Chapter 1 — Introduction and Scope
This manual covers five balun and un-un designs for antenna feedpoint impedance transformation and common-mode current suppression: 1:1 current balun (choke balun), 4:1 Guanella current balun, 6:1 trifilar transmission-line transformer, 9:1 trifilar (for 450Ω end-fed wires), and 1:1 air-core VHF/UHF choke balun.
A balun (balanced-to-unbalanced) interfaces a coaxial line (unbalanced) to a balanced antenna (dipole, loop). An un-un interfaces two unbalanced impedances (e.g., 50Ω coax to 450Ω random wire). Both suppress common-mode currents on the feedline.
Chapter 2 — Theory of Operation
2-1 Current vs. Voltage Balun
A current balun (choke balun) enforces equal and opposite currents in the two antenna elements by presenting high impedance (Z_choke ≥2000Ω) to common-mode current on the coax shield. The differential mode (transmit signal) sees only the low impedance of the coax. A current balun does NOT transform impedance; it only suppresses common-mode.
A voltage balun enforces equal and opposite voltages. It is simpler to build but allows common-mode current if the antenna is not symmetrical. Current baluns are preferred for dipoles and loops; voltage baluns may be used when impedance transformation is also needed.
2-2 Transmission-Line Transformer (TLT)
TLTs (Guanella, trifilar) use transmission-line principles rather than magnetic coupling alone. The characteristic impedance of the transmission line on the core determines the transformation ratio. For a 4:1 Guanella: two equal-impedance lines are connected in series at the input and parallel at the output, giving a 4:1 impedance ratio.
Impedance ratio relationships:
1:1 choke: Z_in = Z_out = 50Ω (no transformation; common-mode choke only) 4:1 Guanella: Z_in = 200Ω balanced → 50Ω unbalanced 9:1 trifilar: Z_in = 450Ω unbalanced → 50Ω unbalanced
Chapter 3 — Equipment and Materials
| Design | Core | Wire | Turns | Freq range |
|---|---|---|---|---|
| 1:1 current (choke) | FT-240-31 | RG-303 coax, 12 passes | 12 | 1.8–30 MHz |
| 4:1 Guanella | 2× FT-140-43 | #16 bifilar, 8 turns | 8 | 1.8–30 MHz |
| 6:1 trifilar | FT-140-43 | #18 trifilar, 10 turns | 10 | 3.5–30 MHz |
| 9:1 trifilar | FT-240-43 | #16 trifilar, 9 turns | 9 | 1.8–30 MHz |
| 1:1 VHF/UHF air-core | None (air core) | RG-58 coax, 6 turns | 6 | 50–450 MHz |
Chapter 4 — Construction
4-1 1:1 Choke Balun (FT-240-31)
- Thread RG-303 (or RG-58) coaxial cable through the toroid 12 times. Keep turns close-wound; secure with cable ties after every 3 turns.
- Connect the input end to an SO-239 or N-type connector (coax center to center pin, shield to shell). Same for the output end.
- Measure choking impedance with NanoVNA: connect one port to the coax center, other port to the coax shield, and sweep 1.8–30 MHz. The |Z| should exceed 2000 Ω across most of the HF band. Core type #31 is optimized for 1.8–30 MHz.
4-2 4:1 Guanella Balun
- Wind two identical 8-turn bifilar windings on two separate FT-140-43 cores. Use color-coded wire to track polarity (dot convention).
- Connect the two windings in series for the balanced (200Ω) port and in parallel for the unbalanced (50Ω) port. Polarity is critical: the start of winding 1 connects to the finish of winding 2 at the balanced port center tap.
- Test: connect a 200Ω resistor to the balanced port; measure 50Ω at the unbalanced port (SWR <1.1:1).
4-3 9:1 Un-Un (End-Fed Wire)
- Wind 9 turns of trifilar #16 AWG on an FT-240-43 core. Three wires wound simultaneously, in the same direction.
- Connect winding A in series with winding B; the junction is the 50Ω (unbalanced input) center tap. Winding C is the 450Ω output winding.
- Test: connect 450Ω resistor to high-impedance port; measure 50Ω SWR at the coax port (<1.2:1 across 1.8–30 MHz).
Chapter 5 — Operating Procedures
- Mount the balun at the antenna feedpoint — not at the radio end. Feedpoint mounting minimizes the length of balanced feedline and the associated common-mode current problem.
- Weatherproof outdoor installations: coat all solder joints and exposed windings with two coats of polyurethane or Plasti-Dip. Wrap the core with self-amalgamating tape before mounting outdoors.
- Check SWR after installation. If SWR is higher than expected, the most common causes are: wrong impedance ratio for the antenna type, poor solder joint on the balun output terminals, or water ingress.
Chapter 6 — Calibration
- Measure impedance transformation ratio: connect a known resistive load (e.g., 200Ω for a 4:1 balun) to the balanced port; measure impedance at the unbalanced port with a NanoVNA. Target: 50Ω ±10%.
- Measure common-mode impedance (choke action): connect NanoVNA port 1 to coax shield at input; port 2 to coax shield at output. Z_CM should exceed 2000Ω across 1.8–30 MHz for type #31 core.
- Measure insertion loss: S21 through the balun into a matched load. Target: <0.3 dB at all HF frequencies.
Chapter 7 — Verification and Acceptance
- Impedance ratio correct to within ±10% at center frequency.
- Common-mode impedance ≥1000Ω at all specified frequencies.
- Insertion loss <0.3 dB at 1.8–30 MHz.
- SWR <1.2:1 at all test frequencies with correct load.
- Power test: key a 100W transmitter into a dummy load through the balun for 30 seconds. No smoke, no temperature rise exceeding 40°C above ambient.
- Log: date, design type, core material, turns, ratio test result, CM impedance, operator.
Appendix A — Core Material Selection
| Material | Best frequency range | Application |
|---|---|---|
| #31 ferrite | 1–300 MHz | Choke baluns (1:1); excellent CMC for all HF |
| #43 ferrite | 1–100 MHz | TLT baluns (4:1, 9:1); HF transformation |
| #61 ferrite | 10–200 MHz | 6M and VHF TLTs |
| #67 ferrite | 50–500 MHz | VHF/UHF choke baluns and TLTs |
| Air core | 50–1300 MHz | VHF/UHF choke (coax coil, no saturation risk) |
Appendix B — Winding Polarity Verification
Use a 1.5V battery and a voltmeter. Connect battery across winding 1. Measure voltage across winding 2. If polarity is correct (dot-to-dot), voltmeter reads positive. If reversed, the windings are connected out of phase and the balun will not function correctly. Reverse one winding's connections to correct.