================================================================================ SCHEMATIC: Station Grounding System and Low-Impedance Ground Bonds TM-LSP-001 Rev A Single-Point Entry Panel, Ground Rod Installation, Bonding Strap Sizing NEC Article 810 / NFPA 780 / ARRL Station Grounding Practices ================================================================================ GROUNDING PHILOSOPHY ───────────────────── The purpose of station grounding is NOT primarily lightning protection. The purposes are, in order of importance: 1. Safety: prevent fatal shock from faulty equipment 2. RF common-mode elimination: single-point RF ground prevents loops 3. Static discharge: continuous low-impedance path for accumulated charge 4. Lightning DIVERSION: guide induced surge energy away from equipment (Note: direct attachment cannot be "protected against" — only diverted) A ground system must be: a) Low-impedance at DC and all operating frequencies (RF bonding) b) Physically continuous (no connections that can corrode open) c) Connected to a single point (star topology prevents ground loops) d) Compliant with NEC 250 (electrical safety) and Article 810 (antenna) THE SINGLE-POINT ENTRY PANEL ────────────────────────────── All cables entering the shack pass through ONE point — the entry panel. At this panel, all cable shields are bonded to a copper bar, which connects to the external ground rod via the shortest possible path. ENTRY PANEL DESIGN: EXTERIOR (OUTSIDE SHACK) │ INTERIOR (INSIDE SHACK) │ ┌─────────────────────────────────────────────────────────────────┐ │ ENTRY PANEL (copper or aluminum plate, 300 × 150 × 6mm) │ │ │ │ [COAX 1] ──[GDT ARR.]──► panel hole ──────────────────► shack │ │ HF feedline 1 │ │ │ ├── BONDING BAR (1/2" copper strap) │ │ [COAX 2] ──[GDT ARR.]──► panel hole ──────────────────► shack │ │ HF feedline 2 │ │ │ │ │ │ [COAX 3] ──[GDT ARR.]──► panel hole ──────────────────► shack │ │ VHF/UHF │ │ │ │ │ │ [ROTOR CABLE] ─────────── surge strip ──────────────── shack │ │ [CONTROL LINES] │ │ │ [POWER LINE] ──────────── MOV suppressor ──────────── shack │ │ │ │ │ MAIN GROUND BOND │ │ │ │ └─────────────────────────────│────────────────────────────────────┘ │ 1/2" × 6" copper strap ≤ 1 meter length (MINIMUM path length!) │ ┌────────┴────────┐ │ GROUND ROD │ │ 8 ft copper-clad│ │ 5/8" diameter │ └─────────────────┘ ENTRY PANEL BONDING BAR SCHEMATIC: GND bar (copper): ──────────────────────────────────────────────── │ │ │ │ │ [ARR1] [ARR2] [ARR3] [ARR4] [POWER] coax1 coax2 VHF UHF MOV outer outer outer outer neutral All arrestor chassis/outer conductors bond to bar via very short (<50mm) lugs. Bar itself connects to ground rod via 1/2" copper strap. ================================================================================ GROUND ROD INSTALLATION ================================================================================ NEC REQUIREMENTS (Article 810.21 and 250.52): - Minimum: single 8-foot (2.44m) copper-clad steel rod, 5/8" (15.9mm) OD - Or: 10-foot (3.05m) rod - Where soil conditions prevent 8-foot depth: L-shape (horizontal run) allowed - Supplemental electrode required if ground resistance > 25Ω HAM BEST PRACTICE (exceeds NEC minimum): - Ground rod: 10-foot (3m) solid copper, 5/8" OD (or copper-clad steel) - Location: as close as possible to entry panel (< 3m distance preferred) - Multiple rods: 2–4 rods in star pattern around shack, bonded together - Rod spacing: ≥ 2× rod length (16 feet / 5m minimum between rods for full effect) - All rods bonded to the same bus via buried #6 AWG insulated copper DRIVING PROCEDURE: 1. Select location: moist soil preferred; avoid rocky ground, septic systems. 2. Wet soil with water before driving if dry (reduces resistance significantly). 3. Drive rod with slide hammer or electric hammer drill (SDS with rod driver). 