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TV pass through on AM21

Discussion in 'DIRECTV HD DVR/Receiver Discussion' started by IDRick, Nov 24, 2008.

  1. IDRick

    IDRick Godfather

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    You make an excellent point VOS. I do have decent head room already. According to tvfool, the NM values for the channels I receive range from 37 to 52 db. I lose part of that due to an attic install but of course add back the gain of the antenna. Conservatively speaking, let's say I lose 50% due to an attic install. My lowest channel drops from 37 to 18.5 db due to attic install but increases to 30.5 after adding back antenna gain. Assuming I lose 4 db per 100 ft of cable run and 3 db for each 2-way splitters, my distribution losses are only 13 db (4 db cable run and 9 db for 1 splitter and 2 AM21's). Perhaps, I'm good to go right now... :grin: However, I do want to build in some overhead due to lower signal with snow on the roof or leaves on the trees in the summer. A small distribution amp may be a wiser choice for me than a pre-amp.
     
  2. RunnerFL

    RunnerFL Well-Known Member

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    I use 1 4 way splitter to feed my AM21, HR20 and 2 for the TV. Works great. I spent a few extra bucks and picked up a really nice splitter, not some cheapy you find at Radio Shack.

    I also put an amp inline.

    It works great.
     
  3. IDRick

    IDRick Godfather

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    Thanks for jumping in with your setup and experience! I see from your sig that you have two dvr's by each tv. If I'm reading correctly, you are doing exactly what I have originally proposed: ant -->4way split --> two runs to each tv. How long are your runs to each tv? Which amp did you purchase and did you tune the signal at location by adding antennuators? Thanks!
     
  4. rudeney

    rudeney Hall Of Fame

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    I use a Radio Shack antenna (http://www.radioshack.com/product/index.jsp?productId=2062075) that has an in-line amplifier. The feed comes into the attic, through its inline amp, then immediately splits five ways (three BR's, kitchen and LR). In the master BR, it's split again, one leg to the HR20, one to the TV. In the living room, it's a 3-way split - HR20, TV and A/V receiver for FM. All other rooms just use one feed to an HR20 or H20. based on the channels I can receive and their signal strengths from TVFOOL, I am getting everything with an -80db or greater power rating.
     
  5. IDRick

    IDRick Godfather

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    Just wanted to follow up and report some information about calculating distribution losses. I was incorrect in stating that a 2-way splitter cut the signal in half. The truth is, the signal level drops by 3 db on the output side of the splitter. For example, if the signal was 30 db going into the splitter, then the signal will be 27db in each output. Based on this information, I will be using three 2-way splitters in my antenna distribution system. According to tvfool, I have very high signal levels in my community and I may be able to distribute to both tv locations without a pre-amp or distribution amp. Should be a fun test this upcoming four day weekend. For those who may want to know more about distribution loss calculation, see URL below: http://www.starkelectronic.com/cmatv.htm
     
  6. IDRick

    IDRick Godfather

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    Good job Russ, you were correct! :)
     
  7. rudeney

    rudeney Hall Of Fame

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    I always counted each "leg" of a splitter as a 4db drop, which is about the same as 100ft of RG6.
     
  8. IDRick

    IDRick Godfather

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    Quoted numbers for a two-way splitter vary by site and probably vary depending on the actual splitter. Using 4db would be a good conservative approach.
     
  9. bakers12

    bakers12 ΔS > 0

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    Chicago -...
    AVS Forum might have information for you. One of their forums is all about local service, OTA or otherwise: AVS Forum.

    I didn't find much about Idaho, but I didn't know where in Idaho you are. I found Idaho Falls if that's any use.
     
  10. hasan

    hasan Well-Known Member

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    VOS's suggestion of an antenna mounted preamp is the best way to go with marginal signals. If you have plenty of signal at the TV end of the coax and then want to make up for splits, do the following:

    1. Purchase a Channel Master CM-7777 antenna mounted preamp.
    2. Put it on the main coax (at the TV end) going to the TV antenna
    3. Put as many splitters as you like on the OUTPUT of the preamp.

    Conventional RF distribution amps have a very poor noise figure and only make the signal weaker (in many cases) than it started out (in terms of signal to noise ratio).

    Using the low noise RF preamp of the CM-7777 makes it an ideal distribution amp. It maintains the noise figure (at least it doesn't make it worse than what exists at the end of your long coax run), thus making it the best possible solution short of putting it at the antenna where it will set your system noise figure to a very low (higher sensitivity) level.

