Is twisted pair considered a transmission line

Discussion in 'Wireless & RF Design' started by dtow1, Aug 9, 2012.

  1. dtow1

    Thread Starter New Member

    Apr 27, 2010
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    So I have been reading a bit about transmission lines and I have a few questions.

    My first question is that I have seen references that say that twisted pair is a transmission line. I have never thought of an Ethernet or Phone line cable as a transmission line. Are they fundamentally using electromagnetic energy to propagate the signal in the same basic principle as a coaxial cable? Or is it more like a mouse where you are just picking up voltage levels.

    My second question is how the signal is received. Is the wave propagation simply helping to keep the current flowing? At the receiving end of the cable I assume it is just sensing the current or Voltage on the cable, and not directly using the Electric and magnetic fields.

    Id love any clarification/books/sources that explain more about whats going on in these.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Every piece of electrical conductor is a transmission line.
     
  3. dtow1

    Thread Starter New Member

    Apr 27, 2010
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    Thanks Mr. Chips. Can you clarify though, does that mean that a twisted pair cable is essentially functioning in the same manner as a coaxial cable? Is the only difference/distinction if it is sending a balanced or a common mode signal(aside from things like how it is shielding from interference, I am interested in transmission). Thanks for any clarification.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    You are correct. A twisted pair is sending complementary signals and therefore has higher common mode rejection at the receiving end.

    You still have to observe transmission line practices such as proper source resistance and load termination to match the impedance of the transmission line.
     
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  5. dtow1

    Thread Starter New Member

    Apr 27, 2010
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    Well, that is pretty interesting, I had never considered that with those types of cables. Thank you very much for replying.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Twisted pair is definitely a transmission line, and it typically has a characteristic impedance of about 100 ohms. it's a little harder to analyze than parallel feed lines (ladder line) because of more lumpy inductance, but all the transmission line equations apply
     
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  7. PRFGADGET

    Active Member

    Aug 8, 2011
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    There can be (and are) many variations of "Twisted Pair" , as stated some have an impedance of 100 ohms , some 300 ohms, some 600 ohms.
    When designing an antenna system , both the terminal impedance of the antenna AND the feed line impedance must be factored in (at least to keep the transmitter happy).
    Twisted pair like open wire feeder will radiate (so will coax if feed to an imbalanced load) unless a line isolator is used.
    "Twisted Pair" is nothing more than parallel line "WITH A TWIST" or spiral the original idea behind the spiral was / is to make the two conductors somewhat self shielding (lotsa theory on that subject).
    There is quite a bit of information on the net , and I would recommend finding one of the "OLDER" (1930's-1950's) handbooks and look at the "transmission line" information there.
     
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