NavList:

George wrote:

"Gary has missed the important difference between an ordinary radio and one
intended for radio direction-finding. Any normal radio has very powerful
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. "
 
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He is correct about the AGC which controls the RF (radio frequency) gain amplifier stage and which tends to even out the changes with rotation of the radio antenna thus also reducing the changes in the level of the audio output (loudness) of the signal. But you can usually get a usable null even with radios with an AGC. Some radios have signal strength meters which merely read out the AGC control voltage, the voltage out put by the AGC circuit which then reduces the gain of the RF stage as the received signal gets stronger. The Grundig does not have a way to disable AGC but it does have a signal strength meter which can help in determining the null. It also has a "DX-LCL" (distant- local) switch that sets the RF gain either high or low and you can sometimes get a better null by setting the switch to "LCL" which then sets the RF gain to the low position. Another radio that works well is the Sony ICF-2010 which has an RF gain control. You can watch the signal strength readout and reduce the RF gain to reduce the signal strength readout and then by rotating the radio you can find the null by watching the readout drop. I adjust it so that only one light is lit at the null so it is easy to see when the signal strength increases as the second light comes on.

Such portable radios can give you usable information to keep the rocks out of your bilge as long as you have assessed the accuracy that is available and allow the appropriate safety margins around your positions. An RDF doesn't need to be particularly precise to allow "homing" on a station (but don't run down the Nantucket light ship again.) But you will have a larger error band if far from the station which should not be a problem far off shore but could be a problem if using a distant station while approaching the shore. Always use the closest transmitter, nearness is more important than angle of cut (within reason.)

gl




George Huxtable 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.