================================================================================ SCHEMATIC: DC-Grounded and DC-Blocked Lightning Arrestors TM-LSP-001 Rev A DC-Grounded: Static Bleed-Off; DC-Blocked: Active Antenna / Bias-T Compatibility ================================================================================ ================================================================================ SECTION A — DC-GROUNDED ARRESTOR (STATIC DISCHARGE PATH) ================================================================================ THEORY — WHY DC GROUNDING MATTERS ─────────────────────────────────── Static charge accumulates on antenna elements and feedlines continuously: - Wind-driven precipitation (snow, rain, dust): generates triboelectric charge - Ionospheric field gradients: can charge a resonant antenna to thousands of volts - Pre-lightning charge buildup: antenna is in the elevated electric field as charged cloud approaches (potential rises to kV range BEFORE the stroke) Without a DC ground path, static charge has no place to go EXCEPT through the receiver front-end when the coax is connected. Even relatively modest static accumulations (1–5 kV) are sufficient to destroy MOSFET front-ends. A DC-grounded arrestor provides: 1. A continuous DC path from antenna center conductor to ground → prevents static accumulation entirely 2. RF choke (high impedance at operating frequency) in the DC path → does not short the RF signal to ground 3. GDT in parallel for fast transient suppression → two-layer protection (slow static + fast surge) HOW THE RF CHOKE WORKS: The RF choke (inductor) has impedance Z = 2πfL. At DC: Z = 0 (short circuit → static bleeds freely to ground) At RF: Z >> Z₀ (high impedance → minimal signal is diverted to ground) For L = 10 µH (RFC): At DC: Z = 0 Ω → DC ground (desired) At 1.8 MHz: Z = 113 Ω → some signal diverted; use larger L At 3.5 MHz: Z = 220 Ω → moderate loss At 7 MHz: Z = 440 Ω → 440 vs 50 Ω: ~21 dB return loss At 14 MHz: Z = 880 Ω → 25 dB return loss For L = 47 µH (RFC): At 1.8 MHz: Z = 531 Ω → minimal loss At 3.5 MHz: Z = 1031 Ω → good for 80M up At 7 MHz: Z = 2.07 kΩ → excellent for 40M up INSERTION LOSS from shunt RFC (in 50Ω line): IL (dB) = 20 × log₁₀( |Z_RFC| / (|Z_RFC| + 25Ω) ) × 2 [approx] For L = 47µH at 1.8 MHz: IL ≈ 20 × log₁₀(531/556) = −0.41 dB (marginal) For L = 100µH at 1.8 MHz: IL ≈ 20 × log₁₀(1131/1156) = −0.19 dB (better) For L = 220µH at 1.8 MHz: IL ≈ 20 × log₁₀(2488/2513) = −0.09 dB ✓ RECOMMENDATION: Use RFC ≥ 220 µH for 160M operation. Use RFC ≥ 47 µH for 40M–10M. Use RFC ≥ 1 µH for VHF. RFC CONSTRUCTION OPTIONS: Option A — Pre-wound toroid choke (recommended): 220 µH: 90 turns #26 AWG on T-130-2 (Mix 2, red, HF): A_L = 27 nH/N²; N = √(220000/27) = 90 turns 47 µH: 42 turns #26 AWG on T-130-2: N = √(47000/27) = 42 turns 1 µH: 6 turns #22 AWG on T-80-10 (Mix 10, black/yellow): Option B — Ferrite bead RFC: For 40M–10M: 10t #26 AWG on FT-37-43 → L ≈ 10 × 557 nH × 100 = 55.7 µH (N² × A_L: A_L = 557 nH for FT-37-43, 1 turn ≈ 557 nH, 10 turns = 100 × 557 = 55.7 µH → adequate for 40M) Option C — Commercial RF choke: Bourns 78F series: 47 µH or 100 µH, axial lead, 1A rated Use for 40M–6M operation; two in series for 160M. ================================================================================ DC-GROUNDED ARRESTOR SCHEMATIC — HF (160M through 10M) ================================================================================ ANTENNA COAX TO SHACK (SO-239 IN) (SO-239 OUT) │ │ ════╪═══════════════════════════════════════════════╪════ center ─────────────────────────────────────────── center │ │ │ RFC1 (220µH, DC-ground choke) │ ├───UUUU────────────────────────────────────────────┤ │ L1 │ │ │ │ │ [GDT1] 90V, 2.5kA │ │ │ │ ════╪═══╪═══════════════════════════════════════════════╪════ shield ─┘ shield │ CHASSIS GND SIGNAL PATH: Center IN → center OUT (direct through; no series components) No insertion loss from signal path elements (RFC is shunt, not series) DC PATH: Center conductor → RFC1 (220µH) → GDT1 (normally open) → chassis GND Static charge: bleeds continuously through RFC1 (DC resistance = coil DCR ~5Ω) Surge: GDT1 fires, bypasses RFC1, handles kA current directly EQUIVALENT CIRCUIT: IN_center ─────────────────────────────────────── OUT_center │ RFC1 (220µH, R_DC~5Ω) │ GDT1 (90V, in parallel with RFC1 from node to chassis) (GDT1 connects center-node to chassis, parallel with RFC1 lower terminal) │ CHASSIS GND Actually more precisely: IN_center ───────────────────────────────────── OUT_center │ ┌──┴──┐ │RFC1 │ 220µH (DC continuity to ground) │ │ └──┬──┘ │ ← This node is at center conductor potential │ ┌──┴──┐ │GDT1 │ 90V sparkover (surge bypass) │ │ └──┬──┘ │ CHASSIS GND ──── ground strap ──── ground rod INSTALLATION NOTE: This is an IN-LINE design. The arrestor is installed at the antenna feedpoint OR at the shack entry bulkhead. If installed at feedpoint: the RFC provides DC ground at antenna base (good). If installed at bulkhead entry: reduces static on the long feedline run (adequate). BEST: Install DC-grounded arrestors at BOTH feedpoint AND bulkhead. ================================================================================ INSERTION LOSS TABLE — DC-GROUNDED ARRESTOR (RFC=220µH) ================================================================================ Freq (MHz) | Z_RFC (kΩ) | IL (dB) | VSWR | Note -----------|-----------|---------|-------|------------------------------ 1.8 | 2.49 | −0.090 | 1.10 | Just within spec at 160M 3.5 | 4.84 | −0.046 | 1.05 | Good 80M 7.0 | 9.67 | −0.023 | 1.025 | Excellent 40M 10.1 | 13.96 | −0.016 | 1.018 | Excellent 30M 14.0 | 19.35 | −0.011 | 1.013 | Excellent 20M 28.0 | 38.7 | −0.006 | 1.006 | Excellent 10M 50.0 | 69.1 | −0.003 | 1.003 | Excellent 6M For 160M operation with IL < 0.1 dB: RFC ≥ 220 µH confirmed. For 80M–10M: RFC = 47–100 µH adequate. ================================================================================ SECTION B — DC-BLOCKED ARRESTOR (ACTIVE ANTENNA / BIAS-T COMPATIBLE) ================================================================================ THEORY — WHY DC BLOCKING IS NEEDED ───────────────────────────────────── Active antennas (wideband receive amplifiers, LNA preamplifiers) and phased array element amplifiers are powered via DC bias injected on the coaxial feedline. Typical bias voltages: 5V, 9V, 12V, 15V DC. A standard GDT arrestor (V_s = 90V) would NOT be triggered by 12V DC bias. HOWEVER: a DC-grounded arrestor would SHORT the DC bias to ground through the RFC, preventing the antenna from being powered. A DC-blocked design is needed. The DC-blocked arrestor: 1. Series capacitor: passes RF, blocks DC bias from grounding 2. GDT: provides surge protection (fires on lightning/ESD, not on DC bias) 3. Bleed resistor: provides high-resistance static discharge path (R_bleed = 1 