On Mar 25, 7:52*am, "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".

The USO has nothing to do with the dispersion. It's just something
that lets you make the measurement, because you can integrate over
100s of seconds, and know that the signal being transmitted didn't
change (much) during that time, so any changes are due to the
propagation.


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 don't know that I'd use a wikipedia article as my primary reference
on this particular topic.

The reason hams don't do EME FM or EME TV is more a matter of link
budgets than propagation characteristics. amateurs are limited in the
maximum output power (e.g. 1500W PEP in the U.S.), and particularly if
they're using amateur antennas (no 300m dish at Arecibo), you don't
have enough radiated power density to send a wideband signal and have
it arrive back at earth with decent SNR. So, hams use narrow band
transmissions and narrow band receivers (so that they get the minimum
noise power in the detection bandwidth). (also, there's a cultural
thing.. the hams that are interested in TV aren't the same hams
interested in moonbounce, so there's not necessarily and motivation to
even try what would be a fairly difficult and expensive stunt)

If you're not limited by amateur budgets or regulatory power limits,
there's no reason you couldn't send FM or TV to the moon and back.
Apollo used FM in their Unified S-band system (granted, one way from
the moon, not a moonbounce). I haven't run the numbers, but maybe if
you had a big enough transmitter, and used something like Arecibo, you
could do moonbounce tv.

We send several MHz bandwidth back from Mars with a 100W transmitter,
a 3 meter dish, and receive it with a 34 meter dish on earth at
greater than 0dB SNR.

A big advantage of AM is that it has simple and cheap transmitters (a
modulated amplifier) and receivers (a diode). If you're spending
billions of dollars to send a probe to Mars, saving (relatively) small
amounts on the radio isn't worth it, especially if you have to spend
large amounts more on solar panels to get the additional DC power
needed.

That said, for short range links in space, FM, FSK, and AM are
considered as viable alternatives, especially where you are looking
for very small, very low mass. The Muses CN rover (which was about
the size of a pack of cards and was going to drive around an
asteroid's surface) was proposed to use a FSK data link, as I recall.
AM for astronaut backup voice comm is also a possibility.