================================================================================ MOTORIZED ROTARY LOOP RDF ANTENNA — TM-ADF-005 Rev A Shielded Magnetic Loop with Servo/Stepper Rotation HF Bands 160M through 10M ================================================================================ LOOP ANTENNA PRINCIPLE ======================== A small loop antenna has a figure-8 (bidirectional) radiation/reception pattern with nulls broadside to the loop plane and peaks along the loop axis. Pattern: G(θ) = cos²(θ) (figure-8 in azimuth) Null bandwidth: ~15°–25° (sharp, high bearing resolution) Bearing accuracy: ±2°–5° with careful null-finding Ambiguity: Two nulls 180° apart (figure-8). Sense antenna resolves which null is toward the transmitter. Sense + Loop (cardioid): G(θ) = 1 + cos(θ) Null only in ONE direction when sense is added. ================================================================================ MECHANICAL DESIGN ================================================================================ ┌──────────────────────────────────────────────────────────────────────────┐ │ ROTARY LOOP ASSEMBLY (Top View) │ │ │ │ ┌─────────┐ │ │ │ Coax │ ← Gap at top (Faraday shield break) │ │ │ to RX │ │ │ └────┬────┘ │ │ │ │ │ ┌───────────┴───────────┐ │ │ │ Bearing / Shaft │ │ │ │ (rotation axis) │ │ │ └───────────┬───────────┘ │ │ │ │ │ ┌────────────────┼────────────────┐ │ │ │ │ │ │ │ │ ┌────────────┴───────────┐ │ │ │ │ │ LOOP (400mm square) │ │ │ │ │ │ Copper tube 4mm OD │ │ │ │ │ │ or heavy copper wire │ │ │ │ │ │ │ │ │ │ │ │ FARADAY SHIELD: │ │ │ │ │ │ Outer copper foil │ │ │ │ │ │ Gap at ONE point │ │ │ │ │ │ (prevents short) │ │ │ │ │ └────────────────────────┘ │ │ │ │ │ │ │ └────────────────┬────────────────┘ │ │ │ │ │ ┌───────────┴───────────┐ │ │ │ NEMA 17 MOTOR │ │ │ │ Belt drive 3:1 │ │ │ │ GT2 20T → 60T │ │ │ └───────────────────────┘ │ └──────────────────────────────────────────────────────────────────────────┘ MOTOR DRIVE RATIOS: Motor pulley: GT2 20-tooth Loop shaft pulley: GT2 60-tooth Reduction: 3:1 Motor steps/rev: 200 × 16 (microstepping) = 3200 steps Loop degrees/step: 360° / (3200 × 3) = 0.0375°/step Position resolution: 0.038° (excellent) Maximum loop speed: 10 RPM (limited by tuning cap follow-time) ================================================================================ LOOP ELECTRICAL CIRCUIT ================================================================================ LOOP ANTENNA (400mm square, copper tube or heavy wire): ┌──────────────────────────────────────────────────────────────────────────┐ │ │ │ ┌──────────────────────────────────────────────────────────────┐ │ │ │ COPPER LOOP │ │ │ │ L ≈ µ₀ × (4a/π) × [ln(2a/r) - 0.774] │ │ │ │ where a = half-side (0.2m), r = wire radius (0.002m) │ │ │ │ L ≈ 2.2 µH at 400mm square, 4mm tube │ │ │ │ │ │ │ └──────────────────────────────┬────────────────────────────── ┘ │ │ │ │ │ Resonating capacitor ── C_tune (varactor or variable cap) │ │ C_resonate = 1/(ω²L) = 1/((2πf)² × 2.2µH) │ │ Example: 7 MHz → C = 1/((2π×7e6)² × 2.2e-6) = 236 pF │ │ │ │ TUNING TABLE: │ │ ┌──────────┬────────────┬──────────────┐ │ │ │ Band │ Freq (MHz) │ C_tune (pF) │ │ │ ├──────────┼────────────┼──────────────┤ │ │ │ 160m │ 1.85 │ 3370 │ │ │ │ 80m │ 3.75 │ 820 │ │ │ │ 60m │ 5.35 │ 402 │ │ │ │ 40m │ 7.15 │ 225 │ │ │ │ 30m │ 10.125 │ 112 │ │ │ │ 20m │ 14.175 │ 57 │ │ │ │ 17m │ 18.118 │ 35 │ │ │ │ 15m │ 21.225 │ 26 │ │ │ │ 12m │ 24.940 │ 19 │ │ │ │ 10m │ 28.85 │ 14 │ │ │ └──────────┴────────────┴──────────────┘ │ └──────────────────────────────────────────────────────────────────────────┘ ================================================================================ BUFFER AMPLIFIER AT LOOP (JFET SOURCE FOLLOWER) ================================================================================ High impedance input (loop terminal) → low impedance 50Ω output Drain ──── +12V ──── RFC (100µH) ──── Bypass (100µF) │ J310 JFET │ Gate ──── to Loop (high Z) │ [10MΩ gate bias to GND] Source ──── 220Ω ──── GND │ ├──── 0.1µF (DC block) ──── 50Ω output coax ──── to receiver │ GND JFET CHOICE: J310 (NPN JFET, Idss=24–60mA) Gate impedance: >10^9 Ω (does not load loop resonance) Gain: ≈0.9 (source follower, slight loss acceptable) Noise figure: ~1.5 dB (adequate for RDF, high Q loop compensates) Mount at loop terminals to minimize capacitive loading ================================================================================ SENSE ANTENNA COMBINER ================================================================================ Sense antenna (short vertical, 1–2m) combines with loop for cardioid pattern: LOOP output ───────────────────────────────────────────────┐ ├──▶ Combined → RX SENSE antenna ──[0°/90°/180° switch]──[level adjust (5kΩ)] ┘ SENSE PHASE SWITCH (3-position rotary or relay): Position 1 (LOOP ONLY): sense disconnected → figure-8 pattern Position 2 (+SENSE): loop + sense in phase → cardioid peak toward NORTH end of loop Position 3 (-SENSE): loop + sense 180° → cardioid peak toward SOUTH end of loop RELAY IMPLEMENTATION: Sense ──[DPDT relay A]──[DPDT relay B]──── to summing resistors (0/90° shift) (0/180° shift) Relay control: GPIO15 (relay A), GPIO4 (relay B) ================================================================================ POSITION SENSOR ================================================================================ OPTION A: 10kΩ potentiometer (simple, lower accuracy) Pot wiper ──── ADC GPIO36 VCC ──── 3.3V, GND ──── GND ADC reading maps to 0–360° (calibrate at known positions) OPTION B: AS5048A absolute encoder (recommended for accuracy) Interface: SPI SPI CLK ──── GPIO18 SPI MISO ─── GPIO19 SPI MOSI ─── GPIO23 SPI CS ───── GPIO5 Resolution: 14-bit → 0.022° per count Magnet: 6mm neodymium magnet mounted on loop shaft ================================================================================ MOTOR DRIVER — DRV8825 ================================================================================ ESP32 GPIO26 ──── STEP ──── DRV8825 ESP32 GPIO27 ──── DIR ──── DRV8825 ESP32 GPIO14 ──── EN ──── DRV8825 (active LOW) MODE0/1/2 ──── GND/3.3V/GND (sets 1/16 microstepping) DRV8825 Vref (current limit): NEMA 17 rated 1.5A → Vref = 1.5 × 0.5 = 0.75V (Vref = I_rated × R_sense = 1.5A × 0.5Ω) Adjust trimpot on DRV8825 module to 0.75V Motor power: 12V from main supply Logic power: 3.3V from ESP32 3.3V pin (logic level must match) Use 10kΩ pullup on STEP and DIR inputs if needed ================================================================================ COMPLETE LOOP RDF SCHEMATIC (SUMMARY) ================================================================================ 12V DC ──┬──[DRV8825]──── NEMA17 motor ──── belt drive ──── Loop shaft │ ↑ │ GPIO26(STEP), GPIO27(DIR), GPIO14(EN) │ ├──[LM7805]──── +5V ──── LNA bias (J310 drain) │ └──[AMS1117-3.3]── 3.3V ──── ESP32, relay logic LOOP ──── J310 buffer ──── [relay: sense combiner] ──── 50Ω coax ──── RX input SENSE ──── [DPDT relay] ──────────────────────────────────────┘ POSITION: AS5048A (SPI) or pot ──── ESP32 ADC36 I2C: GPIO21/22 ──── SSD1306 OLED + QMC5883L compass UART2: GPIO18/19 ──── NEO-8M GPS