Vernall wrote:

Alberto and others,

The most plausible explanation, as already mentioned on the reflector, is
that the lines come from the sampling rate of 8 kHz used by digital
telephone systems.  There could be several telephone exchanges in a city and
while they likely have stable master oscillators, they are not necessarily
"locked".  Telephone systems still use a lot of copper pairs, covering
kilometres of distance, so it would be feasible for some radiation to occur
for any high frequency spectral content in the telephone signal.
[...]

Hello Bob,
  thanks for your message.
Yes, I had read that explanation on the reflector, but I am still puzzled, as I
live in a small village, and the local Telecom told me that  ADSL will be
available here not sooner than at least 6 months. And when they say 6 months,
they mean not less than 12....
I asked my neighbors, but nobody has an ISDN line here. Just plain old analog.
But ADSL and ISDN can be found in a nearby town, about 5 km from here.
Given that today all telephone lines are buried, I am a bit skeptical about the
possibility to receive the 17th harmonic of a signal passing in a buried line
five kilometers from here. But in my life I have learnt that often reality 
defies
intuition and reasoning...

73  Alberto  I2PHD