================================================================================ VHF/UHF REMOTE ELEMENT SWITCHING SYSTEM 70 CENTIMETERS (432.1 MHz) - PIN DIODE ELEMENT SWITCHING COMPLETE IMPLEMENTATION FOR 5-ELEMENT YAGI ================================================================================ OVERVIEW ──────── At 70cm, relay contacts exhibit ~2 nH lead inductance, causing excessive loss and reactance. PIN diodes (MA4P504, HSMP-3814) provide < 0.4 dB insertion loss. This document specifies complete PIN diode switching for all parasitic elements. ================================================================================ PIN DIODE SELECTION FOR 70cm (432.1 MHz) ================================================================================ Candidate devices (all SOT-23 package, suitable for SMD): Part | Vj | Ct | Rs | Vfm | I_bias | F_max | Cost ───────────────────────────────────────────────────────── MA4P504 | 0.7V | 0.6pF| 1Ω | 1.1V @ 20mA | 500 MHz| $0.60 HSMP-3814 | 0.7V | 0.6pF| 1Ω | 1.1V @ 20mA | 500 MHz| $0.55 BAP51-02 | 0.75V | 0.5pF| 1.8Ω| 1.2V @ 10mA | 200 MHz| $0.45 RECOMMENDED: MA4P504 or HSMP-3814 (both equivalent, good availability) Cost: $0.50-0.65 per diode (qty 100) Qty needed: 3 units (one per director element) ================================================================================ COMPLETE PIN SWITCHING NETWORK FOR SINGLE ELEMENT (70cm) ================================================================================ Element stub to boom ↑ From Driven RF Loss @ 432 MHz: 0.4 dB Element Coax Isolation (reverse): > 35 dB │ ┌────o────────────────────────────────┐ │ │ [C_iso] [RFC 47nH] [PIN-D] │ 1µF (0402) MA4P504 │ ╱╱ Murata SOT-23 │ │ LQG15HS47 (anode+) │ │ │ │ │ o─────────┬────────────→ Element stub │ │ │ │ [R_limit] (cathode-) │ 1.5 kΩ │ │ ±1% [C_bypass] │ 1/4W 100 pF, 0402 │ │ │ o──────────── Bias Node (from 2N2222) │ └──────────────────────────────────── RF Ground (boom return) ================================================================================ COMPONENT SPECIFICATIONS ================================================================================ RFC CHOKE (L1): ────────────── Part: Murata LQG15HS47NG00D (0402 SMD, 47 nH) Impedance @ 432 MHz: Z_L = j*2π*432e6*47e-9 = j128 Ω (adequate choke) Q: ~20 @ 1 GHz (low loss) Self-resonant frequency (SRF): 1.2 GHz (above operating frequency, good) Max current: 500 mA (excessive, only 10-15 mA needed, safe) Cost: $0.10-0.15 BIAS CURRENT LIMITING RESISTOR (R1): ─────────────────────────────────── Part: Standard metal-film 1/4W ±1% resistor Value: 1.5 kΩ (provides 8 mA forward bias @ 12V) Power dissipation @ 10 mA: P = 10² × 1500 = 0.15 W (1/4W rated OK) Tolerance: ±1% (ensures consistent bias across all elements) Cost: $0.02-0.05 BYPASS CAPACITOR (C2): ────────────────────── Part: Generic NPO ceramic 0402, 50V+ Value: 100 pF (provides RF bypass on bias node) Impedance @ 432 MHz: Z_C = 1/(2π*432e6*100pF) = j3.7 Ω (good RF bypass) Cost: $0.02-0.03 ISOLATION CAPACITOR (C1): ───────────────────────── Part: Film capacitor or ceramic, 50V+ rating (blocking DC to boom) Value: 1.0 µF (blocks DC bias from feeding to boom feedline) Impedance @ 432 MHz: Z_C ≈ 0.37 Ω (negligible for RF) Cost: $0.10-0.20 PIN DIODE (D1): ────────────── Part: MA4P504 (SOT-23 package) Absolute maximum ratings: - Forward voltage: 1.1V @ 20 mA (typical) - Peak reverse voltage: 6 V (applied -5V for isolation recommended) - Switching speed: < 30 ns (adequate for element switching) Packaging: SOT-23 (3-pin: Anode, Cathode, Substrate) Cost: $0.55-0.70 per diode RELAY DRIVER (For bias control): ──────────────────────────────── Part: 2N2222 NPN transistor (TO-92 package, or BC337 SMD equivalent) Base-emitter voltage drop: VBE ≈ 0.7V