4. Leave 50mm (2") above grade for lug connection. 5. Install listed ground rod clamp (UL listed, copper, two-bolt). 6. Connect #6 AWG solid copper from clamp to entry panel bus. 7. If rock prevents full depth: run 8 feet horizontally at 18" depth. GROUND RESISTANCE MEASUREMENT: Method: Fall-of-potential (3-electrode): requires ground resistance tester. Target: < 10 Ω (excellent), < 25 Ω (NEC minimum). Simple check: measure AC voltage between rod and utility neutral with DMM. Less than 2V indicates good contact. > 5V indicates potential grounding issue. ================================================================================ BONDING STRAP SIZING AND IMPEDANCE ================================================================================ RF GROUNDING PRINCIPLE: At radio frequencies, skin effect causes current to flow on the surface of conductors. A round wire has high RF impedance compared to a flat strap of the same cross-sectional area. NEVER use round wire for RF ground bonds (at any frequency above ~100 kHz). USE COPPER STRAP (braid, strip, or bus bar) for all RF bonds. SKIN DEPTH IN COPPER: δ = √(ρ / (π × f × µ₀ × µ_r)) = 66.1 / √f [mm, f in Hz] At 1.8 MHz: δ = 0.049 mm (49 µm) At 14 MHz: δ = 0.018 mm (18 µm) At 144 MHz: δ = 5.5 µm At 432 MHz: δ = 3.2 µm INDUCTANCE PER UNIT LENGTH: Round wire: L/length = (µ₀/2π) × [ln(2l/r) - 0.75] (high inductance) Flat strap: L/length = (µ₀/2π) × [2l/W × ln(2l/W)] (much lower) Comparison — 1 meter, 12.7mm (1/2") wide: Round 12 AWG wire (2.05mm dia): L ≈ 1.35 µH → Z = 119Ω at 14 MHz 1/2" copper strap (0.5mm thick): L ≈ 0.57 µH → Z = 50Ω at 14 MHz 1" copper strap (1mm thick): L ≈ 0.45 µH → Z = 39Ω at 14 MHz 2" copper strap (1mm thick): L ≈ 0.35 µH → Z = 31Ω at 14 MHz 3" × 1/8" copper bus bar: L ≈ 0.28 µH → Z = 25Ω at 14 MHz LESSON: Wider is dramatically better for RF grounding. Use the widest practical strap for the shortest path. MAXIMUM ALLOWED GROUND STRAP IMPEDANCE: For 30 dB common-mode rejection with 150Ω CM path: Z_ground must be << Z_choke / 30dB = 5000 / √1000 = 158Ω (crude estimate) Rule of thumb: ground strap Z < Z_0/10 = 5Ω for 50Ω coax system STRAP SIZING TABLE: Strap type | Z at 7MHz | Z at 14MHz | Z at 50MHz | Rec. use ------------------------|-----------|-----------|-----------|------------------ #6 AWG round wire, 1m | 38Ω | 75Ω | 268Ω | NOT for RF grounding #6 AWG wire, 0.1m | 4Ω | 8Ω | 27Ω | Minimum acceptable (short) 1/2" strap, 1m | 16Ω | 32Ω | 113Ω | Adequate for grounding strap 1/2" strap, 0.3m | 5Ω | 10Ω | 34Ω | Good (keep short!) 1" strap, 0.3m | 3.5Ω | 7Ω | 25Ω | Good 2" strap, 0.1m | 1Ω | 2Ω | 7Ω | Excellent Copper braid 1", 0.3m | 2Ω | 4Ω | 13Ω | Very good; flexible RULE OF THUMB: Keep ALL ground bonds as SHORT AS POSSIBLE. A 100mm bond is 10× better than a 1000mm bond. Wide and short beats narrow and long every time. ================================================================================ SINGLE-POINT GROUNDING SYSTEM BLOCK DIAGRAM ================================================================================ ANTENNA 1 (HF dipole) │ coax (RG-8X) [GDT ARR. 1]──(chassis bond)──┐ │ coax continues │ │ ANTENNA 2 (VHF yagi) │ │ coax (LMR-195) │ [GDT ARR. 2]──(chassis bond)──┤ │ coax continues │ │ POWER LINE │ │ AC power │ [MOV SURGE FILTER]─────────────┤ │ protected AC │ │ TELEPHONE / ETHERNET │ │ data line │ [ETHERNET ARR.]────────────────┤ │ data continues │ │ ALL GROUNDS MEET HERE ┌──────┴────────────────────────────┐ │ SINGLE-POINT GROUND BUS │ │ (copper strip or bar) │ │ at exterior wall / entry panel │ └──────────────────┬────────────────┘ │ 1/2" copper strap ≤ 1 meter, STRAIGHT (no bends, no coils) │ ┌────────┴────────┐ │ GROUND ROD │ │ 8-10 ft × 5/8" │ └─────────────────┘ CRITICAL RULES FOR SINGLE-POINT GROUNDING: 1. ONE ground rod at the entry point. All bonds radiate FROM this one point. 