================================================================================ VHF/UHF REMOTE ELEMENT SWITCHING SYSTEM PIN DIODE SWITCHING NETWORK - PARASITIC ELEMENT CONTROL For: 1.25M (222 MHz), 70cm (432 MHz), 33cm (902 MHz), 20cm (1296 MHz) ================================================================================ BLOCK DIAGRAM: PIN DIODE RF SWITCHING WITH DC BIAS INJECTION Element ─┬────[RFC]────[PIN DIODE]────┬──── To Boom/Reflector Stub │ (47-100 nH) │ │ ┌───┴─── Bias Network │ │ Boom ────┴─────────────────────────┴──────── Boom Return (RF Gnd) Bias Control (from ESP32 GPIO): +12V ────[2N2222 Transistor]──┬──── Cathode of PIN │ GND ───────────────[100Ω Resistor] ================================================================================ DETAILED SCHEMATIC FOR SINGLE PARASITIC ELEMENT (70cm EXAMPLE: MA4P504) ================================================================================ Pin Diode: MA4P504 (or HSMP-3814) Element Stub to Boom ↑ Insertion Loss: 0.4-0.6 dB (fwd bias) Isolation: > 35 dB (reverse bias) ╔═══════════════════════════════════════════════════════════════╗ ║ ELEMENT RF PATH ║ ╚═══════════════════════════════════════════════════════════════╝ Element Center RFC Choke PIN Diode Boom (SMA or coax) (47 nH SMD) MA4P504 (RF Return) │ │ │ │ o────────────[L]──o────────[D]───────o─────────────────o 0402 SOT-23 │ Lambda/4 stub (optional) │ │ │ │ └─────────────────┴──────────────────┴─────────────────┘ RF Ground Plane (0V) ╔═══════════════════════════════════════════════════════════════╗ ║ BIAS CONTROL CIRCUIT ║ ╚═══════════════════════════════════════════════════════════════╝ From ESP32 GPIO Pin (3V3 logic) │ o───────────[R_base]──────┬──── Base of 2N2222 1.0kΩ │ Collector────┬──── +12V Supply │ Emitter────┬─┤ (Separate supply) GND ──────────────────────┴────[R_e]──────┘ │ 100-470Ω Flyback Diode (1N4148) │ o─────────────┬──[R_current]──┬─ PIN Anode │ 1.5k or 2k │ Bias-T Coupling Network ┌───┴──────────────┤ ┌┴─ PIN Cathode │ │ │ ┌─[C]─┴──────────────────┴──────────────┘ │ 100 pF │ 0402 RFC ─[L]───┴─ (DC blocking on bias line) (see below) BIAS-T COUPLING ON ELEMENT COAX: Element ─────[C_iso]─────●────[L_fc]────[Bias Diode Network] (empty link) │ 100 pF + 47 nH combo ├─[C_bypass]─ GND │ 100 pF ================================================================================ COMPONENT SELECTION BY BAND ================================================================================ BAND | FREQ | PIN DIODE | RFC CHOKE | Bypass Cap | INSERT LOSS ------------|----------|-------------------|-----------|------------|------------- 1.25M | 222 MHz | BAP51-02 | 100 nH | 100 pF | 0.3 dB 70cm | 432 MHz | MA4P504 or HSMP | 47 nH | 100 pF | 0.4 dB 33cm | 902 MHz | SMP1320-074 | 47 nH | 10 pF | 0.6 dB 20cm | 1296 MHz | SMP1320-074 | 22 nH | 10 pF | 0.8 dB PIN DIODE PART NUMBERS (SOT-23 OR CASE-405): ──────────────────────────────────────── BAP51-02: Vj=0.75V, Ct=0.5pF, Rs=1.8Ω, Vfm=1.2V @ 10mA, 200 MHz+ MA4P504: Vj=0.7V, Ct=0.6pF, Rs=1.0Ω, Vfm=1.1V @ 20mA, 500 MHz+ HSMP-3814: Vj=0.7V, Ct=0.6pF, Rs=1.0Ω, Vfm=1.1V @ 20mA, 500 MHz+ SMP1320-074: Vj=0.63V, Ct=0.63pF, Rs=1.0Ω, Vfm=1.0V @ 10mA, 900 MHz+ ================================================================================ BIAS VOLTAGE & CURRENT SPECIFICATIONS ================================================================================ FORWARD BIAS (Element Active - Connected to Boom): ────────────────────────────────────────────── Applied Bias: +12V (from 2N2222 collector rail) Forward Current: 10-20 mA (typical) Voltage Drop: 0.8-1.0V across PIN diode Series R Drop: 10-20mV across 1-2kΩ limit resistor Net 12V supply voltage for bias network: 12V After 1.5kΩ limit resistor at 10mA: ~12V - 15mV ≈ 11.985V applied to diode Series RFC Choke (47 nH) impedance at 432 MHz: Z_L = j*ω*L = j*2π*432e6 * 47e-9 = j128 Ω This is acceptable for DC bias (choke acts as short @ DC). REVERSE BIAS (Element OFF - Isolated): ────────────────────────────────────────── Applied Bias: 0V (GND) or -5V (optional for better isolation) Reverse Current: < 1 µA (leakage, must stay negligible) Isolation Impedance: > 10 kΩ at bias frequency Off-state RF impedance: > 30 dB isolation at operating frequency ================================================================================ PREFERRED PCB LAYOUT (Critical for UHF/Microwave Bands) ================================================================================ 1. Keep all SMD components within 2-3 mm of each other 2. Place RFC choke and bypass cap DIRECTLY at PIN diode cathode 3. RF trace from element to RFC choke should be < 2 mm, wide (50 mil) 4. Bias line to PIN cathode kept separate from RF line, routed orthogonal 5. Ground plane on all layers: both sides of PCB 6. For 20cm band (1296 MHz): consider microstrip bias choke instead of lumped 7. No stubs or dangling traces - all traces terminated Recommended PCB substrate: - FR-4 for 70cm/33cm (acceptable loss) - RT/duroid 5880 for 20cm (lower loss tangent, better stability) - Trace width at 50Ω: ~30 mil on standard 1.6mm FR-4 ================================================================================ MEASUREMENT NOTES ================================================================================ Forward Bias Verification (using network analyzer): ───────────────────────────────────────────────── - Apply +12V to cathode via 1-2kΩ limit resistor - Measure S21 (transmission): should see < -0.5 dB loss - Measure S11 (reflection): should see < -15 dB return loss - Best insertion loss occurs at optimal bias current (~15 mA) Reverse Bias Verification: ────────────────────────── - Ground cathode (0V), or apply -5V bias for maximum isolation - Measure S21: should see < -30 dB isolation at operating frequency - Measure S11: should see > -10 dB return loss (element now looks like capacitive stub, ~0.6 pF to ground) ================================================================================ RELATED DOCUMENTS ================================================================================ - bias_tee_ascii.txt: Detailed RF+DC coupling network schematic - pin_switching_70cm_ascii.txt: 70cm band specific implementation - pin_switching_33cm_ascii.txt: 33cm band specific implementation - pin_switching_20cm_ascii.txt: 20cm band specific implementation - rf_isolation_filter_ascii.txt: Control line filtering and shielding