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Talkative Antennas

Discussion in 'Electronic Repair' started by Ron Hubbard, Nov 6, 2003.

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  1. Ron Hubbard

    Ron Hubbard Guest

    With the glut of sci-fi shows on at the same time, I've set up
    two VCRs and two antennas in a single room. I found that the
    two antennas affect each other: by changing the settings on one
    antenna it will change the picture quality on the other VCR and
    TV.

    I always thought antennas were passive devices and didn't
    radiate any signals. Does anyone know of a way to reduce the
    interference between the two antennas?

    Thanks.
     
  2. However, if you place one antennae in direct line between another and the
    signal source they will affect the quality of the received signal at the
    second antennae. Also, if the antennae are mounted on the same mast the
    answer is that you are relocating both antennae. BTW you do not state if
    these are roof mounted, inside antennae, or what. I presume they are roof
    mounted and separate antennae having absolutely separate transmission leads.
     
  3. Tim Mitchell

    Tim Mitchell Guest

    Well you're sucking the TV signal out of the air with the antenna aren't
    you. Once one antenna has taken its signal, there isn't as much left in
    the air for the other antenna.
     
  4. Jerry G.

    Jerry G. Guest

    There are two factors here. If they are sharing the same feed, the effect
    and match of the antennas will either sum or go differential, depending on a
    lot of complex factors with phase delays. Infact the absolute figures would
    be hard to calculate. When antennas are stacked or worked in an array, there
    is a lot of careful matching that must be done to make sure that their
    impedance is matched, and that the phase track through the spectrum that
    they are to be used.

    If you put two antennas in approximation to each other, they can certainly
    effect each other. This has to do with mutual inductance to some degree.
    Each antenna is resonant and is a tuned circuit. One will have an effect on
    the other, as they may act as a reflector or diverter to each other. In
    most cases they would have to be placed more than four times the wavelength
    from each other in order to not have a measurable effect on each other. In
    practice, this can also be indecisive. If the signal band to be received is
    coming in at 3 meters wavelength, they should be placed at about 12 meters
    from each other.

    There is also the effect of the local oscillator in the receiver radiating
    to some extent back up the antenna. Most of the receiver's local oscillator
    works at about 10 to 20 mw power level. It is supposed to be isolated and
    shielded. But, in many receivers this is not always perfect. In this case
    the local oscillators between two receivers may induce some interference to
    each other depending on the channels being watched, or stations listened to.
    There are many aspects concerning radiated harmonic frequency generation to
    also be considered.

    What I would do for two receivers in the same location, is work out a way to
    share the same antenna. This can be a bit complicated at times, if it is a
    radio, but there will be less problems, than having two antennas in the same
    area, and there will be a space savings.

    --

    I have heard some TV servicemen saying "One antenna is sucking from the
    other". There is an essence of truth here...


    --

    Greetings,

    Jerry Greenberg GLG Technologies GLG
    =========================================
    WebPage http://www.zoom-one.com
    Electronics http://www.zoom-one.com/electron.htm
    =========================================


    With the glut of sci-fi shows on at the same time, I've set up
    two VCRs and two antennas in a single room. I found that the
    two antennas affect each other: by changing the settings on one
    antenna it will change the picture quality on the other VCR and
    TV.

    I always thought antennas were passive devices and didn't
    radiate any signals. Does anyone know of a way to reduce the
    interference between the two antennas?

    Thanks.
     
  5. Sofie

    Sofie Guest

     
  6. Tim Mitchell

    Tim Mitchell Guest

    yes, I was joking.

    I shouldn't do things like that, it will go into Google and people will
    be quoting it in their physics essays.
     
  7. Asimov

    Asimov Guest

    "Ron Hubbard" bravely wrote to "All" (05 Nov 03 22:34:53)
    --- on the heady topic of "Talkative Antennas"

    In order for an antenna to receive it must also transmit. You can't have
    one without the other!


    RH> From: "Ron Hubbard" <>

    RH> With the glut of sci-fi shows on at the same time, I've set up
    RH> two VCRs and two antennas in a single room. I found that the
    RH> two antennas affect each other: by changing the settings on one
    RH> antenna it will change the picture quality on the other VCR and
    RH> TV.

    RH> I always thought antennas were passive devices and didn't
    RH> radiate any signals. Does anyone know of a way to reduce the
    RH> interference between the two antennas?

    RH> Thanks.

    .... No electrons were harmed in the posting of this message.
     
  8. Asimov

    Asimov Guest

    "Sofie" bravely wrote to "All" (06 Nov 03 14:30:10)
    --- on the heady topic of "Re: Talkative Antennas"

    So> From: "Sofie" <>

    So> -----------------------------

    So> You are joking..... right?

    It's really funny too!

    .... Well I defragged my TV and went all the way back to basic cable!
     
  9. JURB6006

    JURB6006 Guest

    In order for an antenna to receive it must also transmit. You can't have
    The only thing it is fed to transmit is leakage of the local oscillator in the
    tuner of the reciever, this should be quite low in the near 100% of recievers
    using at least one TRF stage and even lower when any kind of signal amp is used
    (towards the reciever that is) (I don't know if I was joking)

    JURB
     
  10. Asimov

    Asimov Guest

    "JURB6006" bravely wrote to "All" (08 Nov 03 01:55:51)
    --- on the heady topic of "Re: Talkative Antennas"

    JU> From: (JURB6006)
    JU> The only thing it is fed to transmit is leakage of the local
    JU> oscillator in the tuner of the reciever, this should be quite low in
    JU> the near 100% of recievers using at least one TRF stage and even lower
    JU> when any kind of signal amp is used (towards the reciever that is) (I
    JU> don't know if I was joking)

    The EM wavefront induces a current along the length of the antenna which
    is converted to an EMF at the feedpoint and on down the transmission
    line. However, that same induced current also sets up an EM wavefront
    that interacts with the original but only delayed in time.

    We don't need to talk leakage as note that the 2 local oscillators may
    naturally interact when brought into proximity. I forget what this
    effect is called but the closer they are brought together the stronger
    the tendency to become synchronized if they are nearly the same
    frequency. You can easily observe this effect with a couple toy
    walkie-talkies both set to receive. The noise signal quality changes as
    they are brought closer together.

    Asimov
    ******

    .... Which sparks some mnemonic circuitry.
     
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