================================================================================ SCHEMATIC: PASSIVE FIELD STRENGTH METER TM-FSM-PASSIVE Rev A Germanium Diode Detector, Analog Meter Output Frequency Range: 100 kHz – 500 MHz (broadband, untuned) ================================================================================ OVERVIEW -------- Passive (battery-free) field strength meter using a germanium point-contact diode to envelope-detect received RF. Output drives a 100 μA FSD moving-coil panel meter. Designed for relative field strength comparison at HF/VHF/UHF. No amplification; relies on probe antenna to develop sufficient voltage. Minimum detectable field (50 cm whip): approximately 0.5 mV/m at 7 MHz. COMPLETE SCHEMATIC ------------------ PROBE ANTENNA (J1) | SMA/BNC 50Ω │ (or direct whip banana jack) │ +===SENSITIVITY===+ │ VR1 │ │ 100kΩ LOG │ │ (attenuator) │ +-------+---------+ │ +-------+--------+ │ │ │ L1 (optional)│ Series resonant trap (switchable per band) │ BAND FILTER │ Provides ~6 dB selectivity │ │ +-------+--------+ │ │ RF INPUT NODE │ +-------+-------+ │ │ [C1] [D1 1N34A] 100 pF │ Cathode RF bypass │ │ │ │ [R1 100kΩ] Load resistor │ │ │ └──────────────+───+──── Vdet (DC after envelope) │ [C2 10μF] Integrator / ripple filter │ [R2 10kΩ] Series resistance to meter │ +-----+-----+ │ M1 │ │ 100 μA │ Moving coil panel meter │ FSD │ 50–100 μA full scale └─────+─────+ │ +-----------+ │ [R3] Zero set potentiometer [1kΩ] (series with meter, set zero) │ GND ═══════════════════════ CIRCUIT GND (coax shield/BNC shell) METER SHUNT (if M1 is 50 μA FSD): Add R_shunt across M1 to scale: R_shunt = (V_det_max - V_meter_FSD) / I_meter_FSD For Vdet_max ≈ 1V, I=50μA: R_shunt = 1V / 50μA = 20 kΩ (Adjust R_shunt empirically for desired FSD level) OPTIONAL BAND-SWITCHED TUNED CIRCUIT -------------------------------------- Band switch S1 (1-pole 10-position rotary): Position 1: 160M L=10μH, C=560pF (parallel LC trap) Position 2: 80M L=4.7μH, C=270pF Position 3: 40M L=2.2μH, C=150pF Position 4: 30M L=1μH, C=75pF Position 5: 20M L=560nH, C=47pF Position 6: 15M L=330nH, C=33pF Position 7: 10M L=220nH, C=20pF Position 8: 6M L=100nH, C=10pF Position 9: 2M L=33nH, C=5pF Position 10: WIDE (bypass, all bands via broadband path) Each position connects the probe to an LC parallel resonant circuit tuned to band center. This provides bandpass selectivity (~6 dB/octave rolloff), improving sensitivity to the desired band and rejecting strong out-of-band signals that could saturate the detector. LC values provide resonance: f = 1/(2π√LC) Inductor construction (toroid for high Q): 160M: T-50-6, 31 turns #26 AWG ≈ 10 μH 80M: T-37-6, 21 turns #26 AWG ≈ 4.7 μH 40M: T-37-6, 15 turns #26 AWG ≈ 2.2 μH 20M: T-37-6, 9 turns #26 AWG ≈ 560 nH 10M: T-37-6, 6 turns #26 AWG ≈ 220 nH 2M: T-25-6, 4 turns #28 AWG ≈ 33 nH (air coil acceptable) GERMANIUM DIODE DETECTOR NOTES --------------------------------- D1 = 1N34A (preferred) or 1N60, OA47, OA90, AAZ15 Germanium point-contact or Schottky (not silicon; Vf too high) Forward voltage: ~0.2–0.3V at low current Works at RF levels below 100 mVpp (silicon needs ~300 mVpp) Detector output: Vdet ≈ Vrf_peak - Vf_diode For Vrf = 50 mV peak: Vdet ≈ 50 mV - 30 mV = 20 mV (weak signal) For Vrf = 500 mV peak: Vdet ≈ 500 mV - 30 mV = 470 mV (strong) NOTE: The detector is square-law (Vout ∝ Power) for very small signals (Vrf < ~50 mV) and linear (Vout ∝ Vrf) for larger signals. The meter scale is therefore nonlinear; calibration points are needed. SENSITIVITY / PROBE SELECTION -------------------------------- Probe type Minimum detectable Useful range ---------- ------------------ ------------ 50 cm whip ~1 mV/m at 7 MHz S5 signals Dipole (λ/2) ~0.1 mV/m at 7 MHz S3 signals (connected) Loop 10 cm ~5 mV/m at 7 MHz Near-field only Small loop 2M+ ~0.1 mV/m at 145MHz VHF near-field To detect typical 100W station at 1 km distance on 40M: Free-space E ≈ 5.5 V/m at 1 km from 100W dipole Actual: much less (terrain, buildings); expect 0.1–1 V/m nearby PARTS LIST — PASSIVE FSM -------------------------- Ref Value Description --- ----- ----------- D1 1N34A Germanium diode (point contact) R1 100 kΩ, 1/4W Detector load resistor R2 10 kΩ, 1/4W Series meter resistor R3 1 kΩ trimmer Zero set C1 100 pF, 50V RF bypass (NPO/C0G) C2 10 μF, 16V Integrating capacitor (electrolytic) VR1 100 kΩ log Sensitivity potentiometer M1 100 μA FSD Moving-coil panel meter (50–100 μA FSD) L1-10 various Tuned circuit inductors (see table above) S1 1×10 rotary Band switch J1 SMA or BNC Probe input connector PCB 50×40 mm Single-sided FR-4 ================================================================================