If you have installed external antennas for cellular connectivity, you have probably come across lightning arrestors. You may have been told to fit one. You may have fitted one and not been entirely sure what it was doing.

This guide explains how they work – including the gas discharge mechanism that most product listings wave past with the phrase “air gap technology” – and how to apply them correctly to 4G and 5G antenna installations.

Why External Antennas Are a Problem in Storms

A coaxial cable running from an external antenna into a building is, from an electrical perspective, a conductor entering the structure. If the antenna is struck directly, or if a nearby strike induces a voltage surge in the cable, that energy travels down the coax and into whatever is connected at the other end.

Cellular routers, modems, and connected equipment are not rated for lightning surge voltages. A direct path from an antenna cable to a router input is a direct path from the outside world to a piece of electronics that will not survive the encounter.

The antenna does not need to be struck directly for this to be a problem. A nearby strike – even hundreds of metres away – can induce enough energy in a long coax run to damage equipment. The longer the cable run, the larger the antenna, and the higher the installation, the greater the exposure.

Lightning arrestors sit in the coax path – typically at the point where the cable enters the building – and divert surge energy to ground before it reaches the equipment.

The Gas Discharge Tube: What the “Air Gap” Actually Is

Most coaxial lightning arrestors use a gas discharge tube (GDT) as their primary protection element. This is what people are referring to when they mention the “air gap.”

Inside the arrestor body, there is a small sealed ceramic or glass cylinder – the gas discharge tube. Inside that cylinder, two metal electrodes face each other across a narrow gap. The cylinder is filled with an inert gas mixture, typically argon or a neon-argon blend, at low pressure.

When the surge dissipates and the voltage drops below the sustaining voltage of the arc, the gas de-ionises. The GDT returns to its non-conductive state. This is what “self-resetting” means on product specifications – it can operate repeatedly, though there is a finite operational life measured in discharge events. A GDT that has taken multiple large surges should be replaced.

What the Arrestor Does Not Do

Understanding the limitations is as important as understanding the mechanism.

It does not absorb all the energy

A GDT-based arrestor is fast but not instantaneous. There is a finite response time – typically nanoseconds – during which surge energy continues toward the equipment before the GDT fires. The energy passing through during this period is called the “let-through voltage.” For a direct lightning strike, even let-through voltage can damage sensitive electronics. This is why lightning protection is designed in layers, not as a single device.

It does not work without a proper ground

This is the most common installation error and it makes the arrestor completely useless. The GDT diverts surge energy to ground. If the ground connection is a wire attached to nothing meaningful, or a high-impedance earth, the energy has nowhere to go. It takes the next available path – through your router. The ground connection is not an optional detail. It is the entire mechanism.

It introduces insertion loss

Every component in a coax path attenuates the signal. A quality lightning arrestor at cellular frequencies will introduce 0.1-0.5dB of insertion loss across the operating band. For 4G installations this is negligible. For 5G sub-6GHz it remains generally acceptable. For 5G mmWave above 24GHz it becomes more significant and device selection needs more care.

Types of Protection Element

Connector Types and Frequency Ratings

This is where most people go wrong when buying an arrestor for 5G. Check the frequency specification before ordering – not all devices marketed for “4G and 5G” cover the 5G NR sub-6GHz bands used in the UK.

Connector type for outdoor cellular work: N-type is the standard for professional external antenna connections. SMA and TNC appear on smaller antennas and some routers. The arrestor needs to match the cable connector, or require an adapter – which adds another connection point and potential failure mode.

Where to Install the Arrestor

Position matters. The arrestor goes at the point where the cable enters the building – not at the router, not halfway along the indoor run.

The logic is straightforward: you want the surge diverted to ground before it enters the building. An arrestor at the router position means the surge energy has already passed through the wall penetration and travelled along the indoor run before being diverted – at which point it has had every opportunity to induce damage in adjacent cabling or equipment.

Grounding: The Part That Actually Matters

If there is one thing to take away from this guide, it is this: the ground connection determines whether the arrestor works at all.

Products Worth Knowing

• PolyPhaser American manufacturer, widely regarded as the benchmark for coaxial surge protection in professional installations. The IS series is frequently specified for cellular infrastructure. Properly engineered, not cheap.
• Citel French manufacturer with a strong European presence. The P8AX-MN range covers N-type coaxial protection across cellular bands. Used extensively in telecoms infrastructure.
• Andrew / CommScope The antenna manufacturer’s own protection line. If using Andrew antennas and Andrew feeder cable in a professional installation, their protection devices are a sensible match.
• Proxicast More affordable range, popular in UK IoT and rural connectivity work. Suitable for standard 4G installations. Check frequency ratings carefully for 5G sub-6GHz.
• Times Microwave Primarily known for LMR coaxial cable but also produces protection devices matched to their cable ranges. Relevant if specifying cable and protection together.