================================================================================ bias_tee_ascii.txt — TM-SDR-SCH-003 Rev A Bias-T: DC Power Injection Over Coax for Remote Antenna Amplifiers Dual-band: HF (0.1–30 MHz) and VHF/UHF (30 MHz–3 GHz) versions Document: TM-SDR-SCH-003 Rev A ================================================================================ OVERVIEW -------- A bias-T injects DC power onto a coaxial cable to power remote low-noise amplifiers (LNA), masthead preamps, or active antenna elements. RF signal and DC power share the same coax; separated at each end. Principle: ┌─────────────────────────────────────────────────────────────┐ │ DC IN ──[Lchoke]──┬──[coax]──┬──[Lchoke]── DC OUT (load) │ │ │ │ │ │ RF IN ──[Cblock]──┘ └──[Cblock]── RF OUT (SDR) │ └─────────────────────────────────────────────────────────────┘ RF path: DC blocking capacitor in series (passes RF, blocks DC) DC path: RF choke in series (passes DC, blocks RF) Junction: where RF and DC share coax to antenna KEY SPECIFICATIONS ------------------ HF Bias-T (Version A): RF frequency range: 0.1 MHz – 50 MHz DC voltage: 5 V or 12 V selectable (jumper) DC current max: 500 mA RF insertion loss: < 0.2 dB from 0.1–50 MHz DC isolation to RF: > 40 dB at all frequencies RF isolation to DC: > 50 dB at all frequencies VHF/UHF Bias-T (Version B): RF frequency range: 10 MHz – 3 GHz DC voltage: 3.3 V, 5 V, or 12 V DC current max: 300 mA RF insertion loss: < 0.3 dB from 30 MHz–1 GHz < 0.5 dB from 1–2.4 GHz DC:RF isolation: > 50 dB VERSION A — HF BIAS-T SCHEMATIC (0.1–50 MHz) ---------------------------------------------- ┌─── SMA J1 (RF+DC from antenna) ──────────────────────────────┐ │ │ │ J1 center ──┬──────────────────────── J3 center (RF to SDR) │ │ │ │ │ │ L1 │ │ ├──── node A ──[47µH]──[47µH]── node B ──┐ │ │ │ RF choke pair │ │ │ │ J2 center │ │ │ (DC power in) │ │ │ │ J1 shield ─┴─────────────────────────────────── J3 shield │ │ (GND) (GND) │ └───────────────────────────────────────────────────────────────┘ DETAILED SCHEMATIC — Version A: SMA J1 (ANTENNA): center conductor = RF + DC mixed signal from antenna RF path (series DC-block): J1 center ── C1 (220 pF NP0) ── node RF_OUT ── SMA J3 (RF to SDR) DC injection path: J2 (barrel jack 5.5/2.1 mm) center ──┐ ├── D1 (SS14 polarity protect) ├── F1 (PTC 500 mA) └── node DC_IN DC_IN ── L1 (47 µH) ── L2 (47 µH) ── J1 center DC blocking for RF output: J3 center connected directly to J1 center side of C1 (series cap). The series cap C1 blocks DC from reaching SDR input. Ground: J1 shield = J2 GND = J3 shield = common GND COMPONENT NOTES: C1: 220 pF NP0 — passes RF from 1 MHz up, blocks DC. At 1.8 MHz: Xc = 1/(2π × 1.8e6 × 220e-12) = 402 Ω — some loss. At 3.5 MHz: Xc = 207 Ω — loss present; transformer-coupled LNA input preferred. BETTER CHOICE for 160m: C1 = 1 nF or 10 nF NP0. At 1.8 MHz with 1 nF: Xc = 88 Ω. With 10 nF: Xc = 8.8 Ω. USE 10 nF for 160m/80m; 220 pF for HF-general. L1, L2: 47 µH each, total 94 µH. RF choke at 1.8 MHz: XL = 2π × 1.8e6 × 94e-6 = 1063 Ω. Good isolation. SRF check: 47 µH inductors typically SRF > 10 MHz. Two in series SRF lower. BETTER: use wound toroid (T50-2 or T50-6 core), 40 turns bifilar. T50-2: AL=10 nH/turn², 40T → L = 16 µH (XL=180 Ω at 1.8 MHz). T50-6: AL=25 nH/turn², 25T → L = 16 µH (better at HF). TWO TOROIDS: 32 µH total, XL = 360 Ω at 1.8 MHz. For better isolation, use 3 toroids in series (common HF bias-T design). D1: SS14 Schottky — reverse polarity protection. Forward drop: 0.3V. DC to remote = Vcc − 0.3V. F1: PTC resettable fuse, 500 mA (or 300 mA for LNA-only bias). Trips if remote load shorts or draws excess current. DC SELECTOR JUMPER: JP1 (3-pin, 2.54mm): Position 1-2: routes +5V (from onboard 7805 or USB VBUS) Position 2-3: routes +12V (from external barrel jack) +5V source: USB VBUS directly, or 7805 regulator from 12V supply. +12V source: J4 (2.1mm barrel jack), external 12V wall