================================================================================ SCHEMATIC: Toroidal Common-Mode Choke — HF Bands 160M through 10M TM-CMC-001 Rev A FT-240 and FT-140 Ferrite Toroid Wound Coaxial Chokes Covers: 1.8 MHz (160M) through 28 MHz (10M) and 50 MHz (6M) ================================================================================ THEORY OF OPERATION ─────────────────── A common-mode choke is wound by passing coaxial cable (or twisted wire pair) through a ferrite toroid core. The differential mode (transmit/receive signal traveling on the inner conductor relative to the outer shield) sees essentially zero impedance from the core — both conductors thread through equally, fields cancel. The common-mode current (current flowing on the OUTSIDE of the coax shield, seeking ground via feedline) sees full choking impedance. The core presents a series impedance Z_choke to the common-mode path. Common-mode rejection in dB: CMR = 20 × log₁₀( 1 + Z_choke / Z_load ) where Z_load is the impedance of the path common-mode current wants to take (typically 50–300 Ω for antenna/station ground path) For Z_choke = 1000 Ω, Z_load = 100 Ω: CMR = 20 × log₁₀(1 + 10) = 20.8 dB (significant but not extreme) For Z_choke = 5000 Ω, Z_load = 100 Ω: CMR = 20 × log₁₀(1 + 50) = 34.1 dB (excellent RFI suppression) Minimum useful Z_choke for antenna feedpoint balun: ≥ 1000 Ω Good practice target: ≥ 2000 Ω across entire operating frequency range Excellent: ≥ 5000 Ω (type 31 material, multi-core stacked) ================================================================================ CORE MATERIAL SELECTION ================================================================================ BAND COVERAGE vs. OPTIMUM MIX: Band(s) | f range (MHz) | Best Mix | A_L (nH/N²) | Notes -----------------|---------------|----------|-------------|------------------- 160M only | 1.8 | 75, 77 | 8500, 7500 | High µ LF core 160M + 80M | 1.8–3.5 | 31 | 1800 (FT-240)| Broadband HF 80M – 40M | 3.5–7 | 31, 43 | 1800, 1100 | Overlap region 40M – 10M | 7–28 | 43 | 1100 | Mid-HF optimum 20M – 6M | 14–50 | 43, 61 | 1100, 195 | Upper HF + 6M 6M – 2M | 50–144 | 61 | 195 | VHF entry VHF/UHF | 144–1300 | 61, 52 | 195, 40 | Use sleeve balun CHOKING IMPEDANCE (|Z| in Ω) for FT-240 core with N turns: Mix 31, FT-240-31, A_L = 1800 nH/N²: N=10 turns: L = 100 × 1800 nH = 180 µH f (MHz) | R (Ω) | jX (Ω) | |Z| (Ω) | Note --------|-------|--------|---------|------------------------------------ 1.8 | 450 | 2040 | 2090 | Good 160M choking 3.5 | 750 | 3100 | 3190 | Excellent 80M 7.0 | 1500 | 3900 | 4180 | Peak region 40M 10.1 | 2200 | 3600 | 4220 | 30M best 14.0 | 3200 | 2800 | 4250 | 20M excellent 18.1 | 3800 | 1900 | 4240 | 17M good 21.0 | 3900 | 1200 | 4080 | 15M good 24.9 | 3800 | 600 | 3850 | 12M acceptable 28.0 | 3600 | 200 | 3605 | 10M diminishing 50.0 | 1800 | -400 | 1844 | 6M marginal (use Mix 43 instead) Mix 43, FT-240-43, A_L = 1100 nH/N²: N=12 turns: L = 144 × 1100 nH = 158.4 µH f (MHz) | R (Ω) | jX (Ω) | |Z| (Ω) | Note --------|-------|--------|---------|------------------------------------ 3.5 | 210 | 2450 | 2460 | 80M marginal (prefer Mix 31) 7.0 | 550 | 4200 | 4236 | 40M good 10.1 | 1100 | 5400 | 5511 | 30M excellent 14.0 | 2800 | 5100 | 5819 | 20M peak — best choice 18.1 | 4100 | 4200 | 5874 | 17M peak 21.0 | 5000 | 3600 | 6153 | 15M peak 24.9 | 5800 | 2800 | 6434 | 12M excellent 28.0 | 6000 | 2200 | 6393 | 10M excellent 50.0 | 4800 | 800 | 4866 | 6M very good 144.0 | 1200 | -600 | 1342 | 2M marginal; use Mix 61 above 100 MHz ================================================================================ DESIGN 1: FT-240-31 BROADBAND HF CHOKE (160M–15M) ================================================================================ Core: FT-240-31 (Mix 31), 61mm OD, 35mm ID, 12.7mm height Cable: RG-8X (8.4mm OD), or RG-58 (5mm OD) Turns: 10–12 WINDING PROCEDURE: 10 turns RG-8X on FT-240-31: 1. Cut 2.2m of RG-8X. 2. Mark center of core with tape. 3. Pass coax through core hole; leave 30cm tail for connector. 