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Rain Fade, Has anyone Tried this setup ?

Discussion in 'DIRECTV Installation/MDU Discussion' started by jimmie57, Aug 11, 2013.

  1. jimmie57

    jimmie57 Hall Of Fame

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    Hi,
    South of Houston , TX and had lots of signal loss due to rain today. I had a wild idea and since I am not an electrical guy I have no way of knowing if this would work or not.

    Here you go:
    I have a Slimline 3 SWM dish. What if there were 2 dishes and feed them into a splitter / combiner ? This would result with both signals combining into the one coax that goes into the house.

    Would they gather the signal from each and add strength to the signal ?

    When it was not raining would it overload the system with too much signal ?

    Would one 29 volt power inserter run the both of them ?
     
  2. peds48

    peds48 Genius. DBSTalk Club

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    Since SWM is based on individual "channels" it would not work
     
  3. BobStokesbary

    BobStokesbary Legend

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    +1
     
  4. jimmie57

    jimmie57 Hall Of Fame

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    OK,
    So, how about doing it with the older LNBs with the 4 cables ?
     
  5. Scott in FL

    Scott in FL Godfather

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    Combining receive antennas does work, but you can't do it with a Slimline antenna with multiple, switched LNB's.

    I worked on a project about 20 years ago using an Intelsat antenna located in Australia. It was a C-Band, circularly polarized antenna which actually consisted of two 11 meter antennas (I think... it's been a long time). LNA's were used, not LNB's. I wish I could remember how the signals were combined; I vaguely remember waveguide bringing the output of each antenna feed down to the base, and the signals were combined before the LNA.

    No matter, the signals were combined, with the result that the G/T of the two antennas was about 3 dB higher than a single antenna. I had never heard of anything like that before, and went to my boss to confirm (which he did).

    So, perhaps an interesting tidbit, but no practical use to us. Just thought I'd mention that two receive antennas can, and have been, combined to increase their "size."
     
  6. kenglish

    kenglish Icon

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    Each LNB has it's own local oscillator, so they would never be on the exact same frequency.
    You could go "NASA Style" and use a common LO, but it's probably easier (and cheaper) to just use a much bigger dish, Something like a three or four foot FTA dish would add some headroom to the signal.
    Of course, there will still be times that the signal is completely blocked by extreme weather.
    (Our satellite uplink truck had that kinda problem a couple of weeks ago, They could not make enough power to "punch through" a thunderstorm, and had to just wait for it to move.)
     
  7. jimmie57

    jimmie57 Hall Of Fame

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    Thanks for the answers.

    This idea is just one of the ones that you think it is so easy and it just does not work.
    I remember once many years ago I had a jib crane installed where I worked. It worked well if it was pointing straight out in front of the building support. If it was swung to the side it twisted the building support with a small load being lifted. I then had a bracket made and tied that support to the one right behind it ( support for an addition we made to the building and took out the sheet metal wall between them ) and that would double it's strength.
    Nope, seemed to twist the same amount. It was what is termed "an exercise in futility".
     
  8. slice1900

    slice1900 Well-Known Member

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    Even if this were possible, the extra dish would at best give you double the signal strength, or 3 db. When rain fade knocks out your signal, it drops by at least 30 to 40 db. At best, a 3 db gain from a second dish would add a few seconds of viewing time before rain fade knocked you out, and have your picture return a few seconds earlier.

    You'd need to cover half your roof with dishes to have even a chance at beating rain fade with this strategy. That's basically what those big arrays of radio telescopes are for (like the ones Jodie Foster used to find the aliens in Contact) Except instead of pulling in signals made weak by rain, they're trying to pull in signals that are weak because of the extreme distance they've traveled to get here.
     
  9. HoTat2

    HoTat2 Hall Of Fame

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    Yeah ...

    "Baseline interferometry" I think is the technical terminology.
     
  10. Scott in FL

    Scott in FL Godfather

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    Rain attenuation can be as little as 3 dB, or even less. The other night I watched a movie on the Sony (HD) movie channel at the same time thunderstorms were in our area. The result was minor pixelation for about 10 minutes. The receiver never lost lock. An extra 3 dB would have made a difference that night.

    A 6 dB fade could cause a receiver to lose lock.

    The bottom line is, combining two antennas for an extra 3 dB of gain won't work with switched LNB's. The system in Australia I mentioned was built in the days when low noise amplification was difficult to achieve. Back in the 70's, a less than 100° K LNA for C-Band was very expensive; now you can buy a 20° K LNA off the shelf.
     
