Copyright © 2002-2019 by Harold Melton, KV5R. All Rights Reserved. Rev. 02/19/19.
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 many 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 ~12:1. 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 50Ω.
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 its characteristic impedance is closer to the geometric mean, so 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 geometric mean of the antenna's impedance range, from lowest (odd half-waves) to highest (even half-waves).
See also: my feed-line calculator.
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 Losses
Another popular myth is that antenna tuners are very lossy and waste a bunch of power. Baloney! A T-tuner, properly used, is about 95% efficient, and a switched-L tuner (like most auto-tuners) is about 98% efficient. If your tuner is getting hot, you have something wrong in the antenna system, and are exceeding the design limits of the tuner. A T-tuner becomes quite lossy when the antenna side impedance is low (<50Ω), but when it’s high (>50Ω) the efficiency is over 90%, reaching about 95% at ~500Ω. (See the graph at this article by G3TXQ ).
On the other hand, baluns can be very lossy, 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 need to change the feed-line length (to move the anti-node out of the ham band), and use a balun designed for wide impedance variations.
Traditional baluns, such as the coax-wound toroidal 4:1 Guanella, are not designed to handle the extreme impedance variations of all-band doublets. They tend to arc or saturate and burn up at high power when presented with impedance extremes. Modern balun manufacturers have discovered this and now make 1:1 choke-baluns designed for ladder line feeds. Both DX Engineering and Balun Designs now make “tuner baluns” that are 1:1 current choke/baluns, and MFJ now puts 1:1 bifilar-wound choke/baluns in some models, like the 989D.
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 dipole. If the power in each conductor is equal and opposite, we have complete phase-cancellation, and therefore 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!” Again, ladder line should be balanced, and a good choke/blaun used at ot near the tuner. To further reduce common-mode current that may bring RF into the shack, 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 by induction from the antenna and re-radiate it into the shack. A non-resonant feed-line length will present a high impedance to common-mode current. And, as with any feed-line, it’s best to run it perpendicular to the antenna as far as possible 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.
- “It’s too hard to bring into the shack!” Baloney. There are many waterproof ways to bring ladder-line into any shack. You can drill two small holes through the wall and bring two #12-#14 wires through, then caulk, and connect them to the balanced output of your tuner. Or better, mount your balun outside the wall and come in with a few feet of coax. I like that way better because the coax is easy to disconnect (at a bulkhead fitting) during storms.
- “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 the line 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.