4. Wind 10 turns in same direction (CW looking at core face). Space turns evenly; occupy ~300° of core circumference. Final gap ~60° between first and last turn (avoid exceeding 320°). 5. Leave 30cm tail for second connector. 6. Seal core in weatherproof enclosure (see OpenSCAD design). TURNS COUNT vs. PERFORMANCE (FT-240-31, RG-8X): Turns | Z @3.5 MHz | Z @14 MHz | Max power (CW) | Core temp rise ------|-----------|----------|----------------|--------------- 8 | 2100 Ω | 2750 Ω | 1500W | <10°C @ 100W 10 | 3200 Ω | 4250 Ω | 1500W | <12°C @ 100W 12 | 4500 Ω | 5900 Ω | 1000W | <15°C @ 100W 14 | 5800 Ω | 7200 Ω | 600W | ~20°C @ 100W Note: More turns → higher common-mode impedance but also slightly more differential loss and greater inter-winding capacitance (limits upper freq). RECOMMENDED TURNS BY OPERATING BAND: Operating band(s) | Core | Turns | Coax | Comment ------------------|---------|-------|------------|--------------------------- 160M–80M (CW/DX) | FT-240-31| 12–14| RG-8X | Stack 2 cores for extra Z 160M–40M (general)| FT-240-31| 10 | RG-8X | Standard all-HF choke 80M–20M | FT-240-31| 8 | RG-58 | Compact, adequate for QRP 40M–10M | FT-240-43| 12 | RG-8X | Upper HF preferred 20M–6M | FT-240-43| 8 | RG-8X | Mix 43 advantage above 14 MHz SCHEMATIC REPRESENTATION: COAX IN ──── CM CHOKE ─────────────────────── COAX OUT (inner coax) ══════════════════════════════════ ← signal passes, no change (outer shield - differential mode) Common-mode equivalent circuit: Z_choke shield ──────────/\/\/\/\/──────────────── shield (CM path) (ferrite toroid) (CM path out) The toroid ONLY affects the common-mode (one-way) current on shield. Differential mode (signal between center conductor and shield) unaffected. ================================================================================ DESIGN 2: STACKED FT-240-31 + FT-240-43 WIDEBAND CHOKE (160M–10M) ================================================================================ Two cores stacked (concentric, wound as one unit) provide higher total impedance and flatter response across 1.8–28 MHz. Stack: FT-240-31 (bottom) + FT-240-43 (top), dimensions match Cable: RG-8X, 8 turns through both stacked cores INDIVIDUAL AND COMBINED IMPEDANCE: f (MHz) | Z_31 (8t) | Z_43 (8t) | Z_combined | Improvement --------|-----------|-----------|------------|------------ 1.8 | 1350 Ω | 310 Ω | 1660 Ω | +23% 3.5 | 2000 Ω | 620 Ω | 2620 Ω | +31% 7.0 | 2700 Ω | 1980 Ω | 4680 Ω | +73% 14.0 | 2750 Ω | 3850 Ω | 6600 Ω | excellent 21.0 | 2620 Ω | 5220 Ω | 7840 Ω | excellent 28.0 | 2320 Ω | 5440 Ω | 7760 Ω | excellent Stacking is the recommended approach for multi-band operation (contest station, field day, Field operation QRP). Two cores add only 27g and minimal bulk. STACKING PROCEDURE: 1. Align two cores face-to-face; tape together with self-amalgamating tape. 2. Mark the resulting compound core as one unit. 3. Wind 8 turns of RG-8X through BOTH cores simultaneously (treat as one core). 