Collector-emitter saturation: VCE_sat ≈ 0.2V @ 10 mA Max collector current: 500 mA (more than sufficient) Hfe (current gain): ~100-200 Switching time: < 100 ns (fast enough for bias switching) Cost: $0.05-0.10 per transistor ================================================================================ COMPLETE 5-ELEMENT PIN DIODE ARRAY (70cm) ================================================================================ Each element gets its own PIN diode switching network: Driven Element (Fixed - No switching) ├─ Reflector Element: PIN diode D1 ├─ Director 1: PIN diode D2 ├─ Director 2: PIN diode D3 └─ Director 3: (Optional, via separate GPIO) BIAS CONTROL FROM ESP32: GPIO2 ─[2N2222]─→ Bias Node (D1 reflector) → D1 forward/reverse GPIO4 ─[2N2222]─→ Bias Node (D2 dir1) → D2 forward/reverse GPIO5 ─[2N2222]─→ Bias Node (D3 dir2) → D3 forward/reverse GPIO17─[2N2222]─→ Bias Node (D4 dir3) → D4 forward/reverse (if used) PIN STATE TABLE (432.1 MHz Array, 5-Element Baseline): Pattern | D1(Refl) | D2(Dir1) | D3(Dir2) | D4(Dir3) | State | Gain ──────────────────────────────────────────────────────────────────────── BASELINE 5-EL | FWD | FWD | FWD | FWD | ON | 9.2dBd COMPACT 3-EL | FWD | FWD | REV | REV | ON | 6.5dBd ENDFIRE FORWARD | REV | FWD | FWD | FWD | BEAM | 5dBd ENDFIRE REVERSE | FWD | FWD | FWD | REV | BEAM | 5dBd FWD = Forward bias (+12V applied via GPIO+2N2222) = element active REV = Reverse bias (GND) = element isolated ================================================================================ PIN DIODE BIAS-SWITCHING CIRCUIT (Single Element) ================================================================================ Complete single-element switching node: ESP32 GPIO2 (3V3 output) │ ┌────o────────────────────────────────────┐ │ │ │ ULN2803 (shared for all elements) │ │ Output → +12V relay supply │ │ │ │ ULN2803 OUT(n) ────────────┐ │ │ │ │ │ (or 2N2222 discrete:) [R_base] │ │ │ │ │ GPIO─[R_1k]──→ 2N2222 Base │ 1kΩ │ │ │ │ │ │ GND ─────→ Emitter │ │ │ │ │ │ Collector ────────┴────┐ │ │ │ │ │ +12V Bias Supply │ │ │ │ │ │ │ └─────────[C_coupling] │ │ │ 10 µF │ │ │ │ │ │ [R_limit] │ │ │ 1.5 kΩ │ │ │ │ │ │ [PIN D] pin to Element stub │ (D cathode) │ │ │ [C_bypass] 100 pF │ │ │ Boom GND ================================================================================ DETAILED PCB LAYOUT FOR 70cm PIN DIODE NODE ================================================================================ Constraints: - All components within 5 mm of each other - RFC choke + PIN diode physically adjacent (< 1 mm) - Bypass cap at PIN cathode (< 2 mm) - RF trace width: 50 mil (1.27 mm) at 50Ω - Bias line: separate from RF trace, >2 mm spacing or orthogonal routing Top layer (copper): ┌─────────────────────────────────┐ │ Element Feed (SMA connector) │ │ │ │ │ [C_iso] │ │ 1 µF │ │ │ │ │ o───[RFC L1]────● │ │ │ 47 nH │ │ │ │ [D1] │ │ │ (MA4P504) │ │ GND ─────┴───────────────┤───→ Element Stub │ (continuous plane) │ (to boost/combiner) │ │ [C_bypass] │ │ │ 100 pF │ │ │ (0402) │ │ │ │ │ │ └───────────────o─→ Bias Node (to 2N2222) │ │ │ (+12V Bias from 2N2222) │ │ │ └─────────────────────────────────┘ Bottom layer (Ground plane): ──────────────────────────── Solid GND, via grid every 10 mm ================================================================================ BIAS CONTROL CIRCUIT (2N2222 Transistor) ================================================================================ GPIO from ESP32 → 2N2222 Bias Driver: ESP32 GPIO(2V3) ──[R_base 1kΩ]──→ 2N2222 Base (pin 1) │ Collector (pin 2) ──→ +12V bias to PIN │ Emitter (pin 3) ──→ GND When GPIO = 3.3V (HIGH): 2N2222 Collector → 12V (less drop across bias R) PIN diode forward biased → element active (acts as reflector/director) When GPIO = 0V (LOW): 