================================================================================ SCHEMATIC: Gas-Discharge Tube (GDT) Coaxial Lightning Arrestor TM-LSP-001 Rev A SO-239 and N-Type Panel-Mount Designs Covers: 1.8 MHz (160M) through 1296 MHz (23cm) with band-specific variants ================================================================================ THEORY OF GAS-DISCHARGE TUBES ─────────────────────────────── A gas-discharge tube (GDT) consists of two or three electrodes sealed in a ceramic or glass envelope filled with inert gas (argon or nitrogen) at low pressure (~100 mbar). Under normal conditions the gas is non-conducting and presents essentially open-circuit impedance to the transmission line. When the voltage across the electrodes exceeds the sparkover voltage V_s, the gas ionizes (glow discharge → arc) within ~100 ns. Once conducting, the GDT clamps the voltage to the arc voltage V_arc (~10–25V). After the overvoltage event ends, the gas de-ionizes (follow current ceases and tube self-extinguishes) within ~200 µs. GDT CHARACTERISTICS: V_s (sparkover voltage): 90V, 150V, 230V, 350V, 600V, 1000V DC ratings V_arc (conducting voltage): 10–25V DC (glow-to-arc transition) I_peak (peak impulse): 2.5 kA to 20 kA (8/20 µs waveform) I_followcurrent max: 150A AC (at which self-extinction occurs) Response time: ~100 ns (slower than TVS but handles higher current) Capacitance (at RF): 0.5–1.5 pF (low; excellent for VHF/UHF) Insertion loss at 1 GHz: < 0.05 dB for low-capacitance type GDT vs. TVS vs. MOV COMPARISON: Parameter | GDT | TVS Diode | MOV ------------------|------------|------------|-------- Response time | 100 ns | < 1 ns | 10-50 ns Clamp voltage | V_arc ~15V | V_BR × 1.5 | non-precise Peak current | 2.5–20 kA | 0.5–50 A | 1–10 kA Capacitance | 0.5–1 pF | 5–500 pF | 50–10000 pF Self-resetting | Yes | Yes | Degrades Life (# strikes) | 1000s | limited | limited Cost | Low | Low | Very low Best use | First stage| Third stage| Second stage FOR ANTENNA COAXIAL PROTECTION: GDT is the primary protection element because: 1. Negligible capacitance → negligible VSWR and insertion loss at VHF/UHF 2. Very high peak current capacity → handles direct lightning energy 3. Self-resetting → survives multiple events without replacement 4. Simple coaxial implementation in series signal path ================================================================================ DESIGN 1: HF GDT ARRESTOR — SO-239 / SO-239 THROUGH LINE ================================================================================ Application: 1.8–54 MHz, up to 1500W, weatherproof outdoor installation. Connectors: SO-239 IN (antenna side), SO-239 OUT (coax to shack side). GDT: Epcos/TDK B88069X1140B232 (90V DC, 2.5kA, 1.0pF) or equivalent. Bourns 2040-090TR (90V DC sparkover, axial lead). SCHEMATIC: ANTENNA COAX TO SHACK (SO-239 IN) (SO-239 OUT) │ │ ════╪═══════════════════════════════════════════╪════ center conductor ─────────────────────────── center │ │ │ GDT1 (90V DC sparkover, 2.5 kA) │ ├────┤ ├─────────────────────────────────────── (no connection to out center) │ │ │ ════╪════╪════════════════════════════════════════╪════ shield ──┤ shield │ (both shields connected, outer conductor is GDT return) GND (chassis / enclosure / ground lug) │ └──── #6 AWG or 1/2" copper strap ──── GROUND ROD NOTE ON TOPOLOGY: GDT1 connects IN center conductor to OUTER conductor (chassis ground). The OUT center is directly connected to IN center (through line). GDT1 