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transmission line transformer

Discussion in 'Electronic Basics' started by Charles de Smurf, Jun 5, 2007.

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  1. Hello,


    Winding a transformer with a transmission line is a technique often used
    to improve the bandwidth of a transformer. The transmission line is said
    to "absorb" the parasitics of the transformer. This part I nearly
    understand. Some explanation might still be helpful.

    However, I'm wondering how winding a transformer with a coaxial cable
    might improve the bandwidth of a transformer. I know a coaxial cable is
    a transmission line, but because of the shielding I thought no EM-field
    (or only very little) could leak out to absorb those parasitics ...

    Can someone explain this to me please?


    Thanks.


    Charles
     

  2. I'm not an expert on transmission line transformers, but
    I'll share my thoughts, so someone more knowledgeable can
    correct me.

    In an ordinary two winding transformer, the only energy
    coupling mechanism between the two windings is the common
    magnetic field they share, in the core. But transmission
    lines couple two conductors together with a traveling wave
    that is guided by the pair of conductors. At frequencies
    where little field escapes from the pair, there is no
    external field to be coupled by the transformer core, and
    the transformer would work just as well if the pair were
    just jumbled in space. But, based on the thickness and
    conductivity of the shield conductor, there is some low
    frequency, below which, the net magnetic field (representing
    the current imbalance in the two conductors) leaks out
    through the shield and a transformer core has something to
    work with. It is this low frequency end of the signal
    spectrum that shows improved coupling when you wrap the coax
    around a transformer core.
     
  3. ehsjr

    ehsjr Guest

    I may be misunderstanding you, but I get the impression
    that you think that because it is called "transmission line"
    it must be coax. That is not what is meant in the context
    of transmission line transformers. Sevick wrote a fascinating
    book on the subject, and a paper on it can be found here:
    http://www.highfrequencyelectronics.com/Archives/Feb04/HFE0204_Sevick.pdf

    Ed
     
  4. I know it can be any transmission line, but didn't understand how it may
    work with a coaxial cable, which in theory would behave the same whether
    or not it is winded around a core. John Popelish told me that's because
    of the leaking fields in the low-frequent area.
    Seams interesting, thank you.
     
  5. John Larkin

    John Larkin Guest

    I don't trust Sevick's book. Every single transformer has the same
    high-frequency rolloff, and he built his own test equipment.

    John
     
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