  11. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Changing to the 1.2 meter dish might give you a 6 dB boost.
    This won't eliminate rainfade but may "hold on" longer before you lose the minimum level.

    As mentioned, the LO in each LNB is going to be the show stopper for combining.
    Each would need to be phase locked and then the feed lines would also need to be phase matched to the combining point.
     
  12. slice1900

    slice1900 Well-Known Member

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    You are confusing SNR and signal strength. Rain fade isn't introducing noise, it is attenuating your signal strength - attentuating both the signal and the noise (at least the noise your dish receives, the noise that originates within the LNB is another matter) If you think about a legacy (non-SWM) LNB, it might receive the signal at -20 to -25 db, and a receiver might be able to show a picture as low as -60 db. The only way you got rain fade back in those days if you had a pretty short coax run was due to a pretty massive drop in signal strength due to the rain attenuating the signal. In a SWM system you may have even a bit more headroom depending on circumstances.

    If a storm rolls through and causes pixellation, it is dropping your signal strength far more than 3 db. In fact, it is even dropping your SNR by more than 3 db. Someone recently posted a graph of SNR/CNR to the numbers on Directv's "signal strength" screen. 13 db or so was "100", down to 10 was still mid 80s I believe - where you will clearly still have a perfect picture. You need to drop a lot lower (50s? 40s?) before you start noticing any problems with your picture.
     
  13. Scott in FL

    Scott in FL Godfather

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    Rain does introduce noise. In addition to causing attenuation, rain increases the downlink system noise temperature. The signal-to-noise ratio (or carrier-to-noise ratio) at the output of an LNA or LNB is a measure of the desired signal compared to the undesired noise. The noise consists mainly of the antenna noise and the LNA or LNB noise temperature. At an elevation angle of 40°, a 1.2 meter Ka-band antenna has a noise temp of about 90° K. The LNB will contribute about 110° K. Ignoring the contribution of uplink thermal noise and interference, the system noise will be about 200° K. The antenna temperature is the integrated sky temperature weighted by the antenna gain. At a high angle of elevation, the clear sky temperature is low since the antenna looks at cold space. However, the temperature of liquid water is about 300° K. Thus the rain increases the sky temperature. Therefore, the noise admitted to the earth station receive antenna increases and causes further signal degradation.

    The legacy LNB example you mentioned used the term "db" as an absolute value. dB is not an absolute value; it is a relative value, used to compare two values. dBm is an absolute value. I can not comment on your example because link budgets do not use dBm in their calculations in the uplink or downlink path. However, the length of cable after the LNB (legacy or with an SWM) is largely insignificant because the system noise temperature is set by the LNB and antenna noise. True, the cable length can not be so long that the signal into the receiver is too low, and an SWM will help keep the signal into a receiver from falling below the demodulator's minimum level (as VOS has explained well in the past), but this is of limited help when rain attenuation occurs.

    Finally, pixelation can be caused by a signal strength drop of 3 dB or less. This, of course, depends on how close the demodulator is to losing lock at the time of the signal strength drop. If the demodulator is operating close to its threshold, due to a storm and/or poor antenna pointing, an additional drop of less than 1 dB can cause the video and audio to degrade. Degradations occur when errors are introduced into the demodulated signal, before the decoder. Assuming an HD Ka-band signal, a DVB-S2 demodulator's PER to Es/No graph is extremely steep, due to the powerful error correction characteristics of LDPC inner and BCH outer coding. For 8PSK, 3/4 rate DVB-S2, an increase in PER from 1E-7 to 1E-3 occurs with a drop in Es/No of only 0.1 dB. That is certainly enough to cause pixelation. View attachment 23423

    I agree that a typical rain fade is greater than 3 dB, and if a 3 dB fade causes a demodulator loss of lock or pixelation there is not enough system margin. However, my original post said an additional 3 dB of antenna G/T could be helpful during a rain fade. My Sony movie channel example is valid. Assuming my antenna is pointed correctly (it is), the storms in our area that night caused the C/N, and therefore Es/No, to drop close to the demodulator's threshold. It was operating on the "hairy edge." Additional fading of less than 3 dB caused piexelation. An extra 3 dB of system margin -- C/N or Es/No -- would have helped that night.

    The transponder readouts on your Directv receiver indicate PER, not signal strength (note: Packet Error Rate, or PER, not BER is the correct term for DVB-S2, because DVB-S2 frames carry packets. Likewise, Es/No (symbol), not Eb/No, is the correct term of measurement for DVB-S2). I take my Directv receiver readouts with a grain of salt.
     

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