Center-fed dipoles, loops, Yagis, feed-line cleanup
Use when you mostly need to stop RF on the outside of the coax shield, not change impedance.
NF1J Balun Bench
DIY baluns, UNUNs, chokes, and matching transformers.
A focused builder page for common HF projects: 1:1 current chokes, 4:1 Guanella baluns, 9:1 random-wire UNUNs, and 49:1 / 64:1 end-fed half-wave transformers.
CHOKE Common-mode control
UNUN End-fed matching
TEST Low power first
Builder Safety Notes
Ferrite work is simple, but RF voltage can be serious.
Use these as starting plans. Core mix, core size, wire insulation, duty cycle, antenna impedance, and mismatch all affect heating and safe power.
- Test at low power with a dummy load or known resistor first.
- Weatherproof outdoor boxes and add strain relief on coax and wire terminals.
- Check for core heating after short transmissions before increasing power.
Turns Ratio Helper
Impedance ratio quick calculator
Transformer impedance ratio is approximately the square of the turns ratio. Real antennas are not fixed resistors, so always confirm with an analyzer.
Which One Do I Need?
Fast selection guide
The name tells you two things: balanced vs unbalanced and impedance transformation ratio.
About 200 Ω balanced to 50 Ω coax
Common with some loops, folded dipoles, and balanced-feed antennas when the feed impedance is near 200 Ω.
Non-resonant end-fed or random wire with tuner
A matching aid, not magic. Usually needs a tuner, counterpoise, and often a separate feed-line choke.
End-fed half-wave wire antennas
For EFHW antennas with high feed impedance. Often used with a half-wave wire cut for the desired band.
Build Guides
DIY balun and UNUN plans
Each card includes a clean wiring concept, a starter winding plan, materials, tests, and practical notes.
Most useful first build
1:1 Current Balun / Common-Mode Choke
Use this at the feed point of a dipole, vertical, loop, Yagi, or near the shack to reduce RF coming back on the outside of the coax shield.
Ratio1:1 impedance, choke action
Starter winding8–12 turns coax on FT240-31 or FT240-43 style core
Best useCommon-mode current reduction
Parts: ferrite toroid or ferrite beads, RG-58/RG-316/RG-400 coax, enclosure, SO-239/BNC/N connectors, strain relief.
- Wind the coax turns neatly through the ferrite core.
- Keep turns snug but do not kink the coax.
- Mount in a box with connectors and strain relief.
- Test SWR with a 50 Ω dummy load on the output.
Balanced feed
4:1 Guanella Current Balun
A useful project when the balanced antenna or feed system is near 200 Ω and you want to feed it from 50 Ω coax.
Ratio4:1 impedance, 2:1 turns relationship
Starter windingTwo equal bifilar windings, commonly 8–12 turns each
Best useBalanced loads near 200 Ω
Parts: two ferrite cores or one large binocular/core set, two identical transmission-line windings, enclosure, coax connector, balanced output terminals.
- Make two identical 1:1 current transformer windings.
- Wire the coax side in parallel and the load side in series.
- Keep both windings symmetrical and the same length.
- Test using a 200 Ω noninductive load on the output.
Random wire + tuner
9:1 UNUN
A 9:1 UNUN can make a non-resonant end-fed wire easier for a tuner to match, especially with a counterpoise and feed-line choke.
Ratio9:1 impedance, 3:1 turns relationship
Starter windingTrifilar 8–10 turns on ferrite toroid
Best useRandom wire / non-resonant end-fed with tuner
Parts: ferrite core, three equal insulated wires, wire terminal, counterpoise/ground terminal, coax connector, weatherproof enclosure.
- Twist or bundle three equal insulated wires together.
- Wind 8–10 turns through the core as a trifilar set.
- Wire as an autotransformer for 9:1 impedance transformation.
- Use a tuner and add a counterpoise for more predictable results.
End-fed half-wave
49:1 / 64:1 EFHW Transformer
Used with an end-fed half-wave wire. A 49:1 transformer is commonly wound 2:14 turns; 64:1 is commonly 2:16 turns.
49:12 primary turns, 14 secondary turns
64:12 primary turns, 16 secondary turns
Best useEFHW wire, often around 2,500–3,200 Ω feed impedance
Parts: large ferrite toroid, enamel or PTFE wire, 100 pF high-voltage capacitor optional, antenna terminal, counterpoise terminal, coax connector, weatherproof box.
- Wind two primary turns first, then continue the secondary turns in the same direction.
- Keep high-voltage antenna terminal clearance generous.
- Add an optional high-voltage compensation capacitor across the primary for upper HF bands.
- Test with a half-wave wire and tune wire length before judging the transformer.
Bench Test
Simple tests before the box goes outside
These tests catch bad wiring before you connect an antenna and wonder why nothing tunes.
Load Test
Put a noninductive resistor equal to the transformed load on the antenna side, then check SWR from the radio side.
1:1 → 50 Ω
4:1 → 200 Ω • 9:1 → 450 Ω • 49:1 → about 2450 ΩHeat Test
Transmit briefly at low power, then feel for warming. Increase power gradually only if SWR and temperature stay reasonable.
Low power first
High mismatch can heat ferrite faster than expected.Common-Mode Check
If RF is in the shack, audio buzz appears, or SWR changes when touching coax, add or improve choking on the feed line.
Choke the coax
Current/choke baluns are often the most useful fix.References
Trusted reading for deeper RF design.
These links are included so builders can compare the starter diagrams against deeper technical explanations and commercial examples.