtable
    Date: 2003 Oct 10, 20:47 +0100

    Jan Kalivoda said-
    
    >I wonder, whether the explanation of intermittent fading out of the radar
    >response from the radar reflector on the ground of the phase shift is the
    >main or the only cause for this phenomenon. I had read that radar
    >reclectors themselves have many narrow "dead angles" according to the
    >mechanism of their reflection. Therefore it was stressed in my literature
    >that it is necessary to let the reflector "run", ie. change its position
    >in the horizontal plane so as to rotate the reflection pattern around
    >reflector's vertical axis and not to fix it. George's words "This (phasing
    >out) can happen due to a small shift in the heading of the craft doing the
    >reflecting" hint that he supposes the fixed and ummovable reflector. Am I
    >right?
    
    Response from George-
    
    Well, the traditional corner-reflector, that many small craft use, has a
    polar diagram that's by no means uniform, but when properly fitted (so that
    it's upper corner is in such a position as to "catch rain"), I doubt if it
    shows actual NULLS in its own reflection, just minima. Someone who knows
    more about such things might well correct me.
    
    The nulls that I was referring to arise from the interference between the
    reflection from the corner-reflector and the reflection from the boat.
    
    And Jan is correct: I was considering the situation where somehow the
    reflector was effectively nailed to the boat.
    
    It doesn't have to be that way, though. As Jan suggests, you can dangle a
    reflector, using some sort of bridle, on a line, dropped perhaps from the
    crosstrees, so that it's free to spin around, to and fro (again, in the
    "catch rain" position). The spinning will tend to drive you crazy. Now the
    same null effect will occur, but only occasionally, when the reflector
    happens to be at the wrong angle at the wrong moment, at the time the radar
    beam strikes. So the big-ship will see your vessel vanish on occasional
    radar scans, but reappear on others. That may indeed be a safer state of
    affairs, all things considered. Thanks to Jan for pointing it out.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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