They are fun as a collectors item, and to take apart for study.
Universally, the rotating contact is a stereo phone jack. The ferrite
and coil designs are not all the same. Some are a twin ferrite design,
which looks cool but doesn't work as well as the larger single ferrite
design. The twin ferrite might be a cost reduction technique.

The winding of the coil is also interesting. You would have to google a
bit, but the better ones use two windings (same size) that are combined
to make one loop. There is a good reason for this, which I believe is
to make the coil work well single ended (as opposed to needing a
differential amp). Mine has two large coils and two small ones. Perhaps
the other coils are for different bands.

A real problem with these RDFs is you can't read the dial very
accurately since it is analog. Those beacons are on a 1khz plan, so
unless the beacon can be IDd via Morse code, you can't tell what
frequency you are listening too.

What I like to do is use a shortwave radio with CW filter to find
beacons. Once you find all the beacons you can with just plain AM, I go
to CW. You can find the carrier with CW since it will produce a beat
frequency, which would be a constant tone. Once you have the carrier,
you can use passband tuning to move the CW filter to the sidetone which
has the beacon ID Morse code. [Positive shift for USB, negative shift
for LSB]

Another technique that is a bit more hightech is to use the ARGO
program in beacon mode. Use your shortwave radio in AM mode with a
reasonably narrow filter, say 2kHz, which is what you would use for
sideband. With ARGO running on your PC, it will take a real time FFT in
a manner where you can see the Morse code scroll on by.

NDBs are supposed to go away, but who know. These things seem to have a
life of their own. However, you might want to log a few beacons while
they still exist.
Krackula wrote:

On 9 Feb 2006 13:31:39 -0800, miso@sushi.com wrote:

I have a bendix 555. It's not nearly as good as using a shortwave
antenna (like a Wellbrook with sufficient wire) and shortwave radio.
The filtering in these RDFs isn't so hot. The target market was marine
IIRC.

These are quite common flea market items. FWIW, the Bendix 555 had the
best null out of the units we tried. Other others were a Ray-Jeff and
some other model I don't recall.

There are ferrite based loops such as sold by Quantum, MFJ, etc., that
you can use with a shortwave radio.


Krackula wrote:

some of these  maritime RDF units are probably  an excellent and
economical  RDF unit for those  lowfer air beacons ......

herze one that looked pretty nice ........
http://cgi.ebay.com/ebaymotors/NewMar-Digital-Direction-Finder_W0QQitemZ4610800122QQcategoryZ26444QQrdZ1QQcmdZViewItem

they don't show up in the sw listening NGs or the  receiver  searches
on ebay .. so they usually remain a bargain.  think  some  have a
motor drive on the  rotational antenna on top  and  adjusts it's  self
automatically  as you move around ........ keeping the direction set !

might be good for BCB listening as well !

just a thought ........

roger that .......  thanks for the info about those.
appreciate it !