================================================================================ UNUN 4:1 — RANDOM WIRE IMPEDANCE TRANSFORMER ================================================================================ Purpose: Impedance transformation 200Ω unbalanced → 50Ω unbalanced Type: Voltage transformer (simple 2:1 turns ratio unun) Common-Mode Suppression: None (impedance transformation only) Insertion Loss: 0.08–0.12 dB (single core, no CM choke) Primary Use: Unbalanced random wire antennas, feedpoint matching ================================================================================ CIRCUIT DIAGRAM — 4:1 UNUN (VOLTAGE) ================================================================================ Antenna (Random Wire) │ ├─────────────┐ │ │ 200Ω (other end feedpoint to tuner GND) │ │ ├─[UNUN 4:1]──┤ │ 3:6 │ │ turns on │ │ FT-240-43 │ │ │ ├─────────────┤ │ Coax 50Ω │ ┌───┴────┐ │ Tuner │ │ Input │ │ 50Ω │ └────────┘ ================================================================================ CORE SPECIFICATIONS ================================================================================ HF Configuration (80M–10M): Core: Single FT-240-43 (small form FT-140-43 suitable @ 20M+) Material: Mix 43 (µ=850, AL=1100 µH/100T) Wire: #16 enameled copper Primary: 3 turns (antenna side, 200Ω) Secondary: 6 turns (coax side, 50Ω) Turns Ratio: 2:1 (gives 4:1 impedance transformation) Max Power: 450 W Insertion Loss: 0.08–0.12 dB Band-Specific Configuration: 160M (1.8 MHz): 3 primary, 6 secondary (FT-240-43) 80M (3.75 MHz): 3 primary, 6 secondary 40M (7.15 MHz): 3 primary, 6 secondary 20M–10M: 3 primary, 6 secondary (same configuration broadband) Wire Lengths: Primary: ~40 inches (3 turns) Secondary: ~70 inches (6 turns) Total: ~110 inches ================================================================================ IMPEDANCE TRANSFORMATION ================================================================================ THEORY: Simple voltage transformer (NOT transmission line, NOT bifilar CM choke) Z_out = Z_in / N² CALCULATION: N = N_secondary / N_primary = 6 / 3 = 2:1 ratio Z_out = Z_in / N² = 200 / 4 = 50Ω ✓ VOLTAGE STEP-DOWN: V_out = V_in × (N_primary / N_secondary) = V_in × (1/2) Power: P_in = P_out (minus losses) KEY ADVANTAGE: - Simple winding (standard coil on single core) - Low insertion loss (direct transformer, minimal core loss) - Compact form factor KEY LIMITATION: - No common-mode suppression (impedance transformation only) - Works only for UNBALANCED antennas (random wire, not dipole) ================================================================================ WINDING TECHNIQUE — SIMPLE VOLTAGE UNUN ================================================================================ Materials: - 1× FT-240-43 core - Spool of #16 enameled copper wire - Small vice or mandrel Step 1: Wind primary - Wind 3 turns of #16 wire on core - Keep turns evenly spaced - Strip ~0.3" from both ends (for antenna connection) Step 2: Wind secondary - Wind 6 turns directly on top of primary (no interleaving needed) - Evenly distributed around remaining core space - Strip ~0.3" from both ends (for coax connector) Step 3: Connect antenna - Solder primary wire to antenna feedpoint (one end) - Solder primary return to antenna counterpoise/ground Step 4: Connect coax - Solder secondary center to coax center conductor - Solder secondary return to coax braid/shield Step 5: Insulation - Enamel insulation sufficient for outdoor installation - Potting optional (not critical for voltage unun) ================================================================================ COMMON-MODE BEHAVIOR ================================================================================ IMPORTANT: This is a UNUN (unbalanced-to-unbalanced), NOT a balun. CM Impedance: Z_CM = 2π × f × L_primary = 2π × f × (AL × N_primary²) / 