"Radium" wrote in message
ps.com...
On Jun 29, 9:15 pm, John Smith I wrote:

Radium wrote:

WTF are you thinking when you describe the 20 Khz signal as, "a
pure-sine-wave-tone] with a frequency of 20 KHz and an
amplitude of 1-watt-per-meter-squared"

One square meter of copper wire squared, a squared meter of modulation
xfrmr ... ?

Sorry that should be 1 X [10^-6] Watts-per-m^2

http://www.glenbrook.k12.il.us/GBSSC...nd/u11l2b.html

1 X [10^-6] Watts-per-m^2 is about the loudness of a "normal
conversation" according to the above link.

F-------------------king typos!!!!!!!!!!


Your question sounds like one of a high school physics student
attempting to ask a seemingly logical--yet complex question, and of no
real world value.

Your ability at obfuscation is only mundane ...

If what you say is true, you have an interest, what is the purpose of
your question?

My basic question is if I have an AM receiver which receives signals
on a carrier frequency of Fc, is it mathematically-possible for me to
receive a modulator signal -- on that station -- of a frequency higher
than Fc? If not, then why? If not, then how are the submarines which
use ELFs [Extremely Low carrier Frequencies around 3 to 30 Hz] able to
perform voice communications?

I just stretched the question out to astronomical extremes. I have a
habit of doing that.

Radium

It is not possible to modulate a carrier frequency at a frequency higher
than the carrier frequency. It wouldn't be a carrier frequency then, the
higher frequency would become the carrier frequency by default.

ELF communications are carried out at very slow data rates, only a few
characters per hour at best. Normal demodulation techniques are useless at
these frequencies and messages are received by what amounts to comparing the
noise levels on a given very narrow frequency band over long periods.
Computers are easily capable of performing this task. Messages are generally
sent as 3 character codes which are then looked up in a code book to read
the full text of the message. Each message can take half an hour or more to
send. Only a very limited set of pre arranged messages can be passed but
this is enough to tell a sub to approach the surface and establish line of
sight comms direct to a satellite, when more detailed messages can be passed
securely and at high speeds on higher frequencies (i.e voice and data
communications). Voice comms cannot be passed at VLF or ELF frequencies.

Nuclear subs are extremely autonomous. There is no quick way to establish
communications once they have left port and submerged.

It is possible to communicate at a base band frequency of 0Hz. This is what
happens when you talk down a hard wired telephone or intercom. At a
telephone exchange (switching centre), the signals from each line are
modulated onto a higher frequency for onward transmission down a trunk wire
cable or fibre optic cable. The multiplexed high frequency modulated signals
are down converted back to audio frequencies once they reach the intended
destination.

It is also possible to transmit this signal through the air (at incredibly
low efficiencies and powers). The miles of cables snaking through the
trenches in World War One were so long that messages could be intercepted by
the enemy listening in without any direct connection to the system. A good
ground connection and half a mile of wire rolled out across no mans land was
sufficient to pick up the signals from the other side. Systems were also
discovered which employed two widely separated ground connections and
avoided the need to send men out on a suicide mission to carry wires towards
the enemy trenches.

Mike G0ULI