================================================================================ rfi_sniffer_probe_ascii.txt — TM-RFI-SCH-004 Rev A RFI Near-Field Sniffer Probe Set: H-Field Loop and E-Field Rod Preamplifier Circuit, Shielded Loop Design, and Usage Procedures Document: TM-RFI-SCH-004 Rev A ================================================================================ OVERVIEW -------- Near-field sniffer probes allow you to locate the SOURCE of QRM/RFI by holding the probe near suspected devices and observing the S-meter or spectrum analyzer. When the probe is held near the noise source, the S-meter peaks. Move toward the source until you identify the exact device and coupling point. Two complementary probes: 1. H-field (magnetic) loop: responds to CURRENT in conductors - Sensitive to transformer/inductor switching noise - Directional: null when loop plane faces source - Reject: electrostatic pickup (use Faraday shield) 2. E-field (electric) rod: responds to VOLTAGE on conductors - Sensitive to voltage switching (transistors, switching rails) - Omnidirectional - Use with attenuator to prevent SDR overload PROBE A — SHIELDED H-FIELD LOOP --------------------------------- DIMENSIONS: Loop diameter: 100 mm (convenient for near-field, ~λ/3000 at 10 MHz) Wire: 1mm OD coax (RG-178 or RG-316 miniature coax) Shield gap: 10 mm at top (prevents electrostatic response) WHY FARADAY SHIELD: An unshielded loop responds to BOTH magnetic (H) and electric (E) fields. A Faraday-shielded loop has a copper shield with a gap at the top: - Shield shorts the electric field (E-field voltage → induced current blocked by shield) - Gap prevents shield current from flowing (if no gap: shield is a shorted turn) - Loop wire inside sees only magnetic flux (H-field) through the gap CONSTRUCTION (from coax): 1. Bend 34 cm of RG-178 into 100 mm circle (circumference = π × 100 = 314 mm ≈ 34 cm) 2. Strip 5 mm braid at one end (feed point, bottom) 3. Leave 10 mm gap at top: strip 10 mm braid away but keep center conductor continuous → This gap in the outer shield = electrostatic shield gap 4. Solder feed end to SMA connector: center conductor → SMA center pin braid → SMA ground/shield 5. Seal non-feed end with epoxy (prevent moisture) 6. Optional: wrap loop in heat-shrink tubing for mechanical protection SCHEMATIC: ┌────────────────────────────────────────────────────────────┐ │ 100mm diameter loop │ │ │ │ ←────────── coax shield (Faraday screen) ─────────────→ │ │ |← feed ←──────────────────────── 10mm GAP │ │ │ │ Inner coax center conductor forms the loop throughout │ └────────────────────────────────────────────────────────────┘ SENSITIVITY: Open-circuit voltage of small loop: V_oc = µ₀ × A × N × dH/dt = 2π × f × µ₀ × B × A × N At 10 MHz, B = 1 nT, A = 7850 mm² = 7.85×10⁻³ m², N = 1 turn: V_oc = 2π × 10e6 × 4πe-7 × 1e-9 × 7.85e-3 = 0.62 µV → Preamplifier needed for useful sensitivity. PREAMPLIFIER: The 100mm shielded loop has source impedance of: R_rad + jωL where L ≈ µ₀ × r × [ln(8r/a) − 2] ≈ 290 nH for 100mm loop jωL at 10 MHz ≈ j18.2 Ω Use SPF5189Z or J310 JFET source follower for buffering: Loop terminal ── C_in (47pF) ── SPF5189Z pin 1 (IN) SPF5189Z pin 2 (GND) → GND SPF5189Z pin 3 (OUT+Vcc): → C_out (100pF) → SMA center (output) → L_choke (100nH) → +3.3V supply Gain: +19 dB. NF: 0.6 dB. Effective sensitivity: detects B-field variations of ~0.1 pT at 10 MHz (SNR=0 dB) ALTERNATIVE: Mini-Circuits MAR-6+ wideband MMIC (20 dB gain, 6 mA, 50Ω in/out) MAR-6+ at SMA connector (50Ω matching): place directly at loop feed point. PROBE B — E-FIELD ROD PROBE ------------------------------ Short rod (whip) with buffer amplifier. ROD: 50–100mm stainless steel rod, 2mm diameter. Tip contact: brass SMA inner contact pressed into rod. SCHEMATIC: Rod ── R_protect (1kΩ) ── JFET gate ── [J310 source follower] ── 50Ω output J310 source follower (same as active noise canceller preamp): Drain (+5V via 470Ω) Gate: rod input via 1kΩ Source: 2.2kΩ to