Impedance and Matching

Why impedance matters

Every RF setup is a chain: transmitter → feedline → matching device → antenna → free space. At every link, your signal meets an impedance — the RF “resistance” to current and voltage flow. When these impedances don’t align, part of your power bounces back toward the radio instead of being radiated. The radio senses that mismatch as stress, often cutting power, heating up, or distorting.

Most radios and coaxial cables are designed for 50 Ω. If the whole system — feedline, matchers, and antenna — presents close to 50 Ω with little reactance, your rig is happy and efficient.

Impedance in 60 seconds (with real‑world examples)

• Impedance (Z) has two parts: Z = R + jX where R is resistive (radiation + loss) and X is reactive (energy storage). Radios want roughly R ≈ 50 Ω and X ≈ 0.
• Reflection coefficient (Γ) tells how much signal reflects. A bigger mismatch → bigger |Γ| → more power sloshes back and forth.
• SWR summarizes that reflection into a single ratio at the measurement point (usually the rig).
• Mismatch loss is how much forward power you can’t deliver because of the mismatch — even before considering feedline loss.

• Example — Short vertical with poor ground: Feeds like low R plus capacitive X. Your tuner can cancel the reactance, but the small R means less radiated power and more I²R loss. The SWR meter may be happy; the far‑end signal won’t be.
• Example — EFHW on 40 m: The feedpoint is around 2–3 kΩ. A 49:1 unun brings it close to 50 Ω so the radio can deliver power. You still need a 1:1 current choke after the unun to keep the coax from becoming the counterpoise.
• Example — 40 m dipole used on 20 m with coax: Off resonance, the coax sees high SWR. An internal tuner in the shack will “fix” the rig’s view but the coax between rig and tuner still runs at high SWR. On 20–10 m that added line loss is noticeable; on 160/80 m it’s far smaller (see ATU placement note below).

What the radio actually “sees”

Your radio never “sees” the bare antenna. It sees the antenna transformed by the feedline and whatever devices sit between. A long piece of coax can change how the mismatch appears — sometimes showing a nearly perfect 50 Ω at the shack even when the antenna itself is far off. Adding more coax doesn’t fix the problem; it just moves where the mismatch hides.

The translators: tuners, baluns, and ununs

Antenna tuners (ATUs)

What they do: Use reactive networks (L, T, π) so the radio sees 50 Ω even when the antenna doesn’t. They protect the radio and let it deliver clean RF.

What they don’t do: A tuner does not make your antenna resonate or magically radiate better — it only matches impedance.

Where to place them:

• Best when SWR on the line is high: Put the tuner at (or very near) the feedpoint so the coax between tuner and rig sees ~50 Ω.
• Common and convenient: In the shack. This is fine when the line SWR is modest, or when using low‑loss balanced line.

Frequency‑dependent loss tip: High SWR on coax hurts far more on the upper HF bands. On 20–10 m, coax’s matched loss per foot is higher, so running it at high SWR compounds the loss. On the top bands (160 m, 80 m), matched loss is low; even with a high SWR the added line loss is often much smaller. Place this nuance in your station plan.

Internal vs external: Internal tuners are for small touch‑ups near resonance; external tuners handle wider ranges and more power. Narrow (high‑Q) matches need re‑tuning when you change frequency.

Baluns: enforce current balance

Purpose: A balun exists to force equal and opposite currents in a balanced antenna or line so the system behaves symmetrically and common‑mode RF stays off the outside of your coax.

• Use a 1:1 current balun (choke) wherever a balanced load (e.g., dipole, ladder line) meets unbalanced gear (coax, radio, unun output). This is the “RFI and pattern‑control” component and is needed even if you also have an impedance transformer elsewhere.
• Voltage baluns (autotransformers) enforce equal voltages, not equal currents. They do little to cure current imbalance and are generally not preferred for HF center‑fed antennas.
• About 4:1 at the feedpoint: A fixed 4:1 ratio only makes sense when the antenna actually measures near ~200 Ω at the feedpoint — typically a dipole near ~0.5 λ height over average ground. Most real installations aren’t there, so a default 4:1 often makes things worse. Measure first; otherwise use a 1:1 current balun and let the tuner work.

Behind any unun or voltage transformer, add a 1:1 current choke to stop common‑mode RF from riding the coax into the shack.

Ununs: set voltage/ratio (unbalanced ↔ unbalanced)

Used with unbalanced antennas (end‑fed wires, many verticals). Most ham ununs are voltage autotransformers that provide an impedance ratio but do not provide current balance.