10000 Example @ 80M (3.75 MHz), FT-240-43, 3 turns: L = 1100 × 9 / 10000 = 0.99 µH Z_CM = 2π × 3.75×10⁶ × 0.99×10⁻⁶ ≈ 23 Ω This is POOR CM impedance (need >250Ω for suppression). IMPLICATION: - If antenna has high CM current, this unun does NOT suppress it - Random wire antennas typically have LOW CM current (inherently unbalanced) - So poor CM impedance is ACCEPTABLE (no CM to suppress) CONTRAST with baluns: Baluns are designed for balanced antennas (which generate CM currents) Ununs are designed for unbalanced antennas (which do NOT generate CM currents) ================================================================================ FREQUENCY RESPONSE ================================================================================ FT-240-43, 3:6 turns: Frequency Impedance Ratio Insertion Loss 160M 4.0:1 (200→50) 0.12 dB 80M 3.9:1 0.10 dB 40M 3.8:1 0.09 dB 20M 3.7:1 0.08 dB 10M 3.6:1 0.07 dB Notes: - Impedance ratio stays flat ~4:1 across entire HF band ✓ - Insertion loss DECREASES with frequency (ferrite loss drops) - Excellent broadband response for unbalanced impedance transformation ================================================================================ PRIMARY APPLICATIONS ================================================================================ 1. RANDOM WIRE ANTENNAS: - Unbalanced wire antenna (one end to feedpoint, other to ground) - Impedance typically 200–600Ω (depending on length and height) - 4:1 unun matches ~200Ω feedpoint to 50Ω coax 2. LONG-WIRE ANTENNAS (with appropriate unun ratio): - Non-resonant long wires (>λ/2 length) - Feedpoint impedance ranges from 100Ω (favorable) to 1000Ω (extreme) - 4:1 suitable for moderate impedances 3. BEVERAGE ANTENNAS: - Directional long-wire array - Typical feedpoint impedance: 150–200Ω - 4:1 unun provides good match 4. MAGNETIC LOOP ANTENNAS: - Some loop designs exhibit unbalanced feedpoints - If impedance ~200Ω, 4:1 unun is ideal 5. SIMPLE TUNED ANTENNAS: - End-fed tuned wire (NOT half-wave resonant) - Use unun + tuner for broadband matching ================================================================================ TESTING PROCEDURE — NANOVNA ================================================================================ Setup: Port 1: Coax output (50Ω) Port 2: Primary antenna connection (via 50Ω adapter or 200Ω load) Test Load: 200Ω resistor across primary (simulates antenna) Measurement 1 — Impedance Ratio (with 200Ω load): Sweep 1–30 MHz Expected: Z_out ≈ 50Ω (with 200Ω load on antenna side) S₁₁ <-20 dB (excellent match) Measurement 2 — Insertion Loss: Sweep 1–30 MHz Expected: S₂₁ ≈ -0.08 to -0.12 dB Relatively flat across band Measurement 3 — Phase: Phase delay should be minimal (<10° across band) Indicates good coupling and low leakage inductance ================================================================================ DESIGN EQUATIONS ================================================================================ Turns ratio for 200→50Ω unun: N_ratio = sqrt(Z_in / Z_out) = sqrt(200/50) = sqrt(4) = 2:1 Use 3 primary, 6 secondary (2:1 ratio) ✓ Alternative turns combinations: 2:4 (2:1 ratio, compact) 4:8 (2:1 ratio, larger core distribution) Insertion Loss: L_dB ≈ 0.05 + 0.0015 × f_MHz (FT-240-43, 3:6 configuration) Example @ 14.2 MHz: L ≈ 0.05 + 0.021 = 0.071 dB ✓ Common-Mode Impedance (not applicable for unun): Z_CM = 2π × f × (AL × N_primary²) / 10000 For 3 turns: Z_CM ≈ 23Ω @ 80M (low, but acceptable — no CM signal present) ================================================================================ FAILURE MODES ================================================================================ SYMPTOM: High SWR, poor antenna match ROOT CAUSE 1: Antenna feedpoint impedance NOT 200Ω SOLUTION: - Measure antenna impedance with tuner's built-in SWR meter - If impedance is 300Ω or higher, use 6:1 or 9:1 