adapter. VERSION B — VHF/UHF BIAS-T SCHEMATIC (10 MHz – 3 GHz) --------------------------------------------------------- For SDR wideband use: powers masthead LNA for 2m/70cm/33cm/23cm. Uses SMD components optimized for low parasitic inductance/capacitance. SMA J1 (ANTENNA: RF + DC) SMA J2 (RF TO SDR) J3 (DC power in: USB or barrel) RF PATH: J1 center ── C_blk (100 pF NP0, 0402) ── J2 center Xc at 100 MHz: 16 Ω → good Xc at 1 GHz: 1.6 Ω → good Xc at 10 MHz: 159 Ω → slight loss (for VHF-up bias-T, acceptable) DC INJECTION: J3+ ── D1 (BAT54S SOT-23) ── F1 (PTC 300 mA) ── L_choke ── J1 center L_choke: 1000 nH (1 µH) SMD wirewound (Coilcraft 0805LS-102, SRF > 500 MHz) XL at 100 MHz: 628 Ω XL at 500 MHz: 3142 Ω BYPASS AT DC INJECTION POINT: J3+ ── C_byp1 (10 µF tantalum) ── GND J3+ ── C_byp2 (10 nF NP0 0402) ── GND SHUNT CAPACITORS AT RF/DC JUNCTION (for stability): Antenna side of L_choke ── C_shunt (1 nF NP0 0402) ── GND Prevents self-resonance of choke from creating RF peak. FULL SCHEMATIC (VERSION B, SMD): 100pF NP0 J1 (ANT) ────────┤├──────────────────────── J2 (SDR RF out) center │ C_blk center │ ├── 1µH choke ── PTC F1 ── SS14 D1 ── J3+ (DC in) │ │ │ 1nF NP0 to GND (stability bypass) │ └── 10nF NP0 to GND (RF bypass, broad) J1 shield = J2 shield = J3 GND (common ground) DC VOLTAGE / CURRENT TABLE ---------------------------- Remote Device Typical Vcc Typical Icc Recommended Setting ───────────────────────────────────────────────────────────────────── SPF5189Z LNA 3.0–5.0V 60 mA 5V, 100 mA PTC MGA-86576 LNA 3.0–5.0V 55 mA 5V, 100 mA PTC PSA4-5043 LNA 3.0–5.0V 60 mA 5V, 100 mA PTC ZX60-43+ amplifier 5.0V 95 mA 5V, 150 mA PTC Active whip (HF) 12V 120 mA 12V, 200 mA PTC Wideband preamp 12V 200 mA 12V, 300 mA PTC PA (remote) 12V >500 mA use dedicated power cable MEASURING BIAS-T PERFORMANCE ------------------------------ Equipment: NanoVNA, DC power supply, multimeter. 1. RF insertion loss: NanoVNA Port 1 → J1 (antenna port) NanoVNA Port 2 → J2 (SDR port) DC jack unloaded (no remote device connected) Sweep 0.1–3000 MHz. S21 should be > −0.5 dB across bandwidth. 2. DC:RF isolation: Apply DC to J3 (5V, unloaded). Measure RF spectrum at J1 with SDR. Should see no 50/60 Hz noise or switching noise from power supply. If noise present: add 100 µH + 100 µF LC filter on DC rail. 3. RF:DC isolation: Apply RF signal to J1 (−30 dBm from signal generator). Measure RF at J3 with spectrum analyzer. Should be > 40 dB below input level. 4. Current test: Connect 10Ω resistor to remote end of coax (simulate 500 mA at 5V). Verify PTC fuse does NOT trip (it's a 500 mA polyfuse, rated 500 mA hold). Verify voltage at remote end: 5V − 0.3V (D1) − (I × R_coax). COAX DC RESISTANCE (important for remote LNA powering) -------------------------------------------------------- RG-58 (50Ω): center conductor 0.037 Ω/m, shield 0.006 Ω/m Total loop resistance: ~0.043 Ω/m (one-way: 0.020 Ω/m) Example: 50m RG-58 run to LNA: Conductor resistance: 50 × 0.037 = 1.85 Ω Voltage drop at 100 mA: 0.185 V Power supply needed: 5.0 + 0.3 (diode) + 0.185 = 5.49V minimum For longer runs, use heavier coax (RG-213) or boost supply voltage. BILL OF MATERIALS — BIAS-T (BOTH VERSIONS) -------------------------------------------- Version A (HF): C1: 10 nF NP0 50V 0805 (or 220 pF for VHF use) L1, L2: 47 µH or wound T50-6 toroid, 25T, 16 µH each D1: SS14 Schottky SMA package F1: PTC 500 mA 1812 (Littelfuse 1812L050) JP1: 3-pin header 2.54mm + jumper J1,J2: SMA edge mount 50Ω J3: 2.1mm barrel jack PCB: 50 × 30 mm, 1 oz copper Version B (VHF/UHF): C_blk: 100 pF NP0 0402 C_shunt: 1 nF NP0 0402 C_byp1: 10 µF tantalum case B C_byp2: 10 nF NP0 0402 L_choke: 1000 nH Coilcraft 0805LS-102 SMD D1: BAT54S SOT-23 F1: PTC 300 mA 1206 J1,J2: SMA edge mount J3: 2.1mm barrel jack or USB Micro-B PCB: 40 × 25 mm, 1 oz copper, matched impedance traces ================================================================================