Why a Tuner Won’t Fix a Too-Short Coax Cable
A common assumption in ham radio is that an antenna tuner can "fix" any mismatch between your rig and antenna system. But if your coax feedline is too short, the tuner may not help at all. Here’s why coax length matters and why a tuner can’t make up for it.
Transmission Line Basics
Coaxial cable is more than just a piece of wire. It transforms impedances depending on its electrical length. At certain lengths (like a quarter wavelength), an antenna’s complex feedpoint impedance may be transformed into a value the tuner can easily manage. With a very short coax, the tuner effectively sees the raw antenna impedance — which can be highly reactive, very low, or very high.
Why the Tuner Can't Save You
- No node relocation: Standing waves form on a mismatched feedline. Coax length decides where voltage and current nodes fall. A tuner only matches what it sees at its input; it can’t shift those nodes.
- Extreme currents and voltages: With a short coax, the tuner may face raw impedances. Even if it tunes, the tuner’s components may overheat or arc.
- Stray return currents: On end-feds, OCFs, and random wires, the feedline itself becomes part of the antenna unless you choke it. With no length to place chokes at 0.05–0.25 λ, the tuner won’t fix radiated coax.
Worked Example
Take an 80 m inverted-V with ~20 m of coax:
- At 3.6 MHz, the feedpoint impedance may be ~20 + j150 Ω.
- With 20 m of coax (≈0.25 λ), that transforms into something closer to ~60 Ω — easily handled by most tuners.
- With only 2 m of coax, the tuner sees ~20 + j150 Ω directly, often beyond its adjustment range.
(Indicative — exact values depend on soil, height, and antenna geometry.)
Safe Rule of Thumb
For HF multiband systems, use at least a quarter wavelength of coax at the lowest operating band. That ensures the tuner isn’t exposed to raw, extreme impedances and gives room for choke placement.
Mini-FAQ
- Can I just add more coax? — Yes, but avoid excess loss. Using low-loss coax (e.g., ExtraFlex Bury 7/10/13) keeps efficiency high.
- Will a 1:1 choke help? — Absolutely. Placing a choke at ~0.05 λ and before the shack keeps RF off the feedline.
- Why not let the tuner sit at the feedpoint? — That works, but only if the tuner is outdoor-rated. Most shack tuners aren’t weatherproof.
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