A balun matches a balanced antenna to unbalanced coax; an unun matches unbalanced to unbalanced. Most of them also transform impedance — and there's exactly one equation behind every ratio you've ever seen.
Zsecondary / Zprimary = (turns ratio)2
n = Ns / Npturns ratio (secondary ÷ primary)
Vs = n · Vp a transformer scales voltage by the turns ratio
Vp · Ip = Vs · Is it also conserves power (in = out)
=> Is = Ip / n so current scales the other way
Zp = Vp / Ip
Zs = Vs / Is = (n·Vp) / (Ip/n) = n² · (Vp/Ip) = n² · Zp
=> Zs / Zp = n² and so n = √(Zs / Zp)
That square is everything. Want to drop a load to 50 Ω? The turns ratio you need is just √(Z÷50). Run the numbers and the "weird" ratios stop being weird.
The two words just describe what sits on each side of the transformer:
Balun = BALanced ↔ UNbalanced. Feeds a balanced antenna (a dipole: two symmetric legs, neither tied to ground) from unbalanced coax (centre hot, shield near ground). The two legs carry equal, opposite currents.
Unun = UNbalanced ↔ UNbalanced. Feeds an unbalanced antenna (an end-fed wire or vertical: one wire working against ground/counterpoise) from unbalanced coax. Both sides are single-ended, referenced to ground.
What that means at the workbench:
A balun has two balanced output terminals (one per dipole leg) and wants symmetric windings plus good common-mode choking so the coax shield carries no current. Current-type (Guanella) baluns do this best.
An unun has one hot output terminal for the wire, plus a ground/counterpoise terminal shared with the coax shield. The 9:1, 49:1 and 56:1 here are all ununs — autotransformers with a single hot output.
An unun needs a counterpoise — the wire has to push against something. A dipole balun doesn't: the far leg is the return.
Use the wrong one — say an unun on a dipole — and common-mode current rides the coax: skewed pattern, RF in the shack, noisy receive.
The same core can be wound either way — it's how you reference the outputs that makes it a balun or an unun. The 1:1 is the classic example: a 1:1 balun at a dipole, a 1:1 unun/choke on a vertical or EFHW coax.
The ratios, side by side
Impedance ratio
Turns n = √ratio
Wind it (P:S)
Load it matches to 50 Ω
Typical antenna
1:1
1
1:1
50 Ω
Dipole / vertical (choking, not transforming)
4:1
2
1:2
200 Ω
Off-center-fed dipole, folded dipole
9:1
3
1:3
~450 Ω
End-fed random wire (+ tuner)
49:1
7
2:14
~2450 Ω
End-fed half-wave (EFHW)
56:1
7.48
2:15
~2800 Ω
EFHW on higher/longer installs
Notice 49 = 7² and the only whole-number-friendly way to land near 56 is 15÷2 = 7.5, and 7.5² = 56.25. That's literally why a "56:1" is wound 2:15 — there is no clean √56 in whole turns.
What you'll need
The core, by power level
Power handling tracks core volume, so size the toroid to your rig. These are Type 43 cores for the matching transformers (9:1, 49:1, 56:1):
Your power
Matching-transformer core
Notes
100 W
FT-240-43
Single core for SSB/CW; stack two for RTTY/FT8 or full key-down
50 W
FT-140-43
25 W
FT-114-43
10 W
FT-82-43
Great little QRP core
5 W (QRP)
FT-82-43 or FT-50-43
Smallest practical
Digital / continuous modes derate hard. SSB and CW are only ~25–50% duty cycle, but FT8, RTTY and AM are nearly 100% key-down, so the core sees roughly double the heating. For those, drop one power tier or stack two cores.
For a 1:1 common-mode choke, Type 31 mix gives a higher choking impedance across HF: FT-240-31 for 100 W, FT-140-31 for QRP, or just a clip-on string of Type 31/43 beads on the coax at low power.
Bill of materials — a 49:1 EFHW unun
Toroid core — from the table above (e.g. FT-240-43 for 100 W).
Enamelled (magnet) wire — about 1 m / 3 ft. Use 24–26 AWG for QRP, 22–24 AWG for 100 W (a touch heavier on the 2-turn primary).
Compensation capacitor — 100–150 pF rated ≥ 2 kV: silver-mica or C0G/NP0 ceramic for 100 W, a doorknob cap for high power.
Coax connector — SO-239 (or BNC for QRP).
Weatherproof enclosure — a small ABS/PVC project box.
Terminals — stainless bolts + wing nuts and ring lugs for the antenna wire and the counterpoise.
Strain relief — an eye-bolt or cord grip so the wire's pull isn't on the windings.
Weatherproofing — silicone sealant or self-amalgamating tape for outdoor use.
A 9:1 random-wire unun is the same list minus the capacitor. A 1:1 choke is simpler still: core + coax (or bifilar wire) + box + connectors.