4. Each pass of coax through both holes counts as 1 turn. ================================================================================ DESIGN 3: FT-140-43 COMPACT CHOKE (20M–10M, QRP/portable) ================================================================================ Core: FT-140-43 (Mix 43), 35.5mm OD, 23mm ID, 12.7mm height Weight: 16g (very portable) Cable: RG-58 or RG-174 (fits through 23mm ID easily) WINDING (RG-58, 12 turns FT-140-43): Pass RG-58 through 12 times; total coax used ≈ 12 × (π × 0.03) = 1.1m Maximum turns limited by ID and cable diameter: RG-58 OD = 5mm; ID = 23mm; circumference = 72mm Maximum turns before cable won't fit: ~10–11 turns RG-58 PERFORMANCE FT-140-43, 10 turns RG-58: f (MHz) | |Z| (Ω) --------|-------- 7 | 1950 14 | 3600 21 | 4400 28 | 4200 50 | 2800 Excellent portable QRP choke for 40M–6M backpack operating. ================================================================================ COMMON-MODE REJECTION TABLE (dBs of isolation) ================================================================================ CMR = 20 × log₁₀(1 + |Z_choke| / |Z_CM_path|) Assuming common-mode path impedance = 150 Ω (typical antenna feedpoint/ground): Configuration | f (MHz) | |Z| (Ω) | CMR (dB) ---------------------------|---------|---------|---------- FT-240-31, 10t, 3.5 MHz | 3.5 | 3190 | 26.5 FT-240-31, 10t, 7 MHz | 7.0 | 4180 | 29.4 FT-240-31, 10t, 14 MHz | 14.0 | 4250 | 29.6 FT-240-43, 12t, 14 MHz | 14.0 | 5819 | 31.9 FT-240-43, 12t, 21 MHz | 21.0 | 6153 | 32.3 Stacked 31+43, 8t, 14 MHz | 14.0 | 6600 | 32.9 Stacked 31+43, 8t, 21 MHz | 21.0 | 7840 | 34.4 2× FT-240-31 stacked, 10t | 7.0 | 8360 | 35.0 (double the Z_31 from table) 4× FT-240-31 stacked, 8t | 7.0 | ~16000 | 40.6 Note: 4-core stacks (W1JR style balun) are used for competitive contest stations where maximum common-mode isolation is required. They are not portable-friendly but can be fixed-installed at antenna feedpoints. ================================================================================ POWER HANDLING ================================================================================ Ferrite core power handling is limited by: 1. Core temperature rise (Curie temp of Mix 31 = 230°C; Mix 43 = 200°C) 2. Coax voltage breakdown (not normally limiting at legal power levels) 3. SWR: high SWR concentrates voltage/current at specific points on feed line At 100W CW, 10t FT-240-31, SWR = 1:1: temperature rise < 10°C (safe indefinitely) At 100W CW, 10t FT-240-31, SWR = 3:1: temperature rise < 25°C (still safe) At 1500W CW, 8t FT-240-31, SWR = 1:1: temperature rise < 50°C (safe in cool weather) At 1500W CW, 8t FT-240-31, SWR = 3:1: temperature rise ≈ 80°C (monitor, limit key-down) QRP operation (5W): no thermal concerns with any toroid configuration. ================================================================================ PARTS LIST — TOROID CHOKES ================================================================================ Item | Qty | Designation | Description | Source -----|-----|----------------|------------------------------------|-------- T1 | 1 | FT-240-31 | Mix 31 toroid, 61mm OD | Palomar, Mouser T2 | 1 | FT-240-43 | Mix 43 toroid, 61mm OD | Palomar, Mouser T3 | 1 | FT-140-43 | Mix 43 toroid, 35.5mm OD (portable)| Palomar, Mouser | 2.5m| RG-8X | 0.77 cm² RG-8X coax, per choke | CommScope, Ham store | 2.0m| RG-58 | Thinner coax for compact/QRP choke | any J1,2 | 2 | PL-259/SO-239 | UHF connectors, silver | Amphenol J_N | 2 | N-type (opt) | N-type for weatherproof install | Amphenol | 1 | Enclosure | See openscad/enclosure_choke_hf | 3D printed | | Self-amal. tape| Weatherproof coil wrapping | 3M or Scotch ================================================================================