• 9:1 unun: For “random wire” antennas feeding a wide‑range tuner. Provide a counterpoise or real RF ground and a 1:1 choke after the unun.
• 49:1 or 64:1 unun: For EFHWs (feedpoint ≈ 2–3 kΩ → ≈ 50 Ω). Always add a 1:1 choke a few meters down the coax to keep RF off the braid.
• Avoid “ratio by habit”: Pick a fixed ratio only when the feedpoint impedance warrants it (by measurement). Otherwise match with a tuner.

Feedlines: coax vs open‑wire

• Coax (50 Ω): Simple and shielded, but loss rises with frequency and gets worse when operated at high SWR. Keep long, high‑SWR runs off coax when you can (use a remote tuner).
• Ladder line / open‑wire: Very low loss even with high SWR. Don’t “transform” the line to 50 Ω ahead of a tuner; the smart play is: balanced line → current balun (for balance) → balanced tuner (or tuner + external current balun on its output). Choose line length to avoid extreme impedances at the tuner (lengths near odd ¼‑waves can be troublesome; multiples of ½‑wave tend to “repeat” the antenna’s impedance).

Simple match math (for the curious)

Getting a good match in practice

• Start with the antenna — size and placement matter more than any tuner trick.
• Choke first, transform second. Stop common‑mode RF with a 1:1 current balun at feedpoints and/or just after any unun or transformer.
• Let the tuner do the matching. On balanced systems with ladder line, use a current balun for balance and a tuner for the impedance work — don’t “fix” ladder line with a 4:1 by default.
• Place the tuner wisely — remote at/near the antenna if the coax would otherwise run at high SWR (especially on 20–10 m).
• Keep coax short when SWR is high — long lossy runs waste power, most noticeably on the higher HF bands.
• Respect power ratings — ferrites heat under mismatch; check CCS vs PEP specs.
• Measure, don’t guess. Use an analyzer or SWR bridge to confirm you’re fixing the antenna, not hiding the mismatch.

Quick rules of thumb

• Low SWR at the rig ≠ efficient antenna — it only means the radio is well matched.
• Balanced loads prefer balanced lines — and a current balun to enforce current symmetry.
• Use a ¼‑wave transformer only when you want a fixed, narrowband match: ZT = √(Zin × Zout).
• L‑networks are lowest‑loss; T and π types trade a bit of efficiency for wider range.
• Pick a 4:1 (or any ratio) only after you’ve measured the feedpoint impedance. Otherwise stick with a 1:1 current choke and a tuner.

Common pitfalls

• Using a 9:1 unun for an EFHW — EFHWs need 49:1 or 64:1 plus a 1:1 choke after it.
• No counterpoise on end‑fed wires → coax shield becomes the return path → RFI.
• Believing in “magic coax lengths.” Line length only moves where the mismatch shows up.
• Relying on tiny internal tuners for big jobs — they have limited range and duty cycle.
• Defaulting to a 4:1 at the feedpoint without measurements — it’s rarely the right choice unless the antenna actually sits near ~200 Ω (often requires proper height).

Starter checklist

• 1:1 current balun / choke at feedpoints and after any unun or voltage transformer, rated for your power.
• Only the impedance transformer you actually need (e.g., 49:1 EFHW, 9:1 random‑wire) — chosen by measurement, not habit.
• External tuner with enough matching range — balanced if you run ladder line.
• Quality feedline + weatherproofing.
• Proper counterpoise or radials for end‑fed / vertical antennas.

Micro‑glossary

• Impedance (Z): Resistance + reactance seen by RF, measured in Ω.
• SWR: Standing‑wave ratio at your measurement point; lower is kinder to your rig.
• Return loss: How much power is not reflected — higher (dB) is better.
• Current balun (choke): Forces equal and opposite currents; blocks common‑mode RF.
• Voltage transformer (balun/unun): Enforces a voltage ratio (impedance step), but doesn’t ensure current balance.
• Unun: Unbalanced ↔ unbalanced impedance transformer; usually voltage‑type in ham gear.
• ATU/Tuner: Network that lets the radio see ≈ 50 Ω even when the antenna doesn’t.

The bottom line

Your radio wants a 50 Ω, non‑reactive partner. If the antenna can’t provide that on every band, use the right translators — a current balun to keep RF where it belongs, a properly chosen unun/transformer only when the feedpoint impedance calls for it, and a tuner placed where it prevents (not hides) loss. Measure, adjust, and you’ll keep more of your watts in the air and out of the shack.

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