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    Re: Radio direction-finders. was: Re: LORAN-C to be shut down.
    From: George Huxtable
    Date: 2009 Dec 6, 00:10 -0000

    Apache Runner wrote-
    
    "Gary's suggesting using a cheap radio with a loopstick antenna - which
    definitely works.   I've done this myself and will attest that if you are
    willing to do your own plotting you can do a fairly good job.   The Grundig
    is a good example."
    
    ==========================
    
    Response from George-
    
    Yes, it will work-sort of, under some circumstances. But if you happen to be
    close to a radio direction-finding transmitter, the combination of strong
    signal and automatic gain control means that any null will be so narrow you
    are likely to miss it. Removing the automatic gain control, and providing
    adjustable gain control in its place, allows a clear null in those
    conditions. That's the way in which a dedicated RDF receiver differs from a
    radio.
    
    Why would anyone need RDF when so close-in to a transmitter? Let me tell of
    a little experience, perhaps 20 years ago, in days before GPS: we didn't
    have Decca either.
    
    Caught out by sudden descent of really thick fog, on a Channel crossing to
    Cherbourg, we were homing in on the radio beacon, which was thoughtfully
    placed at Fort de l'Oest, on the East Corner of the Western entrance. We had
    heard, without seeing, a cross-Channel ferry, which had just left the
    harbour, and felt its wash. On that fort there's also a 3-blast fog signal,
    which was getting progressively louder and louder. It had started to get
    quite deafening, and we know we were very close. The entrance is only half-a
    mile wide. The danger was in the likelihood of us hitting that Eastern wall,
    below the fort, in homing on the beacon, or running on to the rocks close to
    its foot. Although we had been getting good nulls on the RDF, they then
    started to get worse and worse. Until we tried swinging the receiver, not
    just in a horizontal plane, but angling it up at the same time. And we
    suddenly discovered that the true null occurred when the receiver was tilted
    upward at about 40 degrees. That was due to the transmitting antenna being
    mounted on a mast, overlooking the entrance.
    
    As that track seemed to be taking us much closer to the fort than we
    intended, we shied away from that side of the entrance. We got into
    Cherbourg harbour without seeing either side of the entrance, and felt our
    way into the inner harbour, and a snug berth for the night.
    
    On the way out next day, we took a look at the geometry of that antenna, and
    estimated, from the elevation angle of our null, that we must have been
    within a couple of boat's length of the wall, but had seen nothing of it.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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