Copyright © 2002-2015 by Harold Melton, KV5R. All Rights Reserved.
Why Use Ladder-Line?
To efficiently feed a non-resonant multi-band antenna.
First, let's dispel the greatest myth in antenna theory: Antennas must be “resonant” to be efficient. Baloney! It just ain’t so!
Please recognize that an antenna need not be resonant in order to be an effective radiator. There is in fact nothing magic about having a resonant antenna, provided of course that you can devise some efficient means to feed the antenna. Many amateurs use non-resonant (even random-length) antennas fed with open-wire transmission lines and antenna tuners. They radiate signals just as well as those using coaxial cable and resonant antennas, and as a bonus they usually can use these antenna systems on multiple frequency bands.ARRL Antenna Book, Ch. 2
As long as the length of the antenna is at least a half-wavelength at its lowest intended frequency, its efficiency is well over 90%, just like a resonant dipole. The problem is getting power to it—coax is very lossy (due to dielectric heating) unless terminated into its characteristic impedance, and this effect is what leads most hams to erroneously believe that non-resonant antennas are inefficient. But the problem isn’t non-resonance, it’s high SWR on coax.
On the other hand, ladder-line does not suffer from high losses at high SWR, so may be effectively used to feed an antenna that may, at various frequencies, present the feed-line with any SWR from 1:1 to ~10:1. So, with ladder-line, you can completely forget about resonance and SWR, until you get to the radio, where you use a tuner to make the match to 50 j0 ohms.
To compare mismatched feed-line losses we have to start with the antenna’s feed-point impedance, and the line’s impedance, then calculate the SWR, and finally, the loss of each feed-line-type at a given frequency and length.
For a worst-case example, feeding a voltage node (like running 40 meters on an 80 meter dipole), let’s say the feed-point impedance is 3500 ohms. With 100 feet of RG-8 coax at 7 MHz, that's a whopping 65:1 SWR, with a total loss of 78%. With 600-ohm open-wire line, the SWR is only 5.8, and the loss is 3%! Then, if we switch to 80 meters, the impedance is 50 ohms, the SWR is ~12:1, and the loss is 7%. In this case, 450-ohm line would be even better, because the SWR only varies from about 9:1 at 50 ohms to 7.7:1 at 3500 ohms. The total losses for 100 feet of 450-ohm windowed ladder-line, at 9:1 SWR, ranges from 5% at 3.5 MHz, to 14% at 28 MHz, and again, that’s at the worst-case mismatch points.
So we see that ladder-line is not only better for non-resonant antennas because of its much lower loss at high SWR, but also because its characteristic impedance places it nearer the center of the antenna's impedance range, from lowest (odd half-waves) to highest (even half-waves).
See also: my feed-line calculator -- you can have hours of fun with that!
Handy formula: VSWR = (1+r)/(1-r), where r = (Zl-Zo)/(Zl+Zo), where Zl=load impedance and Zo=line impedance, in ohms.
Tuner and Balun Loss Myths
Another popular myth is that antenna tuners are very lossy and waste a bunch of power. Baloney! A T-tuner is about 95% efficient, and an L-tuner (like most auto-tuners) is about 98% efficient. If your tuner is getting hot, you definately have something wrong in the antenna system, and are exceeding the design limits of the tuner.
On the other hand, baluns can be very lossy or very efficient, depending on the design and how they are used (or misused). As with tuners, if they are getting hot, they are wasting power, and you either need to change the feed-line length, or buy a better balun. Note that there are many baluns being sold that aren’t even real baluns, like the 4:1 current balun with only one toroidal core. Any real current balun will have two cores, and be well-engineered.
Many hams refuse to use it because they are afflicted by common misconceptions:
- “Ladder-line radiates!” Baloney! Ladder-line does not radiate any more than does coax, if feeding a balanced antenna, like a center-fed wire. If the power in each conductor is equal and opposite, we have complete phase-cancellation, and thus, no RF radiation. (This is NOT true for off-center-fed antennas, like end-feed, or the various Windom-type feeds, where feed-line radiation is significant enough to warrant a bunch of power-wasting chokes.)
- “I tried it once, and it messed up my TV, my computer, and filled the shack with RF!” The trick here is simply to make sure you use a length of ladder-line that is not a multiple of a half-wavelength on any band. Lengths like 40, 80, and 110 feet work well. A resonant length of ladder-line, just like the shield of coax, will pick up RF (common-mode induction) from the antenna and re-radiate it into the shack. However, while the coax shield is grounded, giving induced currents a place to go, ladder-line is not grounded, so there's no place for induced currents to go, except for reflection, if the ladder-line is near a resonant length. A non-resonant length will present a high impedance to common-mode current reflections. And, as with any feed-line, it’s best to run it perpendicular to the antenna as far as you can so the magnetic fields from each half of the dipole will cancel each other instead of inducing common-mode current in the feed-line.
- “It’s too hard to work with! You have to keep it away from metal!” Well, yes, a few inches or so. The general rule is: at least twice the width of the line. It’s easy to make stand-offs from small PVC pipe. Ladder-line can cross a metal edge, like a window sill; you just don’t want to run it right against metal for any significant length. Like, 8 feet right against metal would be too much at 28 MHz, where you'd have ¼-wave of inductive coupling.
- “It’s too hard to bring into the shack!” Baloney. There are many waterproof ways to bring ladder-line into any shack. One is shown later in this article (brass screws through a plastic window). You could also use parallel coaxes to make your entry, or simply mount your balun outside the wall and come in with a few feet of coax. If your wall is non-metallic, you can simply drill two small (~1/8") holes and bring two #12-#14 wires through, then caulk, and connect them to the balanced output of your tuner with banana plugs. Simple!
- “I can’t buy a lightening arrestor for ladder-line!” So just make them yourself, as shown later herein, using sparkplugs.
- “It flops around in the wind, and it breaks too easy!” (a) Windowed line should be twisted about one twist for every two feet to prevent wind-induced oscillations. (b) Make a good feedpoint connection, with proper strain-relief. It doesn’t hurt to wrap it over the top of your feedpoint insulator and then secure it to itself with cable ties. Also, the 14-gauge stranded is much more reliable than the 18-gauge solid line.
If you run an all-band dipole (with a tuner in the shack), you need ladder-line. Coax is very lossy when operated at high SWR. It’s easy to lose 75% of your power in your coax when operating on bands where the non-resonant dipole presents a high feedpoint impedance to the feedline.