================================================================================ SCHEMATIC: TRANSMISSION-LINE DIRECTIONAL COUPLER — VHF/UHF BANDS TM-COUP-001 Rev A Microstrip coupled-line coupler: 50–1300 MHz (6m through 23cm) Coupling: −20 dB / −30 dB versions Directivity: >25 dB (50–500 MHz), >20 dB (500–1300 MHz) Insertion loss: <0.1 dB (50–500 MHz), <0.2 dB (500–1300 MHz) ================================================================================ REFERENCE DOCUMENTS: sch_detector_swr_output.txt — Detector and SWR calculation circuits rf_power_monitor_esp32.ino — Firmware enclosure_vhf_coupler.scad — SMA-connector enclosure ================================================================================ SECTION 1 — TRANSMISSION-LINE COUPLER THEORY ================================================================================ A coupled-line directional coupler consists of two parallel transmission lines in close proximity (coupled region), with four ports: INPUT, THROUGH, COUPLED, and ISOLATED. PORT 1 PORT 2 IN ───────────────── THROUGH ))))))))))))))) ───────────────── ISOLATED COUPLED PORT 4 PORT 3 Electrical length: λ/4 at center frequency (quarter-wave coupler) Coupling occurs from Port 1 to Port 3 (backward coupling in TEM structures) Isolation at Port 4 (input to isolated) ideally = infinity EVEN/ODD MODE ANALYSIS: Z0_even (Z_oe): characteristic impedance of the even mode Z0_odd (Z_oo): characteristic impedance of the odd mode Coupling coefficient: C = (Z_oe − Z_oo) / (Z_oe + Z_oo) For Z0 = 50Ω system impedance: Z_oe × Z_oo = Z0² = 2500 Coupling values: −10 dB: C=0.316 → Z_oe=90.9Ω, Z_oo=27.5Ω (tight; large gap change needed) −20 dB: C=0.100 → Z_oe=55.3Ω, Z_oo=45.2Ω (practical; standard PCB etch) −30 dB: C=0.032 → Z_oe=51.6Ω, Z_oo=48.5Ω (loose; small gap; less sensitive) −40 dB: C=0.010 → Z_oe=50.5Ω, Z_oo=49.5Ω (very loose; difficult to achieve) ================================================================================ SECTION 2 — PCB MICROSTRIP DESIGN ================================================================================ SUBSTRATE: FR4, εr=4.4, h=1.6mm, copper 35µm (1oz), tan_δ=0.02 SINGLE MICROSTRIP 50Ω LINE: Width w = 3.0mm (calculated), effective εr_eff ≈ 3.26 Guided wavelength λ_g = 300mm / (f_GHz × √εr_eff) At 144 MHz (2m): λ_g = 300/(0.144 × 1.806) = 1154mm → λ/4 = 289mm (too long!) At 432 MHz (70cm): λ_g = 300/(0.432 × 1.806) = 385mm → λ/4 = 96mm ✓ At 1296 MHz (23cm): λ_g = 300/(1.296 × 1.806) = 128mm → λ/4 = 32mm ✓ SOLUTION FOR VHF (50–222 MHz): Lumped-element approximation using LC section or use meander-line structure to fit λ/4 within PCB constraints. OR: Use coaxial-line coupler (no PCB fabrication needed for field use). COUPLED MICROSTRIP GEOMETRY (−20 dB, 432 MHz center): Line width (w): 3.0mm for 50Ω Gap (s): Calculated from required Z_oe/Z_oo For Z_oe = 55.3Ω, Z_oo = 45.2Ω on FR4: s ≈ 1.2mm (using Hammerstad-Jensen synthesis) Length: λ_g/4 at 432 MHz = 96mm COUPLED LINE CROSS-SECTION: ← w=3.0mm→ ←s=1.2mm→ ← w=3.0mm → ┌─────────┐ ┌─────────┐ │ LINE 1 │ gap │ LINE 2 │ └─────────┘ └─────────┘ ──────────────────────────────────── FR4 substrate (1.6mm) ──────────────────────────────────── Ground plane (bottom copper) COMPLETE PCB LAYOUT (432 MHz, −20 dB, 50mm × 35mm board): ┌─────────────────────────────────────────────────────┐ │ J1(SMA) ─[50Ω line]─[coupled section 96mm]─[50Ω]─ J2(SMA) │ │ ↕ 1.2mm gap │ │ GND ─[50Ω line]─[coupled section]─[50Ω]─ J3(SMA) │ │ └───── J4(SMA) │ │ All corners mitered at 45° for low VSWR │ └─────────────────────────────────────────────────────┘ PORT ASSIGNMENT: J1: Input (IN) J2: Through (main line output — antenna or dummy load) J3: Coupled (forward power sample — to detector) J4: Isolated (terminated with 50Ω to GND — improves directivity) IMPORTANT: Port 4 (isolated) MUST be terminated with 50Ω. Open or mismatched Port 4 destroys directivity. Use 51Ω 0805 SMD soldered directly to J4 pad. ================================================================================ SECTION 3 — COAXIAL BRANCHLINE COUPLER (FIELD-EXPEDIENT VHF) ================================================================================ For field use without PCB fabrication, a coaxial sleeve directional coupler is practical for 50–500 MHz. Uses sections of semi-rigid or standard coax. HAIRPIN COUPLER (50–300 MHz): ─────────────── 50Ω coax (λ/4) ─────────────── IN J1 ──────┬──┘ └──┬── OUT J2 │ │ │ Coupled loop (1T around coax) │ │ ┌──────────────────────────────┐ │ └───┤ 1-turn loop of RG-316 coax ├──────┘ └──────────────────────────────┘ │ FWD/REF detector Loop diameter: ~λ/20 at lowest frequency Loop coupling: ~−20 dB (empirically calibrated) COAXIAL-LINE TRANSMISSION COUPLER (preferred, broadband): Uses outer conductor of a coaxial coupling section adjacent to main line coax. MAIN LINE: RG-8 or RG-213 (50Ω, any length) COUPLING LINE: RG-316 semi-rigid, length = λ/4 at center freq Physical construction (for 144 MHz): 1. Strip outer jacket from 250mm of main line coax at coupling point. 2. Lay coupling coax parallel to inner conductor, insulated by Teflon tape. 3. Both coax shields soldered to common ground at each end of coupling section. 4. Coupling coax center conductor → FWD port (matched end); → REF port (far end). 5. Far end of coupling coax: terminate with 50Ω to ground → ISOLATED port. ================================================================================ SECTION 4 — WIDEBAND MULTI-SECTION COUPLER DESIGN (50–1300 MHz) ================================================================================ A single λ/4 section has 3 dB bandwidth of about 0.5 octaves (1.4× frequency ratio). For multi-band coverage, use a 3-section multi-stub approach or a parallel-coupled structure that spans 50–1300 MHz (4.6 octaves). MULTI-OCTAVE APPROACH — COAXIAL SLEEVE: The Stockman/Schiffman approach uses a constant-impedance terminated coupler rather than resonant coupling. Performance is limited in directivity (15–20 dB) but bandwidth is excellent (DC to > 3 GHz). CONSTRUCTION: INNER CONDUCTOR: 3mm copper or brass rod, 50mm long, threaded M3 at each end OUTER CONDUCTOR (shield): 10mm ID brass tube, same length, coaxial with inner CENTER GAP: 3.5mm annular space between inner (3mm OD) and outer (10mm ID) COUPLING ELEMENT: 1.5mm OD copper wire loop, length 20mm, parallel to inner conductor 50Ω GEOMETRY VERIFICATION: Z0 = (138 / √εr) × log₁₀(D/d) Air (εr=1): D=10mm, d=3mm → Z0 = 138 × log₁₀(3.33) = 138 × 0.522 = 72Ω Adjust: d=4mm rod → Z0 = 138 × log₁₀(2.5) = 138 × 0.398 = 55Ω ≈ 50Ω Use 4mm OD brass/copper inner rod in 10mm ID outer shield. COUPLING COEFFICIENT vs. ELEMENT LENGTH: Element length / total length | Approx coupling --------------------------------|---------------- 0.1 (2mm of 20mm) | −40 dB 0.2 (4mm of 20mm) | −34 dB 0.4 (8mm of 20mm) | −28 dB 1.0 (full 20mm length) | −20 dB (approx) ================================================================================ SECTION 5 — BAND-SPECIFIC COUPLER DESIGNS ================================================================================ BAND GROUP 1: 6M / 2M / 1.25M (50, 144, 222 MHz) Coupler type: 3-section lumped-LC + coupled microstrip hybrid PCB dimensions: 80mm × 40mm Center frequency design: 144 MHz ±3 dB bandwidth: 40–300 MHz (covers all three bands) Coupling: −20 dB ±1.5 dB across band Connector: SMA female (PCB edge mount) Ground plane: full bottom copper pour BAND GROUP 2: 70cm / 33cm (432, 902 MHz) Coupler type: Microstrip coupled line, single-section PCB dimensions: 50mm × 30mm Center frequency design: 620 MHz (geometric mean of 432 and 902) λ/4 length at 620 MHz on FR4: 67mm → fold 180° (hairpin) to fit 35mm Coupling: −20 dB ±2 dB across 432–902 MHz Connector: SMA female BAND GROUP 3: 23cm (1296 MHz) Coupler type: Microstrip coupled line PCB dimensions: 30mm × 20mm Center frequency: 1296 MHz λ/4 length: 32mm Coupling: −20 dB ±1 dB at 1296 MHz ±100 MHz Connector: SMA female (end-launch) ================================================================================ SECTION 6 — DIRECTIVITY AND FREQUENCY-COMPENSATED DESIGN ================================================================================ DIRECTIVITY LIMITING FACTORS IN MICROSTRIP: 1. Phase imbalance: Microstrip is not a pure TEM mode. Even and odd modes travel at slightly different velocities due to unequal E-field distribution in substrate/air. Solution: Use capacitive compensation at each end of coupled section. Add C_comp = (1/Z0) × (1/ω) × (1 − √(εr_eff_odd/εr_eff_even)) × ... Practical: Add 0.5–2 pF SMD capacitors between coupled lines at each end. Empirically: Start with C_comp = 1 pF; adjust for best directivity. 2. Substrate thickness variation: Use consistent FR4 source for reproducibility. 3. Connector VSWR: Use quality SMA connectors (SWR < 1.05 to 6 GHz). Cheap SMA connectors are the #1 cause of poor directivity at UHF. DIRECTIVITY vs. FREQUENCY: 50 MHz: >30 dB (TEM mode good; connectors dominate) 144 MHz: >28 dB (excellent) 432 MHz: >25 dB (good) 902 MHz: >22 dB (acceptable; phase error increases) 1296 MHz: >18 dB (marginal; add phase compensation) ================================================================================ SECTION 7 — PARTS LIST (VHF/UHF TRANSMISSION LINE COUPLER) ================================================================================ FOR PCB VERSION (432 MHz BAND GROUP 2): PCB 1 50mm × 30mm, FR4, 1.6mm, 2oz copper, HASL finish Gerber files: generate from layout per dimensions above J1 1 SMA female, edge-launch (Amphenol 901-9875-RFX or equiv) J2 1 SMA female, edge-launch J3 1 SMA female, edge-launch (coupled port) J4 1 SMA female, edge-launch (isolated port) R_iso 1 51Ω, 0402 SMD, 0.1W (terminates isolated port) C_comp 2 1 pF, 0402 SMD, NP0/C0G (phase compensation, adjust empirically) D1 1 BAT85 Schottky SMD (forward detector) D2 1 BAT85 Schottky SMD (reflected detector) R_L1 1 2.2kΩ, 0402 SMD (FWD detector load) R_L2 1 2.2kΩ, 0402 SMD (REF detector load) C_det 2 10 pF, 0402 NP0 (detector filter at UHF) COAXIAL CONSTRUCTION VERSION: Rod 1 4mm OD × 50mm brass rod (inner conductor) Tube 1 10mm ID × 50mm brass tube (outer conductor / shield) Coupling 1 1.5mm OD × 25mm copper wire (coupling element) J1-J4 4 SMA female chassis (panel mount, 4-hole flange) R_iso 1 51Ω, 1W metal film (isolated port termination) ================================================================================