Uzytkownik "Wimpie" napisal w wiadomosci
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On 24 mar, 18:42, "Szczepan Bialek" wrote:
"Wimpie" napisal w
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On 24 mar, 10:53, "Szczepan Bialek" wrote:
"This finding had practical applications for telegraph communications.
For
example, Heaviside actually solved one of the biggest problems
affecting
long distance telegraph and telephone communication in
1887 -distortion.
It
was known that different frequencies travel with different speeds on a
long
cable. For example, the low bass frequencies in a voice signal travel
faster
than the high treble frequencies. When the cable is long enough, the
frequencies smear, and both voice and telegraph signals become garbled
noise. Heaviside used his equations to show that if inductances (i.e.,
a
small coil of wire) were added along the length of the cable, the
distortion
could be reduced."
From:http://www.ieeeghn.org/wiki/index.php/Oliver_Heaviside

" It was known that different frequencies travel with different speeds
on
a
long cable".

Is the same in air and space?
S*

Hello Szczepan,

Search for the effective permittivity of media with free electrons
(plasma). You will see that the effective permittivity changes with
frequency, hence the phase velocity. Don't look strange to find
apparent permittivities below the value for vacuum.

Just ignore DC magnetic field as this complicates that math
significantly.

Hello Wim,

So you confirm that in plasma is the same as in metal.

No


But what with the space. The AM should be better than FM to communicate
with
the Mars.
Is/were FM used for long distances?

As power is limiting factor, a modulation scheme with coherent
detection and digital decoding will give best performance (best Eb/N0
ratio for certain BER) I think. So it is not just a question of AM or
FM/PM, but more how it is processed at the receiver. Processing power
changed over time, so theoretically the best method may not be used
because of technical limitations.

Were done the proper experiments in the early years of radio?
S*

Best regards,

Wim

I am simple asking if radio people have trouble with the fact that the speed
of waves are frequeny dependent.
I am interesting with the real radio waves in the real media. Here is an
example Pulsars are spinning neutron stars that emit pulses at very regular
intervals ranging from milliseconds to seconds. Astronomers believe that the
pulses are emitted simultaneously over a wide range of frequencies. However,
as observed on Earth, the components of each pulse emitted at higher radio
frequencies arrive before those emitted at lower frequencies. This
dispersion occurs because of the ionised component of the interstellar
medium, which makes the group velocity frequency dependent

S