on the ongoing driftnet beacons-
The Non Directional Beacon service in the domestic 200-400 KHz band can and
does produce harmonics in our band. The antennas used on the NDB transmitter
is often a T hat vertical which is rather effective at 1.8 MHz. One of the
locals here is a maintenance contractor for a couple NDBs, and I hear many
stories on the failure modes (lightening and rifle fire being the most common)
Specs on the NDB is power values of 10, 25, 50, 100, 200W or 1KW, and
identify at a 5 to 15 ident loops per minute with an apparent tone freq of
1200Hz, and 85% apparent modulation. The NDB transmitter does NOT have a
tone in between the ID cycle. On Beacons where voice mod is not used, the
transmitters do NOT have a conventional modulator and do have a linear PA.
The ID is created by mixing (!!) a second carrier into the PA input. This
carrier is offset (high) by 1200 Hz, and keyed at the cw rate!. Obviously,
failure or mis- adjustment can easily produce a BIG mess!
These signals can be quite a hassle to identify with a narrow receiver. The
modulation products will have an unconventional power distribution at the
harmonics, and you can easily hear what appears to be only a cw signal
significantly separated from the "carrier" The NDB modulation scheme can
also lead to the "negative" id - when you listen to the carrier itself, and
the "mixing" carrier is mis- adjusted, it will appear to "capture" the
carrier producing a "negative" id.
Almost ALL domestic NDBs identify with a 3 letter combination which usually
does not include a number. The NDBs are all continuous carrier
transmitters. There are relatively few other types of transmitters left in
the domestic service- in AK and VE, there are a few high power beacons still
in service- these have a steady tone, but are no longer the A & N system
"LF Range" of the 50s and 60s.
I am told that most all of the foreign LF beacons are conventional CW- a
carrier for some seconds, with a cw ID in a loop- the carrier is usually on
for one minute, and a cw id which requires a bfo or cw rx to hear. I am not
aware which countries use these commonly.
There is an old marine system which may still have transmitters in some
parts of the world. There are usually 3 transmitters which operate in a one
minute on 2 minutes off cycle - a steady carrier followed by the id followed
by the next station in the string. There used to be several of these sets
of stations on the west coast. most are decommissioned now, but certainly
could be still in operation in other parts of the world.
I am told that most domestic NDB transmitters will be decommissioned by 2000.
Guess I have blown my bandwidth allocation for a while. Hope this info is
of some use.
73 merry christmas, cu in the BB test, robin, wa6cdr/n6ll/xz1n
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