NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Radio direction-finders. was: Re: LORAN-C to be shut down.
From: John Huth
Date: 2009 Dec 5, 17:39 -0500
From: John Huth
Date: 2009 Dec 5, 17:39 -0500
??
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.
For fun, I purchased discrete components and am trying to see how well I can make one from scratch. I found a neat ganged variable capacitor that can do 0 to 800 pF.
What was tough was finding a ferrite rod. I looked all over the internet, and what I finally found was from a company that sells parts for a contraption that supposedly turns water into hydrogen for powering cars. It's a bit of quackery - if you want, google "Stanley Meyers" - but for whatever it's worth, you can get decent ferrite rods from these guys. I was just calculating how many turns of wire to get the right inductance this afternoon - speak of the devil.
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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.
For fun, I purchased discrete components and am trying to see how well I can make one from scratch. I found a neat ganged variable capacitor that can do 0 to 800 pF.
What was tough was finding a ferrite rod. I looked all over the internet, and what I finally found was from a company that sells parts for a contraption that supposedly turns water into hydrogen for powering cars. It's a bit of quackery - if you want, google "Stanley Meyers" - but for whatever it's worth, you can get decent ferrite rods from these guys. I was just calculating how many turns of wire to get the right inductance this afternoon - speak of the devil.
On Sat, Dec 5, 2009 at 7:39 AM, George Huxtable <george@hux.me.uk> wrote:
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@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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