"msg" wrote in message
...
Greetings:

Seeking opinions on an old, commonly heard signal (AM) on HF, of some
considerable power, with a very memorable sound.

This will be a bit difficult to describe and I wonder if some folks
would have audio recordings of spectrum surveillance from the 1960s
for an actual sample, but in lieu of that I will try to convey a
sense of what it was. I had always assumed that it was photo
transmissions by the wire services, but listening to modern equivalents
I have doubts; it actually sounds more like the early telephone
voice scrambling systems that were acoustically coupled to the handset,
and which produced a continuous noise irrespective of voice level.
The sound on the air was much like hearing several P51 fighters
approaching at high airspeed and props not in phase; the spectral
power is mostly in the range of about 200 Hz to 500 Hz with a
varying heterodyne of several Hz. It had a bandwidth of at least
25 kHz (can't say more precisely due to the cheap receivers I had at
the time). I don't remember the frequencies, but at the time I would
have been concentrating on monitoring 3 Mhz to about 12 Mhz, and these
signals were strong at all hours on perhaps a dozen different frequencies.
I always regarded them as annoying QRM.

I did little SWLing from the mid '70s until somewhat recently, and of
course there is nothing like this heard nowadays.

At the time, my QTH was about 25 miles from the largest National Guard
training camp in the midwest and there were four USAF bases within 200
miles
as well, so it could well have been some modulation mode used by the
military.

I would really enjoy knowing if others remember a signal like this
and knew its origins.

Regards,

Michael

I remember what you are talking about. When I was
a preteen in the late 60's using a cheap shortwave
receiver we heard things such that we naturally said,
"That sounds like an airplane." Sounded like someone
left the mike open in the cockpit of an airplane. We
suspected that they probably weren't airplanes because
there never was a voice and who would just transmit
airplane noise.

Most likely they were, as others have said, VFT or
something like it. They were almost certainly a
Frequency Division Multiplexing or Frequency Diversity
system, i.e. multiple carriers stacked in frequency,
each modulated (with either PSK or FSK) and
shifted in time relative to each other. The diversity
is usefull against the frequency selective multipath
fading of HF propagation. If multipath causes
a dropout of one carrier the info is still available
in the other carriers.
I'm a little surprised to think they had that technology
back then but maybe I shouldn't be.

Ionospheric multipath fading can also give it
a doppler-like sound, as if it were airplanes diving
and banking.

You can sometimes hear signals like you describe
today. There is something like it now at
11455 kHz, s5, 04:55 utc, in socal.

I hear VFT at 11010.55 and 11499 kHz
occasionally.
I have demodulated them down to bits and then
run autocorrelations. The autocorrelations were
flat indicating that they are encrypted.

There are many utes using OFDM these days.
They sound similar but sound more flat,
more whitenoise-like. I've heard them at:
4.28480
4.5905
4.81020
5.0175
6.39250
6.39850
6.42320
6.4345
6.76525
6.84272
8.4884
8.541
8.553
8.6255
8.646
12.7243
12.805
13.41050
16.9435
17.07750
17.08250
17.098

all in MHz, all USB.

If you find any old recordings I'd be interested in
hearing them too.

--
rb