================================================================================ VHF/UHF REMOTE ELEMENT SWITCHING SYSTEM BIAS-TEE NETWORK: RF+DC COUPLING FOR PIN DIODE BIAS INJECTION For PIN Diode Element Switching (1.25M through 20cm) ================================================================================ FUNCTION ──────── The bias-tee allows DC bias voltage to be applied to the PIN diode cathode while maintaining RF signal path with minimal loss. Used at the element feed point for each remotely-switched parasitic element. ================================================================================ BASIC BIAS-TEE NETWORK ================================================================================ Element (from boom center) │ │ ┌────o────────────────┐ │ │ │ RF signal path │ │ (element) │ │ │ │ ┌────o─────── To PIN Diode Network │ │ ├───[C_iso]──────┤ │ (1.0 µF) │ │ low freq │ │ block DC │ │ │ │ ┌────o────── Bias Control (+12V, 0V, or -5V) │ │ │ ┌──[RFC]──┐ │ │ (47-100 nH) ┤ │ │ SMD ├──[R_limit]─┬─ PIN ANODE │ │ 0402 ┤ (1.5k) │ │ └──────────┴─ PIN CATHODE │ │ │ ┌────o────── DC Bias Supply/Control (2N2222 output) │ └────────────────────────── RF GND (Boom) ================================================================================ COMPONENT SELECTION & FUNCTION ================================================================================ 1. C_ISO (Isolation Capacitor: 1.0 µF ceramic or film) ───────────────────────────────────────────────── Purpose: Block DC bias from feeding back to boom/feedline Impedance @ 50 MHz: Z_C = 1/(2π × 50e6 × 1e-6) = 3.2 Ω (excellent RF pass) Impedance @ 1296 MHz: Z_C = 1/(2π × 1296e6 × 1e-6) = 0.12 Ω (excellent) DC blocking: Infinite impedance to DC Part: KEMET C315C105M5U or equivalent 1.0µF ceramic 50V+ Placement: At element feedthrough, immediately before RFC choke 2. RFC Choke (47-100 nH SMD 0402) ────────────────────────────── Purpose: Block RF from feeding into bias control network; act as RF short Impedance @ 50 MHz: Z_L = j*2π*50e6*47e-9 = j14.8 Ω (small, acceptable) Impedance @ 432 MHz: Z_L = j*2π*432e6*47e-9 = j128 Ω (moderate, tuned) Impedance @ 1296 MHz: Z_L = j*2π*1296e6*47e-9 = j384 Ω (high, provides choke) Selection by band: 50-144 MHz: 100 nH (larger choke acceptable, lower SRF) 222-432 MHz: 47 nH (balance between DC/bias and RF choke) 902+ MHz: 22-47 nH (tighter control, but watch SRF) Part: Murata LQG15HS47NG00D (47 nH, 0402, 1.5 GHz SRF) Alternative: TDK MLZ1005R47M (47 nH, low cost, 1.0 GHz SRF) 3. R_LIMIT (Bias Current Limiting: 1.5-2.2 kΩ) ────────────────────────────────────────── Purpose: Set forward bias current to 10-20 mA; prevent excessive drain current Forward current @ 12V, 1.5kΩ: I = 12V / 1500Ω = 8 mA (acceptable, conservative) Forward current @ 12V, 1.0kΩ: I = 12V / 1000Ω = 12 mA (target, typical) Forward current @ 12V, 470Ω: I = 12V / 470Ω = 25.5 mA (high, risky for lifetime) Selection: Use 1.5kΩ to 2.2kΩ for 10 mA forward current Part: Standard metal-film 1/4W 1.5kΩ ±1% Power dissipation @ 10 mA: P = 10mA² × 1500 = 0.15 W (safe, 1/4W resistor OK) For -5V reverse bias (optional, for maximum isolation): Reverse current @ -5V, 1.5kΩ: I = -5V / 1500Ω = -3.3 mA (leakage acceptable) 4. C_BYPASS (Bypass Capacitor: 10-100 pF) ────────────────────────────────────── Purpose: Bypass high-frequency RF noise, improve bias line impedance Impedance @ 50 MHz: Z_C = 1/(2π*50e6*100pF) = 32 Ω (modest RF bypass) Impedance @ 1296 MHz: Z_C = 1/(2π*1296e6*100pF) = 1.2 Ω (excellent RF bypass) Selection: 100 pF for lower bands (50-432 MHz), 10-47 pF for UHF (902+ MHz) Part: Generic 100 pF NPO ceramic 0402, 50V+ ================================================================================ DETAILED SCHEMATIC: PIN DIODE BIAS-TEE FOR 70cm EXAMPLE (432 MHz) ================================================================================ Element Coax from Boom Bias Control (from 2N2222 Output) │ │ o────────────────────┬──────o │ │ C_ISO [RFC 47nH] 1.0µF SMD 0402 ╱╱ │ │ [R_limit] │ 1.5kΩ │ 1/4W ±1% │ │ Element ─────────────────┬─┴──── PIN Anode (+) Stub to │ Boom ─┐ (RF Path) PIN Diode │ (minimal │ │ loss) (SOT-23 or │ Cathode ←──────┐ │ │ │ │ (-) (-) [C_bypass] │ 100 pF, 0402 │ │ Boom Return (GND / 0V RF) │ └──────────────────────────── RF Ground Plane SERIES IMPEDANCE AT 432 MHz: ─────────────────────────── C_ISO @ 432 MHz: Z ≈ 1/(2π × 432e6 × 1e-6) = 0.37 Ω (negligible) RFC @ 432 MHz: Z ≈ j × 128 Ω (moderate reactance, ~0.3 dB loss) R_limit @ 432 MHz: DC resistor, acts as pure resistance (negligible @ RF) Total series Z: ~130 Ω (causes ~0.3-0.4 dB insertion loss at 50Ω system) This is acceptable for parasitic element switching. If loss is too high, reduce RFC value to 22 nH and increase C_bypass to 470 pF. ================================================================================ ALTERNATIVE DESIGN: MICROSTRIP TRANSMISSION LINE CHOKE (20cm, 1296 MHz) ================================================================================ At 1296 MHz with SMD components, parasitics dominate. Consider microstrip implementation on PCB: Element ─────[C_iso 1µF]─────●────── Microstrip Line (λ/4 high-Z) │ Bias Node │ Microstrip λ/4 HIGH-IMPEDANCE (acts as quarter-wave choke) Z = 100 Ω → 200 Ω tuned Length @ 1296 MHz: λ/4 ≈ 44 mm Width: ~2-3 mm on 1.6mm FR-4 │ [C_bypass 10pF] │ +12V Bias Input Design equations (50 ohm system, FR-4 substrate): Characteristic impedance of microstrip: Z_0 = 200 Ω (target) For λ/4 line at 1296 MHz: Length = 299.792 / (4 × 1.296) = 57.8 mm in free space In FR-4 (ε_r ≈ 4.7): L = 57.8 / √4.7 ≈ 26.6 mm (accounting for dispersion) Quarter-wave choke behavior: Z_in = Z_0² / Z_load At RF: Z_load ≈ small (bias resistor ≈ 0 at RF) Z_in ≈ 200² / (negligible) ≈ HIGH IMPEDANCE (effectively open circuit for RF) At DC/bias: λ/4 line acts as transmission line, low impedance for DC ================================================================================ EXPERIMENTAL VERIFICATION (With Network Analyzer) ================================================================================ 1. INSERTION LOSS TEST: ───────────────── - Terminate bias-tee output with 50Ω load (no PIN diode connected) - Measure S21 from input to output - Forward bias (apply +12V to bias node): should see < -0.5 dB loss at operating freq - Reverse bias (ground bias node): should see < -0.5 dB loss (RF path unaffected) 2. IMPEDANCE MATCHING TEST: ──────────────────────── - Apply +12V bias (diode conducting, low impedance) - Measure S11 (reflection): should see < -15 dB return loss - Adjust C_bypass value if reflection peaking is observed 3. ISOLATION TEST (PIN Diode Reverse Biased): ────────────────────────────────────────── - Ground bias node (0V or -5V) - Measure S21: should see > -30 dB isolation at operating frequency - This verifies PIN diode is switching OFF properly ================================================================================ PCB LAYOUT CRITICAL POINTS ================================================================================ For 70cm (432 MHz) and above: 1. Keep all SMD components within 3 mm of each other Use schematic trace layout guides, not long PCB traces 2. Minimize trace lengths: C_ISO → RFC → PIN Diode: < 5 mm total path length 3. Ground plane on top/bottom: Both layers reference to ground at multiple points Avoid long via-to-pad traces 4. Bias line routing: Keep bias line >2 mm away from RF path to minimize coupling Route orthogonal to RF line if possible Use ground trace as shield between RF and bias 5. SMA Connector placement: Element input: North of RFC choke Bias input: South of RFC choke (opposite side) Ground: Connected to both connectors, merged at single point 6. Optional: Solder a 4-pin JST connector for element bias input JST XH 2.5mm pitch (common in RC/FPV applications) Allows field replacement without unsoldering ================================================================================ FREQUENCY-SPECIFIC BIAS-TEE COMPONENTS ================================================================================ BAND | RFC Choke | C_ISO | C_bypass | Z_RFC @ f | Notes ──────────────────────────────────────────────────────── 1.25M(222M)| 100 nH | 1 µF | 100 pF | j28 Ω | Conservative 70cm(432M) | 47 nH | 1 µF | 100 pF | j128 Ω | Moderate loss 33cm(902M) | 47 nH | 1 µF | 47 pF | j265 Ω | Tighter control 20cm(1296M)| 22 nH * | 1 µF | 10 pF | j90 Ω* | * Use ustrip choke (*Microstrip λ/4 preferred for lowest parasitics) ================================================================================ COMPONENT PROCUREMENT ================================================================================ RFC Chokes (0402 SMD): - Murata LQG15HS47NG00D: 47 nH, $0.08-0.15 each (Digi-Key, Mouser) - TDK MLZ1005R47M: 47 nH, $0.06-0.10 each - Würth 742 792 047: 47 nH, $0.05-0.08 each For 22 nH: Murata LQG15HS22NG00D ($0.10-0.15) Capacitors: - C_ISO 1.0 µF: KEMET C315C105M5U ($0.12-0.25) - C_bypass 100 pF: Generic NPO 0402 ($0.01-0.05) - C_bypass 47 pF: Generic NPO 0402 ($0.01-0.05) - C_bypass 10 pF: Generic NPO 0402 ($0.01-0.05) Resistors: - 1.5 kΩ ±1% metal-film 1/4W ($0.01-0.05) - 1.0 kΩ ±1% metal-film 1/4W ($0.01-0.05) PIN Diodes: - BAP51-02: Digi-Key (~$0.50 each) - MA4P504: Digi-Key (~$0.60 each) - SMP1320-074: Digi-Key (~$0.40 each) ================================================================================ RELATED DOCUMENTS ================================================================================ - pin_diode_switching_ascii.txt: Complete PIN diode circuit overview - rf_isolation_filter_ascii.txt: Control line filtering - pin_switching_70cm_ascii.txt: 70cm band implementation - pin_switching_20cm_ascii.txt: 20cm band implementation (microstrip)