Ladder Line is not Evil

Discussion in 'Amateur Radio News' started by VK6FLAB, Oct 14, 2017.

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  1. VK6FLAB

    VK6FLAB Ham Member QRZ Page

    1280px-TransmissionLineDefinitions.png
    Foundations of Amateur Radio #123


    The way we connect our antenna to our radio depends on a number of different factors. If you've come through the ranks recently, like I have, it's probable that you've only ever considered using COAXIAL cable. It's a single conductor, surrounded by some type of insulator, which in turn is surrounded by a conductive shield, which is protected by another layer.

    There are variations where the shield has multiple layers, including layers of foil and braid, so-called quad-shield COAX, and there are variants that have several cores, sometimes two sets of COAX connected side-by-side and so on.

    In many ways, COAX is an invention of convenience, which has several compromises as a result, loss over distance, termination issues, twisting and deformation and others. It's compact, less susceptible to external interference, it's relatively easy to route to its destination and if you treat it well, it's easy to carry around, but it's not the only way you can feed an antenna.

    You may have come across the term Ladder Line or Twin-lead, or Twin-feed line. You may also have heard horror stories associated with this "ancient" - well at least in Amateur Radio Terms - technology. Essentially, Ladder Line is two conductors, side-by-side, evenly separated by spacers. It's in use all over the place. If you look up at your power-lines in the street, or the high-voltage lines on top of towers, you'll notice that those are essentially Ladder Line.

    You've no doubt been told that you need to keep Ladder Line away from everything, in order for it to work, but that's not actually what's needed. What's required is that both conductors are exposed to the same fields. This means that if you're running the Ladder Line through a metal window, you need to ensure that both lines get the same amount of exposure to any nearby metal, you might put a slight twist in the Ladder Line, or you might put it in the middle between two bits of conductor, like a metal window frame.

    You might also have been told that Ladder Line radiates. It does, but only if the antenna you're feeding isn't balanced, because what actually happens is that the two sides of the antenna don't cancel each other out and the difference is radiated by the Ladder Line. It's worse if the Ladder Line is some resonant length on the frequencies on which you're using the antenna, because it will receive the signal from your antenna and re-radiate that too.

    It really means that you need to pay attention, but the cost of that attention pales into insignificance, if you think of the benefits.

    You might recall that your radio is most happy when it's transmitting into a 50 Ohm load. One of the measurements associated with that is an SWR reading of 1:1. This has come to be interpreted as: "You need a 50 Ohm antenna in order for it to radiate." and that's not actually true.

    All antennas will radiate, and as long as they are at least half a wave length long, their efficiency will be about 90%. The problem isn't the antenna, it's how you feed the antenna. As I said earlier, if you're a relatively new amateur like me, you might have put one and one together and decided that you need to feed it with 50 Ohm COAX and that your antenna needs to be 50 Ohm. If you do that, it will work, but it's not the only way.

    The reason you need to have a 50 Ohm feed-point to plug your COAX into, is because the COAX has a lot of loss if there is a feed-point mis-match. The higher the mis-match, the higher the loss. For example, using an 80m Dipole on 40m might mean an SWR of 65:1. This has about 80% loss on 100 feet of RG-8 COAX at 7 MHz. All you're doing is heating up COAX.

    However, if you were to feed it with 600 Ohm Ladder Line, the loss might only be around 3%. Before you start getting out the calculator to prove my maths wrong, this isn't about maths, it's about the difference between Ladder Line and COAX.

    COAX is wonderful as a tool, but Ladder Line should not be consigned to the annals of history, because in many, many situations it out-shines COAX. The combination of the thickness of the conductor and the separation width, determines its native impedance. There is lots of documentation online, including calculators on spacing and thickness, so you can build your own.

    I've seen lots of different types of spacers, from watering tubes cut to length holding the wire with cable-ties, to bits of Perspex, to cutting board plastic and others. The simplest and cheapest one I've seen to date was last weekend, made from two earth wires separated by strips of garden edging that comes in rolls. You cut off little strips, drill two holes, feed the wire through and if you're feeling that it needs permanency, glue them in place.

    Of course you can buy the stuff, but it's getting harder and harder to find at reasonable cost, so experiment a little.

    Ladder Line, it's not evil, it might surprise you and you will have another feather in your cap when you go out portable to set-up a field station.

    I'm Onno VK6FLAB
     
    K1JFJ, N6PJB and VK4HAT like this.

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