Twin lead characteristic impedance

Discussion in 'General Technical Questions and Answers' started by CT2FZI, Apr 21, 2010.

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  1. CT2FZI

    CT2FZI Ham Member QRZ Page

    'Morning to all :)

    Is there a way with a antenna analyzer to measure the characteristic impedance of a ladder line/twin feed cable?

    I would like to build a W3DZZ but I think that if I feed it with a balanced line the results would be better.

    Lets say that I would like to feed it with speaker wire, 2mm diameter. How can I calculate the best length of this cable in order to work at least OK in the 80M and 40M band where the W3DZZ shines more?

    I will add a 1:1 current balun at the end of this balanced feed-line not only to make the transition to a small length of coax but also to help the ATU in other bands.

    Note: There is a great article about coaxial cable here, I learned a lot just by reading it, but I guess not enough ;)

    --> http://www.allaboutcircuits.com/vol_2/chpt_14/1.html
     
  2. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Best way to do this is to measure r.f. current at two different places in the line....preferably about a quarter wavelength apart. Get an assortment of non-inductive load resistors. Select the load resistance that causes both current meters to read the same thing. The load resistor will be equal to Z0 under this condition.

    Eric
     
  3. CT2FZI

    CT2FZI Ham Member QRZ Page

    Hi Eric, and thanks for your reply.

    Although what you said makes sense to me in a theoretical way I do not know how to put it in practice.

    In other words, an antenna analyzer is OK to input rf current?
    A multimeter is ok to measure the rf current?
    How can I exactly use the twin line and the meters?
    Is there any other method using a 50ohms dummy load and some non-inductive load resistors?

    Thanks in advanced!
     
  4. W8JI

    W8JI Ham Member QRZ Page


    yes there is.

    The manual for the MFJ259 or 269 tells you how.

    You put a small carbon variable resistor potentiometer across the far end, and adjust the value so when you sweep the frequency over a wide range the SWR changes the smallest amount.

    The value of the resistor is the impedance of the transmission line.
     
  5. CT2FZI

    CT2FZI Ham Member QRZ Page

    Thanks!


    Hello W8JI, Mr RAUCH Jr,

    Thank you so much for the "eye opener", I felt that like a tap in the back and a "Dinozzo" [copyright to NCIS] :D :D

    Mental note to CT2FZI: do not ask before reading the instructions manual twice.



    I transcript here the user manual chapter regarding my question, if someone else needs it:

    [Copy from the MFJ-259B user manual, that I own]

    7.3 Impedance of Transmission Lines or Beverage antennas

    The impedance of transmission lines between a few ohms and 650 ohms can be directly measured with the MFJ-
    259B. Lines of higher impedance can be measured if a broadband transformer or resistance is used to extend the
    MFJ-259B’s range. Select any measurement mode that indicates resistance (R=) and reactance (X=).

    If a balanced line is used, operate the MFJ-259B only from internal batteries. Keep the MFJ-259B a few feet
    away from other conductors or earth, and do not attach any wires (other than the feedline) to the unit. Use the
    ANTENNA connector’s shield for one lead and its center pin for the other. Two wire balanced lines must be
    suspended in a fairly straight line a few feet away from metallic objects or ground.

    Coaxial lines can lay in a pile or coil on the floor. Internal or external power can be used, and the MFJ-259B can
    be placed on or near large metallic objects with no ill effects. Coaxial lines connect normally, with the shield
    grounded.

    Beverage antennas must be directly connected to the MFJ-259B.

    Using fixed resistances:

    1.) Terminate the line or antenna in a non-inductive resistance somewhere around the expected value.

    2.) Connect the transmission line or antenna directly to the MFJ-259B "ANTENNA" connector. Adjust the
    frequency (near the expected operating frequency) until the lowest resistance and lowest reactance is
    measured.

    3.) Record the impedance value.

    4.) Adjust the frequency until the highest resistance and lowest reactance is measured.

    5.) Multiply the highest resistance by the lowest resistance, and find the square root of the result.

    Example: The highest resistance is 600 ohms, the lowest is 400 ohms. 400 x 600 = 240,000. The square
    root of 240,000 is 490. The impedance is 490 ohms.

    Using a potentiometer or resistor decade box:

    1.) Connect the MFJ-259B to one end of the system (in this case you can use a broadband matching transformer).

    2.) Adjust the frequency and note only the SWR change.

    3.) Adjust the termination resistance until the SWR remains as constant as possible with very large frequency
    changes around the operating frequency range.

    4.) The resistance of the termination resistor is the surge impedance of the system.
     
  6. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I'd like to add the results of my testing several samples of "300" ohm twin lead using the small variable resistor method (directional coupler/scope and pulse gen variation on the method).

    I have measured the Zo for TV twin lead to be as low as 230 ohms and, I think, one sample was over 300 ohms. The point being that they were all over the place:)

    The low winner was from JSC (?)or some brand name starting with a J. They are in New Jersey. They didn't reply to my e-mail that was sent to the address posted on their site.

    This made me wonder about new people trying to build pocket J antennas.
    For my needs the Zo is not that critical but the higher Z would be best.

    In regard to testing balanced impedances, have been slowly working on a balun and a measuring technique on and off since December when this area of concern came up. I have met with good results only about 2 weeks ago. In general, I measure the balun's characteristics and then remove its influence with some simple arithmetic leaving the impedance across the balanced line's input. I stole the balun design out of a Wifred Caron designed impedance bridge article from years ago.

    73

    Pete
     
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