Ladder Line Page 3

Copyright © 2002, 2006 by Harold Melton, KV5R. All Rights Reserved.

Spacing and Impedance

Don’t worry about it. A non-resonant antenna will present a feed point impedance of 10 to 5,000 ohms, with plus and minus reactance, at various frequencies, so who cares about the exact feed line impedance? Matching the antenna to the feed line simply has nothing to do with efficiency (unless you’re using coax).

Anything from 1 to 6 inches is acceptable spacing. 1-inch #14 line is 370 ohms. 1-inch #18 line is 450. 6-inch #12 is about 600. It just isn’t at all critical - and don’t let any geezer or guru tell you different! The spacing should not be over 1 percent of the highest-frequency wavelength, and that’s the only real consideration with ladder-line spacing. On the other hand, it needs to stand-off from metal at least 2-3 times the line's width, so wider lines need longer stand-offs. I think 3" open line, with 9-12" stand-offs, is probably a good place to start.

Length

As mentioned above, parallel feeders can pick up RF from the antenna and transport it into the shack as common-mode current (this means the two wires acting as one, in-phase). The way to avoid this is to avoid resonant lengths of ladder-line, and, if possible, bring the line away from the feedpoint perpendicular to the antenna (90 degrees) for a far as possible before turning parallel to one side of the dipole. Setting your bend-point will also help you take up slack when using a non-resonant length, without having to cut off the excess.

In other words, measure your total run, then increase that to the next available “good” (non-resonant) number, then route the line to take up the slack. A 300-foot (or 100 meter) open-reel tape measure is handy, and Harbor Freight has them for about $30.

Lengths to avoid (in feet): 32, 65, 96, 130, and 260 - and multiples of any of those. Don’t let them make you buy 100 feet when you know that’ll be too close to 96! Make them sell you 110 feet, for example.

Good lengths: Somewhere around 40, 80, 110, etc. Say you saw it here. If in doubt, consult the ARRL Antenna Book.

Continued…

10 thoughts on “Ladder Line Page 3
  1. Sir, superb feedline write-ups. Like you, I’m astonished at ik1mnj’s use of LL. What a guy!

    Question: I will be feeding a U (or C?) shaped doublet with ladder line soon. Unfortunately, there is no other way of achieving the feedline path other than to creep along the ground, a few inches off the deck, almost underneath the antenna will be around 25 and 30 feet high. Primary bands for this doublet will be 160m and 80m.

    I’d love to go perpendicular, but parallel is the only way. Am I in for serious trouble?

    Regards, Callum.

    • NAAAA! It’ll probably be okay.

      I ran a 160M dipole with ≈75′ of ladder line parallel and 20′ under a leg and it was okay, at least on 100W.

      33-1/3,
      —kv5r

      • Thanks fella. I’m going to be up around 500 watts RTTY, but I’ll give it a go anyway!

        Finally Harold, I think you might know the answer to this one. I want to run another LL feedline to the attic. It’s a complex route and I already have 6 x coax lines in a bundle I installed years ago, mostly unused. I’d like to use 2 of these feeders in parallel for about 10 feet (which is part of the bundle), just where the complex route starts and ends up through some roof tiles etc. I can either extend this pair of coaxes to the tuner and ground the braid to the back of the AT4K (big fat tuner) and then when I get to the attic, split it back to LL, or I have the option of starting with LL, going to parallel coax for the difficult route and then back to LL again. Of interest, I could ground the braid to a redundant earthing system I have that nothing else is connected to to get rid of any electrostatic build up? Many conflicting reports here across the internet. I’m starting to consider just building a new route just for the attic project..!

        Cheers, Callum (Birmingham, England)

        • Hi Callum,
          I think you could extend 2 coaxes to the tuner and ground the shields there, with no problems.
          At the other end, where you connect the LL, tie the shields together but do not ground them.
          That’s the usual way STP cable works, you have 2 parallel conductors, with shield(s) grounded at 1 end.
          If you run LL from your coaxes to the tuner you’re just asking for more RF in the shack.
          73,
          –kv5r

  2. Harold hi, i’m ik1mnj Henry, I read your interesting articles on the open wire line, I fell in love with this type of power antennas to the point that I removed all coaxial cables that fed all my antennas 6 m. to 40 m. and I fed my antennas with 450 homs windowed open wire line, duly kept away from the boom of about 10 cm, with different brackets of insulating material, all power supplies from their radiators, go to an intermediate point on the boom where I put a switch 10 positions made with 16 amp plug relay, from this point (length equal for all bands) descends into the station outside the wall where I put a 4: 1 balun, and go into the station with a few meters of coax RG213, just a hard job !! see my web site, the whole system works very well but I have met some problems still not completely solved, these are my questions: 1 ° when calculating the non-resonant length for the various bands I have to take into account the velocity factor of the line, that in the case of 450 homs should be 0.90, 2nd in my case because I have different lengths from the respective feed points to the switch, it becomes difficult to obtain a non-resonant length that works for all bands, of course, I wrote down the lengths added together for each band, from the feed point of each band up to the balun, what could I do? use different lengths of RG213 cable from the balun on the radio station? (in this regard I would like to have your opinion on my work, thanks in advance) thanks for your interest in Harold, 73′

    • Hi Enrico,
      I located your nice web site and your ladder-line page with all the photos. That’s one impressive antenna system! And probably the most complex use of LL in the world. I would not have thought it possible to feed so many directional antennas on one boom with LL. Your experience is far beyond mine — I have only fed single and crossed dipoles, and a 2-meter yagi, with LL.

      1. On velocity factor: I think that VF is not considered when determining non-resonant lengths of LL in common-mode (in-phase, acting as a single wire). VF is only considered in differential mode. In common-mode there’s almost no dielectric interference, well, it’s like a single insulated wire, so about .98-.99.

      I think in your case getting all those lengths to be non-resonant in common-mode will be impossible (many bands and many more harmonics). If your problem is RF in the shack, try a choke on the RG-213 (~10 close-fitting ferrite toroids stacked), near the balun. That should remove RF from the outside of the shield.

      You should also choke your relay and rotator cables; everything that comes from tower to shack can act in common-mode to bring RF indoors.

      If that isn’t enough you might also need to use snap-on chokes on various cables in the shack that connect to the affected equipment. There is also the whole topic of eliminating ground loops.

      2. LL crossing rotation point: Just make a larger C-shape of LL. If the C-loop is large enough it won’t touch the mast when turned, it will half-spiral around the mast when turned 180° from center.

      Please let us know how it all works out!
      73, –kv5r

  3. What can be done wit excess length if the natural length to the back of the tuner happens to be 65 feet? Can the extra 15 feet of ladder line needed to get to 80-foot total length be looped or coiled some way without detrimental effect?

    • Yes, the slack can be hung in a large, wide loop, but no don’t ever coil it.
      You might put a couple eye-hooks several feet apart under an eve, or tie some nylon string through window line in a horizontal run and tension it, either way, turning the slack into a big U-shape, or even several U’s, as needed. Just don’t coil or roll it up, you don’t want it touching (or near) itself to where the magnetic fields would interfere with each other.
      73,
      –kv5r

  4. Does the good length and bad length the same for 450 ohm ladder line? Or is there a formula to use to calculate the appropriate length.

    • Yes, the lengths given should be OK for any kind of parallel-wire feedline. The formula would be 492/f=feet, where is f is frequency in megahertz, and you pick a frequency that is right between ham bands, to get a non-resonant feedline length. Then you also check the odd harmonics (f times 3, 5, 7) to make sure they are also between bands.

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