limits the voltage ON the center conductor to V_arc (~15V) relative to chassis ground when a lightning surge appears. Both SO-239 outer conductors are bonded to the enclosure chassis. The chassis is bonded to the station ground system. THROUGH-LINE COAXIAL DIAGRAM: ──────────────────────────────────────────────────── CENTER CONDUCTOR │← IN pigtail →│ GDT center node │← OUT pigtail →│ │ [GDT1] 90V sparkover │ ──────────────────────────────────────────────────── OUTER CONDUCTOR / CHASSIS Full physical layout (top view, PCB or point-to-point): [SO-239 IN]──────────┬──────────────[SO-239 OUT] outer ───────┤(chassis)──── outer center │center GDT1 │ CHASSIS │ GND LUG ──► ground strap ──► ground rod ================================================================================ DESIGN 2: VHF/UHF GDT ARRESTOR — N-TYPE / N-TYPE THROUGH LINE ================================================================================ Application: 50–1296 MHz. Connectors: N-type (superior weatherproofing vs. UHF type; required above 300 MHz). GDT: EPCOS B88069X1140B232 (90V, 1.0pF) — specify low-capacitance variant. Maximum usable frequency at < 0.1 dB insertion loss: ~1.5 GHz with 1pF GDT. INSERTION LOSS FROM GDT CAPACITANCE: The GDT shunt capacitance C_GDT forms a low-pass filter with the 50Ω system: f_−3dB = 1 / (2π × Z_0/2 × C_GDT) [for shunt C in 50Ω line: Z_0/2 = 25Ω] f_−3dB = 1 / (2π × 25 × 1×10⁻¹²) = 6.37 GHz Insertion loss at frequency f (f << f_−3dB): IL (dB) ≈ 20 × log₁₀(1 + (f/f_−3dB)²/2) ≈ 0.0024 dB at 144 MHz (negligible) ≈ 0.022 dB at 432 MHz (negligible) ≈ 0.098 dB at 1296 MHz (just within 0.1 dB spec) For 2-GDT configuration (bi-directional protection), capacitances add: C_total = 2 × 1pF = 2pF → f_−3dB = 3.18 GHz → IL at 1296 MHz = 0.33 dB (marginal) USE SINGLE GDT for 1296 MHz installations. N-TYPE THROUGH-LINE SCHEMATIC: [N-TYPE IN] ─────────────────────────────── [N-TYPE OUT] outer (chassis) outer (chassis) center ─────────┬──────────────────────── center │ [GDT1] 90V sparkover, 1.0 pF Epcos B88069X1140B232 │ CHASSIS / GROUND │ N-type M/F grounding lug or chassis stud │ #6 AWG or 1/2" copper strap │ GROUND ROD N-TYPE CONNECTOR SPECIFICATIONS (at the arrestor): Use silver-plated N-type with PTFE dielectric. IP67 or IP68 rated (with O-ring face seal). Amphenol 82-836 (N-female panel), 82-835 (N-male panel). Maximum RF power: 1500W at 450 MHz (50Ω resistive load). ================================================================================ DESIGN 3: DUAL-STAGE GDT (HF + VHF COMBINED) — N-TYPE ================================================================================ Two GDTs in parallel with different V_s ratings: GDT1: 90V sparkover — handles main lightning surge GDT2: 230V sparkover — backup / redundant protection Connected in parallel between center conductor and chassis ground. DUAL GDT SCHEMATIC: CENTER CONDUCTOR ────────┬────────────────────── CENTER CONDUCTOR │ ┌──[GDT1]──┐ GDT1: 90V DC, 2.5kA (Epcos B88069X) │ │ GDT2: 230V DC, 2.5kA (backup) └──[GDT2]──┘ │ CHASSIS Benefits: redundant protection; if GDT1 fails open, GDT2 provides backup. If GDT1 fails shorted (rare but possible): GDT2 is destroyed, but coax is open-circuited (fail-safe: attenuates signal rather than passing destructive V). ================================================================================ SPARKOVER VOLTAGE SELECTION GUIDE ================================================================================ V_s (DC) | V_s (AC