How to measure SWR of ladder line?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3RW, Feb 20, 2020.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    Last edited: Feb 21, 2020
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    You are a book worm.

    Prove it to me using a Ohm meter.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    Actually, Professor Mike, Phd EE, Retired

    Have you got a Ohmmeter that uses a RF source instead of a 9Vdc battery?
  4. AI3V

    AI3V Ham Member QRZ Page

    What? Did your "efhw" just gain ANOTHER part?

    You just can't seem to grasp that the problem with unbalanced antennas inducing a current into the feedline is caused by the two halves of the antenna- even if you somehow magically "force" equal power into both- is caused because one side of the antenna is now closer to the feedline than the other.

    In the same vein, its now useless to add another "choke" at a random point -that is at the transmitter, because the feedline will re-radiate that current on it into any other wires around.

    As I have explained to you many times, your misunderstanding of the issue is because your simulation software treats coax as three parallel wires, that do not interact with each other, instead of a coaxial structure.

  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Ignorance is bliss!
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Mike, I agree with you on coax effectively having a third conductor to RF via skin effect.

    But I'd disagree that a current balun looking into balanced feed line cannot force balanced currents on that feed line. It's the same mechanism by which a current balun (aka common mode choke) forces current balance at a dipole feed point looking into the somewhat unbalanced antenna elements. When the CMC creates a high impedance to the I3 term that might otherwise flow down the outside of the coax it forces a balanced condition on the antenna elements due to Kirchoff's Current Law. If the current imbalance persisted on the antenna elements after the CMC was added at the feed point there would have to be another path to explain where the common mode current flowed or KCL would be violated.

    It seems that the situation is exactly the same for a current mode tuner balun looking into two wire balanced feed line. If you have the tuner or coax connected to one side of the CMC and balanced feed line connected to the other and the CMC has sufficiently high choking impedance at the test frequency, the CMC effectively forces a balanced current condition looking into the balanced feed line. If not where does the common mode current (the residual of the vector sum of the RF currents on the balanced feed line) go without violating KCL?

    The way I look at it, current baluns (aka CMCs) with sufficiently high choking impedance force balanced currents on their output terminals. Voltage mode (aka flux coupled) baluns force balanced voltages on their output terminals. Seems to me that applies just as well to currrent baluns feeding nominally balanced feed line with some degree of imbalance as it does for a current balun feeding nominally balanced antenna elements with some degree of imbalance.
    K2XT likes this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    I didn't mean to imply that you cant choke balanced line. I was trying to explain that placing a choke on coax effecively blocks current flow along the "third conductor".
    K7TRF likes this.
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Fair enough, I must have misread your earlier post.
  9. AA5MT

    AA5MT Ham Member QRZ Page

    If you have a tuner that has a 4:1 output, you can use this for direct drive. These are usually voltage baluns. The balun is center tapped. Take loose the center tap connection and separate the wires. Take one of the separated wires on the input side and put it to ground. Attach the other wire to the input connection from the tuner section, which usually parallels coax 1. You will have to figure out if there is another connection for direct feed vs through the tuner. The remaining two wires are connected to the balanced line output, which is actually now balanced with a 1:1 current balun. I don't know why no company makes a tuner with both types of output.

    I hope I said that right; It's been a long time since I did this.

  10. K3RW

    K3RW Ham Member QRZ Page

    I've wondered in some cases if I should somehow bypass the 4:1 balun in the tuner and instead put in a 1:1 or for very low impedances even a 1:4.

    I'll have to measure it first to see if that even makes sense to do at all. But kicking down a 25ohm impedance even lower seems BAD.

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