On 25 mar, 15:52, "Szczepan Bialek" wrote:
*"Wimpie" napisal w ...
On 25 mar, 10:17, "Szczepan Bialek" wrote:





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).

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.

They used the "Ultrastable oscillator".
They measure the ions and electrons density in the interstellar medium. So
the band dispersion is obvious and the only remedy is the "Ultrastable
oscillator".

Sorry, I don't see the link between "ultrastable oscillator" and
dispersion. Good oscillators in both transmitting and receiving
equipment are required to get the lowest possible Eb/No for your
digital modulation scheme.


So AM and FM are quite opposite. *It seems to me that FM is not the best for
the long distances.

It seems to me that here is a confirmation of that: ""Because of the low
signal-to-noise ratio, as with amateur-radio practice,
EME signals can generally only be detected using narrow-band receiving
systems. This means that the only aspect of the TV signal that could be
detected is the field scan modulation (AM vision carrier). FM broadcast
signals also feature wide frequency modulation, hence EME reception is
generally not possible. There are no published records of VHF/UHF EME
amateur radio contacts using FM." From:http://en.wikipedia.org/wiki/TV_and_FM_DX

I do not understand this but guess.


Low bit rates, result in low required power, so it is true that with
the advent of low bitrate digital schemes, EME is within the reach of
more radio amateurs. As these modes use narrow bandwidth, dispersion
effects are negligible when looking to the demodulation of the
received signal (as far as I know). Even amateurs using satellite
communication with analog voice, don't have to modify their
demodulators.

Wim
PA3DJS