2. NO separate ground rods for equipment inside the shack. (Multiple rods → different potentials during surge → ground loop damage) 3. ALL cable shields bonded to the SAME bar at the SAME point. 4. STRAIGHT strap from entry panel to ground rod (no coils, no loops). 5. AC safety ground (from utility panel) bonds to the SAME ground rod system. (NEC 250.106: antenna ground and power ground must be bonded together) ================================================================================ IN-SHACK BONDING ================================================================================ Inside the shack, all equipment must be bonded to a SINGLE RF ground bus: RADIO 1 ──── #6 AWG or braid, <0.5m ────┐ RADIO 2 ──── braid, <0.5m ──────────────┤ LINEAR AMP ─ braid, <0.5m ──────────────┤ COMPUTER ─── braid, <0.5m ──────────────┤ TUNER ────── braid, <0.5m ──────────────┤ ROTATOR CTL─ braid, <0.5m ──────────────┤ │ IN-SHACK RF BUS (copper strap on desk edge) │ 4" copper braid or 2" strap │ ENTRY PANEL GROUND BUS │ ground rod IN-SHACK BUS: 1" × 0.05" copper bus bar (readily available as "copper busbar") Run along back edge of operating desk. All equipment chassis bond to bus via shortest possible braid. Bus connects to entry panel ground via 2" copper strap through wall. ================================================================================ ROTOR AND CONTROL CABLE SURGE PROTECTION ================================================================================ Control cables (rotor, remote switches, etc.) also carry surge energy. Protect with: Bourns 1812-16/32 multi-line surge protector, or individual TVS diodes (SMBJ15CA) on each conductor. ROTOR CABLE PROTECTION SCHEMATIC: ROTOR CABLE IN ──┬──────────────────────────────────── ROTOR CABLE OUT (each conductor) │ (each conductor) [TVS] 15V bidirectional, per conductor │ CHASSIS GND (bonded to entry panel bus) Use a 6-conductor or 8-conductor terminal strip with TVS diode per pin. Mount in weatherproof enclosure at shack entry. Typical rotor cable: 8-conductor, 22AWG. TVS per wire: SMBJ15CA (15V, 600W, bidirectional). TYPICAL CONTROL LINE VOLTAGES AND TVS SELECTION: Control line type | Voltage | TVS V_BR | Notes ------------------|---------|----------|--------------------------- Rotor (AC) | 24V AC | SMBJ27CA (27V) | Must be AC-tolerant Rotor (DC) | 12-24V | SMBJ30CA (30V) | DC control lines Band switch | 12V | SMBJ15CA (15V) | Logic-level PTT/keyer | 5-12V | SMBJ12CA (12V) | Remote coax relay | 12V | SMBJ15CA (15V) | Audio/speaker | ≤ 1V AC | SMBJ5.0CA (5V) | Very sensitive ================================================================================ PARTS LIST — GROUNDING SYSTEM ================================================================================ Item | Qty | Value / Description | Source ------|-----|-------------------------------------------|------------------ ROD1 | 1 | 10-ft copper-clad steel rod, 5/8" OD | Electrical supply ROD2 | 1 | 10-ft (supplemental, recommended) | Electrical supply CLAMP | 2 | Acorn Ground Rod Clamp, UL listed, #6 AWG | Electrical supply BSTR1 | 3m | 2" × 0.050" copper strip (entry → rod) | Metal supplier BSTR2 | 2m | 1" × 0.050" copper strip (in-shack bus) | Metal supplier BRAID | 2m | 3/4" tinned copper braid | eBay, RF Parts LUG1 | 10 | #6 AWG two-hole copper lug | Electrical supply LUG2 | 10 | #10 ring terminal, copper, large OD | Electronic supply SCRW | 20 | #10-32 SS machine screws + lock washers | PANEL | 1 | 300×150×6mm aluminum or copper plate | Metal supplier / 3D print TVSHF | 10 | SMBJ15CA | Mouser TVSAC | 4 | SMBJ27CA (rotor AC lines) | Mouser MOV_AC| 1 | Littelfuse V275LA10BPX (275V MOV, AC line)| Mouser ================================================================================