HF Antenna Feedline Length Guide – Avoid SWR Problems

Yes—feedline length can materially affect a multiband HF station. Electrical length sets how mismatched antenna impedances are transformed, where common‑mode currents thrive, and how much extra loss your coax accumulates under SWR. Here’s a practical guide to picking lengths that behave well and where to add chokes so your tuner—and your neighbors—stay happy.

Impedance Transformation Basics

With standing waves on the line, the impedance you see at the rig depends on the line’s electrical length (physical length × velocity factor, VF). Quarter‑wave sections flip impedances; half‑wave sections repeat impedances.

• Avoid odd λ/4 multiples referenced to your lowest band. These invert impedances at the shack and can force the tuner into ugly corners.
• “Safe” lengths are those that are not near odd λ/4 on the lowest band. Half‑wave (λ/2) repeats the antenna impedance—often more predictable.
• Quarter‑wave calculator: λ/4 (m) ≈ (75 × VF) / f(MHz). Example (80 m, 3.5 MHz, VF = 0.86): 18.4 m (electrical). Steer clear of ~18 m, 55 m, 92 m…; lengths near 36 m or 72 m behave more benignly.

Note: If your coax run is already fixed, a good choke and slight antenna‑side adjustments (fold/trim, small length change) can shift problem spots away from your favorite bands.

End‑Fed & Off‑Center Systems

EFHW, EFOC, OCFD, and “random wire” antennas are inherently imbalanced and tend to drive common‑mode currents (CMC) onto the coax shield. The shield length then acts like a stray counterpoise—strongly length‑dependent.

• Treat the choke as the feedpoint: Measure “feedline length” from the first 1:1 CMC choke. The coax before that is part of the antenna.
• Choose non‑resonant shield lengths: Avoid making the shield near λ/4, λ/2, λ, … on active bands (pre‑choke), or it will radiate and detune things.
• Double‑choke long runs: One high‑CMC choke at the feed, a second at the shack bulkhead often calms stubborn CMC.

Balanced Lines (Ladder/Open‑Wire)

Doublets with tuned feeders thrive when you pick line lengths that keep the tuner in range on all target bands.

• Avoid exact odd λ/4 electrical lengths from tuner to center of the doublet; those can land the tuner at extreme impedances with high reactive components.
• Good practice: If a band tunes “touchy,” add/remove 0.05–0.1 λ of line (electrical) and re‑try. Small shifts often move impedances into friendlier territory.

Feedline Loss Under SWR

High SWR multiplies coax loss—especially at higher frequencies and longer runs. Two levers help:

• Reduce SWR on the line: Favor near‑resonant/semi‑resonant antennas on key bands; keep incompatible bands on ladder line when practical.
• Use better coax, shorter runs: Low‑loss cable and tidy routing can save more dB than the next amplifier step.

Quick Planning Rules

• Pick line length from the lowest band up: Make it not an odd λ/4 (electrical) at that band.
• Place a high‑CMC 1:1 choke at the antenna feed (or transformer) and again at the shack entry for problem systems.
• Re‑check after rain: Water changes VF and loss slightly; if a system becomes marginal only when wet, beef up choking or nudge lengths.
• Don’t chase “magic” lengths: The right length is the one that works with your antenna and site. Use the analyzer, then lock it in.

When Length Really Matters

• End‑feds & long wires: Coax length prior to the first choke changes pattern, noise, and SWR.
• OCFD/Off‑center feeds: Impedance seen at the shack varies widely with line length—pick a stable zone and stay there.
• Multiband verticals: The line can act as part of the return path; avoid shield resonances and add a choke.
• Doublets with ladder line: Small ladder‑line trims often fix “won’t tune” bands.

Interested in more technical content? Subscribe to our updates for deep‑dive RF articles and lab notes.

Questions or experiences to share? Contact RF.Guru.