MΩ: drains static slowly; too high to significantly load the DC bias supply; negligible effect on RF signal) DC BIAS COMPATIBILITY: V_s (GDT) must be GREATER than the DC bias voltage × safety margin: V_bias_max = 15V (high-end bias) Required V_s ≥ 15V × 3 (safety margin) = 45V minimum Use V_s = 90V: passes 0–40V DC without spurious triggering. Check: RF peak voltage + DC bias must not approach V_s. At 100mW into 50Ω: V_RF_peak = 3.16V; total max = 3.16 + 15 = 18.2V << 90V ✓ BLOCKING CAPACITOR SELECTION: C_block must have low reactance at operating frequency: X_C = 1/(2πfC) For HF (1.8 MHz): X_C < 1Ω → C > 1/(2π × 1.8e6 × 1) = 88 nF → use 100 nF For VHF (50 MHz): X_C < 0.1Ω → C > 32 nF → 47 nF adequate For UHF (432 MHz): X_C < 0.01Ω → C > 37 nF → 47 nF adequate INSERTION LOSS from series C_block in 50Ω line: IL (dB) ≈ 20 × log₁₀( 1 / √(1 + (X_C/25)²) ) [C in series with 50Ω] For C = 100 nF: At 1.8 MHz: X_C = 0.88Ω → IL = 20×log₁₀(1/√(1+(0.88/25)²)) = −0.013 dB ✓ At 3.5 MHz: X_C = 0.45Ω → IL = −0.003 dB ✓ All HF bands: IL < 0.015 dB ✓ Capacitor type: NP0/C0G 100 nF, 200V DC rating (handles bias + RF peaks) Use X5R at 100V if NP0 100nF unavailable; avoid Y5V/Z5U (large TCC) ================================================================================ DC-BLOCKED ARRESTOR SCHEMATIC — HF + ACTIVE ANTENNA ================================================================================ ACTIVE ANT. COAX TO SHACK / BIAS-T (SO-239 IN) (SO-239 OUT) │ │ ════╪═══════════════════════════════════════════════╪════ center ──[C_block 100nF]────────── [C_block 100nF]── center │ blocking cap │ blocking cap │ │ │ node A │ │ [R_bleed] │ │ 1 MΩ │ │ │ │ │ [GDT1] │ │ 90V, 2.5kA │ │ │ │ ════╪═══════════════════╪═══════════════════════════╪════ shield ─────────────────┘──────────────────────── shield (chassis GND) ALTERNATE SIMPLIFIED VERSION (single C_block, IN side only): IN_center ──[C_block 100nF]──────────────────── OUT_center │ [R_bleed 1MΩ] ← static drain │ [GDT1 90V] ← surge protection │ CHASSIS GND This version: DC from IN side is blocked by C_block. OUT_center is direct connection to node between C_block and GDT. DC bias from OUT (shack/bias-T side) passes through to antenna: OUT_center → (no blocking cap on OUT side) → antenna via C_block Wait: C_block blocks DC from going from OUT to antenna as well. WRONG: Need DC to pass from shack side to antenna side! CORRECT TOPOLOGY for DC-powered active antenna: DC flows: SHACK BIAS-T → OUT connector → center conductor → ANTENNA RF flows: ANTENNA → IN connector → C_block → OUT connector → SHACK Therefore: C_block must be on ANTENNA (IN) side ONLY. OUT side connects directly to center conductor without blocking. GDT on antenna-side of C_block to ground (protects antenna + blocks DC from GDT). CORRECT SCHEMATIC: IN (ANT) ──[C_block 100nF]──────────────────── OUT (SHACK) center │ center (DC from bias-T passes through) │← Node A [R_bleed 1MΩ] │ [GDT1 90V] │ CHASSIS GND - DC from bias-T: OUT_center → Node_A → antenna through (C_block = DC BLOCKED!) PROBLEM: C_block is on antenna (IN) side; it blocks DC going TO the antenna. ACTUAL CORRECT TOPOLOGY: The C_block should be on the SHACK (OUT) side to block DC from the arrestor's GDT ground path, while allowing DC to pass to the antenna: IN (ANT) ──────────────────[C_block 100nF]──── OUT (SHACK) center │ center │← Node B (connects to antenna center) [R_bleed 1MΩ] + [GDT1 90V] to CHASSIS GND This