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AM21 Sensitivity Reduced - Why?

Discussion in 'DIRECTV General Discussion' started by gcd0865, Jun 22, 2009.

  1. tigerwillow1

    tigerwillow1 Legend

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    Jan 26, 2009
    As an "enhancement" to this suggestion, this is the Winegard HDA100. Look also at the HDA200, only a few bucks more. In addition to more gain (and :-( higher noise figure), it has an adjustable attenuator. I used the HDA200 with my AM21 and found that it was just as bad receiving signals that were too strong as those that were too weak. The amp had to be adjusted "just right" for me to pull in the stronger and weaker stations at the same time. I've posted before about the AM21 being a "weak" receiver, with it not picking up at all stations that were solid on two other receivers split off of the same antenna. With a Dish DVR now using the same connection that the AM21 formerly was, OTA reception is much more reliable, and the gain setting on the amp doesn't make a bit of difference except for changing the reading on the signal meter.
     
  2. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Dec 9, 2006
    You bring up a good point [that I don't agree with :lol:]

    First: not all solutions will work in every situation.

    You can over amplify the signals for the strong stations as you're trying to boost the weak stations.
    Dynamic range of the tuner [any tuner] is what comes into play here.

    The HDA200 has a 4.5 dB NF and a max gain of 24 dB
    The HDA100 has a 3 dB NF and gain [fixed] of 15 dB

    So the question sort of comes down to how much gain do you need for the weak signals verses how much noise are you adding, thus reducing the noise margin, which is what this is all about.

    The best solution is using a larger antenna, as its gain doesn't add any noise.
    The next is to use a low noise amp, and add another amp if you need more gain for distribution losses [cable & splitters].

    The HDA200 gain ranges from 6 dB [max attenuation] to 24 dB.
    At its minimum gain, you've got 6 dB, but you're adding 4.5 dB NF
    Not the best solution is it?
    It would be better at about 16 dB of gain, as the noise margin would now be close to the HDA100, and it would then work better to the 24 dB output, "if" you needed that much gain.

    Now if I needed more gain than the HDA100, adding a http://www.solidsignal.com/pview.as...gain-(SKY38323)&c=Amplifiers&sku=610370581759

    Might be a better choice.
    The combined NF would be < 3.5 dB and the total gain would be ≈ 26-36 dB.
    Again not every solution is best for every situation.

    And if you're dealing with trying to find the sweet spot between the strongest and weakest signals, it might only need one of these:
    http://www.solidsignal.com/pview.as...ion-1296F&c=Signal Variators&sku=853748001293

    YMMV
     
  3. tuff bob

    tuff bob Icon

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    Mar 4, 2007
    That's real cheesy to use that as an exception. It a receiver and its got DirecTV written on it.
     
  4. jcwest

    jcwest Legend

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    May 3, 2006
    In Mobile Al. during the summer "storm season" we frequently have extremely large afternoon/evening storms passing by, hence the need for OTA backup to satellite locals.

    I got the AM21 right after I got my first HR21 box (2007 I think) our NBC channel is on the fringe of constant strong signal.
    Added the Channel Master 7777 signal amp and have never looked back.
    Signal via the AM21 has been rock solid every since.

    J C
     
  5. gov

    gov Legend

    1,101
    50
    Jan 11, 2013
    A rule of thumb from 'sparky school':

    Connect one TV to your antenna (no splitters, no amps) and the stations you get are the stations you are going to get when you're done. RF amps just let you drive more tuners (with appropriate splitters), they don't drag signal out of the noise.

    But, remember, for me 'sparky school' was before digital anything. LOL! Maybe ATSC is a little more forgiving ??
     
  6. Davenlr

    Davenlr Geek til I die

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    Sep 16, 2006
    Not actually true. By using a mast mounted preamp, any stations above the noise floor at the antenna terminals, will be present after the often rather high coax losses, at the TV. Without the preamp, stations you otherwise would be able to receive, might get lost in the attenuation of the coax. This is especially true when using RG59, or if the stations are on UHF band (which most ATSC stations are).

    You are correct in that the use of amps on the indoor end of the coax only serve to boost the signal enough to pass through splitters, but dont improve the incoming signal at all.
     
  7. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Dec 9, 2006
    Seems we all have our take on this.

    I don't see either as "wrong", but I still see more to what both are trying to say.

    The noise floor starts at about -106 dBm, so you need 15-16 dB SNR/CNR for ATSC.

    Your best [primary] gain comes for the antenna.
    From here the loss of the transmission lines drops signal, but not noise power.

    Using an amp generally defines the system NF, as loss/gain is linear, but cascaded NF is anything but linear, as you can find here: http://www1.sphere.ne.jp/i-lab/ilab/tool/NF_e.htm
     
  8. bflora

    bflora Legend

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    Nov 5, 2007
    Amps never improve the S/N ratio at any given point in the line but they can help preserve S/N ratio 'downstream'.
     
  9. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Dec 9, 2006
    This is true, and the only thing you can play with is what is "upstream" and where downstream starts.
     

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