Power cord causing antenna interference

Power cord causing antenna interference


This is a discussion on Power cord causing antenna interference within the DTV | HDTV Reception and Antenna Discussion forums, part of the Over-the-Air (Antenna TV) category.

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  1. #1
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    Power cord causing antenna interference

    We have a low VHF channel (channel 3) and have discovered that if we have our computer power cord plugged into our laptop, it makes the channel unwatchable. Unplug it from the computer, and the channel goes right back to perfect. Any ideas?

    By the way, thanks to everyone for helping getting our antenna system working. We have ditched our $70/month satellite and are enjoying our free tv!! Our setup is a low vhf antenna on the roof, cm 8 bay in the attic, both plugged into a cm 7777 preamp, and then split to two televisions. We have also tried plugging the computer into different outlets, and no matter where we plug it in we get the interference on both tvs.

  2. #2
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    mtb,

    That really shouldn't happen. I think it probably is directly related to the Power Cord it's self.
    Might try another (replacement} Cord, and see if it doesn't effect things as the first does.
    I'd check polarity on the first cord, sounds like it may be reversed.
    If that's the case, you can replace the Plug, straightening out the crossed/reversed connection.


    Have a good Day !
    S.W.

  3. #3
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    Living in WBRA's coverage area, I know how much of a pain in the neck their signal is. Every little thing interferes with it, especially random electronics. The management can't seem to be convinced to move it to UHF like they should, claiming that low-VHF is just wonderful. They even want to program Mobile DTV on it, so people with 7-foot long antennas on their cell phones can enjoy it I guess.

    (Honestly, I used to watch PBS all the time, but now watch pretty much none because I'm simply tired of dealing with WBRA. I still watch PBS when I'm at school in Charlottesville though; I love WHTJ.)

    But I agree with SWHouston, we have plenty of computers and none of them interfere with the WBRA signal, it's usually things with electric motors like vacuum cleaners, blenders, etc. How old is this laptop power cord? There could be something wrong with it and it could be causing interference on its way to dying.

    - Trip
    Last edited by Trip; 11-23-2010 at 04:38 AM.
    N4MJC

    Comments are my own and not that of the FCC (my employer) or anyone else.

    RabbitEars

    "Ignorance and prejudice and fear walk hand in hand..." - Rush "Witch Hunt"

  4. #4
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    It's not the cord, but the power supply on the cord. Check into a replacement, also see if you can borrow a laptop to see if it causes interference too.

    Computers, their power supplies and peripherals can cause a lot of interference. One of the first "useful" things ever done with a personal computer was to use this interference to make "music".

  5. #5
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    Welcome back mtmb,

    Although not directly associated to your interference issue, my desktop computer throws out all sorts of RF intereference that makes it very hard to receive most AM radio stations inside my home. If you have a hand-held AM 'transistor-radio' it can be used like a bloodhound dog to pinpoint where the 'noise' is coming from. Simply tune the radio off-channel and walk it around like a divining rod looking for water.

    Odds are, the power supply (wall wart) is a 2-prong non-grounded version in a plastic case and that's where the interference is coming from. You may be able to find a replacement wall wart online, but I would prefer to adapt something like this: http://cgi.ebay.com/Micronta-22-120A...ly-/2005219970

    I don't know what your laptop needs for power, but this 'box' could be internally altered to provide a lower voltage if necessary. I have an identical Radio Shack Micronta power supply and although it uses a 2-wire power-plug, it does not toss RF interference into the air.

    Jim

  6. #6
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    Channel 3 issues and laptops !!

    Quote Originally Posted by mommytomyboys View Post
    We have a low VHF channel (channel 3) and have discovered that if we have our computer power cord plugged into our laptop, it makes the channel unwatchable. Unplug it from the computer, and the channel goes right back to perfect. Any ideas?

    By the way, thanks to everyone for helping getting our antenna system working. We have ditched our $70/month satellite and are enjoying our free tv!! Our setup is a low vhf antenna on the roof, cm 8 bay in the attic, both plugged into a cm 7777 preamp, and then split to two televisions. We have also tried plugging the computer into different outlets, and no matter where we plug it in we get the interference on both tvs.
    You may have to replace the power supply. It may have a bad solder joint that could be the culprit. All electronic devices emit some type of electro-magnetic field when in operation, and this ends up causing interference to surrounding devices. Try this experiment. Move the computer as far away as possible, and then wrap the power supply in several wraps of Aluminum Foil and see if the signal comes back.

    I have seen this very same problem actually on channel 3, and also on UHF when my Dell Inspiron Laptop (The very same one I use for signal strength tests) would cause me to loose all channels on an Emerson 19" LCD TV plugged into the same outlet as the DTV set. If I moved the laptop power supply to a different outlet, all would then be fine.

    You may also try keeping the cords on the power supply cables from being extended to full length, as the length of the power cord could be contributing to the problem by radiating on a certain frequency due to its length and the frequency of the electro-magnetic field being generated by the laptop power supply.

    This actually shows the lack of RF filtering and shielding in the computer supply and with the DTV receiver itself.
    WE ARE NOT SHEEPLE !!

  7. #7
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    Quote Originally Posted by Jim In Seattle View Post
    I have an identical Radio Shack Micronta power supply and although it uses a 2-wire power-plug, it does not toss RF interference into the air.

    Jim
    And the Radio Shack Micronta power supply was most likely made when FCC part 15 was actually observed by most manufacturers !! A lot of laptops use 19 volts or so, and the actual electronics of the power supply are in the middle of the cords, giving it a chance to act just like a transmitter going into an antenna.

    Hey, that's a problem when you have an RF generating device that has two long conductors attached to each end, with one carrying AC power and the other carrying DC power. That kinda sounds a little like a radio transmitter to me !!

    It would be interesting to use an RF spectrum analyzer to see exactly what frequencies this thing is spouting out !! Switching type computer power supplies are notorious RF generators, and the frequencies can be all over the spectrum with this type of supply. A lot of these cheap Chinese electronics don't even submit for FCC part 15 testing any more.
    Last edited by FOX TV; 11-23-2010 at 06:54 AM.
    WE ARE NOT SHEEPLE !!

  8. #8
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    Quote Originally Posted by mommytomyboys View Post
    We have a low VHF channel (channel 3) and have discovered that if we have our computer power cord plugged into our laptop, it makes the channel unwatchable. Unplug it from the computer, and the channel goes right back to perfect. Any ideas?
    I'd try a common mode choke on both power cords (assuming that there are two) where they connect to the power supply. Suitable chokes can often be salvaged from an old VGA monitor cable.

    The short version: http://www.thiecom.de/pdf/MFJ-701.pdf

    The verbose version: http://www.yccc.org/Articles/W1HIS/C...S2006Apr06.pdf

    You can also wrap aluminum foil around the case of the power supply.
    Last edited by Tower Guy; 11-23-2010 at 07:16 AM.

 

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