================================================================================ SCHEMATIC: PROBE DESIGNS FOR FIELD STRENGTH METER TM-FSM-PROBES Rev A Telescoping Dipole, Small Loop, Calibrated Terminated Dipole, Shielded Loop (Directional), VHF/UHF Yagi-Stub Probe ================================================================================ PROBE 1 — TELESCOPING WHIP / ADJUSTABLE DIPOLE ------------------------------------------------ The simplest broadband probe. A single telescoping whip antenna connected to the FSM input via coax. The antenna is extended to approximately λ/4 for the band of interest. QUARTER-WAVE WHIP LENGTHS: Band f(MHz) λ/4 physical (92% VF) ---- ------ --------------------- 160M 1.85 38.4 m (not practical; use short loaded) 80M 3.75 19.0 m (not practical) 40M 7.15 9.9 m (long whip or wire extension) 30M 10.125 7.0 m 20M 14.175 5.0 m 17M 18.118 3.9 m 15M 21.225 3.3 m 12M 24.940 2.8 m 10M 28.500 2.5 m 6M 50.000 1.4 m (telescoping: 7 × 20 cm sections) 2M 144.000 0.49 m (48 cm) 70cm 432.000 0.16 m (16 cm) 33cm 902.000 0.077 m (7.7 cm) PRACTICAL PROBE: 70 cm 7-section telescoping Retracted: 25 cm (pocket-size) Extended: 175 cm (maximum; covers 40M with reduced sensitivity) Section material: brass or stainless, 6/5/4/4/3/3/3 mm OD DIPOLE VERSION: Two telescoping elements extending in opposite directions from a balun/sleeve. Provides balanced input and approximately 6 dB more sensitivity than single whip. CENTER CONNECTOR FOR DIPOLE: [LEFT ELEMENT]────[SLEEVE BALUN]────[RIGHT ELEMENT] │ coax shield coax center (left element) (right element) │ BNC/SMA to FSM input SLEEVE BALUN (for dipole): λ/4 sleeve of 1/2" copper tubing over coax feedline Electrically: λ/4 shorted stub (bottom grounded to coax shield) Creates high impedance at top, preventing coax current from feeding back down the shield. Sleeve length for 2M: 50 cm / 4 = 12.5 cm minimum (Use the band with the shortest sleeve for portable operation) PROBE 2 — SMALL SINGLE-TURN LOOP (HF FIELD SENSING) ------------------------------------------------------ A small loop antenna responds to the magnetic field component (H) of the electromagnetic wave. Used for near-field and directional sensing. The probe is omnidirectional in its plane and shows a figure-8 null pattern on its axis. LOOP SPECIFICATIONS: Shape: Circular, 10 cm diameter (shielded) or 20 cm (unshielded) Turns: 1 (single-turn; simpler construction, lower noise) Material: 3 mm OD copper tubing or 12 AWG bare copper wire Shield: Faraday shield (gap at top prevents electric field pickup) Copper foil or 1/4" copper tubing, ~95% coverage, gap at top LOOP SCHEMATIC: Faraday shield (copper tubing, continuous except for top gap): ┌─────────────────────── TOP GAP ───────────────────────┐ │ │ LOOP │ ╔════════════════════════════════════════════════╗ │ WIRE └──╢ Single conductor loop (copper tubing 3mm OD) ╟───┘ ╚══════════════════╤═════════════════════════════╝ │ (bottom; loop terminals) ┌─────┴─────┐ │ │ Shield GND Loop hot │ │ GND Coax center │ 50Ω coax to FSM input (short: <30 cm, RG-174) SENSITIVITY: Open-circuit EMF: V_oc = 2π × f × μ₀ × H × N × A At 7.15 MHz, N=1, A=π×0.05² = 7.85×10⁻³ m²: V_oc = 2π × 7.15×10⁶ × 1.257×10⁻⁶ × H × 7.85×10⁻³ = 0.443 × H millivolts per A/m Since E = 377 × H (free space): V_oc = 1.17 × E μV per V/m With ERA-3SM preamp (20 dB gain) in probe housing: Output ≈ 11.7 μV per V/m at 7 MHz AD8307 detects to −75 dBm = 0.89 μV at 50Ω Minimum detectable field: ~0.08 V/m at 40M (loop probe) DIRECTIONAL PATTERN: Null axis = loop axis direction Max response = broadside to loop plane Null depth = >20 dB for well-constructed shielded loop Use: Rotate loop for null to find direction TO transmitter (null points at transmitter and away; 180° ambiguity) PROBE 3 — CALIBRATED TERMINATED DIPOLE ----------------------------------------- For accurate field strength measurements (in V/m or dBμV/m), a calibrated antenna with a known antenna factor (AF) is required. The terminated folded dipole provides a known, flat impedance over a 10:1 bandwidth. CALIBRATED TERMINATED DIPOLE SCHEMATIC: [Element 1: 2.85 m each side for 20M] ←────────────────────────────────────→ ╔═══════════════════════════════════════╗ ║ Element 1 ║ Element 1 ║ ╚════════╤══════════════════╤═══════════╝ │ │ 100Ω termination Feed point (non-inductive) │ at each far end BNC/SMA output │ │ GND (shield) Coax center │ [optional 4:1 balun] │ [SMA to FSM input] SIMPLE TERMINATED DIPOLE: Each dipole arm terminated at its tip with 100–200Ω