================================================================================ BALUN 6:1 TRIFILAR — 300Ω WINDOW LINE MATCHING ================================================================================ Purpose: Impedance transformation 300Ω balanced → 50Ω unbalanced Type: Trifilar current balun (three wires wound together) CM Suppression: Excellent (ferrite + trifilar symmetry) Insertion Loss: 0.15–0.20 dB (single core, good balance of loss/size) Primary Use: Folded dipoles, 300Ω open-wire feedlines, broadband matching ================================================================================ CIRCUIT DIAGRAM — 6:1 TRIFILAR BALUN ================================================================================ ┌────────────────────────────┐ │ 300Ω BALANCED FEED │ │ (window line, open-wire) │ └──────────┬─────────────────┘ │ ┌────────┴─────────┐ │ │ Leg A Leg B │ │ ┌──┴──────┐ ┌──────┴──┐ │ Trifilar│ │ Trifilar │ │ 5 turns │ │ 12 turns │ │ on │ │ (5:12 = │ │FT-240-43│ │√6:1 ratio) │ core │ │FT-240-43 │ └──┬──────┘ └──────┬──┘ │ │ ├────────┬─────────┤ │ Coax │ ┌─────┴────┐ │ 50Ω Out │ │ (unbal. │ │ to coax)│ └──────────┘ ================================================================================ CORE SPECIFICATIONS ================================================================================ HF Configuration (80M–10M): Core: Single FT-240-43 (some designs use FT-140-43 @ 20M+) Material: Mix 43 (µ=850, AL=1100 µH/100T) Wire: #16 enameled copper Turns: 5 primary, 12 secondary (exact 2.4:1 ratio) Total turns on core: 5+12 = 17 wire passes (= 17 turns bifilar secondary) Max Power: 450 W Insertion Loss: 0.15–0.20 dB Band-Specific Configuration: 80M (3.75 MHz): 5 primary, 12 secondary (FT-240-43) 40M (7.15 MHz): 4 primary, 10 secondary (FT-240-43) 20M (14.2 MHz): 3 primary, 7 secondary (FT-140-43 adequate) 10M (28.4 MHz): 3 primary, 7 secondary (upper HF limit) Wire Lengths: Primary: ~60 inches (5 turns, large core) Secondary: ~120 inches (12 turns) Total: ~180 inches per balun ================================================================================ IMPEDANCE TRANSFORMATION THEORY — TRIFILAR ================================================================================ The 6:1 balun uses TRIFILAR configuration (three separate wire paths). TURNS RATIO INTERPRETATION: N_ratio = 5:12 = 1:2.4 Z_ratio = N_ratio² = (2.4)² ≈ 5.76 ≈ 6:1 (nominal 6:1) VOLTAGE TRANSFORMATION: V_out / V_in = 1 / 2.4 ≈ 0.42 (voltage step-down) V_in × I_in = V_out × I_out (power conservation) IMPEDANCE TRANSFORMATION: Z_out = Z_in / N² = 300 / 5.76 ≈ 52Ω ✓ TRIFILAR ADVANTAGE: Three wire paths provide balanced transformation Each leg of antenna sees equal impedance and phase No imbalance → minimal CM current generation ================================================================================ WINDING TECHNIQUE — TRIFILAR ================================================================================ The trifilar configuration is more complex than bifilar. Materials: - 1× FT-240-43 core - 3× spools of #16 enameled copper wire - Small vice and wire-guiding jig Step 1: Prepare three wire strands - Cut three equal lengths (~180 inches each) - Stripe ends ~0.5" (for connection) - Do NOT twist together yet (keep parallel) Step 2: Wind three wires simultaneously - Hold all three strands parallel - Feed through core hole together - Wrap around core in synchronized pattern - Each "pass" = 3 turns (one per strand) - 5 passes = 5 turns per strand = 15 total wire turns Step 3: Separate and reconnect - Group wires 1+2 as "primary" (lower impedance) - Group wire 3 + center-tap as "secondary return" - Primary connections: Solder wires 1+2 to antenna legs A and B - Secondary center tap: Between primary coil and secondary - Secondary output: Solder wire 3 to coax