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Transmission Line Demonstrators

Discussion in 'Electronic Design' started by chris7007, Aug 10, 2007.

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  1. chris7007

    chris7007 Guest

    Hi, my name's Chris and currently finishing my college degree in EE. I
    was really interested in leaving some sort of legacy for my department
    and my professor suggested building a Transmission Line Demonstrator
    for her RF and EMC class. I looked up online and could only find 2
    sites, one that sold them, and another that gave a very short

    Has anyone here ever tried to build one or has any idea what it
    entails, as well as any references anyone might have? The help is
    greatly appreciated. This isn't for a project by the way, its just one
    thing I want to do for my school
  2. John Larkin

    John Larkin Guest

    A piece of rope works pretty well. It can demonstrate termination,
    reflection in both polarities, loss, dispersion, stuff like that, and
    you can *see* what's going on.

  3. LVMarc

    LVMarc Guest


    You can build a very nice legacy device by making a coaxial air line.

    this would be two tubes one centered within the other one. by selecting
    the id an od you can very close 50 ohm structure, a structure 60- 42
    ohms would be great.

    place a suitable rf connector on each to terminate the air line. the
    connector wail enable connecting sources or vna on one end, and loads,
    or test device on the other end.

    In the middle of the air txline make a small hole 3/8 od for inserting
    voltage current or power flow probes.

    I would make the air line 5-12 inches to make is useful an practical to
    handle mount etc.

    Just measuring the tx lines impedance and loss tangent is an intersing
    exercise for graduate students.

    undergrads can watch what happens for matched and mis matched
    conditions,,,, others may use it to set up a know field at the hole
    access pint and then use this field to calibrate field sensor that
    other graduate students can make to obtain various bandwidths,
    sensiivites, etc.

    Lots of fun, I have done this myself..
    Best regards,

    marc Popek and follow the links to the Center for energy research

  4. I built a Lecher line demo with a trivial 150MHz tube generator for school
    somewhat 25 years ago. It is very simple and illustrative. Still working in
    our days.
    BTW, doing that with modern RF transistors would be a lot more complicated.

    Vladimir Vassilevsky
    DSP and Mixed Signal Consultant
  5. Bob

    Bob Guest


    Hmmm...interesting idea. I can see how you'd show a pulse into a short
    (getting -A coming back with zero amplitude at the end) but how would you
    demonstrate the pulse into an open (getting +A coming back and 2A at the

    In the mean time, I'm off to Home Depot to get some rope.


  6. Hang it.
  7. Bob

    Bob Guest

    I beg your pardon? Why don't you go...oh, I see.

    Yeah, that's what I was thinking, too. Okay, now I'm really gonna go to Home

  8. Tom Bruhns

    Tom Bruhns Guest

    The best I've seen, though tedious to make, is a torsion system.
    There's a central rod, with connectors at each end, and cross pieces.
    The impedance depends on the moment of the cross pieces; the system I
    saw used uniform center rods, I believe. (It was a lonnnnggg time
    ago, in Bell Labs educational films.) It's especially impressive if
    you paint the ends of the cross pieces with fluorescent paint...

    I'd say there were perhaps four cross pieces per foot, with central
    rods about three or four feet long. The system could be made from
    plastic rods or wooden dowels.

    The advantage over rope is that you can demonstrate specific
    reflection properties, and you can make tapered lines to match
    impedances over a range of frequencies. The system in the film had a
    motor that could be used for sinusoidal excitation; it just needs to
    pull up and down on the end of a cross piece. It must also have had
    loads, though I don't remember them specifically.

    The images were so vivid to me that I still remember them when
    thinking about transmission line problems.

    If you decided to do it that way, you could perhaps enlist the help of
    some other students and make it a more interesting team effort.

  9. John Larkin

    John Larkin Guest

    As JP says, you can hang it. But propagation is nicer with some
    tension on the rope, so "terminate" the end into a longish piece of
    fine string or fishing line. That's not a true open, but it is a
    transition to a much higher impedance line, so you will get an almost
    100% non-inverted reflection.

    Hey, you can even "capacitively" load a section of the line, lumped or
    distributed, and see the wave slow down in that section.

  10. Robert

    Robert Guest

    Like this one?


    "You see, the behavior of radio waves has always been hard for me to keep
    straight in my mind; yet one must understand this subject thoroughly in
    order to have a clear knowledge of such things as resonance,
    impedance-matching, standing wave ratio, and antenna theory. I was talking
    to my high school science teacher back home about it recently, and he
    suggested that I build a wave machine as described by Dr. John N. Shive,
    Director of Education and Training of the Bell Labs, in his little book
    called Similarities in Wave Behavior. This booklet tells how to build the
    machine and describes several experiments that can be performed with it.
    Well, Carl built one, and that's what we want to show you tonight."


  11. Bob

    Bob Guest

    That is excellent. What a simple idea, and should be very effective at
    demonstrating the fundamentals. I've located Dr. Shive's book at a local
    bookstore and I'll pick it up tomorrow ($10 for the teacher's edition).


  12. And also a much more uniform impedance. A hanging rope has
    an increasing tension as you go up toward the anchor, and
    the impedance is a function of the mass per length and
    tension at any point.
  13. Guy Macon

    Guy Macon Guest

    I was playing around with a bunch of repelling magnets in a
    horizontal clear plastic tube a while back, and they behaved
    a lot like a transmission line.
  14. Ross Herbert

    Ross Herbert Guest

    I assume you are not referring to high voltage or "power transmission
    lines"? EE could be Electrical Engineering or Electronics Engineering.

    If you have some basic electronics test gear and a roll of coax then
    you only need to apply the theory;

    Or this

    Is this the demonstrator you found?
  15. Ross Herbert

    Ross Herbert Guest

  16. Buy some extra rope to hang the Donkey and the 'Morphmister', while
    you're at it.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  17. John Larkin

    John Larkin Guest

    But think of the hassle of disposing of the giant, smelly, bloated
    carcasses. The toxic waste fees would be outrageous.

  18. John Larkin

    John Larkin Guest

    Which brings up the interesting idea of demonstrating the wave
    behavior in a tapered line.

  19. Crack that whip.
  20. Bob

    Bob Guest

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