Why Minimum Coax Length Matters for HF Antennas
Minimum Coax Lengths for Multiband Antennas: The ¼-Wave Rule Explained
For multiband antennas — especially those fed through a 4:1 current balun or transformer — a very short coax run between the feedpoint and the shack can cause instability, stray RF in the station, and unpredictable SWR readings. That’s why we advise using at least a ¼-wavelength of 50 Ω coax on the lowest band of operation as a safe minimum.
Why Multiband Systems Are Different
In a monoband setup, coax mainly acts as a transmission line. But in multiband antennas — off-center-fed dipoles, loops, long wires, and “no-tuner” hybrids — the feedline becomes part of the antenna system. These antennas rarely present a perfectly balanced, purely resistive 50 Ω impedance on every band. The coax shield inevitably carries some common-mode current, effectively acting as a third radiator.
When the coax is very short (less than about 0.1 λ), that third radiator extends directly into your shack. The result? RF feedback, false SWR shifts when you move cables, and sometimes even mic bite or erratic tuner behavior.
The Role of a ¼-Wave Section
A ¼-wave of 50 Ω coax moves the first voltage and current hot spots away from the operating position. Electrically, this shifts the strongest E/H-field region toward the antenna, keeping the shack at a lower RF potential. It also improves choke effectiveness and stabilizes impedance readings across multiple bands.
- Improved RF hygiene: The rig, microphone, and accessories remain at a quieter RF potential.
- Reduced SWR “twitchiness”: Small frequency changes no longer cause erratic meter swings.
- Better common-mode suppression: Chokes at the feedpoint work more efficiently when they see a realistic shield impedance instead of the near-zero impedance of a very short coax stub.
¼-Wave ≠ ¼-Wave Transformer
This recommendation is not about impedance transformation. A true ¼-wave transformer intentionally uses a different characteristic impedance (for example, 75 Ω) to match two specific impedances. The ¼-wave of 50 Ω coax discussed here is purely for stability and RFI control — not matching. It ensures consistent performance across bands, regardless of impedance variations at the antenna.
Recommended Practice
- Use at least one electrical ¼-wavelength of 50 Ω coax on the lowest band of operation (e.g., ~20 m for 80 m antennas, ~10 m for 40 m antennas).
- Place a 1:1 current choke right at the feedpoint, before the coax run toward the shack.
- Route the feedline cleanly away from conductive structures to avoid pattern distortion.
- Use quality coax (RG-213, LMR-400, or equivalent) to minimize loss on higher bands.
Mini-FAQ
- Is this about reducing SWR? — No. The SWR at the rig may remain similar; the benefit is system stability and reduced RFI in the shack.
- Can I use longer coax? — Yes. Longer lengths can further stabilize readings, but line loss rises; use low-loss cable if extending runs.
- What if I use a tuner at the shack? — The tuner behaves more predictably when it isn’t directly exposed to the feedpoint’s raw impedance extremes.
- Does this apply to end-fed wires? — Absolutely. End-fed half-waves, multiband wires, and hybrids with 4:1 or 9:1 transformers all benefit from ≥¼ λ of 50 Ω feedline and a good choke at the transformer.
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