On Mar 25, 5:54*am, Wimpie wrote:
On 25 mar, 10:17, "Szczepan Bialek" wrote:



Uzytkownik "Wimpie" napisal w ...
On 24 mar, 18:42, "Szczepan Bialek" wrote:

"Wimpie" napisal w
...

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

Try to find document "Descanso4--Voyager_new.pdf" (very likely the
first result in google). This describes the Voyager communication
system. It is now more the 10 light hours from us (as far as I
know).

The DESCANSO web site (http://descanso.jpl.nasa.gov/) has a variety of
reports and books available for online/download/free use that discuss
a lot of this stuff. Pretty much all the missions JPL has done in the
past few decades have a "design and performance" report out there that
describes the radio system and how it works. The monograph series has
a bunch of books on some general aspect (like large antennas or
autonomous radios).

the IPN progress reports (which has had various names over the years)
is more a peer reviewed journal of record for various work in space
communication, and all of it is freely downloadable, too.


As far as I know, they don't equalize to correct for in band
dispersion (due to wave propagation). Maybe other people have better
info on this.


No compensation is made for dispersion.. the bandwidth of the signal
is so narrow compared to the carrier frequency that dispersion in
propagation isn't a factor. The bumps in the group delay
characteristics of the radio components are orders of magnitude
greater. To minimize that, we tend to use designs that keep the
signal in the nominal center of the IF passband.

Where dispersion becomes interesting (and is actually measured) is
when comparing signals at two separated frequencies, e.g. 8.4 GHz and
32 GHz. Or, for a closer to earth example, L1 & L2 for GPS (separated
by a couple hundred MHz at 1500 MHz) allow for compensation for
ionospheric delays.