unun instead - If impedance is 100–150Ω, use 2:1 unun (1:2 turns ratio) ROOT CAUSE 2: Unun winding incorrect (turns count wrong) SOLUTION: - Unwind and recount turns - Verify connection (antenna to primary, coax to secondary) ROOT CAUSE 3: Counterpoise/ground connection poor SOLUTION: - Random wire antennas REQUIRE good ground connection - Add additional radial ground wires (extend ground plane) - Verify ground connection is low-impedance (<1Ω to earth) SYMPTOM: Excessive heating ROOT CAUSE: Unun is transformer, not CM choke — heat indicates high current SOLUTION: - Verify antenna is not shorted - Check for intermittent connections - Reduce TX power if antenna current excessive ================================================================================ POWER HANDLING ================================================================================ FT-240-43 single core, 3:6 unun: Continuous (CW): 450 W @ 50°C ambient Peak (SSB): 700 W (transient) Thermal rise: ~15°C @ 400W (low loss = minimal heating) The 4:1 unun runs very cool (compared to baluns) because: - Single ferrite path (low core loss) - No long winding (lower wire resistance) - Simple voltage transformation (high efficiency) ================================================================================ CONSTRUCTION NOTES ================================================================================ Mechanical Assembly: - Compact form factor (can fit in 2"×2"×1" enclosure) - Mount at antenna feedpoint or between tuner and antenna - Stainless steel hardware (corrosion resistance) Weatherproofing: - Enamel insulation adequate for outdoor use - Potting optional (helps with moisture, not critical) - PVC tape wrap sufficient field protection Field Installation: - Mount as close to antenna feedpoint as practical - Minimize coax run from unun to tuner - Keep antenna wire and ground separate (avoid shielding) Spare Parts: - Keep extra FT-240-43 cores - Stock #16 enameled wire - Pre-made connectors (SO-239, BNC) ================================================================================ COMPARISON: 4:1 UNUN vs OTHER UNUN RATIOS ================================================================================ Ratio Input Z Output Z Turns Core Use Case ────────────────────────────────────────────────────────── 2:1 100Ω 50Ω 1:2 FT-140 Low-Z antennas 4:1 200Ω 50Ω 3:6 FT-240 Most random wire 9:1 450Ω 50Ω 4:12 FT-240 Ladder line 16:1 800Ω 50Ω 5:20 FT-240 Long-wire 49:1 2450Ω 50Ω 3:21 FT-240 EFHW 64:1 3200Ω 50Ω 5:40 FT-240 Extreme impedance RECOMMENDATION: For unknown antenna impedance: Use antenna tuner to measure first, then choose appropriate unun ratio. For typical random wire: 4:1 unun is safe starting point ================================================================================ RANDOM WIRE ANTENNA BASICS ================================================================================ What is a random wire? - Any length of wire erected between two points (not resonant) - Impedance varies with length, height, frequency - Low feedpoint impedance if near resonance length Advantages: - Simple construction (single wire, not balanced) - Cheap (just wire and insulators) - Works multiple bands (non-resonant design) - Small footprint (can zigzag along fence or building) Disadvantages: - Requires good ground/counterpoise - Higher losses (not resonant, radiation resistance low) - Impedance unpredictable (depends on frequency and wire path) - Requires antenna tuner for all-band operation Counterpoise design: - Single ground wire (return path for RF current) - Or: Multiple radial wires (ground plane, 15–30 radials @ /4 length) - Or: Connection to earth ground (ground rod at feedpoint) - CRITICAL: Ground must be low impedance (<1Ω @ operating frequency) ================================================================================