Stop Cutting Coax to “Fix” SWR — The Smith Chart Doesn’t Lie

Updated October 17 2025 — validated technical overview

TL;DR

• SWR at the antenna doesn’t change with coax length — only the impedance your rig “sees.”
• Lossy coax can make a bad SWR look better by burning some reflected power as heat.
• Short, low-loss HF coax runs with a tuner typically add less than 1 dB extra loss — negligible for RX and modest for TX.
• If you’re using coax intentionally as a transformer, it becomes monoband. Place the choke at the end of the transforming section.
• Only open-wire fed doublets truly “need” a tuner in the feedline path. Most other antennas benefit from feedpoint matching.

SWR, Impedance, and What Actually Changes

A transmission line both delivers energy and transforms impedance. On a lossless line, SWR is constant along the length — only phase changes, rotating your impedance point around the Smith chart.

On real coax, the reflected wave is attenuated. The reflection coefficient at the rig shrinks by the one-way loss L_dB:

|Γin| ≈ |ΓL| × 10–LdB/10

Longer or lossier coax therefore shows a nicer SWR — not because the antenna improved, but because the coax warmed up.

How “Better” SWR Costs You Power

Takeaway: It takes roughly 3 dB (one-way) — half your power — to make a 5:1 look like 2:1 at the rig. The “fix” is heat loss, not improved radiation.

Acceptable SWR-Related Loss in Practice

Quarter-Wave Behavior & the Monoband Trap

Coax length doesn’t change the SWR circle, but moves the impedance around it. A quarter-wave (0.25 λ) section inverts resistance — high ↔ low. Because electrical length depends on frequency (and velocity factor), coax transformers are monoband by nature. Compute using VF (0.66–0.85 typical) before cutting.

Tuners, Coax Loss, and When to Worry

Once a tuner matches the transmitter, standing waves remain on the line. The additional loss depends on the line’s matched loss and |Γ|:

ΔLdB ≈ –10 log₁₀((1–|Γ|²)/(1–|Γ|²·10–2L/10))

• L = 0.2 dB (one-way) + 5:1 SWR → ≈ 0.5 dB total
• L = 0.5 dB (one-way) + 10:1 SWR → ≈ 2 dB total

For HF RX, a 2–3 dB total loss is inconsequential. For TX, it’s a bit of warm coax — still better than no contact.

Coax as Transformer — Place the Choke Wisely

When coax is part of the transformation, the common-mode choke belongs after the transforming section — right at the feedpoint. Inside the coax you want differential-mode transformation; outside you want no RF current on the braid.

Where Tuners Belong — and Where They Don’t

• Doublets on ladder line: tuner in-line is fine (balanced type or current balun + tuner).
• Most antennas (dipoles, verticals, EFHW, loops): match at feedpoint instead — it improves efficiency and pattern stability.

Long Coax to “Tame” SWR? A False Friend on HF

Some operators deliberately add extra coax to “lower” their SWR reading. In reality, that added length just increases loss and hides the reflection — it does not improve efficiency.

If you have a proper tuner (or more correctly a transmatch) in your shack, it already matches the entire antenna system (feedline + antenna) to the rig’s 50 Ω port with minimal loss. Here’s why:

• The transmatch operates where voltage and current are moderate, avoiding the high-reactive points that cook coax.
• It provides true impedance correction via reactive elements — not resistive loss.
• Loss in a decent HF transmatch is typically under 0.2 dB, far below what long coax sections waste as heat.

By contrast, “taming” SWR with extra coax only converts reflected power into warmth. The antenna current and pattern remain identical — only your coax loss increases. Use your tuner to match, not your feedline to hide the mismatch.

Loss Depends on Frequency — Not SWR

Another common myth: that coax “loses more” when SWR is high. In truth, the line’s fundamental loss is determined by its construction (diameter, dielectric, conductor quality) and operating frequency — not the SWR value itself.

The attenuation of coax arises from two mechanisms:

• Conductor loss — proportional to √f due to skin effect;
• Dielectric loss — roughly proportional to frequency (and dielectric’s tan δ).

These losses exist even with a perfect 1:1 match. When SWR rises, the forward and reflected waves travel back and forth, so the energy makes multiple passes through the same lossy medium. That’s why the total loss increases slightly — not because SWR “adds loss,” but because the power simply travels a longer path inside the same cable.

At HF, this effect is tiny. A 10:1 mismatch on a short, low-loss line might raise the total attenuation by less than 0.5 dB. The dominant factor remains frequency — not the standing-wave ratio.

Why SWR Meters Mislead

Ham-grade meters infer SWR from forward/reverse power with limited directivity. Expect errors on high-SWR or complex loads. A VNA or lab coupler reveals the true story.

Quick Rules of Thumb

• Changing coax length rotates impedance; loss reduces apparent SWR.
• Loss depends on frequency, not directly on SWR — mismatches only make the signal travel farther within the same loss curve.
• “Better” SWR may mean hotter coax, not a better antenna.
• Short low-loss HF coax + tuner = fine, even with ugly SWR.
• Put the choke where matching ends.
• Only balanced-fed doublets need a feedline tuner.

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