Using Ladder Line Length to Achieve Minimum SWR

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W5DXP, Nov 7, 2018.

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  1. W2VW

    W2VW Ham Member QRZ Page

    Cecil, Thanks for putting this out to the amateur public. I'm a huge fan of non physically resonant antennas.

    Everyone who has learned what all hams should eventually know is aware that a resonant antenna does not radiate one iota better than a non-resonant antenna.

    Coax is for kids.

    One thing that jumps off the screen here in the photos is ladder line coupling where it shouldn't due to close proximity to other ladder line and wiring.

    It may not be happening but the picture angles look like multiple violations of ladder line installation rules.

    Am I seeing things?

    73,
    Dave W2VW
     
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    lol

    Now a days you need a sign that says do not climb.

    Squirrels love ladder line.
     
    Last edited: Nov 8, 2018
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Here's a Smith Chart plot of the feedpoint impedances for my 39 ft dipole. They all fall inside the magic green doughnut such that the magnitude of resistance looking into the ladder line at a current maximum at the XMTR will be between 20 ohms and 100 ohms for all bands ensuring that the SWR50 will be less than 2:1.
    39SCFPs.png

    "A picture is worth a thousand words". For a VF=0.83 feedline, like I have measured for mine, any feedline length halfway between the black dots will be a worst case length for matching to 25-100 ohms.
    39DipImax.png
     
    AI3V likes this.
  4. W5DXP

    W5DXP Ham Member QRZ Page

    This method depends on the Z0 of the parallel line to transform impedances close to 50 ohms at a current maximum point on the feedline. Given that the feedpoint impedances of the dipole are fixed, that makes the impedance transformation one-dimensional, i.e. the only control we have over the impedance seen by the XMTR is the Z0 of the parallel line. With that in mind, here are the calculated SWR50s that the XMTR will see for 300, 450, and 600 ohm feedlines feeding the 39 ft dipole.

    Band, 300, 450, 600
    10m, 1.7:1, 1.2:1, 1.9:1
    12m, 1.4:1, 1.6:1, 2.8:1
    15m, 1.3:1, 1.8:1, 3.1:1
    17m, 1.1:1, 1.8:1, 2.8:1
    20m, 1.6:1, 1.9:1, 2.1:1
    30m, 1.4:1, 1.2:1, 1.1:1

    Since my goal is no-tuner operation with an SWR50 of less than 2:1, you can see why 600 ohm feedline is unsatisfactory. 450 ohm line is not bad but may not cover the entire 20m band with an SWR50 of less than 2:1. That's why I chose the 300 ohm feedline.
     
    AI3V likes this.
  5. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    It isn't that non resonant antennas radiate, it is the pattern that it puts out.

    If an antenna is putting out power in the wrong direction, all the tuning isn't going to make it work any better or worse.
    Having a known length of antenna and knowing what the pattern should be is part of the solution to being able to communicate with people in the area they are in and not have power going where it does no good.

    But even with all the resonant and non resonant antennas, having chokes to keep the RF where it belongs is very important, and just because we didn't use them in the past doesn't mean we don't need them now.

    As for coax being for kids, I would like to see any open wire transmission line in cars, trucks and airplanes, not to mention when I have to run it underground.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Or near your computer, if using digital modes.... (Both directions: coax picks up less RFI radiated from the computer stuff, and coax is much less likely to crash the computer due to RF radiated by the open-wire line.)
     
  7. W2VW

    W2VW Ham Member QRZ Page

    Check out my logbook. Much of the FT-8 was worked with open wire line. A lot of the AM, same. There's plenty of audio gear and compression that will not tolerate RF in the shack.

    People run balanced feeders within a few feet of computers and audio gear all the time with success.

    The problems happen when the balanced line installation does not adhere to a few simple rules and/or shack end is at a voltage maximum.

    Open wire line can be made to radiate near nothing. It does take some care and a lot of people who try it for the first time have problems. This is especially true when one replaces coax with balanced line to a physically resonant antenna with near 1/4 electrical wavelength of transmission line.


     
    AG5CK likes this.
  8. W5DXP

    W5DXP Ham Member QRZ Page

    I run ladder line directly into my shack. There's about 20 ft of it in my ladder line length selector leaning against a wall about four feet from my computer. With an older XCVR (IC-756PRO) there's a rat's nest of cables, USB RS232, USB sound dongle, CI-V cable, keyboard cable, mouse cable, antenna analyzer cable, and SG-500 amp ... I have no ferrite on the cables and have no RF problems when running 300 watts on digital modes while chasing DX.

    Where I do have RF problems is in my GMC pickup where I don't use ladder line but cannot run more than 20 watts digital. All cables in the pickup have pounds of ferrite on them. Go figure.
     
    W2VW and AG5CK like this.
  9. KI7AAR

    KI7AAR Premium Subscriber QRZ Page


    Thanks, Cecil. I love your research and passion for matching with ladder line. The “Optimum Line Length” webpage is a great resource. Is that yours? I used it when setting up a ZB.. and it was really helpfull. I’m also on the band wagon that antennas do not need to be resonant to work well as long as they are properly matched.

    The reason for my interest in 600 ohm ladder line is for a second attempt on an 130’ 80 meter doublet. The first doublet never got ironed out because I had to move on short notice shortly after erecting it. I was trying to understand the relationship of the 600 ohm ladder line length on the matching when it had to come down. A shame that I never got on the air with it because it was at 90’ and it probably would have kicked tail.

    Anyway, in the Pacific Northwest it gets a little wet at times and I’ve read that 300 and 450 ladder line don’t behave all that well when wet. So that’s why the 600 ohm was (is) my line of choice for doublets. If you have any tools for 600 ohm line length calculation they would be most welcome.
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    Yes, Dave, you are seeing things. The pictures are two-dimensional and space is three-dimensional. All 450 ohm ladder line is at least five inches distant for all other 450 ohm ladder line. I sometimes use three inch spacing for 300 ohm twinlead.

    IMAXGRAF works for 300, 450, and 600 ohm line to locate the current maximum points. It's just that the impedance at the 600 ohm current maximum points often doesn't yield an SWR50 less than 2:1. You could just as easily build 300 ohm open wire feedline as you could 600 ohm open wire feedline which would tend to improve your rain problem. We get lots of rain here in East Texas during certain times of the year (like now). The beauty of having an adjustable length of ladder line is that you just change that length when it rains. It rained last night and my SWR on 40m went from 1.2:1 to 2.3:1. I just shortened my ladder line by two feet and the SWR dropped to 1.5:1. With JS8 and FT8, I just ignore the additional losses due to rain.
     
    Last edited: Nov 9, 2018
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