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…

23 thoughts on “Ladder Line Page 3
  1. So i am making one of these long loop antennas. i can fit 537′ of wire where it is going. i am building the true open ladderline and i bought a 2:1 balun, from there about 6ft of coax to the amp. Will the 2:1 balun work? it said “loop antenna matcher” on the description.

    • A full-wave loop has a free-space feed-point impedance of about 100 Ohms (but much lower when only a fraction of a wavelength above ground), and a 2:1 would make even lower. I would suspect that a 2:1 balun marketed as a “loop antenna matcher” would be for a relatively small and high loop (like the driven element of a quad array), where the feed-point would actually be close to 100 Ohms, and the feed-line is 50-Ohm coax.

      Feeding a big full-wave horizontal loop with ladder-line is the best way, but you need determine what you’ll actually need to tune it, assuming you’ll be operating it on several bands.

      I recommend that you contact Balun Designs’ owner and ask him. He’s probably gonna say get the 1171 and a big ol’ antenna tuner. I just installed an 1171 for my open-wire-fed 80 meter doublet (used as a non-resonant multi-band with a tuner) and very happy with it. See: http://www.balundesigns.com/baluns-for-loops/ .

      73, kv5r

      • I think your right a 1:1 common mode choke but I may also need a transformer. If I just have the 1:1 balun or current choke I will have 50 ohms impedence(coax) going to that window line that is about 600 ohms.That may over heat that 1:1 balun. Going to coax I think the match may need to be closer. If I added a 12:1 transformer it would be 600 ohms to 50 ohms correct? The 1:1 to take care of any common mode current, and the transformer to go from the coax to 600ohm window line?

        • No, you don’t need a transformer. All you need is the 1:1 choke balun.

          It’s a common misconception. The characteristic impedance of a feed-line is only meaningful when the line is terminated in a non-reactive (resonant) load with a feed-point impedance the same as the line, i.e., no standing waves. For example, a resonant folded dipole (300 ohm feed-point), fed with 300-ohm LL, and a 6:1 balun at the radio, would be a proper use of a transformer balun. But that’s not frequency agile.

          The impedance you’ll have at the LL to coax transition will vary widely (~25-4500 ohms) depending on the length and height of the antenna, the length of the feed-line, and the operating frequency. It’ll zoom up and down as you spin the dial.

          In any feed-line with standing waves, the impedance varies from low to high every quarter-wave along the feed-line. That’s what standing waves are — impedance variations along the feed-line.

          For any non-resonant multi-band antenna, the SWR will be high, the impedance will widely vary, and the characteristic impedance of the line is meaningless.

          Well, except that a 600-ohm line that varies from 25-4500 ohms is a LOT better than a 50-ohm line that does the same! A 600-ohm line at 4500 is a 7.5:1 mis-match ratio, but a 50-ohm line at 4500 is a 90:1! That’s why 450-600 ohm lines are far better for high SWR applications; 450 is near the geometric mean of the expected impedance variation.

          As for burning up the balun, well, that’s why you need a “tuner” or “ATU”-type balun; they are designed to handle the very wide impedance variations seen in non-resonant multi-band ladder line fed doublets and loops. I push 1kW through a Balun Designs 1171 from 80 to 10 meters and it it doesn’t even warm up.

          73, –kv5r

  2. Hi there – this is a great resource. Thank you. I have a quick question. I currently have a 44 foot doublet fed with 51 feet of 300 ohm slotted twinlead – via a 1:1 current balun with a short 1 foot jumper into an LDG Auto Tuner. It tunes 80(!) to 12 down to 1:1 swr, but NOT 10m (can’t find a match on 10m better then 3:1 – ironically tunes fine on 27mhz). Can you suggest an alternative ladderline length to incorporate 10m, bearing in mind my 44 foot doublet length? Thank you – I truly believe doublets fed with ladderline are the way forward. 73

    • Hey Tim!
      Yeah I have that problem with mine, too. Hard to tune on 10M. A don’t know how to calc your antenna, but a + or – 8 feet or so will rotate your impedance about 90 degrees on 10 and may put it within range of your tuner.
      I need to do the same with mine! Drag out the dang ol ladder, soldering gun, tape; yeah, maybe I’ll do that, next week! 😉
      If you do find a good number please report it back here for all to benefit.
      I’m working on a new page about ladder line length but my own system is giving me fits so it’ll be a while. Good ol’ experimentation!
      73, –kv5r

  3. Thank you for providing so much valuable information on your site. I shared your NVIS article with several hams. I think it’s one of the best, if not the best I found online regarding NVIS.

    As far as this topic, I would appreciate it if you could confirm what I think happens in my case. I have an OCFD that I feed into a 6:1 impedance transformer. The SWR right under the balun is about 6:1. My coax is about 15 ft long. Based on your calculator, that’s a roughly 10% power loss. If I lengthen the coax, the power loss increases (rather rapidly). So far, so good.

    My question is this: does the impedance transformer play any role in power loss? My guess is that any power loss would be pretty small, depending on how well the balun was built. Am I correct in assuming this? Thanks.

    • Yes, all baluns have some loss. It could range from negligible to terrible, depending on the balun design, mis-match on the load side, frequency, etc. You shouldn’t have a 6:1 SWR on the coax side, that would seem to indicate you’re using the wrong ratio balun or need to move your OFC feedpoint.
      But I’m no balun expert! It’d be better to ask someone at DX Engineering or Balun Designs.

      • Thank you for the reply. Yes, I must have the wrong ratio balun. Unfortunately I can’t move the feedpoint (HOA limitations). My options are to use a tuner to take care of the SWR (my current setup), or experiment with rolling my own balun and see if I can bring down the SWR.

  4. Thanks for sharing all your expertise and experiences.
    I am a new General Class ham w/o an Elmer to guide me so I try to research all that I can on the Internet.
    Perhaps you could comment on my situation using window feed line. I am using a W5GI “mystery” antenna on which the maker has installed a window ladder line with some sort of a connector to convert the ladder line to coax. I have used this antenna with success by connecting the coax connector on the feedline directly to my MFJ939 auto tuner. The feedline comes through a double hung window and is only inside the shack for about 2 feet. There have been no apparent problems with the antenna tuner or any equipment.
    I recently bought a “my panadapter” so now I am running my laptop computer alongside my transceiver. I am now finding that on the 20m band, and higher frequencies, that the computer is affected by my transmissions. I assume that it is RFI. I am not sure how to find out just what it happening and then how to mitigate the problem.
    Any words of practical wisdom you can offer will certainly be appreciated.
    I also note that you are in favor of a voltage balance at the output of the tuner, but I am not sure to exactly to tell if my tuner has one.
    Thanks,
    Bill
    KD8WMJ

    • Your 939 doesn’t have an internal balun. Get a good dual-core current balun, outdoor type, run a short coax to it outside. That’ll reduce your RFI. If still getting RFI, get some snap-on chokes and put them on your computer’s cables. Also make sure your radio & tuner are well grounded.
      73, –kv5r

      • Hi,
        Thanks. Just one last point about the dual core current balun. Should it be 1:1 or 4:1 or other? I assume it should be 1:1 as the antenna builder should have taken care of impedance marching in the included ladder line to coax connector.
        Bill
        KD8WMJ

  5. 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

  6. 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

  7. 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

  8. 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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