Finding the Best Feedline Length for a Doublet
I recently got an email from a reader asking for the “most optimal” feedline length for a doublet fed with ladder line or window line. My answer was simple: there is no universal magic number. A doublet fed with balanced line is a system, and the feedline is part of the impedance transformation.
On a multiband doublet, the impedance at the shack end is not simply the antenna feedpoint impedance. It depends on the antenna itself, the line’s characteristic impedance, the line’s velocity factor, and the line’s electrical length. That is why one feedline length can be easy to tune while another becomes miserable, even though the same doublet is still hanging in the same place.
That is also why balanced-line folklore can be misleading. A feedline length that works beautifully for one station may be awful at another, even if both antennas appear similar on paper. Change the line type, change the velocity factor, change the height, change the routing, and the transformed impedance at the tuner changes too.
Measure After the 1:1 Balun
For this job, the analyzer should look at what the station side of the system actually sees. The practical way to interface a typical 50 Ω analyzer to a balanced doublet and feedline system is through a proper 1:1 current balun or choke. In practice that means: analyzer, very short coax jumper, 1:1 current balun, then the balanced line.
Take the tuner out of the circuit while measuring. That way you are looking at the raw R + jX that the tuner will have to deal with. That is the useful measurement. The question is not whether the feedline itself feels happy. The question is what load your tuner and balun are being asked to survive.
A proper 1:1 current balun is important here. Use a real current-balun or choke arrangement, typically bifilar transmission-line style, not a simple mirrored voltage-balun arrangement. The goal is to keep current balance under control while you measure the system honestly.
Keep the balanced line routed exactly as it will be used in real operation, and keep it away from metal gutters, mast sections, downspouts, wiring, and other conductive structures as much as possible. Routing changes the result.
What to Watch on the Analyzer
On most modern analyzers, the useful screens are the SWR view, the impedance view, and the resistance/reactance view. Sweep every band you care about, then write down the values at the exact frequencies you actually use. Do not rely only on band centers. If you operate the CW end, the SSB end, and one or two digital frequencies, note those specific points.
The key mistake is optimizing a doublet feedline by chasing the lowest SWR on one favorite band and calling the job finished. That is not the real target. The real target is a feedline length that presents tuner-friendly impedances across all the bands you care about.
As a rule of thumb, good candidate lengths are the ones that keep resistance away from the extreme ends and keep reactance from becoming wild on the troublesome bands. Very low resistance, very high resistance, or huge reactance are the readings that usually create pain. Balanced line is forgiving about SWR, but the tuner and balun still have to handle the resulting voltage and current stress.
Practical mindset: do not ask, “Which length gives the prettiest SWR dip?” Ask instead, “Which length gives the least obnoxious set of R and X values across the bands I actually use?”
Electrical Length Matters More Than Physical Length
This is where many feedline-length discussions go wrong. They talk only in feet or meters and forget that the line must be evaluated in electrical length, not just physical length. Velocity factor matters because it changes the electrical length of the line.
A half-wave line repeats the load impedance. A quarter-wave line transforms it strongly. That is why electrical length matters so much. Two feedlines that are physically similar can behave quite differently if their velocity factors are not the same.
Useful formulas:
line wavelength (ft) = 983.6 × VF / f(MHz)
line wavelength (m) = 299.79 × VF / f(MHz)
1/8-wave line length (ft) = 122.95 × VF / f(MHz)
1/8-wave line length (m) = 37.47 × VF / f(MHz)
candidate odd-1/8 lengths (ft) = (1, 3, 5, 7...) × 122.95 × VF / flow(MHz)
candidate odd-1/8 lengths (m) = (1, 3, 5, 7...) × 37.47 × VF / flow(MHz)
For example, on an 80 through 10 meter doublet using 450 Ω window line with a velocity factor of 0.91, the electrical 1/8-wave length at 3.5 MHz is about 31.97 ft (9.74 m). The odd multiples are about 31.97 ft (9.74 m), 95.90 ft (29.23 m), 159.84 ft (48.72 m), and so on.
With true open-wire line at roughly 0.97 velocity factor, those starting lengths become about 34.07 ft (10.39 m), 102.22 ft (31.16 m), 170.37 ft (51.93 m), and so on. That is a real difference. It is also why copying someone else’s “magic 90-foot” or “magic 27-meter” feedline only works if their line type, velocity factor, and lowest operating band happen to match yours.
Feet and meters are only meaningful after the velocity factor is known. Without that, you are comparing physical lengths while the tuner is reacting to electrical ones.