2N2222 Collector → 0V (grounded via emitter) PIN diode reverse biased → element isolated (acts as free director or OFF) DC Biasing calculation: ────────────────────── Forward bias current (desired): I = 10 mA Across 1.5kΩ limit resistor: V_R = 10 mA × 1.5kΩ = 15V But we only have +12V, so: I = 12V / (1.5kΩ + PIN diode ~1Ω) ≈ 8 mA This is conservative but acceptable (min insertion loss ~0.35 dB) For more aggressive bias (higher current): Use 1.0 kΩ resistor: I = 12V / 1kΩ = 12 mA (0.4 dB insertion loss) Use 0.68 kΩ resistor: I = 12V / 0.68kΩ = 17.6 mA (0.35 dB loss, higher risk) ================================================================================ ELEMENT STUB LENGTH & RESONANCE ================================================================================ For PIN diode switching at 70cm, use a short stub (< λ/4) at boom connection: Element length (from NEC2): 0.342 m (half-wave) Stub length at boom: λ/4 = 0.694 / 4 = 0.173 m (short stub) Stub function: - When PIN forward biased (on): element shorted to boom via stub - Stub acts as short circuit → element becomes reflector - Stub impedance match: λ/4 line with ~70 Ω characteristic impedance Stub implementation: - Aluminum tube or tubing, ~6.35 mm OD - Length: 0.170-0.175 m (tuned empirically with NanoVNA) - Mounted directly below driven element ================================================================================ MEASUREMENT & VERIFICATION (Network Analyzer) ================================================================================ Test setup: VNA Port 1 → Element feed (at C_iso) VNA Port 2 → 50Ω load (or antenna port) Bias voltage: Apply +12V to PIN cathode S21 (Insertion Loss): ──────────────────── Forward bias (+12V): S21 ≈ -0.4 dB (acceptable) Reverse bias (0V): S21 ≈ -0.4 dB (RF path unaffected by OFF state) S11 (Reflection): ──────────────── Forward bias (+12V): S11 ≈ -15 dB @ 432 MHz Reverse bias (0V): S11 ≈ -8 dB @ 432 MHz (element looks capacitive) ================================================================================ PARTS LIST (Single Element, 70cm) ================================================================================ Qty Part Number | Description | Value | Cost ────────────────────────────────────────────────────────────────── 1 MA4P504 | PIN diode, SOT-23 | - | $0.55 1 Murata LQG15HS47 | RFC choke, 0402 SMD | 47 nH | $0.12 1 Metal-film resistor| Bias limit resistor | 1.5 kΩ | $0.05 1 NPO ceramic 0402 | Bypass capacitor | 100 pF | $0.02 1 KEMET C315C105M5U | Isolation capacitor | 1.0 µF | $0.15 1 2N2222 or BC337 | Bias driver transistor | - | $0.08 1 1N4148 | Flyback diode | - | $0.03 TOTAL (per element): ~$1.00-1.20 TOTAL (5 elements): ~$5.00-6.00 in PIN diode components ================================================================================ ASSEMBLY NOTES ================================================================================ 1. Solder C_iso capacitor first (large coupling cap at element entry) 2. Solder RFC choke immediately next to element stub connection point 3. Solder PIN diode (SOT-23) with cathode facing toward bias control 4. Solder bypass capacitor directly at PIN diode cathode 5. Wire bias control (2N2222 output or ULN2803 output) to PIN anode 6. Add ferrite bead on bias line (100Ω @ 432 MHz) 7. Use shielded twisted pair if control line > 1 meter from main PCB ================================================================================ RELATED DOCUMENTS ================================================================================ - pin_diode_switching_ascii.txt: General PIN diode overview - bias_tee_ascii.txt: Bias-tee coupling network details - rf_isolation_filter_ascii.txt: Control line shielding - yagi_70cm_baseline_5el.nec: NEC2 antenna model