How to measure SWR of ladder line?

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

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  1. AA4BQ

    AA4BQ Ham Member QRZ Page

    I deleted my message because I was addressing a point that was much earlier in this thread and that would be confusing instead of helpful. Sorry.
     
    Last edited: Aug 6, 2020
  2. AA5MT

    AA5MT Ham Member QRZ Page

    For this special value below 50 ohms, your tuner should tune it ok. Most tuners will go to at least 10:1 on the swr, whether it is above or below 50 ohms. Most MFJ models will handle 30:1. This is before any extra balun insertion.

    Tom
     
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  3. KI4IO

    KI4IO Ham Member QRZ Page

    "...kicking down a 25 ohm impedance even lower seems bad...:

    Yeah, with my new nanoVNA I measured my 4" open wire feed-line on 80 meters with and without a 4:1 BALUN.
    my 40 meter dipole with NO BALUN - 11.2 ohms and 7.04uH
    my 40 meter dipole with 4:1 BALUN - 1.9 ohms and 157uH

    1.9 ohms is a big ask for an antenna coupler. On the higher bands, it's probably not an issue which my
    measurements confirm. A 1:1 BALUN seems a better overall choice.

    Jerry W
    KI4IO
    Warrenton, VA
     
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  4. WR2E

    WR2E Ham Member QRZ Page

    Also note that the post just before yours was from February... so yeah, much earlier! ;)
     
  5. W4HWD

    W4HWD Platinum Subscriber Platinum Subscriber QRZ Page

    Sorta, I guess...

    Most tuners do better with a mismatch into high impedance loads. For instance, my KAT500 can handle a mismatch up to 10:1 at 600w into a high impedance load (meaning a mismatched load GREATER than 50 ohms). It can only handle a 5:1 mismatch on a low impedance load at 600w (meaning < 50 ohms). Now if the load is 3:1 or better, it’s rated for 1kW either way.

    Seems a HIGH impedance mismatch is better, especially considering the fact that most of the time, ladder line is used as a feed or a matching stub for non resonant multiband wire antennas. The antenna system won’t really achieve resonance anywhere, so why bother measuring SWR at all?
     
    Last edited: Aug 6, 2020
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Losses in tuners/baluns/wires/coax are proportional to current squared (i^2). It takes high voltage and low current to drive high Z loads, so losses are much lower than if the load Z is low, especially when R << 50Ohms
     
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  7. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I determined the SWR on my 300 ohm twin lead by measuring the impedance looking up toward the antenna thru a 1:1 balun with high CM impedance. Normalize this Z to the characteristic impedance of the twin lead. Plot this Zn on the Smith Chart. Get the radius of Zn with a divider. Then mark this off on the SWR scale and read it off. This is the SWR on the twin lead relative to the characteristic Z of the twin lead. I was able to determine the electrical length of the 1:1 balun to take that into account for the first measurement. Sounds like a mess but after some practice it is not a problem. Use an old GR impedance bridge and you get a lot of practice:) BTW the Nanovna is pretty inaccurate if the Z is > about 300 ohms so I use the GR if the Z is above 300 ohms.

    In regard to adjusting the current on balanced line to be equal, what about the phase relationship between them? You could have equal currents but if they are out of phase, you have common mode current.

    In regard to transmatch losses I thought the worst cases were when the Z at its terminals is low causing high current resulting in higher I^2 R loss in the wiring and inductor and even the caps (higher circulating currents in general.....after all the transmatch creates a resonant circuit condition....AKA a conjugate match).
     
  8. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    I measure balance and current on my open feeders like the Old/timers taught me!!

    15967480666231175813396178016325.jpg
     
  9. KD0CAC

    KD0CAC Ham Member QRZ Page


    It's called skim-affect - the higher the frequency the thinner the current penetrates the conductor - to the point with coax and the average frequencies hams use the shield becomes 2 conductors - inside & outside - making 3 electrical paths .
    Example - look at all the RF conductors / wire that is copper coated , over aluminum , steel , ect. - this thin layer is where the RF is conducted .
     
  10. K3RW

    K3RW Ham Member QRZ Page

    Sounds like a NOOB thing but I will admit, I was puzzled to read about LOW impedance antennas and how antenna transmatches struggled with them. How does one even get a low impedance antenna in the first place?!

    Well, mount a 160m dipole at 10ft and thats one way for a low(er) one!

    I have a 1:4 balun (not a 4:1) and actually wanted to design a non-yagi antenna to utilize it. I've done 12ohm yagis on VHF, but couldn't think of even one wire LOW impedance antenna to use it on.

    Problem with a low impedance is knowing around what value to expect. A dipole 'can be' almost any value between a few ohms to a hundred ohms, despite being between nominally 35 to 70ish.

    Every antenna I could design (that I knew of) except a 160m dipole at 3ft had higher than 50 ohm impedances already... except a 12 ohm yagi. But for HF thats a big yagi for most bands. No idea what to do with it except that it was formerly used with a cobweb folded dipole and we already have a replacement.
     

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