allows: DC from OUT → C_block → Node_B → antenna (WRONG again — C blocks DC) ── RESOLUTION: TWO-CAPACITOR TOPOLOGY ───────────────────────────────────── For active antenna applications, use a SPLIT design: IN (ANT) ──[C_block_IN]──────────────────[C_block_OUT]── OUT (SHACK) center 100nF │ 100nF center │← Node_mid [R_bleed 1MΩ] │ [GDT1 90V] │ CHASSIS GND DC path: OUT_center → C_block_OUT (BLOCKED) → Node_mid → C_block_IN (BLOCKED) → ANT_center DC CANNOT pass through — this is WRONG for bias injection. ── CORRECT APPROACH FOR BIAS-T COMPATIBLE ARRESTOR: ────────────────────── The bias-T (inductor + capacitor) is SEPARATE from the arrestor. The arrestor uses standard DC-grounded design; the bias-T provides DC injection. See Section C — Combined Bias-T + Arrestor schematic below. ================================================================================ SECTION C — COMBINED BIAS-T + ARRESTOR (ACTIVE ANTENNA SYSTEM) ================================================================================ A complete active antenna protection system uses: 1. Standard GDT arrestor at antenna feedpoint 2. DC-blocking capacitor at the arrestor's GDT ground side 3. RF choke (bias-T) at the shack end for DC injection COMPLETE SYSTEM BLOCK DIAGRAM: [ACTIVE ANTENNA AMP] │ coax (DC bias + RF together) │ ┌───────┴───────────────┐ │ ARRESTOR AT │ │ ANTENNA BASE │ │ │ │ ANT ──────────── ANT │ │ center shield│ │ │ │ │ [GDT] 90V, │ │ [C_blk] 1nF (DC blk) │ ← prevents DC from shunting to ground via GDT ground │ │ │ │ chassis (float) │ │ bonded via C_gnd │ │ (1nF HV cap to gnd) │ └───────────────────────┘ │ coax (DC bias + RF) │ long coax run │ ┌───────┴───────────────┐ │ BIAS-T INJECTOR │ │ (in shack) │ │ │ │ RF ──[C_bypass]────── RF OUT │ 1nF (to radio, RF only) │ DC ──[RFC_bias]────── │ 47µH │ │ combined DC+RF │ to antenna coax └───────────────────────┘ BIAS-T SCHEMATIC (in-shack): RF PORT ─────[C_dc_block 1nF]────── RF+DC OUTPUT ──── coax to antenna │ [RFC 47µH] ──┘ │ DC POWER (+12V) ───────────────────────── RF signal passes through C_dc_block; DC is blocked from going back to RF port. DC passes through RFC (high impedance at RF); RFC prevents RF from going to DC supply. Both appear on the combined output. ================================================================================ PARTS LIST — DC-GROUNDED AND DC-BLOCKED DESIGNS ================================================================================ DC-GROUNDED HF ARRESTOR: Ref | Qty | Value/Part | Description | Notes -------|-----|----------------------|----------------------------|-------- RFC1 | 1 | 220 µH toroid | DC-ground RF choke, 160M | See winding RFC1 | 1 | 47 µH choke | DC-ground RF choke, 40M+ | Bourns 78F GDT1 | 1 | B88069X1140B232 | 90V GDT surge bypass | Mouser J1,J2 | 2 | SO-239 silver | Panel mount connectors | DC-BLOCKED ACTIVE ANT ARRESTOR: GDT1 | 1 | B88069X1140B232 | 90V, 2.5kA GDT | R_bleed| 1 | 1 MΩ 2W carbon film | Static drain resistor | C_gnd | 1 | 1 nF 2kV ceramic | HV cap for chassis float | J1,J2 | 2 | N-type panel female | IP67 connectors | BIAS-T (IN-SHACK): RFC_bias| 1 | 47 µH ferrite bead | Bias injection choke | C_block| 1 | 1 nF NP0 100V | RF bypass on DC port | J_RF | 1 | SO-239 female | RF input connector | J_OUT | 1 | SO-239 female | RF+DC output | J_DC | 1 | 2.1mm barrel DC jack | DC power input | ================================================================================