Strange OTA antenna behavior

Discussion in 'DIRECTV Installation/MDU Discussion' started by RxMan1, Mar 17, 2016.

  1. RxMan1

    RxMan1 Godfather

    352
    0
    Jun 1, 2005
    I have two AM21s attached to two receivers. I have a large antenna in my attic which feeds these. The coax comes into an equipment room and then splits and goes another 6 feet to each am21.

    I get 95-100% on every channel except one. In my case that channel is NBC.

    Here is the strange part. At times I will have a signal of 90% and above then out of the blue it will drop to 7% or even zero%. I have learned if I disconnect the coax where it comes into closet and then reconnect I will once again get a decent signal. This strange behavior is only on this channel. All others remain fine even when this one drops out.

    Any ideas?
     
  2. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

    42,754
    378
    Dec 9, 2006
    I have a bunch [which may be useless :lol: ]
    You might have a reflection that is canceling the NBC signal.

    On another forum was a 2 TV setup that only worked when both TVs were tuned to the same channel.
    It looks to have been a bad match at one of the TVs causing the signal to be reflected back up the coax.
    The solution was to change the splitter for a directional coupler [tap] so the reflected signal ended up in a load and not canceling the forward signal.

    This might not be your case but "playing with" cabling and loads might give you some ideas.
     
  3. Jim148

    Jim148 Godfather

    375
    7
    Jun 22, 2004
    Well, the first idea would be to mount the antenna outside. Of course, I say that knowing that you probably mounted it in the attic for a reason. However, I, too, have an antenna in the attic and know what you are going through. If possible, trying moving the antenna a very short distance. And when I say short distance, I mean literally a few inches. It could be up or down, right or left, forward or backwards. There is no practical way to know what will work until you try it.

    Also, this is classic MPDI (Multi Path Distortion Interference). Like the other person said, it is some signals canceling other. Some receiver handle MPDI better than others. If moving it slightly doesn't work, consider trying a variable attenuator. Ironically, sometimes "less is more" when it comes to signal. So, attenuating the signal can actually improved reception sometimes!
     
  4. DoyleS

    DoyleS Icon

    1,029
    8
    Oct 21, 2002
    El Dorado...
    With limited information it is hard to give any help.
    Can you state your location so we can see what the map is for channels in your area. It could be as simple as a slight change in pointing direction if NBC is on a different tower than the other stations.
    Removing the splitter and seeing if the problem still occurs on a single AM21 could be another possibility indicating a problem with the splitter.
     
    1 person likes this.
  5. NR4P

    NR4P Dad

    6,565
    334
    Jan 15, 2007
    Sunny Florida
    In addition to VOS' comment, my first inclination is multipath. Inside antennas have that problem often.
    Point the antenna in a different direction may help. Even just 10 degrees. But as also pointed out here, get the antenna outside and probably will eliminate the issue.
     
  6. carl6

    carl6 Moderator Staff Member DBSTalk Club

    12,816
    1,106
    Nov 15, 2005
    Seattle, WA
    I'll agree with the multipath suggestion. With analog, that would result in blurred images, ghosting, etc. With digital, it basically just kills the signal, there isn't enough forward error correction to overcome the errors. The other thing to consider is the "signal levels" on DirecTV boxes are not true RF level indicators, but are more along the lines of an expression of bit error problems. That further supports the likelihood of multipath.
     
  7. RxMan1

    RxMan1 Godfather

    352
    0
    Jun 1, 2005
    Thank you for all of the replies. I live at zip code 79407.
    I have moved my antenna from several inches to several feet. The signal used to be high 70s until its most recent position which gives 90s. That is until it drops completely out.
    I do think I have some sort of multipath problem. Sometimes the satellite signal (not ota) will pixelate at very consistent intervals. Unplugging the swim and then plugging back in corrects this.
    I looked up the directional coupler but need to read more. If this replaces the splitter does one coax get connected to the tap connection?
    Is this something that can be purchased at Radio Shack?

    Thanks again
     
  8. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

    42,754
    378
    Dec 9, 2006
  9. RxMan1

    RxMan1 Godfather

    352
    0
    Jun 1, 2005
    Ha. Not very many Rat Shacks left.
    Sorry for my ignorance. I simply pick one of the AM21s to be connected to the tap port?
     
  10. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

    42,754
    378
    Dec 9, 2006
    I wouldn't call it "ignorance" is it should only be not knowing strange stuff.

    The couple works where the through port [out] is almost 3 dB lower and the "tapped" port is 6±1.5dB.

    Unlike a "splitter" that has equal outputs, the couple doesn't "but" also has an internal termination for signals going backwards through the coupler

    [​IMG]
     
  11. RxMan1

    RxMan1 Godfather

    352
    0
    Jun 1, 2005
    OK. That makes sense. So I simply hook it up like this and see what happens?


    [​IMG]
     
  12. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

    42,754
    378
    Dec 9, 2006
    "Yep"
     
  13. RxMan1

    RxMan1 Godfather

    352
    0
    Jun 1, 2005
    A local electronics shop has a 4 port tap. In and Out plus 4 tap ports.
    Would this work or be useless in this application?

    Thanks once again.
     
  14. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

    42,754
    378
    Dec 9, 2006
    It might NOT be what you want.

    "The thing is"....
    For a 4 port tap to be of any use, the coupled ports have to have twice the loss.

    The tap I linked to was a 6 dB tap.
    The same folks make a 4 port "but" it's a 12 dB tap: http://www.skywalker.com/Products/Skywalker-Signature-Series-SWQ12-Quad-4-way-Tap-12db__SKY2342-12.aspx
     
  15. NR4P

    NR4P Dad

    6,565
    334
    Jan 15, 2007
    Sunny Florida
    Did the signal drop out completely when it was 70%?
     
  16. alnielsen

    alnielsen Godfather

    748
    24
    Dec 31, 2006
    ...- .....
    Maybe the origination signal and it's reflection is too strong. A dB drop may cut the reflection to a point where it isn't causing a problem anymore. It's kind of a trial to see what will work best.
     
  17. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

    42,754
    378
    Dec 9, 2006
    there's a whole bunch of maybes and yes it will take several/many "try and sees".
    Unless the towers are fairly close, "too much" signal seems like a rare case.

    Why I started with reflections "in the coax" is because disconnecting and reconnecting the coax seems to have an affect.
    Multi-path shouldn't react this way.
     
  18. NR4P

    NR4P Dad

    6,565
    334
    Jan 15, 2007
    Sunny Florida
    Very good point. Wouldn't the signal have to be very strong like within a few miles of the towers for reflections to act that significantly?
     
  19. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

    42,754
    378
    Dec 9, 2006
    Let's not mix things up.
    There's two things here:
    #1 too much power where the tuner saturates [attenuation/padding helps this]
    #2 Multipath [reflections] which aren't power related as they can be weak signals "but" be from two different paths to the antenna. This could be a direct signal from the tower [far away] and another reflected off a building [or anything else] that has a slightly different path to the antenna and as such reaches the antenna with a different phase. As these two combine one degrades the other. Moving the antenna can degrade one of the signals and thus have less problems with the two, since the delta between the two becomes greater.

    So have I got everyone lost by now?
    !rolling
     
  20. RxMan1

    RxMan1 Godfather

    352
    0
    Jun 1, 2005
    I replaced the splitter with a tap yesterday afternoon.
    So far so good. Time will tell if this is a permanent fix.
     

Share This Page

spam firewall

Advertisements