[linux-dvb] [Q]: is it possible to get different signal strengths for different transponders on the same sat?

Morgan Tørvolt morgan.torvolt at gmail.com
Sun Nov 26 16:41:44 CET 2006


On 25/11/06, manu <eallaud at yahoo.fr> wrote:
>         Hi all,
> I get VERY different signals strengths on transponders on the same sat
> (with TT S-1500), is it normal? If not what could be the diagnostic?
> Thx
> Manu
>

Hi Manu. I have been working with this some time. Not anymore though,
but I do remember a thing or two. Please excuse my nested parenthesis
and such.

I am not sure everyone can follow this either unfortunately, and most
people will not need to know.

There are quite a few possibilities for this. Firstly, I would expect
that the signal level is reduced at higher frequencies ( L band
frequency on the cable that is ), secondly, the satellite does have a
coverage-area and different "spots". If you are at the edge of one
spot, and dead center in another, you will of course get better
reception from the muxes from the one spot. Satellites usually
(always?) has different antennas for horizontal an vertical, even the
rotational stabilized ones like Thor 2 and 3 at 1W, so there could be
a difference between the polarities, and even completely different
spot coverage. If you are receiving a hemi beam ( usually means
covering half of the visible globe from the satellite (North/South or
East/West). Very expensive bandwidth..) you will get lower signal
level. If your antenna azimuth and elevation makes the signal
penetrate very much atmosphere (like in Norway, where the antenna must
be pointing more to the horizon than straight up), you will get a
frequency dependant attenuation. Also, the opening angle of the
antenna is wider at lower frequencies, so you will get the opposite
effect from that, but that effect is of course not dependent upon
elevation. On the other hand, if the antenna is not pointed correctly
(on large antennas, say more than 1.2meters you should also point your
antenna at the correct time of day, as the satellite wanders around in
an figure 8 on the sky, and two times a day it crosses center. If you
point in one of the extremes you will get bad reception most of the
day), the decreased opening angle will give you less gain in the
antenna at higher frequencies. There are still many more reasons for
problems also. A very common one is not having the polarization right.
If there is a very strong mux (or god forbid, an analog channel) on
the opposite pol, you will get good signal level, but bad signal/noise
ratio. Also, stray carriers from the thousands and thousands of
satellite links going over satellite sometimes causes problems, but
usually the providers notice and fix it. I have had arguments with
some that did not though... These are just some reasons. Wideband
transponders have different problems. AGC(automatic gain control)
transponders can also have their own problems.

So, to specify more on your problem:

Problem: High L band frequency problems
Answer: Get a shorter cable, signal amplifier or bigger dish.
Sometimes a new LNB will help, but rarely.

P: Fluctuating signal trough the day.
A: Repoint your antenna. Find some tool on the Internet to figure out
what time of day is the best for doing it. Jens Sætre (or Satre if you
want English) has some nice online tools for this here:
http://www.satellite-calculations.com/  Need to get 11 parameter
ephemeris data from the satellite operator though.

P: Random muxes have poor signal strength.
A: Check if the randomness is in accordance with the spot coverage
from your satellite.
ex:http://www.eutelsat.com/satellites/13ehb6_popd.html  You find links
to these on lyngsat. May also be caused by different signal strength
on different transponders. This is quite common. Sometimes, on
wideband transponders, one have to prioritize one mux over another if
you have two on one transponder. Sometimes also on small transponders
with low symbolrate muxes.

P: Still random signal strength, but also fluctuating on one mux
A: Fasten your antenna better, it is affected by the wind. If not, cut
down the tree that sways in to block the path :-)

There is always the possibility of hardware failure. Diseqcs can be a
real killer if they are poorly designed, or malfunctioning. Usually
there is a more logical explanation though. Give some more info, maybe
I can help out some more. More importantly, don't worry about signal
strength. What makes the signal quality is the signal/noise ratio (or
rather Eb/No, but most non professional equipment cannot measure that,
signal/noise ratio is a very reliable substitute unless you have noise
peaks or stray carriers in the mux you receive).

Regards
-Morgan-

PS. Sorry for spamming you Manu, I forgot to send this to the mailing list

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