rms) | Application | Concern ----------|--------------|-------------------------------------|--------------------------- 75V | 53V rms | Low-voltage DC antennas (active) | May arc on FM deviation peaks 90V | 64V rms | Standard HF coax (recommended) | Standard choice 150V | 106V rms | Coax with DC (rotor/power injected) | DC blocks needed if < 150V 230V | 163V rms | 12VDC active antenna supply | Use with 12V bias-T 350V | 247V rms | Higher DC bias systems | 600V | 424V rms | High-power HF (1.5kW into SWR >3) | Peak voltage on high-SWR line PEAK RF VOLTAGE ON TRANSMISSION LINE: At 1500W into 50Ω resistive load: V_peak = √(2 × P × Z) = √(150,000) = 387V peak This would TRIGGER a 350V GDT during normal transmit at 1500W! CRITICAL: V_s must be > maximum peak RF voltage on the line during transmit. For 1500W / 50Ω: V_peak = 387V → use V_s ≥ 600V for high-power use. For 100W / 50Ω: V_peak = 100V → use V_s ≥ 150V. For QRP 5W / 50Ω: V_peak = 22.4V → V_s ≥ 90V safe. NOTE: Under high SWR conditions, voltage can be much higher: At 1500W with SWR = 3:1: V_peak_max = 387 × √3 = 670V → V_s ≥ 1000V for safety! RECOMMENDED V_s by power level and SWR: P ≤ 100W, SWR ≤ 2:1: V_s = 150V ✓ (recommended standard) P ≤ 500W, SWR ≤ 2:1: V_s = 350V P ≤ 1500W, SWR ≤ 1.5:1: V_s = 600V P ≤ 1500W, SWR ≤ 3:1: V_s = 1000V (reduced protection margin) QRP ≤ 10W, any SWR: V_s = 90V (maximum protection) ================================================================================ INSERTION LOSS AND VSWR SPECIFICATIONS ================================================================================ For single GDT (1.0 pF) in 50Ω line: Frequency | Insertion Loss | VSWR | Return Loss ----------|---------------|--------|------------ 1.8 MHz | < 0.001 dB | 1.001 | 67 dB 3.5 MHz | < 0.001 dB | 1.001 | 67 dB 7.0 MHz | < 0.001 dB | 1.001 | 67 dB 14.0 MHz | < 0.001 dB | 1.001 | 67 dB 28.0 MHz | < 0.001 dB | 1.001 | 67 dB 50.0 MHz | < 0.001 dB | 1.001 | 67 dB 144.0 MHz | < 0.002 dB | 1.003 | 51 dB 432.0 MHz | < 0.022 dB | 1.027 | 41 dB 902.0 MHz | < 0.055 dB | 1.06 | 31 dB 1296.0 MHz | < 0.098 dB | 1.09 | 27 dB All specifications exceed requirements (< 0.1 dB IL, < 1.1:1 VSWR) for a single 1.0 pF GDT. ================================================================================ PARTS LIST — GDT ARRESTOR DESIGNS ================================================================================ Item | Qty | Part / Value | Description | Source ------|-----|---------------------------|------------------------------|-------- GDT1 | 1 | EPCOS B88069X1140B232 | 90V DC, 2.5kA, 1.0pF | Mouser GDT1 | 1 | Bourns 2040-090TR | 90V DC sparkover, axial | alt. GDT_HV| 1 | EPCOS B88069X5140B502 | 350V DC, 5kA, 1.0pF | high power GDT_HV| 1 | Bourns 2040-600TR | 600V DC sparkover | 1.5kW J_IN | 1 | Amphenol SO-239 (UHF-F) | Panel mount, silver | HF version J_OUT | 1 | Amphenol SO-239 (UHF-F) | Panel mount, silver | HF version J_IN | 1 | Amphenol 82-836 | N-type female panel | VHF/UHF J_OUT | 1 | Amphenol 82-836 | N-type female panel | VHF/UHF LUG | 1 | #10 copper lug, 2-hole | Ground lug on chassis | STRAP | 1 | 1/2" × 6" copper strap | Ground bond to enclosure | ENCL | 1 | Waterproof enclosure | See openscad/mounting_brackets|3D print ASA SEAL | 1 | Silicone RTV | Seal around connector flanges| BOLT | 4 | M4 × 10mm SS | Connector mounting | GSKT | 2 | Copper/silver washer | RF-grounding connector gasket| ================================================================================