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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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