================================================================================ BALUN 1:1 CURRENT CHOKE — AIR-CORE COAX (VHF/UHF) ================================================================================ Purpose: Common-mode suppression for VHF/UHF balanced antenna feeds (2M–20cm) Common-Mode Impedance: 5000+ Ω @ 145 MHz, 3000+ Ω @ 432 MHz, 2000+ Ω @ 1.3 GHz Insertion Loss: 0.03–0.05 dB (minimal ferrite losses at microwave frequencies) ================================================================================ CIRCUIT DIAGRAM (Air-Core Transmission Line) ================================================================================ Antenna (Balanced Feed) │ ┌───┴────┐ │ │ Leg A Leg B │ │ ┌──┴──┐ ┌──┴──┐ │ RG58│ │ RG58│ │ wrapped on │ 3.5" PVC form │ 8 turns helical └──┬──┘ └──┬──┘ │ │ ├────┬───┤ │ Coax to Receiver │ 50Ω IN ================================================================================ CORE SPECIFICATIONS — AIR-CORE TRANSMISSION LINE ================================================================================ Construction: - PVC pipe or phenolic rod: 3.5", 3", 2.5", or 2" diameter (depending on freq) - Coax cable: RG-58 for 2M–70cm, RG-174 for 1.25M–70cm - Turns: 8 (2M/1.25M), 8 (70cm), 6 (33cm), 5 (20cm) Frequency Band Details: 2M (145 MHz): Form: 3.5" (88 mm) diameter PVC Coax: RG-58 (0.193" OD) Turns: 8 turns in helical spiral Spacing: ~0.5" axial pitch Velocity Factor: 0.66 (RG-58 standard) CM Impedance Target: 5000+ Ω @ 145 MHz Insertion Loss: <0.1 dB 1.25M (223 MHz): Form: 3.5" PVC Coax: RG-58 Turns: 8 CM Impedance: 8000+ Ω Insertion Loss: <0.08 dB 70cm (432 MHz): Form: 3.5" PVC Coax: RG-58 or RG-174 Turns: 8 CM Impedance: 6000+ Ω Insertion Loss: <0.05 dB 33cm (902 MHz): Form: 3" PVC Coax: RG-58 Turns: 6 CM Impedance: 3500+ Ω Insertion Loss: <0.04 dB 20cm (1296 MHz): Form: 2.5" PVC Coax: RG-58 Turns: 5 CM Impedance: 2200+ Ω Insertion Loss: <0.02 dB (minimal) ================================================================================ WINDING TECHNIQUE — AIR-CORE COAX CHOKE ================================================================================ Materials needed: - PVC pipe (Schedule 40) or phenolic insulating rod - Appropriate diameter form (see above) - RG-58 or RG-174 coax - Hose clamps (optional, for securing form) - Masking tape (optional, for marking spiral) Construction steps: 1. Prepare form: - Cut PVC to length (6–8 inches for most VHF) - Deburr edges with sandpaper - Mark starting point with masking tape 2. Determine pitch (axial spacing): - For N turns over L-inch length: pitch = L / N - Example: 8 turns over 4" length = 0.5" pitch - Uniform spacing critical for phase coherence 3. Wind coax in helical spiral: - Start at one end, lay down tape line (reference mark) - Wrap coax around form in uniform spiral - Maintain consistent tension (not too tight, avoid crushing) - Complete N turns, ending opposite from start - Secure end with masking tape or low-strength adhesive 4. Check for shorts: - Inspect coax jacket — ensure no cracks or abrasions - Continuity test: Center to braid should be OPEN (>10 MΩ) 5. Final assembly: - Solder/crimp connector to one end (feed input) - Solder/crimp balanced pair connections to opposite end - Optional: Potting (silicone preferred for thermal dissipation) ================================================================================ COMMON-MODE IMPEDANCE (Air-Core TL Choke) ================================================================================ An air-core choke works as a transmission-line transformer. The impedance derives from the series inductance of the coax itself, not ferrite permeability. Inductance calculation (distributed TL): L ≈ 0.2 × d_form × N² µH where d_form in inches, N = number of turns Example: 3.5" form, 8 turns L ≈ 0.2 × 3.5 × 64 = 44.8 µH (rough estimate) Z_CM @ 145 MHz = 2π × 145×10⁶ × 44.8×10⁻⁶ ≈ 4070 Ω ✓ Common-Mode Impedance vs Frequency: Rising steeply with frequency (inductive slope, ~20 dB/decade until SRF) SRF typically 1–3 GHz for most air-core chokes (flat-top response) ================================================================================ DESIGN RULES — AIR-CORE CHOKES ================================================================================ Rule 1: Diameter selection - Target CM impedance: Z_CM ≥ 5 × Z₀ = 250 Ω minimum - Larger diameter → higher inductance → lower turns needed - For 2M: 3.5" is standard (easy to obtain PVC) Rule 2: Turn count and spacing - Too few turns: insufficient inductance (Z_CM too low) - Too many turns: increased capacitance (lowers SRF) - Optimal: Even distribution, ~0.5" to 1" spacing Rule 3: Coax choice - RG-58: 0.195" OD, standard for 2M–70cm - RG-174: 0.095" OD, good for tight forms (33cm+) - RG-8: 0.405" OD, rarely used (bulky) Rule 4: Form material - PVC: Low-cost, widely available, low loss @ VHF/UHF - Phenolic: Better dimensional stability (tighter tolerances) - Avoid: Metal forms (increases loss, shields coax) ================================================================================ FREQUENCY RESPONSE ================================================================================ Typical air-core 8-turn choke on 3.5" form: Frequency Z_CM (Ω) Attenuation Phase 145 MHz 5000–7000 20–25 dB 0–5° 223 MHz 8000–10000 25–30 dB 0–10° 432 MHz 6000–8000 20–28 dB 0–20° 1296 MHz 2000–3000 15–20 dB 0–40° Insertion Loss (typical): 0.02–0.05 dB @ 2M (RG-58, 8 turns) 0.03–0.06 dB @ 70cm 0.02–0.04 dB @ 33cm Negligible @ 20cm ================================================================================ IMPEDANCE TRANSFORMATION (1:1) ================================================================================ Theory: Air-core chokes are pure 1:1 (no impedance transformation). Like ferrite chokes, they suppress CM currents by presenting high series impedance. Key difference from ferrite: - NO frequency-dependent permeability loss - Insertion loss dominates (skin effect + dielectric loss in coax jacket) - Thermal stability excellent (no core saturation) - Works above GHz where ferrite becomes lossy ================================================================================ TESTING PROCEDURE — NANOVNA ================================================================================ Setup: 1. Port 1 → Choke input (center conductor, first end) 2. Port 2 → Choke output (center conductor, opposite end) 3. Braids can be loosely bundled but kept separate (minimal coupling) 4. Perform SOLT calibration Measurement 1 — S₂₁ (insertion loss): Sweep 100 MHz–2 GHz Expected: Rising response (high-pass, inductive) Typical: -0.02 to -0.08 dB across VHF, slightly higher at UHF Measurement 2 — S₁₁ (input reflection): Port 2 open circuit Expected: Highly reactive (inductive impedance) Phase: ~90° (capacitive coupling minimal) Measurement 3 — Phase delay: S₂₁ phase should be <10° across band Verify no resonances (local dips in magnitude = SRF approaching) ================================================================================ FAILURE MODES ================================================================================ SYMPTOM: Low SWR but pattern distortion ROOT CAUSE: CM choke not wound correctly (inadequate inductance) SOLUTION: - Verify winding: count turns, measure spacing - Re-measure with NanoVNA: Z_CM should be >1000 Ω @ operating frequency - If Z_CM <500 Ω, add form diameter (move to larger PVC) SYMPTOM: Insertion loss higher than expected (>0.2 dB) ROOT CAUSE: Coax damaged, connector quality poor, or loss measurements made with non-calibrated NanoVNA SOLUTION: - Inspect coax for cracks or kinks - Resolder connectors (cold solder = high resistance) - Use quality N-connector plugs (stainless contact preferred) SYMPTOM: Choke heating during transmission ROOT CAUSE: Extremely high currents or coax damaged (partially shorted) SOLUTION: - Check continuity: center-to-braid should be OPEN - Reduce TX power temporarily - Replace coax if damage suspected SYMPTOM: Broadband CM suppression poor (only works on one band) ROOT CAUSE: Air-core choke has narrow impedance peak near SRF SOLUTION: - Use combination: Large diameter choke (lower freq) + air-core (high freq) - Or use ferrite choke for 2M, air-core for 70cm separately ================================================================================ ASSEMBLY NOTES ================================================================================ Mechanical: - PVC form is non-conductive (no RF shielding needed) - Can be mounted directly to antenna or mast - Lightweight (~50 grams for typical 2M choke) Environmental: - PVC stable -40°C to +60°C (outdoor rated) - UV exposure: PVC will yellow/degrade over years; paint recommended - Potting optional: silicone rubber or polyurethane acceptable Field modifications: - If Z_CM too low: Add turns (reduces pitch, more compact) - If Z_CM too high: Remove turns (increases pitch) - Form diameter can be modified by careful unwinding/rewinding Cost comparison: Air-core: ~$5 (PVC + coax segment) Ferrite 1:1: ~$15 (FT-240 core + connector) Commercial air-core choke: $30–80 ================================================================================ BAND RECOMMENDATIONS ================================================================================ 6M (51 MHz): Not recommended (air-core too bulky, ferrite 1:1 preferred) 2M (145 MHz): Primary recommendation: 8-turn air-core, 3.5" form, RG-58 Z_CM target: 5000+ Ω ✓ 1.25M (223 MHz): Standard: 8-turn air-core, 3.5" form Alternative: 6-turn on 3" form (more compact) 70cm (432 MHz): Standard: 8-turn air-core, 3.5" form, RG-58 Compact: 6-turn on 3" form Z_CM: 6000 Ω typical 33cm (902 MHz): Recommended: 6-turn on 3" form, RG-174 Z_CM: 3500+ Ω 20cm (1296 MHz): Recommended: 5-turn on 2.5" form, RG-174 Z_CM: 2200+ Ω (lower, but adequate for 1.3 GHz) ================================================================================