================================================================================ common_mode_choke_ascii.txt — TM-RFI-SCH-002 Rev A Common-Mode Chokes for Coax Feed Lines and Signal Cables Ferrite Core Selection, Winding Tables, Impedance Specifications Document: TM-RFI-SCH-002 Rev A ================================================================================ OVERVIEW -------- Common-mode current on coax braid is the primary mechanism by which: 1. RFI enters the shack from the antenna feed line (noise antenna effect) 2. Transmitter RF enters the shack (RF in the shack) 3. Radiation from the feed line distorts antenna patterns 4. Computer and appliance noise couples to the antenna system A common-mode choke (also called "choke balun" or "1:1 current balun") presents high impedance to current flowing on the outside of the coax braid while presenting near-zero impedance to the differential signal inside the coax. THEORY: COMMON-MODE vs. DIFFERENTIAL-MODE ------------------------------------------- Coax cable carries two current paths: 1. DIFFERENTIAL MODE: Current flows from transmitter (+) on center conductor, returns via inner surface of braid. This is the desired signal path. CMC does NOT affect this path. 2. COMMON-MODE: Current flows in the same direction on both conductors (both forward on center AND on outside of braid). This is the interference. CMC presents high impedance (Z_CM = 2πf × L_CM) to block this. Effect of common-mode current on noise: - Coax acts as a receiving antenna for any common-mode noise source - At 40m (7 MHz): λ/4 = 10.7m — typical 10m feedline is significant antenna - Common-mode current creates radiation resistance: effective receive area FERRITE CORE SELECTION GUIDE ------------------------------- Different ferrite mixes have different impedance vs. frequency characteristics. Choose core mix to match the frequency range of interference. Mix 31 (Ferroxcube 3S4 / Fair-Rite 31): Complex permeability: µ' peaks ~1–10 MHz range Best for: 1–30 MHz (HF bands), especially medium-wave and broadcast Color code: gray (Fair-Rite) FT-240-31: 1600 R_loss at 7 MHz (most loss = most noise absorbed) FT-240-31: 800 Ω at 14 MHz, 300 Ω at 30 MHz Mix 43 (Ferroxcube 4C6 / Fair-Rite 43): Best for: 1–300 MHz (wide HF + low VHF) More resistive loss in 10–100 MHz range Color code: black (Fair-Rite) FT-240-43: 1000 Ω at 10 MHz, 1400 Ω at 30 MHz, 1200 Ω at 100 MHz Mix 61 (Fair-Rite 61): Best for: 10–200 MHz (upper HF + VHF) Higher Q — more inductive, less resistive (not ideal for noise absorption) Color code: off-white/cream Mix 75 (Fair-Rite 75): Best for: 0.1–5 MHz (LF/MF, medium-wave broadcast interference) Very high µ, very lossy at low frequencies Best choice if primary noise source is AM broadcast RECOMMENDATION BY PROBLEM: 40m/80m/160m noise primarily: FT-240-31 (best mix) HF broadband noise: FT-240-31 (primary), FT-240-43 (secondary stage) VHF noise on coax: FT-240-43 USB/Ethernet cable noise: Mix 31 snap-on clamps LED driver noise (broadband): FT-140-31 or clip-on Fair-Rite 0431167281 COAX CHOKE WINDING TABLE -------------------------- For a given feed line coax and frequency, select winding parameters. Impedance guideline: Z_CM > 1000 Ω required at problem frequency for >20 dB suppression. Z_CM > 5000 Ω achieves >34 dB suppression. COAX THROUGH TOROID WINDING ("W2DU" style): Thread coax through hole of FT-240-xx toroid N times. L_CM ≈ N² × AL (nH), where AL = nH/turn² for the core at frequency. FT-240-31 AL values (approximate, from Fair-Rite data): At 1.8 MHz: AL ≈ 1200 nH/T² (very lossy, use resistive model) At 7 MHz: AL ≈ 800 nH/T² At 14 MHz: AL ≈ 400 nH/T² At 30 MHz: AL ≈ 200 nH/T² BETTER: Use impedance |Z| tables from Fair-Rite or Jim Brown's (K9YC) data. The published |Z| = R + jX where R is the resistive (absorbing) component. FAIR-RITE FT-240-31 IMPEDANCE PER SINGLE TURN: (Multiple turns on same core: multiply single-turn Z by approximately N²) 1.8 MHz: |Z|₁T ≈ 180 Ω (R: 120 Ω, X: 130 Ω) 3.5 MHz: |Z|₁T ≈ 350 Ω (R: 250 Ω, X: 240 Ω) 7.0 MHz: |Z|₁T ≈ 700 Ω (R: 500 Ω, X: 490 Ω) 14 MHz: |Z|₁T ≈ 1100 Ω (R: 850 Ω, X: 700 Ω) 28 MHz: |Z|₁T ≈ 1500 Ω (R: 1300 Ω, X: 700 Ω) CHOKE DESIGN: N = 8 turns on FT-240-31: |Z| scales as N² × |Z|₁T per core (approximately): 7 MHz: |Z|_8T ≈ 64 × 700 = 44.8 kΩ ← excellent, >40 dB suppression 3.5 MHz: |Z|_8T ≈ 64 × 350 = 22.4 kΩ ← excellent 14 MHz: |Z|_8T ≈ 64 × 1100 = 70 kΩ ← excellent Note: actual winding-to-winding capacitance limits high-frequency performance RECOMMENDED CHOKE CONFIGURATIONS BY BAND/APPLICATION: APPLICATION A — HF Feed Line Choke, 160m–10m: Core: 2× FT-240-31 