resistors. The termination absorbs the traveling wave, providing a nearly resistive impedance of ~100Ω over a wide bandwidth. With a 2:1 balun, the 50Ω FSM input sees a reasonable match. ANTENNA FACTOR (AF) for terminated dipole: AF ≈ 14 dB/m (approximately constant ±3 dB from 2–30 MHz) Calibrate against a known reference for traceable measurements. REFERENCE CALIBRATION: Place probe at 3m from a calibrated half-wave dipole fed with a known power P_watts (measured on power meter, e.g., the TM-DL-001 directional coupler). From Friis: E(V/m) = sqrt(30 × P × G_t) / d Where: P = transmit power in watts G_t = transmit antenna gain (numeric, not dB): 1.64 for dipole d = distance in meters Measure FSM reading (dBm) with known E_field applied. AF_dBm = E_dBuVm − V_dBuV (difference defines the AF) Record AF at each band for use in firmware lookup table. PROBE 4 — SHIELDED LOOP FOR DIRECTION FINDING ----------------------------------------------- Used with the CYD Direction Finder screen (Screen 2). The loop is rotated manually; the CYD firmware records peak and null bearings referenced to a compass. COMPONENTS: [RG-58 coax, 40 cm, formed into 12.7 cm (5") diameter loop] [Gap at top of loop (cut outer shield, leave inner conductor intact)] [Coax connector at bottom of loop handle] [Optional: small ferrite bead at gap to improve null depth] CONSTRUCTION FROM COAX (detailed): Step 1: Cut 55 cm of RG-58 (or RG-316). Step 2: Strip 1 cm of outer jacket from the center point; cut through the braid shield only at this point (top gap). The center conductor continues unbroken through the gap. Step 3: Form into a circle: 12.7 cm diameter (5 inch). The gap goes at the top. Step 4: At the bottom, terminate the two coax ends: Join the two braid shields together (both to GND). Join the two center conductors together (single-turn loop). Step 5: Bring the bottom feed to a BNC connector: Center = loop wire, Shield = braid GND. DIRECTIONAL NULL PATTERN: At null: >25 dB rejection on the loop axis Usable HF bands: 40M through 10M (loop is electrically small) At 2M and above: loop is no longer small; pattern degrades. For VHF DF, use the yagi stub probe (Probe 5). HANDLE/MOUNTING: 3D print loop housing (see probe_housings.scad) Rotating compass ring on housing basebase Bearing readout: manual or BLE compass module (HMC5883L) If HMC5883L: I2C on GPIO22/21 (check CYD availability) PROBE 5 — 2M/70CM DIRECTIONAL STUB YAGI (VHF/UHF PROBE) ---------------------------------------------------------- For VHF (2M, 1.25M) and UHF (70cm, 33cm) field strength work. A simple 3-element stub Yagi provides 6 dBd gain with a clear directional pattern for source location. 3-ELEMENT 2M YAGI DIMENSIONS: Reflector: 1.05 m (approx λ/2 × 1.05) Driven: 0.97 m (approx λ/2) Director: 0.91 m (approx λ/2 × 0.95) Boom length: 0.52 m (total) Element spacing: reflector-driven = 0.2 m, driven-director = 0.12 m FOLDED DIPOLE FEED (driven element): Impedance: ~200Ω → use 4:1 balun for 50Ω Or use γ-match for 50Ω direct coax connection SMA OUTPUT: Mini-coax RG-316 from folded dipole to SMA connector Mount SMA at rear of boom, connect to FSM 3-ELEMENT 70CM YAGI DIMENSIONS: Reflector: 0.35 m Driven: 0.325 m Director: 0.305 m Boom: 0.17 m, element spacing 0.07 m each NOTE: Print probe_housing.scad "yagi_boom" for 3D printable boom with press-fit element holes (3 mm aluminum rod elements). PROBE AF LOOKUP TABLE (for firmware) --------------------------------------- #define NUM_BANDS 15 struct BandInfo { const char* name; float freq_MHz; float AF_whip_dBm; // 50cm telescoping whip float AF_dipole_dBm; // λ/2 dipole (tuned) float AF_loop_dBm; // 10cm shielded loop }; BandInfo bands[NUM_BANDS] = { // name f(MHz) whip dipole loop {"160M", 1.85, 32.0, -30.6, 56.0}, {"80M", 3.75, 26.0, -24.6, 50.0}, {"40M", 7.15, 20.0, -18.6, 44.0}, {"30M", 10.125, 17.0, -14.9, 41.0}, {"20M", 14.175, 14.0, -12.5, 38.0}, {"17M", 18.118, 12.0, -10.0, 36.0}, {"15M", 21.225, 10.0, -8.5, 34.0}, {"12M", 24.940, 9.0, -7.0, 33.0}, {"10M", 28.500, 8.0, -6.5, 32.0}, {"6M", 50.000, 6.0, -3.0, 30.0}, {"2M", 144.000, 3.0, 7.2, 27.0}, {"1.25M", 222.000, 2.0, 11.5, 25.0}, {"70cm", 432.000, 1.0, 17.2, 23.0}, {"33cm", 902.000, 0.0, 22.8, 21.0}, {"20cm", 1296.000, -1.0, 26.2, 19.0}, }; NOTE: AF values above are approximate and must be verified against a calibrated reference antenna at each band. Store user-calibrated AF values in EEPROM, overriding the defaults. ================================================================================