GND; output via 1µF NP0 cap ATTENUATION: E-field probe can pick up very strong near-field voltages. Add SMA 20 dB attenuator at output before connecting to SDR to prevent overload. Signals >+10 dBm will damage RTL-SDR input. Always use attenuator with E-field probe. PROBE HANDLE AND CABLE ------------------------ HANDLE: 3D printed grip (see rfi_sniffer_handle.scad) Cable: RG-178 miniature coax, 1–1.5m, SMA(M) to SMA(M) Tip connection: strain relief sleeve, loops at front end of handle Power: LiPo cell 100mAh (for preamp) OR phantom power via bias-T from SDR BIAS-T PHANTOM POWER: RTL-SDR v3 has built-in bias-T (software enabled): rtl_biast -b 1 (from command line, or in SDR++ settings) Provides +4.5V at 100mA via coax center conductor. Add 100nF blocking cap + 100nH choke at probe to separate RF from DC. USAGE PROCEDURE — RFI HUNTING ------------------------------- STEP 1: IDENTIFY PROBLEM FREQUENCY Using SDR or receiver, tune to the interference frequency. Note: does it appear at a specific frequency? Multiple harmonically-related spikes? → Harmonic series: switching power supply (fundamental = PS switching frequency) → Single carrier: oscillator, clock signal → Broadband hash: motor brush arcing, plasma arc → Pulsed: microcontroller running periodic task → S-meter sync with video: HDMI cable, LCD backlight STEP 2: LOCATE NOISE SOURCE BUILDING/ROOM From your antenna, rotate a directional antenna (Yagi, loop, or beam). Note bearing of maximum noise. This points toward noise source. STEP 3: ROOM HUNT WITH H-FIELD PROBE Move through room holding H-field probe near: - Power supply bricks and wall warts - Computer motherboards (near CPU and GPU VRM) - LED light fixtures and dimmers - Smart meters (outside wall) - Solar inverters / battery charge controllers - Ethernet switches and routers - TV sets and monitors (near power supply area) S-meter peaks when probe near source. STEP 4: DEVICE-LEVEL IDENTIFICATION Once room identified, disconnect appliances one at a time. Monitor S-meter: when noise floor drops significantly, last-disconnected = culprit. STEP 5: COUPLING PATH IDENTIFICATION Once source identified: - Check if noise conducts via mains wiring (power line filter may help) - Check if noise radiates from device body (shielding may help) - Check if noise couples via data cables (ferrite clamps help) Use E-field probe to identify high-voltage switching rails. Use H-field probe to identify high-current switching nodes. STEP 6: APPLY REMEDIATION AND VERIFY Apply fix (ferrite clamp, filter, replacement component). Re-measure noise level to confirm improvement. Log: device, frequency, cure applied, improvement achieved. PROBE CALIBRATION (ROUGH) --------------------------- For relative measurements only (identifying sources, not absolute calibration): 1. Set receiver to CW, narrow BW (500 Hz). 2. Tune to clean frequency (no known signals). 3. Record baseline S-meter level. 4. Move probe near each suspect device. 5. Record S-meter change above baseline. 6. Largest delta = primary noise source. For quantitative field strength measurements, use: - Calibrated EMC antenna (not covered in this document) - Spectrum analyzer with calibration factor - LISN (Line Impedance Stabilization Network) for conducted EMC measurements BILL OF MATERIALS — SNIFFER PROBE SET ----------------------------------------- H-Field Loop: 1m RG-178 or RG-316 miniature coax 1× SMA(F) right-angle PCB connector 1× SPF5189Z or MAR-6+ MMIC preamp 1× 100nH Coilcraft 0402CS RF choke 1× 100pF NP0 0402 cap Handle: see rfi_sniffer_handle.scad E-Field Rod: 100mm × 2mm stainless steel rod 1× J310 JFET 1× 2.2kΩ, 1× 470Ω, 1× 1kΩ resistors 1× 1µF NP0 cap Handle + PCB: integrated in rfi_sniffer_handle.scad Common: 2× SMA(M)-to-SMA(M) 1.5m RG-178 cable assemblies 1× 20 dB SMA attenuator pad (protect SDR from strong near-field) 1× LiPo 100mAh 3.7V (or use RTL-SDR bias-T at 4.5V) ================================================================================