center Step 4: Alternative winding (2+1 split) - Some designs: Wind 5 turns total, then add 12 turns of separate secondary - Less balanced, but works if primary/secondary isolated by ferrite Step 5: Weatherproofing - Trifilar is delicate (three separate wires) - Potting recommended (protects against environmental damage) - Use silicone (flexible, won't crack enamel) ================================================================================ COMMON-MODE OPERATION — TRIFILAR ================================================================================ Differential (desired): Each wire carries different signal components Three-phase-like distribution cancels external fields Z_differential ≈ 50Ω (design impedance) Common-Mode (unwanted): All three wires carry identical current Ferrite core impedance: Z_CM = 2π × f × L_core Example @ 80M (3.75 MHz), FT-240-43, 5 turns primary + 12 turns secondary: Total turns on core: effectively "17 turns" for CM path L_CM ≈ 1100 × 17² / 10000 = 31.8 µH (higher than simple coil) Z_CM = 2π × 3.75×10⁶ × 31.8×10⁻⁶ ≈ 750 Ω (excellent!) DESIGN BENEFIT: Trifilar provides better CM suppression than bifilar or voltage balun Due to longer physical winding path on single core ================================================================================ FREQUENCY RESPONSE ================================================================================ FT-240-43, 5:12 turns (with adaptive turns for higher frequencies): Frequency Turns Z_in/Z_out Z_CM Loss 80M 5:12 300→50 750 Ω 0.20 dB 40M 4:10 300→48 520 Ω 0.18 dB 30M 4:9 300→47 480 Ω 0.17 dB 20M 3:7 300→45 380 Ω 0.15 dB 15M 3:7 300→45 350 Ω 0.13 dB 10M 3:7 300→45 320 Ω 0.11 dB Note: Z_in varies slightly across band (300Ω nominal at each frequency) Design adjusted by turns count per band ================================================================================ PRIMARY APPLICATIONS ================================================================================ 1. OPEN-WIRE 300Ω FEEDLINES: - G5RV multiband antenna (80M–10M) with ladder line - Folded dipole antennas (nominal 300Ω impedance) - High-efficiency broadband systems 2. WINDOW LINE ANTENNAS: - Commercial 300Ω twin-lead or window line - Broad bandwidth (low loss, matching across multiple bands) 3. BALANCED MATCHING NETWORKS: - Antenna tuner with balanced input port - Tuner sees 300Ω load, outputs 50Ω coax 4. BROADBAND DIPOLE ARRAYS: - Coupled dipoles operating as balanced system - 6:1 balun converts to standard 50Ω 5. DELTA LOOP ANTENNAS: - Some delta loops exhibit ~300Ω feedpoint impedance - 6:1 transformation provides low-loss match ================================================================================ TESTING PROCEDURE — NANOVNA ================================================================================ Setup: Port 1: Coax 50Ω output Port 2: Primary input (via 50Ω adapter to maintain reference impedance) OR: Use test load (300Ω resistor across primary) Measurement 1 — Impedance Ratio (300Ω load): Sweep 1–30 MHz Expected: S₁₁ <-20 dB (excellent match) Z_in ≈ 300Ω (verify with 300Ω load connected) Measurement 2 — Insertion Loss: Sweep with 300Ω load Expected: S₂₁ ≈ -0.15 to -0.20 dB across band Measurement 3 — Phase balance (trifilar check): Measure phase of three wires @ primary Expected: Equal phase within ±10° (indicates balanced winding) Measurement 4 — Common-Mode Impedance: With primary open, measure Z_CM to ground Expected: Z_CM >300Ω @ design frequency, rising with frequency ================================================================================ FAILURE MODES ================================================================================ SYMPTOM: Very high SWR (>3:1) on 300Ω antenna ROOT CAUSE 1: Trifilar winding incorrect (one wire misaligned) SOLUTION: - Carefully