A Very Good Starting Point
A very practical starting point is an odd multiple of 1/8 wavelength on the lowest operating band. That is not a law of nature, but it is a sensible first cut because it tends to avoid the exact half-wave and quarter-wave cases that can land you in unhelpful impedance transformations.
From there, let the analyzer decide. If one band is still ugly, add or subtract line in steps related to the lowest-band 1/8-wave value, or in smaller trial sections if you want finer control. Re-sweep and compare. Because the transformation repeats every half-wave electrically, you do not need to wander through random lengths forever. Move methodically and watch what happens to R and X.
A Practical Analyzer Routine
- Install the antenna first. Put the doublet at its final height and route the balanced line exactly as it will be used.
- Connect the measurement chain correctly. Use analyzer, short coax jumper, 1:1 current balun, then the balanced line.
- Remove the tuner. Measure the raw impedance the tuner would otherwise have to match.
- Calibrate if your analyzer supports it. If you can move the reference plane to the far end of the jumper, do it.
- Sweep every operating band. Record SWR, resistance, and reactance at the exact frequencies you care about.
- Compare candidate lengths. Start near odd 1/8-wave electrical lengths based on the lowest band and the real velocity factor of your line.
- Choose the best compromise. Pick the length that gives the most tuner-friendly set of impedances across the bands you actually use, not the prettiest graph on a single band.
If your analyzer lets you change system impedance, leave it at 50 Ω for this exercise. You are not trying to ask, “How happy is the feedline as a 450 Ω line?” You are asking, “What load will my unbalanced station side see through the 1:1 balun?” That is the load the tuner must match.
You Can Also Measure the Real Velocity Factor of Your Line
If you are not sure the published velocity factor matches your exact ladder line or window line, you can measure it. A known-length sample makes this easy. In most real doublet work, the published value is good enough to choose starting lengths. But when you are trying to push a stubborn band into a tuner’s comfort zone, measuring the real velocity factor can save a lot of blind trimming.
Velocity factor shortcuts:
For the first open-circuit resonance:
VF = 4 × L × f / c
VF ≈ L(ft) × f(MHz) / 245.9
VF ≈ L(m) × f(MHz) / 74.95
For the first short-circuit resonance:
VF = 2 × L × f / c
VF ≈ L(ft) × f(MHz) / 491.8
VF ≈ L(m) × f(MHz) / 149.9
Worked Example in Imperial and Metric
Say your lowest operating band is 80 meters and you are using 450 Ω window line with a published velocity factor of 0.91. Using the formulas above, your first good trial length is about 31.97 ft (9.74 m). The next odd-multiple candidate is about 95.90 ft (29.23 m).
If you already have, for example, about 100 ft (30.5 m) of line installed, you are close to that second odd-1/8-wave candidate and it is worth measuring before cutting anything. If instead you have only 65 ft (19.8 m), you are sitting at a very different electrical length, so the impedances at the shack end may land in a very different place.
That is the real lesson: think in electrical length first, then check what that means in both feet and meters for your actual feedline.
Common Mistakes to Avoid
- Do not optimize only for SWR. On balanced line, tuner-friendly impedance matters more than a pretty SWR plot.
- Do not measure through the tuner. The tuner hides the raw impedance you actually need to understand.
- Do not forget velocity factor. “Thirty feet” or “ten meters” means very little without knowing the line’s electrical behavior.
- Do not copy random internet lengths. A feedline length that works somewhere else may be electrically different in your installation.
- Do not change routing between tests. Height, spacing, and nearby objects affect the result.
Bottom Line
The “best” feedline length for a doublet is not a universal number. It is the length that transforms the antenna’s multiband impedance into something your tuner and balun can handle. Start with the real velocity factor of your line, begin near an odd multiple of electrical 1/8 wavelength on the lowest band, and measure at the station side of a proper 1:1 current balun or choke with the tuner out of circuit.
Then optimize by watching resistance and reactance across every band you actually use. That approach is repeatable, explainable, and far more useful than folklore.
Mini-FAQ
- Is there one ideal feedline length for every doublet? — No. The best length depends on the antenna, the line type, the velocity factor, the routing, and the bands you want to tune.
- Should I optimize only for the lowest SWR? — No. The real goal is a tuner-friendly combination of resistance and reactance across all the bands you actually use.
- Should I show both metric and imperial lengths? — Yes. It makes the article more useful, but the real calculation must still be based on electrical length and velocity factor.
- Where should I connect the analyzer? — At the station side of a proper 1:1 current balun or choke, with the tuner removed from the circuit.
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