49:1 or 64:1 Unun
Matches the very high impedance (~2,500–3,500 Ω) at the end of a half-wave wire down to 50 Ω. One wire, no tuner, works on 40/20/15/10 m. The POTA favorite.
KENTUCKY PORTABLE CLUB • BUILD IT YOURSELF
UNUNS, BALUNS & CHOKES
Wind your own end-fed transformer, 9:1 unun or coax choke for a few dollars. Which core, how many turns, how to wire it, and how to test it before you take it to the park.
START HERE
Which One Do You Need?
All of these are coils of wire on a ferrite ring ("toroid"). What changes is the ratio and how they're wired.
9:1 Unun
Brings a "random" length wire into range of your tuner. Needs a tuner and a counterpoise, but works on almost any band with one wire.
1:1 Choke / Balun
Stops RF from flowing back down the outside of your coax — the cause of RF in the shack, hot mics, noisy receive and odd SWR. Put one on dipoles, verticals and end-feds.
4:1 Balun
For off-center-fed dipoles and some loops (~200 Ω). Best built as two 1:1 chokes: inputs side by side, outputs in series (a "Guanella" current balun).
BUILD PLANNER
Plan Your Build
Pick what you're building and how much power you run. You get the core, turns, wire, parts and a test resistor value.
Ratings here are conservative starting points. Always do the heat test (below) before a long run — a hot core means go bigger.
HOW IT CONNECTS
Wiring Diagrams
Each "turn" is one pass of wire through the center hole of the core. Count passes, not wraps around the outside.
Tapped Autotransformer
One wire, 14 passes through the core. Pull out a little loop after turn 2 and scrape it — that's the tap. Start → coax shield and counterpoise. Tap → coax center. End → antenna wire. The 100 pF capacitor goes across the coax connector. (64:1: same thing with 16 turns.)
Three Wires Twisted (Trifilar)
Twist three colors together and wind them as one — 9 passes. Then join them in a chain: A start → shield and counterpoise. A end + B start → coax center. B end + C start → joined to each other only. C end → the wire. The coax sees one wire, the antenna sees all three: 9× the impedance.
Two Cores, Two Wires Each
Wind two cores the same: two wires side by side, 10 passes. On the radio side the cores are in parallel: both A's → coax center, both B's → shield. On the antenna side they're in series: core 1 B joins core 2 A, and the two leftover ends are your balanced outputs.
Coax Through a Toroid
No separate wire — the coax is the winding. Pass it through the hole 8–14 times (see the recipes below), spread evenly around the ring, and keep the two ends apart. Connectors on both ends and it goes in-line anywhere.
Dots mean wires are joined (solder them). Lines that cross without a dot don't touch. Colors are just to tell the wires apart — use any colors you have.
MORE RATIOS
Other Transformers You Might Need
Less common, but they come up as soon as you try receive antennas, loops or random wires.
Isolation Transformer (3 : 9 turns)
Two separate windings that never connect: 3 turns to the coax, 9 turns to the antenna. That's a 9:1 impedance step-up, and the isolation keeps coax-shield noise out. Receive only, so thin magnet wire on a small mix 73 or 43 core is plenty. Turns helper
4:1 Unun
Two wires side by side, 10–12 turns (bifilar), like the 9:1 but with two wires:
- A start → coax shield / counterpoise
- A end + B start → coax center
- B end → the antenna wire
Use it on a vertical or a wire that's around 200 Ω. It's not for balanced antennas. For an OCF dipole or loop, use the 4:1 Guanella above.
Bifilar Current Balun
Same job as the coax choke, but with screw terminals for the dipole legs. Wind 10–12 bifilar turns on an FT-240-31 (FT-140-43 for QRP):
- Wire A: coax center → one dipole leg
- Wire B: coax shield → the other leg
Use twisted pair or PTFE wire so the two wires stay close and evenly spaced.
2:1 Match
The easiest 2:1 has no core: a quarter-wave of 75 Ω coax. The loop calculator gives the length. Commercial 2:1 ununs also work. Either way, add a 1:1 choke after it.
COMMON-MODE CHOKES
Choke Recipes
Aim for at least 1,000 Ω of choking — 3,000–5,000 Ω is excellent. These are proven starting points; a NanoVNA lets you check yours.
| BANDS | CORE | COAX & TURNS | POWER |
|---|---|---|---|
| 40 – 10 m | FT-240-43 | 8–10 turns RG-8X or RG-58 | 100 W |
| 80 – 10 m | FT-240-43 | 10–12 turns RG-8X or RG-58 | 100 W |
| 160 – 80 m | FT-240-31 | 12–14 turns RG-8X or RG-58 | 100 W |
| 160 – 10 m | FT-240-31 + FT-240-43 (two chokes in series) | 10–12 turns on each | 100 W |
| 20 – 6 m | FT-240-61 | 8–10 turns RG-8X or RG-58 | 100 W |
| QRP, 40 – 10 m | FT-140-43 (or FT-114-43) | 10–12 turns RG-316 or RG-174 | 10–20 W |
| Quick fix | Clip-on mix-31 beads | Several snapped on the coax at the feedpoint | Any |
Dipoles & Verticals
Right at the feedpoint, where the coax meets the antenna. A second choke at the radio end helps with RF in the shack.