    In simple terms, use the CM-7777. At the antenna it is the perfect solution. At the TV end, it is the best possible distribution amp to make up for splitter losses.

    Tip: If the amp you are going to use doesn't specify its noise figure (and if it does and is more than 4 dB), you can be pretty certain it stinks. People have gotten away with them, but they are a very poor choice.

    I have 8 splits off my output, and signals range between 100% and 83% on all channels on my AM21. I have a good signal to start with, and the CM-7777 as a distribution amp keeps it that way.
     
  11. hasan

    hasan Well-Known Member

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    Ogden, IA
    Loss on a good 2 way splitter is specified at 3.7 dB. The one's I have measured are in that range. 4 way splitters are just under 8 dB.

    3 dB is 1/2 power
    6 dB is 1/4 power
    10 dB is 1/10 power

    you can interpolate from there. A perfect 2 way splitter (there is no such thing) is 3 dB, practical ones are 3.7 dB as above.

    ...and all this talk about loss needs to consider that the quality of the signal is less a function of gain than system noise figure, which is controlled by three factors:

    1. All (coax/splitter) losses ahead of the TV or first RF amp (preamp)
    2. Preamp noise figure
    3. TV front end noise figure.

    (of course, the antenna itself establishes the amount of signal before getting to #1 above, hence there being no substitute for a good antenna in the first place)

    Ideal:

    ANT > short coax (5') Antenna Mounted Preamp > Main Coax Run > Splitters > TVs

    2nd choice:

    ANT > Main Coax Run> Preamp (same as above) > Power Inserter > Splitters > TVs

    (of course, install the power inserter properly and split off the TV side of the splitter, not the powered coax side)
     
  12. MartyS

    MartyS New Member

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    I did the same thing and it works like a charm.
     
  13. IDRick

    IDRick Godfather

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    Very interesting and informative post Hasan! I like it. The CM-7777 is highly recommended at the avsforums and high def forum. My concern is potential overload if I use it as pre-amp. NM from tvfool range between 37 and 56 for the six stations that I receive with my antenna. Two of them are so strong that I had 85% signal with the antenna laying on the ground! My two lowest NM stations are 50% on the tv signal meter when the antenna is mounted in the attic. Their signal rating will probably increase after I add the reflector to the DIY 4-bay antenna. ADD IN: I performed all my tests with a single tv connected to the antenna with a 25 ft 5.9 cable. I had not finetuned placement of the antenna nor added the reflector. Distribution losses would be very small with my test set up.

    I'm interested in your recommendations. Should I try the CM-7778 instead since it has lower gain than the CM-7777? Should I consider the Winegard HDP269 since it is designed for reducing potential overload issues? Do recommend a 4-way splitter in my case (2 runs to 2 tvs) or three 2-way splitters (split after pre-amp and split at each tv)? Thanks for your assistance!

    BTW, this thread does have relevance. I can't switch to D* until I have an OTA system in place. D* does not provide locals in my area and we are DMA 163 so it could be a long while before available from D*.
     
  14. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Hasan is a great source.
    You "levels" are really "RF levels" but a bit error rate, which can be more "enough or not enough".
    If you cable runs aren't that long [lossy] and your gain from the antenna is high enough, then it does sound like the lower gain pre-amp would be better. I don't think you can "swamp" the pre-amp, but simply run into it's limiting and with a [good] low noise figure, have plenty of signal to noise ratio.
     
  15. IDRick

    IDRick Godfather

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    I'm really a newb at this stuff. Haven't had a physics class since Nixon was President.... Yikes! From my reading on-line, it appears that one can overload the tv tuner if one uses too large a pre-amp in combination with high signal levels at the antenna. Classic symptoms of overloading includes signal level increasing when the antenna is pointed away from the tower and decreasing when pointed towards the tower. An overloaded tuner can totally lose the picture and have a 0 signal reading on the tv meter. I want the best I can have with an attic mounted antenna. :)
     
  16. bobnielsen

    bobnielsen Éminence grise

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    Bainbridge...
    I doubt that any of the new discoveries in physics have any bearing on what is happening here (LBJ was president when I took my last class) :)

    It's probably best to try it without any amplification and only add it if necessary. Some amplifiers have gain controls, which may help.
     
  17. IDRick

    IDRick Godfather

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  18. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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  19. IDRick

    IDRick Godfather

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  20. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    I've found a map to know just what is in my way. I think the last time I went to TVfools and fillout my address, they simply came back with some of these: :lol:
     

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