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    Radio direction-finders. was: Re: LORAN-C to be shut down.
    From: George Huxtable
    Date: 2009 Dec 5, 12:39 -0000

    Gary wrote about radio direction-finders-
    
    "An ADF will work on a boat but they aren't cheap, the readout is only
    marked every 5 degrees and the antenna has to be mounted somewhere. If
    you want RDF capability just by an inexpensive digitally tuned
    portable radio that covers the LF band such as the Grundig G5 which
    also covers HF and has SSB capability so you can get your time
    signals  too. These all have ferrite rod internal antennas which are
    highly directional. Get one and tune a distant station. Then orient
    the radio in different attitudes and rotate the radio until you get a
    null which will let you know the orientation of the ferrite rod. Then
    you can use the edge of the radio to indicate the direction to the
    station. Place it on top of a universal plotting sheet to use as a
    compass rose placed on a table or nav station desk and rotate the
    radio to get a null. You may want to make a calibration table for it.
    Don't worry about the lack of a sense antenna which are really only
    needed by an ADF since a human can easily determine which is the
    correct bearing, the 180 degree ambiguity, which is a big problem for
    an ADF, is not a problem for a human."
    
    Gary has missed the important difference between an ordinary radio and one
    intended for radio direction-finding. Any normal radio has very poerful
    Automatic Gain Control (AGC). This is intended to counteract differences in
    received signal strength by turning up the gain to counteract it, so all
    stations sound about equally strong. In doing that, it is fighting against
    the very changes in signal strength that you are trying to detect when
    swinging the antenna. As a result, the apparent signal level doesn't change
    until you get right to the null-point, when it gets overcome by noise.
    
    In a dedicated RDF receiver, it's possible to disable the AGC control, and
    it's usually replaced by a knob by which the gain to be held constant at a
    suitable level for the received signal as the receiver is swung. That makes
    all the difference.
    
    In the days of discrete components, and receivers that were supplied with a
    circuit diagram, in used to be easy to get in and unhook the AGC connection,
    and cobble-in a potentiometer to allow knob-adjustment instead. You could
    even add a meter to show the signal level. Not so easy now, I imagine.
    
    George.
    
    contact George Huxtable, at  george{at}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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