1:1 current balun / choke
1:1 — the odd one out: no impedance change
Turns 1:1 | Type 43 toroid (FT-240-43) or a string of beads
A 1:1 doesn't transform impedance — its job is to present a high impedance to common-mode current so RF stays off the outside of your coax shield (which otherwise radiates, distorts your pattern, and bites you with RFI in the shack).
Wind 10–14 turns of RG-316/RG-400 coax through an FT-240-43, or 11–13 bifilar turns of enamelled wire.
Goal: a choking impedance above ~1–4 kΩ across the bands you use. More turns helps the low bands; too many adds stray capacitance that hurts the high bands.
Use it at the feedpoint of dipoles and inverted-Vs, and on the coax a metre back from any EFHW/vertical.
1:1 means equal turns on each side — no tap, no impedance change. The whole job is choking common-mode current.
In practice you build it as 10–14 turns of coax (or bifilar wire) passed through the core — that is the 1:1 current balun. It simply blocks current from flowing on the outside of the shield.
4:1 balun
4:1 — 200 Ω down to 50 Ω
Turns 1:2 (n = 2, 2² = 4) | FT-240-43
Guanella (current, preferred): two equal transmission-line windings of ~100 Ω; join them in parallel on the 50 Ω side and in series on the 200 Ω side. Best common-mode rejection.
Ruthroff (voltage, simpler): one bifilar winding of 10–12 turns, cross-connected. Cheaper and smaller; fine through ~30 MHz.
Use it for off-center-fed dipoles (~200 Ω) and folded dipoles. Add a 1:1 choke behind it for clean common-mode control.
The 1:2 turns ratio that makes 4:1. Shown as a tapped autotransformer (a 4:1 unun); for a balanced OCFD you'd wind the Guanella current-balun version of the same ratio.
The same 1:2 winding on the core — orange is the primary the coax taps across.
Twist three enamelled wires together (trifilar) and wind 8–10 turns, then connect the three windings as a 3:1 autotransformer.
A non-resonant end-fed wire sits in the rough neighbourhood of a few hundred ohms; the 9:1 lands it near 50 Ω so your tuner has an easy job on every band.
Always give it a counterpoise (the wire needs something to push against). This is the match for the random-wire antenna in the modeler.
1:3 turns → 9:1. Drawn as a tapped autotransformer; in practice it's wound trifilar — three wires twisted together and joined to give the 1:3 ratio.
On the core: the coax taps across the lower third of the winding, the wire takes the full length.
Autotransformer: wind a 14-turn secondary of 24–26 AWG enamelled wire, and tap the coax centre conductor 2 turns up from the ground end. The shield and the counterpoise go to that ground end; the antenna wire goes to the far end of the 14 turns.
Add a 100–150 pF capacitor (rated ≥ 2 kV: silver-mica or C0G) across the 50 Ω input. It cancels the transformer's leakage inductance so 10–15 m don't go high on SWR.
Add a short counterpoise (~0.05 λ) on the ground side. This is the feed for a half-wave end-fed (EFHW).
For more power, stack two cores or move to Type 31; bigger/more core = more flux headroom = less heat.
Wiring: a 14-turn winding with the coax centre tapped 2 turns up from the ground end. The 100–150 pF cap sits across the 50 Ω input (the 2-turn primary).
The same thing on the toroid: wind 14 turns (orange = the first 2, the primary). Coax centre to the 2-turn tap; coax shield + counterpoise to the ground end; the half-wave wire to the far end.
56:1 unun
56:1 — ~2800 Ω down to 50 Ω
Turns 2:15 (n = 7.5, 7.5² = 56.25) | same core, wire and cap as the 49:1
Built exactly like the 49:1 but with a 15-turn secondary tapped at 2 turns. The single extra turn pushes the match toward a higher feed impedance.
Why bother? A real EFHW feedpoint isn't always 2450 Ω — raise the wire higher, or run certain band sets, and it drifts up toward 2800–3000 Ω. Matching to 56:1 (or 64:1 = 8:14…16) then gives a flatter SWR than a 49:1 would.
Keep the same 100–150 pF compensation cap and a short counterpoise.
Identical to the 49:1 but a 15-turn secondary tapped at 2: 15÷2 = 7.5, and 7.5² = 56.25:1.
On the core: 15 turns, the first 2 (orange) are the primary the coax taps across.
Cores, wire & testing
Core mix: Type 43 ferrite is the HF default (good 1–30 MHz). Type 31 handles higher power and the low bands better but gets lossy up high. Use FT-240 size for ~100 W, FT-114 for QRP.
Wire: enamelled (magnet) wire for the transformers; PTFE-jacketed wire or thin coax for 1:1 chokes. Spread windings evenly around the core.
Test it: terminate the high-Z side with a non-inductive resistor of the target value (2450 Ω for a 49:1) and sweep with an analyzer/VNA — you should read close to 50 Ω with low SWR across HF. If it climbs on 10 m, tweak the compensation cap.
Tune the antenna, not the transformer: on an EFHW you set resonance by trimming the wire. If the core runs hot on key-down, the match is off or the core is undersized.