HF Antenna Feedline Length Guide – Avoid SWR Problems

Many operators cut coax to “magic” lengths hoping to fix SWR. The truth: for resonant antennas, it doesn’t matter. For multiband or end-fed systems, it sometimes does—but for very different reasons. Let’s separate myth from physics and show when feedline length really matters.

Debunked – “Cut coax for better SWR”

On a properly resonant monoband antenna that’s near 50 Ω, feedline length doesn’t change the true SWR. The mismatch is determined by the antenna, not the cable. SWR only appears to change because longer coax adds loss, hiding reflections as heat.

• SWR is invariant on a lossless line. Length only moves where voltage and current peaks occur.
• Coax loss makes SWR readings look better—but power is simply being dissipated.

Quarter-wave “transformers” use different-impedance cable (e.g., 75 Ω) to achieve a match. A quarter-wave of 50 Ω coax doesn’t perform that trick.

Why It Sometimes Does Matter – Common-Mode Current

Your coax carries two currents:

• Differential mode (DM): the intended signal flow—equal and opposite currents inside the cable.
• Common mode (CM): unwanted current on the outside of the shield—turning the feedline into a radiator.

When CM exists, changing coax length changes the antenna system’s behavior. SWR shifts, noise rises, and RF creeps into the shack.

Fix: Add a quality 1:1 current choke at the feedpoint and route the feedline away from the antenna. Once CM is suppressed, coax length becomes irrelevant again.

Multiband Antennas with 1:4 UNUN

Multiband “random wires,” OCFs, and certain loops use 4:1 transformers and can show 1.5:1 SWR across many bands—but they’re not purely resistive or balanced. Common-mode still lurks.

• Use at least ¼-wave of 50 Ω coax (on the lowest band) between the UNUN and shack.
• Place a 1:1 current choke at the UNUN output (coax side).
• Add a second choke near the rig if SWR or noise varies with feedline routing.

This quarter-wave section helps stabilize the impedance transformation and shifts the high-field region away from the operator. It’s not an SWR transformer—it’s RF hygiene.

EFHWs – The Special Case

End-fed half-waves (49:1 or 64:1) feed at a high-impedance, high-voltage point and rely on a return path. Without a counterpoise, the coax shield becomes that path and radiates.

• Place the first choke ~0.05 λ from the transformer (measured in free-space, not multiplied by VF).
• Optionally add a second choke ~¼ λ away for extra isolation.
• If possible, add a short counterpoise at the transformer to offload the feedline.

This two-choke layout dramatically reduces stray RF in the shack and stabilizes readings across bands.

Handy Length Calculations

Use these formulas to size your coax sections:

• ¼-wave coax section (inside cable): L(ft) ≈ (246 × VF) / f(MHz)
• 0.05 λ spacing for first EFHW choke (outside shield): L(ft) ≈ 49.2 / f(MHz)

Band	Freq (MHz)	0.05 λ (ft)	¼ λ RG-58 (VF 0.66)	¼ λ RG-8X (VF 0.78)	¼ λ LMR-400 (VF 0.85)
80 m	3.5	14.1	46.4	54.8	59.7
60 m	5.3	9.3	30.6	36.2	39.5
40 m	7.1	6.9	22.9	27.0	29.5
30 m	10.1	4.9	16.1	19.0	20.7
20 m	14.2	3.5	11.4	13.5	14.7
17 m	18.1	2.7	9.0	10.6	11.6
15 m	21.3	2.3	7.6	9.0	9.8
12 m	24.9	2.0	6.5	7.7	8.4
10 m	28.5	1.7	5.7	6.7	7.3

(For multiband 1:4-UNUN systems, base your ¼-wave coax on the lowest band you plan to operate.)

Quick Decision Guide

System Type	Recommended Feedline Strategy
Monoband (50 Ω match)	Any 50 Ω length works. Add 1:1 choke at feedpoint.
Multiband with 1:4 UNUN	¼-wave of coax (lowest band) → 1:1 choke → coax to shack. Add 2nd choke near rig if needed.
EFHW (49:1/64:1)	First choke at ~0.05 λ, second at ¼ λ. Add short counterpoise.

Key Takeaways

• For resonant monoband antennas: any length of 50 Ω coax is fine once you choke it properly.
• For multiband 1:4-UNUN systems: a ¼-wave coax run on the lowest band plus a good choke keeps SWR stable.
• For EFHWs: choke placement defines your counterpoise. Use the 0.05 λ + ¼ λ rule.

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