EFHW Antennas
The coax shield is part of an end-fed's counterpoise. Put the choke a short way down the coax — about 0.05 wavelength (roughly 7 ft on 40 m) from the transformer — or at the radio end.
Air-Wound "Ugly Balun"
6–8 turns of coax about 6" across, taped together. It only works well on one or two bands and is weaker than ferrite — fine in a pinch.
PICKING A CORE
Ferrite Mixes & Core Sizes
Part numbers read FT-(outside diameter in hundredths of an inch)-(mix). An FT-240-43 is 2.4" across, mix 43.
| MIX | BEST FOR | USE IT FOR |
|---|---|---|
| 31 | 160 – 30 m (low HF) | Chokes for the low bands; pair with 43 for full-HF chokes |
| 43 | 80 – 10 m (general HF) | EFHW and 9:1 transformers, most HF chokes — the go-to mix |
| 52 | 40 – 6 m | Alternative to 43 for chokes, a little better up high |
| 61 | 20 m – VHF | Upper-HF/6 m chokes and low-loss transformers |
| Powdered iron (-2 red, -6 yellow) | Tuned circuits | Traps, filters and some 9:1 ununs (T-130-2, T-200-2). Low loss, but much lower inductance — needs more turns |
0.82" core
QRP only (~10 W SSB/CW, ~5 W FT8). 24–26 AWG wire. Tiny and light for SOTA.
1.14" core
QRP to ~25 W. 22–24 AWG. Good middle ground for backpack builds.
1.4" core
Up to ~50 W SSB/CW (~25 W FT8) on an EFHW. 20–22 AWG. Most portable POTA builds.
2.4" core
100 W and up (~50 W FT8 on an EFHW). 16–18 AWG. Two stacked for more power.
THE KNACK
Winding Tips
- Use magnet wire. Enameled copper ("magnet wire") is standard. Scrape or sand the enamel off the ends — or use solderable enamel and a hot iron — then tin them.
- Protect the wire. Ferrite edges can nick enamel. A wrap of PTFE (plumber's) or Kapton tape on the core first helps on bigger builds.
- Count passes through the hole. Every time the wire goes through the center, that's one turn. Mark the start with tape so you don't lose count.
- Spread turns evenly. Cover about ⅔–¾ of the ring and leave a gap between the start and end, where the voltage difference is highest.
- Twist bifilar/trifilar wires first. Twist 3–4 twists per inch before winding so the wires stay together. Use different colors (or mark the ends) so you can tell A, B and C apart.
- Pick the right capacitor. EFHW: 100–150 pF across the coax connector, rated 1 kV for QRP and 3 kV or more at 100 W. Ceramic or silver-mica. It flattens SWR on the high bands.
- Box it. A small project box, an SO-239 or BNC, and stainless bolts or binding posts for the wire and counterpoise. Add an eye-bolt for strain relief so the wire tension pulls on the box, not the winding.
- Seal it. A dab of hot glue or silicone holds the core still. Drill a tiny drain hole at the bottom for condensation.
- Careful at the end of an EFHW. The far end of the wire (and the transformer output) runs at high voltage — hundreds to thousands of volts at 100 W. Keep it out of reach while transmitting.
BEFORE THE PARK
Testing Your Build
Dummy Load Check
Put a resistor equal to 50 × the ratio across the output and check SWR on each band with an analyzer or NanoVNA:
- 49:1 → 2,450 Ω (2.4 kΩ works)
- 64:1 → 3,200 Ω (3.3 kΩ)
- 9:1 → 450 Ω (470 Ω)
- 4:1 → 200 Ω
Use plain carbon or metal-film resistors, low power only. Under 1.5:1 across your bands is a good build.
Is the Core Big Enough?
Hook it to the real antenna, send a steady carrier (or FT8) for 30–60 seconds at your normal power, then feel the core. Warm is OK; hot means losses — use a bigger core, a second core, or less power.
Measure the Choke
With a NanoVNA, connect the coax shield ends of the choke across the analyzer (S21 "through" or series measurement) and read the impedance across your bands. Over 1,000 Ω is working; over 3,000 Ω is great.
More: Transformer turns calculator · Antenna wire lengths · Random-wire length finder · SWR basics
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