unwind - Re-wind ensuring all three wires synchronized - Check with ohmmeter (all three should have similar resistance) ROOT CAUSE 2: One wire of trifilar strand broken SOLUTION: - Check continuity on each of three wires - If broken, unwind and repair the broken strand SYMPTOM: Imbalance detected (pattern distorted) ROOT CAUSE: One wire path has higher loss than others SOLUTION: - Check wire connections for cold solder joints - Verify enamel insulation intact on all three strands - Consider re-soldering primary connections SYMPTOM: Excessive heating ROOT CAUSE: High CM current flowing (antenna imbalance) SOLUTION: - Verify antenna is symmetrical (equal leg lengths) - Add secondary 1:1 choke if heating persists - Check for RF in shack (indicates CM current) ================================================================================ POWER HANDLING ================================================================================ FT-240-43 single core, continuous operation: 450 W @ 50°C ambient Thermal rise: ~20°C @ 400W Thermal rise: ~35°C @ 700W For higher power (>500W): Option 1: Use FT-240-43×2 stacked (doubles core area, halves loss) Option 2: Reduce number of turns (lowers core loss, increases SRF) Option 3: Potting with high-thermal-conductivity compound ================================================================================ DESIGN EQUATIONS ================================================================================ Turns ratio for 300→50Ω transformation: N_ratio = sqrt(Z_in / Z_out) = sqrt(300/50) = sqrt(6) ≈ 2.45 Use N_primary : N_secondary = 1 : 2.45 Practical: 5:12 (ratio = 2.4, impedance ratio = 5.76 ≈ 6) Alternative combinations: 4:10 (ratio = 2.5, Z = 6.25) 3:7 (ratio = 2.33, Z = 5.43) — use for higher frequencies Insertion Loss: L_dB ≈ 0.1 + 0.002 × f_MHz (single FT-240-43) Example @ 14.2 MHz: L ≈ 0.13 dB Common-Mode Impedance: Z_CM = 2π × f × L_total where L_total accounts for ALL turns on core (primary + secondary) ================================================================================ CONSTRUCTION NOTES ================================================================================ Mechanical Assembly: - Trifilar is delicate (do not stress wires during installation) - Support balun on antenna mast or feedpoint bracket - Keep three wires separate (avoid crushing) Potting Recommendation: - Silicone potting highly recommended for trifilar - Protects enamel from UV and environmental moisture - Allows thermal expansion without cracking (unlike epoxy) Weatherproofing: - Enclosure: Plastic or aluminum, with O-ring seal - Drain hole: 1/16" for moisture prevention - Cable glands: Stainless steel, with strain relief Field Deployment: - Mount at antenna feedpoint (minimize high-impedance feedline length) - Counterpoise or ground plane recommended (improves CM suppression) - Use open-wire or window line (do NOT use coax on primary) Spare Parts: - Keep extra FT-240-43 cores (can retro-fit existing winding) - Stock #16 enameled wire (can rewind if needed) - Pre-made connectors (SO-239, BNC) for field repair ================================================================================ COMPARISON: 6:1 TRIFILAR vs OTHER BALUNS ================================================================================ Feature 6:1 Trifilar 4:1 Guanella 9:1 Trifilar ────────────────────────────────────────────────────────── Impedance 300→50Ω 200→50Ω 450→50Ω CM Suppression Good (750Ω@80M) Excellent Fair (500Ω) Insertion Loss 0.15–0.20 dB 0.20–0.25 dB 0.25–0.35 dB Winding Ease Moderate Difficult Difficult Power Handling 450 W 350–400 W 400 W Cost ~$15 ~$18 ~$16 Recommended Window line, Balanced 450Ω ladder Use Case folded dipole feedlines line (G5RV) ================================================================================