Why Ladder Line Length Matters Between Tuner and Antenna
One of the most common myths in amateur radio is that if you run open-wire or ladder line directly to your tuner, the length of that line “doesn’t matter.” In reality, the line length always matters, because transmission lines transform impedance depending on their electrical length. Ignoring this can mean the difference between a smooth, low-loss match and a tuner that struggles under stress.
Impedance Transformation
A ladder line is not just a “lossless pipe.” Every length of feedline has a characteristic impedance (typically 300–600 Ω). If the antenna presents an impedance different from that, the feedline transforms it as a function of line length. The formula is:
Zin = Z0 × (ZL + jZ0tanβl) / (Z0 + jZLtanβl)
Where:
- Zin = impedance seen at tuner
- ZL = antenna impedance
- Z0 = line impedance
- βl = electrical length (radians)
This means a 1 m change in ladder line length can shift the tuner’s load from an easy 200 Ω to a painful 2000 Ω.
Why Tuners Care
Modern tuners have limited ranges, usually around 20–1500 Ω. If your feedline length puts the transformed impedance outside this window, the tuner fails or runs with poor efficiency. By adjusting line length, you can “steer” the impedance into the sweet spot.
Practical takeaway: Keep ladder line either deliberately short (a few meters) or deliberately long (multiple half-wavelengths). Avoid arbitrary mid-lengths, where impedance peaks and nulls make matching difficult.
Why Losses Stay Low
Even if SWR along the ladder line is high, its extremely low loss keeps efficiency high. That’s the beauty of open-wire feed: it tolerates mismatch better than coax. But efficiency doesn’t save you from tuner stress — and that is where line length optimization comes in.
Practical Guidelines
- Never cut ladder line to “random” lengths. Use a quarter-wavelength minimum as a safe starting point.
- If your tuner struggles, add or subtract several meters until the SWR curve shifts into range.
- Keep runs as straight as possible, away from gutters and metal siding to reduce coupling losses.
Mini-FAQ
- Does SWR on ladder line cause loss? — Not much. Loss is negligible compared to coax, even at high SWR.
- Should I aim for a specific length? — Avoid exact half-wavelength multiples; they can reflect extreme impedances at the tuner.
- Is coax better with a tuner? — No. Coax losses multiply rapidly with SWR. Ladder line is the low-loss choice.
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