KENTUCKY PORTABLE CLUB • ANTENNA GUIDES
One wire, one feedline, every band. How doublets fed with ladder line work, the popular lengths (88 ft, 44 ft, G5RV, ZS6BKW), how to pick a feeder length your tuner will like, and why the "lossy" feedline usually isn't.
A doublet is a dipole you don't cut for one band. On most bands the feedpoint is a bad match, sometimes very bad. With coax, that mismatch burns power in the cable. With ladder line it barely costs anything, and the tuner at the radio takes care of the rest.
Ladder line (450 Ω "window line" or 300 Ω twin-lead) has very little loss to begin with. Even at an SWR of 10:1 on the line, 100 ft of window line usually loses less than 1 dB. The same SWR on RG-58 can lose most of your power. Try the numbers
An 88 ft doublet with ladder line and a decent tuner works 80 through 10 m. A 44 ft doublet covers 40–10 m. No traps, no links, no band changes in the field.
Ladder line is balanced, so it needs a balanced tuner or a balun at the tuner. It also has to be kept away from metal. And some feeder lengths put an impedance at the tuner that it can't match. The checker helps you avoid those.
All are center-fed wires. The difference is the length and how they're fed.
Doublet: ladder line all the way to the tuner. This is the most flexible and the lowest loss. G5RV / ZS6BKW: a cut-to-length piece of ladder line, then coax. It's easier to bring indoors, but there's more loss on the bands it doesn't match.
| 88 FT DOUBLET | 2 × 44 ft legs. Ladder line to a tuner. 80–10 m (60 m too). The classic all-band wire. |
| 44 FT DOUBLET | 2 × 22 ft legs. 40–10 m, and 80 m with a good tuner (it's short there). Fits most campsites and a 31 ft pole as an inverted V. |
| 135 FT DOUBLET | 2 × 67.5 ft. 160 m with a tuner, strong on 80/40. Needs room. |
| G5RV | 102 ft top + 34 ft open-wire section (about 30.5 ft of 450 Ω window line, or 28.5 ft of 300 Ω twin-lead), then coax. Low SWR on 20 m. A tuner is needed on most other bands. |
| G5RV JR | 51 ft top + 17 ft open wire (about 15.3 ft of window line). 40–10 m with a tuner. |
| ZS6BKW | 93.5 ft top + about 40–42 ft of 450 Ω window line, then coax. Often under 2:1 on 40, 20, 17, 12 and 10 m without a tuner. Trim the matching section for your height. |
Use the checker below to try your own lengths. Inverted-V or flat, these all work. The ends just need to stay out of reach.
Some combinations of leg length and ladder line length put a very high impedance at your tuner on a band. That makes it hard to tune, and it's hard on the tuner. Pick your antenna and feeder, see which bands look good, and get a better feeder length if one band is bad.
A rule of thumb, not a model: it tracks where the voltage peaks land on your feeder. Real height, ground and line spacing move things a little.
Why doublets use ladder line, in one table. Same band, same length, same SWR on the line. Only the cable changes.
Matched-loss figures are typical published values for dry line. Wet or dirty twin-lead is much worse.
Ladder line is balanced. Most tuners and every radio are unbalanced (coax). Something has to bridge the two.
A true balanced tuner (link-coupled, or a balanced L-network) takes ladder line straight in. It has the widest range and the best balance. They cost more, but nothing beats one for a doublet.
Put a 1:1 current balun right at the tuner's output (or outside the window), then a short coax jumper to the tuner. 1:1 handles the widest range of impedances. A 4:1 balun helps when the impedance is high on most bands, but it can make the low-impedance bands harder.
Internal tuners in portable rigs typically only match up to about 10:1. Pair them with a 4:1 balun and pick the feeder length with the checker. Or carry a small external manual tuner. That's the real portable doublet secret.
A 44 ft doublet on a fiberglass pole is one of the best all-band portable antennas there is. No band changes, no links.
Further reading: The ZS6BKW wire antenna
ONE WIRE. EVERY BAND.
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