stacked (tape together before winding) Turns: 8 turns of RG-8X (small enough to fit through FT-240 hole) RG-8X OD: 7.5mm. FT-240 hole: 25mm dia. → fits 8T through 25mm hole Method: coil 8 turns inside 3D-printed housing (see choke_housing.scad) APPLICATION B — HF Feed Line Choke (larger coax RG-8/RG-213): Core: Snap-on type with larger aperture: Fair-Rite 0443167251 (large) Or: 4× FT-240-31 stacked with RG-213 (OD 10.3mm) threading Turns: 5–6 turns (larger coax, fewer turns possible) APPLICATION C — VHF/UHF Feed Line Choke (RG-8X on FT-240-43): Core: 2× FT-240-43 Turns: 6 turns (fewer at VHF — VHF needs less impedance but SRF concerns) Verify SRF (self-resonant frequency) above operating frequency. SRF check: use NanoVNA, find frequency where |Z| drops (capacitive resonance). APPLICATION D — Computer USB Cable: Core: Fair-Rite 0431167281 (snap-on, 7mm aperture, Mix 31) Method: snap over USB cable at BOTH ends (device end and host end) If noise persists: use 2 clamps at each end, or make 2 turns through one clamp APPLICATION E — Ethernet Cable: Core: Fair-Rite 0431167281 (snap-on, 7mm aperture) Method: snap over Ethernet cable near shack equipment Better: use shielded Cat6A (STP) with shielded RJ45 connectors Ground shield at ONE end only (avoid ground loops) APPLICATION F — LED Driver Power Cables: Replace with ferrite bead snap-ons on both AC power lead and DC output lead Core: Mix 31 or Mix 75 (if LED noise primarily below 5 MHz) If LED noise persists: replace LED bulb/driver with "amateur-grade" low-noise type (Look for EN55015 Class B certified LED drivers) SCHEMATIC — COAX FEED LINE CHOKE ----------------------------------- OPTION 1: Toroid Threading (most common for HF) [TRX] ───── [Coax, PL-259] ─────── [Choke] ──────── [Antenna] │ │ ┌──────────────────┐ │ FT-240-31 × 2 │ │ (stacked/taped) │ │ │ │ Coax threaded │ │ 8 turns through │ │ each core hole │ └──────────────────┘ (inside 3D printed housing) Mount at: antenna feed point OR: base of feed line at shack entry OPTION 2: Choke Using Coax Wound on Coil Form ("ugly balun") For large coax that won't thread through toroids: ┌────────────────────────────────────────────┐ │ Coil 8–12 turns of coax (RG-8) on │ │ 4" diameter PVC form or 3D-printed core │ │ Turns touching, neat winding │ │ Tie with cable ties; weatherproof │ └────────────────────────────────────────────┘ "Ugly balun" or "coax balun": the distributed capacitance of the coiled coax creates a resonant circuit. At resonance, |Z| is very high. Self-resonant frequency: fSRF = 1 / (2π√(L_coil × C_coil)) For 8 turns of RG-8 on 4" form: fSRF ≈ 7–10 MHz (tune with extra/fewer turns) Less effective than toroid choke but easier to make with large coax. SCHEMATIC — MULTI-POINT GROUNDING AND CHOKE PLACEMENT ------------------------------------------------------- OPTIMAL PLACEMENT: Antenna ─── AT ANTENNA FEED POINT: Choke A (FT-240-31, 8T) │ │ Coax run (outside, weatherproofed) │ Shack entry ─── AT SHACK ENTRY: Choke B (FT-240-31, 8T) │ │ Short jumper inside shack │ Transceiver ─── AT TRANSCEIVER: Choke C (optional, FT-240-43, 4T) for VHF THREE-CHOKE SYSTEM benefits: - Choke A: stops noise from shack from going up feed line - Choke B: stops noise on outside of coax from entering shack - Choke C: secondary protection for VHF path SINGLE-CHOKE minimum: install Choke A at antenna feed point. If RF-in-shack problem: ALSO install Choke B at shack entry. IMPEDANCE vs. FREQUENCY — PERFORMANCE SUMMARY ------------------------------------------------ All values for 2× FT-240-31, 8 turns, RG-8X coax: Frequency |Z| (kΩ) R (kΩ) X (kΩ) Suppression (dB) ───────────────────────────────────────────────────────────── 1.8 MHz 8.4 5.6 6.4 40 dB (good for 160m) 3.5 MHz 16 11 11 43 dB (good for 80m) 7.0 MHz 28 20 20 47 dB (excellent 40m) 14 MHz 40 31 26 48 dB (excellent 20m) 28 MHz 44 39 20 47 dB (excellent 10m) 50 MHz 30 (est) 28 12 45 dB (good 6m) 144 MHz 12 (est) 12 4 38 dB (good 2m, FT-240-43 better) NOTE: At high frequencies, winding self-capacitance limits performance. For 2m/70cm, use fewer turns on FT-240-43 for better SRF margin. BILL OF MATERIALS — HF COAX CHOKE (PER CHOKE) ------------------------------------------------ Qty Part Spec Source ──────────────────────────────────────────────────────────── 2 FT-240-31 2.4" OD ferrite toroid Amidon, kitsandparts.com 1 RG-8X ~2m for 8 turns any RF supplier 2 PL-259 SO-239 compatible DX Engineering 1 Choke housing 3D printed see choke_housing.scad 1 Self-amalgamating tape 50mm ×0.5m weatherproof joints ================================================================================