7.4 Lightning Protection and Station Security

7.4 Lightning Protection and Station Security

ACMA Exam Focus: Syllabus items 10.20 and 10.22 — recall that it is good practice to install lightning protection on antennas, to disconnect antennas from radio equipment before a thunderstorm and to never operate an amateur station during a thunderstorm; and that an operable amateur station must not be accessible to unauthorised persons.

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Part 1: Lightning (10.20)

Why Your Antenna Is a Target

An amateur antenna is usually the tallest, best-earthed metal object on the property, which is precisely the profile a lightning discharge is looking for. A direct strike delivers tens of thousands of amperes with a rise time measured in microseconds. Nothing in an amateur station survives that; the object of lightning protection is to keep the energy outside the building and to keep people alive.

Most damage, though, is not from direct strikes. It comes from induced surges — a strike hundreds of metres away couples into feedlines, rotator cables, mains wiring and telephone or data lines, and arrives at the back of the transceiver as a fast, high-voltage transient.

Australia gives you plenty of opportunity to find out. The northern wet season, roughly November to April, brings near-daily storms across the Top End and the tropical north, with Darwin recording more thunder-days than any other capital. The south-east has a spring and summer storm season from about September to March, and severe storms are common along the eastern seaboard. Treat lightning as a seasonal certainty, not a freak event.

Building Protection Into the Installation

• Earth the mast or tower directly. Run a heavy conductor — copper strap or large cross-section cable — down the outside of the structure to one or more driven earth electrodes. Keep the run as short and straight as possible: lightning current will not negotiate tight bends, and a sharp bend invites the discharge to jump sideways to something else.
• Bond the coax outer conductor where the feedline leaves the structure, and again at a single-point earth entry panel on the outside wall where all feedlines enter the building. Every feedline, the rotator cable and any control cable land on that one panel.
• Fit gas discharge tube (GDT) coaxial arrestors on the entry panel. A GDT is a sealed spark gap: it is effectively invisible to normal RF voltages, but it ionises within nanoseconds when a surge reaches a few hundred volts and shunts the energy to the earth bar.
• Bond everything to one earth. Any earth electrode installed for the mast or the RF earth must be bonded back to the building's main earthing system — the MEN (Multiple Earthed Neutral) connection at the switchboard — with a heavy conductor. Separate, unbonded electrodes sit at wildly different potentials during a strike, and the difference flashes over through your equipment and possibly through you. This is the single most misunderstood point in amateur earthing.
• Protect the mains and data feeding the shack with surge protection as well; a surge that cannot get in via the coax will happily use the power lead.
• The Australian standards to be aware of are AS/NZS 1768 for lightning protection and AS/NZS 3000 for wiring. Anything that connects to the MEN system or the switchboard is licensed electrical work.

Why Disconnection Beats Every Arrestor

An arrestor is designed to divert induced surges. It cannot absorb a direct strike, and even when it operates it clamps to some voltage well above zero. A physical air gap of a metre between a coax connector lying on the floor and the back of your transceiver is a better insulator than anything you can buy.

So the syllabus rule is disconnection, and it is done properly:

1. Unplug the feedlines, rotator cable and control cables from the equipment.
2. Put those connectors somewhere useful — into grounded bulkhead sockets on the entry panel, in a metal box, or outside the shack entirely — not coiled on the desk beside the radio, where a flashover will simply jump the gap.
3. Unplug the mains leads and any network cable too.
4. Do it when the forecast is issued or the storm is still distant, not when the thunder gets loud. Strikes can precede the rain by many kilometres.

Never Operate During a Thunderstorm

This is an absolute in the syllabus, and it is about the operator, not the equipment. Do not transmit, do not sit at the operating position listening, and do not stand near the feedline entry point. If you can hear thunder, you are close enough to be struck. Wait until the storm has clearly passed — the widely used rule is 30 minutes after the last thunder — before you reconnect anything. And never climb a mast, raise an antenna or handle a feedline with a storm anywhere in sight.

Part 2: Station Security (10.22)

An operable amateur station must not be accessible to unauthorised persons. The syllabus states it plainly, and the reasons split neatly into compliance and safety.

Compliance. A callsign is personal to the qualified amateur. An unqualified person may only operate under the direct supervision of a qualified amateur; unsupervised, they are transmitting unlawfully and doing so under your callsign. Anything they say, any band they wander onto and any interference they cause comes back to you as the person responsible for the station. The consequences run from an embarrassing complaint to interference with emergency or aeronautical services.

Safety. An amateur station left powered up and connected is a hazard to the untrained. A linear amplifier holds lethal charge in its capacitors even after it is switched off; heatsinks and valve envelopes are hot; open-wire feeders and low antennas produce RF burns; and a child who keys a transmitter has no idea which of those things is dangerous.

The habit that satisfies almost all of this takes about fifteen seconds on the way out: master switch off, microphone away, antenna disconnected at the entry panel, door locked. It closes the security requirement of 10.22 and most of the lightning and fire risk at the same time.