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Trying to sort out rain fade issues - transponder map anywhere?

Discussion in 'DIRECTV General Discussion' started by carillon, Jul 6, 2013.

  1. P Smith

    P Smith Mr. FixAnything

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    let me ride to the point ...
    [​IMG]
    so, decreasing Vc (carrier signal) while have same noise floor will produce much lover CNR value

    we just can't get same CNR if signal drops
     
  2. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    By "we" you must mean the receiver.

    The CNR is still on the coax, but it's the receiver that is reducing it's detection.

    A CNR of 10 dB is the same whether it's -30dBm /-40 dBm, -60 dBm /-70 dBm, or -70 dBm / -80 dBm..
     
  3. P Smith

    P Smith Mr. FixAnything

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    well, I can't see how the noise floor will float with a weather condition ...
    say you have SS=90 (on screen) - could you give the CNR/noise numbers for non-SWiM setup, then say a storm is came - what would be SS, CNR and noise figures ?
     
  4. slice1900

    slice1900 Well-Known Member

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    Is that noise coming from the LNB or is it simply present in the environment? (i.e. some combination of spillover from other satellites, cosmic background radiation and whatever else)

    If it is the result of compromises in the design of the LNB to keep the cost low, one would think someone would have at some point built a "better" LNB for those customers who might consider buying a $500-$1000 LNB (MDUs, sports bars) if it gave them significantly better resistance to rain fade.

    Dunno how much it would need reduce noise to go that extra mile to eliminate rain fade entirely. Given that even a 2 meter C band dish isn't completely immune to rain fade, it would need to reduce the noise floor by more than the ~10db gain you'd have if it were possible to use a 2 meter dish with a Directv LNB. Maybe 15 to 20 db? That's probably not even remotely possible, or even if it is such an LNB might cost far more to make than anyone would ever want to pay.
     
  5. HoTat2

    HoTat2 Hall Of Fame

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    Back when I was in college studying Radcom systems, I was always trained that the all important "C/N" ratio (actually C+N+I/N+I ratio, with the "I" for "interference") was determined at the feedhorn input comprising the strength of the received satellite compared to the noise and interference received from the external environment of space and earth along with the antenna noise generated by the dish itself.

    The C/N was then degraded somewhat by the receiver system's overall noise figure, primarily determined by the noise contribution of first amplifier in the LNB.

    But beyond that I was taught it was like VOS stated, the C/N was essentially constant after the LNB regardless of actual RF signal level. However the receiver naturally has a minimum signal sensitivity level (typically less than -65 dbm for DIRECTV receivers I understand) where even with an acceptable C/N the receiver can't respond to it.

    However, in deference to P Smith's position, if I was taught wrong on this issue, I'm more than willing to change my thinking here even at this latter stage in my life. :)
     
  6. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    First off, there looks to be "noise" on the SAT signal.
    A TP can measure -20 dBm and have a 11 dB CNR so the noise power is -31 dBm.
    You can cover the LNB and have -40 dBm of noise.
    "If" the LNB was the only source of noise, and the signal was -20 dBm, you'd have a 20 dB CNR [-20/-40] and we don't.

    To make "a better mousetrap" it looks like the LNA, on the input of the LNB, would need to have more gain, but this would bite you in the butt on a good day with a higher output.

    If the LNA was a log amp, this might work by limiting the max output while improving the gain at low levels.
    "The problem is", there may not be a large enough market for this change in design..
     
  7. peds48

    peds48 Genius. DBSTalk Club

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    I will provide a picture once I have it
     
  8. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    With the max input of the tuner chip spec'd at -20 dBm, a SWiMLNB with -21 dBm output is getting close.
     
  9. slice1900

    slice1900 Well-Known Member

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    Well, if the -20 dbm spec is sort of a worst case spec, where some receivers may accept -15 dbm or perhaps even less, and the -21 dbm output is a worst case overly powerful LNB with the spec trying to hit let's say -25 dbm or so, I don't think its a real problem.

    How many people will have a single receiver directly connected to a SMW LNB over a very short cable run with no splitters at all where the potential for overlap between an overly powerful LNB and a receiver with no headroom over the specified ability to handle strong signals becomes a problem? If there were a dozen potentially affected customers I doubt Directv's engineers would be overly concerned, versus the number of people with overly long runs or too many splitters that would be out of spec if the LNBs conservatively output @ -30 dbm who get brought into spec by padding the LNB power a bit over what a SWM multiswitch outputs.
     
  10. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    I think the more you get to know what & how DirecTV engineers do things, you might find that they wouldn't design something that would knowingly affect a customer.

    Over driving the tuner chip isn't something any one wants, since it causes distortion.
    This is a real problem and why the bandstop filter is needed with a receiver without a DECA, that's connected to a coax that has DECA, even though it's out of band for the tuner.
     
  11. Diana C

    Diana C Hall Of Fame DBSTalk Club

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    People trying to aim their dish, or people tailgaiting, or people with RVs.

    I'm just saying... ;)
     
  12. peds48

    peds48 Genius. DBSTalk Club

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    as promised
     

    Attached Files:

  13. HoTat2

    HoTat2 Hall Of Fame

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    Just curious peds48;

    Was this reading taken with the SWiMLNB in alignment or "peaking" mode, without any receivers connected. In which case the SWiM's internal AGC is disabled?

    Or the SWiMLNB's output when receivers were connected, in which case the AGC circuit was enabled?
     
  14. peds48

    peds48 Genius. DBSTalk Club

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    just AIM to SWM LNB in peaking mode
     
  15. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    TP 25 looks to be a spot beam if the TP list is correct:

    25

    D8 @101W

    101

    024

    0028

    54

    Denver 59 ION (Virt)

    What do you get for other TPs off 101 that are CONUS?
     
  16. HoTat2

    HoTat2 Hall Of Fame

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    Nah ...

    Its a CONUS beam one VOS;

    Spotbeams tps, at 101 are only even numbered and are at 4, 12, 18, 20, 26, and 28.

    However, since that is just the output in peaking mode, isn't it likely that the AGC will still limit the output to -30 dbm or below for normal operation when the receivers are connected?
     
  17. peds48

    peds48 Genius. DBSTalk Club

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    Did not install any DirecTV today, just 4 "wildblues" will post more pictures next time I do DirecTV. looks like tomorrow will be another wildblue day
     

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