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Please help-Antenna questions

Discussion in 'Electronic Basics' started by Eric R Snow, Oct 18, 2003.

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  1. Eric R Snow

    Eric R Snow Guest

    Greeting Radio Folks,
    I have looked all over usenet and our local library but can't seem to
    find the answers... I work in an all metal building, sort of like a
    Quonset hut. I wear hearing protection all day that has a radio built
    in. With the big roll-up door closed FM radio reception is terrible.
    Is there some kind antenna I can use to gather signals outside the
    shop and re-radiate them inside? Something passive would be best but
    active would also be acceptable. And since I listen to several
    different stations it also needs to broadcast all stations at once
    because I need to stay at whatever machine I'm running and can't run
    back and forth to change channels constantly. Hope I'm not asking too
    much...
    Thank You,
    Eric R Snow,
    E T Precision Machine
     
  2. If you search Google for "Faraday shield", it may help you better
    understand what's happening. You need to get signal from the outside to
    the inside of the building. The easiest way to do this is with an
    external antenna. Since you probably don't want to drag a wire around
    with you all day, you may have success with using two FM antennas
    connected together, one outside and one inside. The outside antenna
    will pick up signals and pass them thru the cable to the inside antenna
    which will then re-radiate the signal. This is a passive system and
    will result in some signal loss, but that can be overcome to an extent
    by using a good outside antenna with some gain and perhaps an inline
    20-30dB booster amp. I doubt you'll need the amp if you use something
    resembling resonant antennas. Since you're in a shielded room your
    receiver will be quite sensitive since the only signals present are the
    ones generated inside the building, therefore it should work fine IMO.
    Depending upon the strength of the stations you are trying to hear, you
    may have success by simply passing some coax outside and exposing a
    couple of feet of the center conductor at each end.

    michael
     
  3. dB

    dB Guest


    Yes. You need two ordinary f.m. broadcast band antennas. Mount one
    outside the building and one inside. Join them with whatever cable
    they were designed to be usd with. The signals received on the
    outside one will be re-radiated by the inside one.
     
  4. John Fortier

    John Fortier Guest

    I've actually used the two antenna method of getting a wanted signal inside
    an electronically sealed space, in my case a steel module on an offshore oil
    production platform where UHF communications ere required but where the
    intrusions of volatiles would have been disasterous..

    There is another method of diseminatin the internal siganal and that is
    leaky feeder. Connect the external antenna to the internal space, with or
    without the booster amplifier, using a length of deliberately poor co-axial
    cable, leavingthe end of the cable unterminated. This has the advantage of
    allowing you to string the cable into all the areas of the internal space
    you will be occupying, without th intrusion of a VHF antennal into the
    internal space.

    It's important to stand the leaky feeder off from the conductive
    construction of the building hanging the cable from the ceiling using tie
    wraps is quite effective. About 6" of separation should be sufficient.

    You will need to experiment a bit, since the internal reflections inside the
    building will affect the impedance of the cable. A leaky feeder system may
    actually work better with the feeder cable terminated in its intrinsic
    impedance, although this is not common.

    Also, you can expect nodes and antinodes to form inside the building where
    reflections add to each other or cancel each other. Your reception may vary
    as you move around.

    John
     
  5. Fred Abse

    Fred Abse Guest

    Google for "passive deflector". may not be quite what you want, but will
    point you the right way.
     
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