[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 20:09:37 CET 2006

On 26/11/06, manu <eallaud at yahoo.fr> wrote:
> Le 26.11.2006 10:59:48, Morgan Tørvolt a écrit:
> > 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-
> >
> Well many thanks for the detailed explanations. I still need to dig

You are most welcome. As I do have some experience with this, though
mainly with antennas of 5-18m in diameter, I am available for
questions. I am not contributing alot in other areas on the
mailing-list, so feel free to bug me with anything I can help with =)

> through them a bit more but at least I can give you this new detail: I
> Observed that the transponders with good signal (SNR does not vary
> much, but it could be that it is not really measured) are the vertical
> ones, the bad ones are horizontal. So it could be, as you said, a

The 18V could be a cause of problems if you have a very long cable to
your LNB. If the voltage drops too low, it could fail to keep
horizontal, and with results as you describe. It does of course depend
largely on the LNB also. I bought a new one for some friends of mine
because the el-cheapo LNB they bought did not like the ~25meter cable
lenght and the probably somewhat weak power from the tuner of theirs.

> reception problem, but it could also be either a driver problem badly
> setting things for horizontal transponders, or the LNB does not like

Is there difference in the drivers other than 18V/13V when changing

> the voltage my TT S-1500 sends to it (I think it is 18V for horizontal).

I also have a TT-1500, and have had some problems with diseqc. It
seems not to actually send the commands sometimes, but I am uncertain
about how things work here. We have distributed satellite signal here,
and I am connected to a diseqc that I unfortunately have no control
on. The guy responsible for the shit have no clue eighter. Do you know
of a way of checking if the diseqc command was actually accepted? Will
I get a diseqc failed message? I guess I will open another "thread"
with this if i don't manage to sort it out.

> I will go on checking things and let you know if somethings turns up.
> Thx
> Emmanuel


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