Grounding and Lightning Protection
Grounding and Lightning Protection
A properly grounded and surge-protected antenna installation protects people, equipment, and buildings from the destructive effects of direct lightning strikes and the more common (but still damaging) induced transients from nearby strikes. This page covers the components and procedures for a compliant, effective 915 MHz LoRa antenna grounding installation.
Why Ground Your Antenna Installation?
The goal of antenna grounding is threefold:
- Lightning protection: Provide a low-impedance path to earth for direct strike energy, bypassing protected equipment.
- Static dissipation: Continuously bleed off static charge that accumulates on isolated metal structures, preventing equipment damage from static discharge.
- Safety: Ensure that if a cable or mast becomes energized (e.g., power line contact), the structure is grounded, protecting anyone who touches it.
Note: Grounding does not prevent lightning from striking. It controls where the energy goes when a strike occurs - to ground, not through your radio.
Ground Rods
The earth electrode (ground rod) is the interface between the grounding system and earth. NEC (National Electrical Code) Article 810 (for antenna systems) and Article 250 (general grounding) specify requirements:
- Minimum rod specifications (NEC 250.52): 5/8" diameter, 8-foot length, copper or copper-clad steel. Two rods required if single rod resistance exceeds 25 ohms.
- Preferred rod: 3/4" diameter, 10-foot copper-clad steel for lower resistance in dry soils.
- Installation: Drive rod vertically into soil. Where rock prevents full depth, rod may be installed at a 45° angle or in a horizontal trench per NEC 250.53.
- Connection: Use a listed ground rod clamp (not a hose clamp). Connect with minimum #6 AWG solid copper (NEC 810.21) or larger conductor.
- Bonding to building ground: The antenna ground rod must be bonded to the building's primary grounding electrode system per NEC 810.21. Do not create an isolated "antenna ground" disconnected from the main service ground - this creates dangerous voltage differences between grounded objects during a strike.
Bonding Conductors
The bonding conductor (ground wire) connects the antenna mast, cable shield, and equipment ground to the earth electrode:
| Component | Minimum Wire Size (NEC) | Recommended | Notes |
|---|---|---|---|
| Antenna mast to ground rod | #10 AWG copper | #6 AWG solid copper | Must be mechanically protected if exposed to physical damage |
| Coax shield ground at entry | #17 AWG | #10 AWG | Ground coax shield at the building entry point (NEC 810.21) |
| Bonding to building electrode | #6 AWG | #6 AWG solid copper | Connects antenna ground rod to main building ground |
Run bonding conductors in as straight a path as possible. Every bend in the conductor adds inductance, which increases impedance to fast-rise lightning transients. A ground wire with many bends is far less effective than a straight run, even if the same gauge.
Lightning Arrestors at 915 MHz
A lightning arrestor (also called a surge protector, coaxial surge protector, or gas discharge tube protector) is installed inline in the coaxial feedline, typically at the building entry point where the cable enters a weatherproof enclosure. It provides a low-impedance path to ground for surge energy while remaining essentially transparent to normal 915 MHz signals.
Types used at 915 MHz:
- Gas discharge tube (GDT) type: Contains a sealed gap filled with an ionizable gas. Remains open (no conduction) at normal voltages; ionizes and conducts to ground when voltage spike exceeds breakdown voltage (typically 90 - 200 V). Returns to non-conducting state after transient passes. Excellent RF transparency; virtually no insertion loss.
- Solid-state (transient voltage suppressor) type: Uses TVS diodes to clamp voltage. Faster response than GDT but higher capacitance. At 915 MHz, higher capacitance can cause reflections; look for units specified for 900 - 1000 MHz with insertion loss under 0.5 dB.
- Hybrid GDT + TVS: Best of both; GDT handles bulk energy, TVS handles fast rising edge. More expensive but preferred for high-value installations.
Recommended Arrestors for 900 MHz LoRa
| Model | Type | Connectors | Insertion Loss @ 1 GHz | Notes |
|---|---|---|---|---|
| Polyphaser IS-50NX-C2 | GDT | N-female both ends | <0.1 dB | Industry standard; bulkhead mount; requires grounding lug |
| Proxicast LAN-Cell (ProteX) | GDT | N-female both ends | <0.2 dB | Lower cost alternative to Polyphaser |
| Citel P8AX-900 | GDT | N-female both ends | <0.3 dB | DC-blocked version available for bias-T applications |
| Times Microwave CBA-LGNS | GDT | N-female both ends | <0.1 dB | 2-stage gas tube; good energy handling |
Installation Procedure
- Install the ground rod at or near the building entry point. Drive to full depth. Connect the ground lug from the ground rod to the building's main electrode system with #6 AWG copper.
- Mount the arrestor at the building entry point - the location where the outdoor coaxial cable transitions from outside to inside the building. Mount it on a grounding panel or use a bulkhead mount penetration.
- Bond the arrestor ground lug directly to the ground rod with the shortest possible #10 AWG or heavier copper conductor. Every inch of extra length adds inductance and reduces protection effectiveness.
- Ground the mast separately. Run a #6 AWG conductor from the mast base directly to the ground rod. Bond at a second lug on the ground rod or a listed bonding clamp.
- Connect outdoor cable from antenna to the antenna (outdoor) port of the arrestor.
- Connect indoor cable from the equipment (indoor) port of the arrestor to the LoRa radio or gateway.
- Verify continuity: With an ohmmeter, verify that the mast, cable shield, and arrestor ground lug all measure under 1 ohm to the ground rod.
NEC Requirements Summary
Key NEC articles applicable to antenna grounding (2023 NEC):
- Article 810.21: Grounding of outside antenna systems - conductors, electrode requirements, bonding to building electrode system.
- Article 810.20: Surge protector installation location and specifications for receiving antenna systems.
- Article 250.52/250.53: Grounding electrode and installation requirements.
- Article 250.94: Bonding of separately derived systems (requires interconnection of ground electrodes).
Disclaimer: This page provides a general overview for reference. Always consult the current edition of the NEC and any applicable local amendments. Installation must be performed by or under the supervision of a licensed electrician where required by local jurisdiction. Radio amateur and commercial operations may have additional FAA and FCC tower requirements beyond NEC scope.