Automatic Coax Disconnects: One Layer, Not a Lightning-Protection System
Automatic Coax Disconnects: One Layer, Not a Lightning-Protection System
A motorised air gap can isolate radio equipment from one conductive path. It cannot replace risk assessment, an external LPS, equipotential bonding, coordinated surge protection or control of every service entering the building.
An automatic coax disconnect can be genuinely useful, particularly at a remote or unattended station. The engineering mistake is not using one; it is promoting the switch from an equipment-isolation layer to a complete lightning-protection strategy.
Safety and legal note: Lightning protection is a life-safety and building-safety discipline. Apply the current rules adopted in your jurisdiction and involve a competent lightning-protection designer or electrician. In Belgium, begin with the current official AREI/RGIE publications from FOD Economie. Do not add an isolated earth rod, alter an LPS conductor, or move a required bond because an RF arrangement appears more convenient.
What an Automatic Disconnect Does
The True Ladder Line Lightning Detection and Protection System, associated with Gary K7EMF, is presented by its manufacturer as an automated controller that can monitor lightning data, separate four coax connectors mechanically, inhibit PTT, wait for a clear interval and reconnect. The current product page also describes optional control of mains outlets, contactors and relays.
These functions come from the manufacturer's documentation rather than independent test data. That documentation does not state a standardised lightning-impulse withstand rating for the open connector assembly, a direct-strike current rating, insulation-coordination data, or evidence that the complete installation complies with IEC 62305. The safe conclusion is therefore limited: the unit provides automated operational isolation, while its contribution during a lightning event depends on verified ratings and the complete installation.
A defensible description: the device is an automatic operational isolator for selected station circuits. Its protection contribution depends on where it is installed, the actual open-gap impulse withstand, pollution and moisture, cable routing, nearby metalwork, bonding, SPDs, all other incoming services and the lightning exposure of the structure.
The Standards Describe a Coordinated System
The current IEC 62305 series divides the problem into connected tasks. Part 2 addresses risk management. IEC 62305-3:2024 covers protection against physical damage and life hazard by means of an LPS, including touch and step voltage. IEC 62305-4:2024 covers surge-protection measures for electrical and electronic systems inside a structure.
For customer antenna installations, ITU-T K.71 supplies a narrower decision framework. It addresses risk classification, mains-contact protection, mast and cable-screen bonding, entry bonding and selection of SPDs. K.71 is aimed principally at network operators and does not replace the national rules or a building-specific IEC 62305 assessment.
Classify the structure, antenna location, incoming lines, occupancy, possible losses and whether an LPS is required.
Coordinate air termination, down conductors, earth termination, separation distances and equipotential bonding.
Bond services at the entry and coordinate suitable SPDs, routing, shielding and isolating interfaces across protection zones.
A coax disconnect can contribute to the third step. It does not perform the first two.
What Opening the Coax Can—and Cannot—Do
| Function | What an automatic disconnect may contribute | What it does not establish |
|---|---|---|
| Normal operation | Disconnects selected feedlines and can inhibit transmit before deliberate reconnection. | That the antenna, mast, feeder or building is safe during a storm. |
| Induced or conducted surge | May reduce coupling into the radio through the opened path if its impulse withstand and layout are adequate. | That the gap will not flash over or that another cable will not carry the surge. |
| Direct lightning | May be one isolating interface in a designed system. | A direct-strike rating, current path, equipotential bonding or an LPS. |
| Remote automation | Acts without an operator being physically present. | Safety independence from power, sensors, networks, software, mechanics or maintenance. |
An air gap is not an infinite impedance during a lightning impulse. Flashover voltage depends on geometry, distance, edge shape, humidity, contamination, altitude and the rate and polarity of voltage rise. Long leads and wide physical loops add inductive voltage. Nearby conductors can bridge an apparently generous gap through side flash or capacitive coupling. A visual connector separation therefore cannot be converted into a universal lightning rating without test data and installation constraints.
“Fail-safe when unpowered” also needs careful wording. A device that parks in its preferred state after loss of control power is operationally fail-safe with respect to that one mechanism. It is not automatically fail-safe against welded contacts, mechanical obstruction, a failed actuator, a lightning impulse, incorrect bonding, an unprotected control cable or an unmaintained earth-termination system.
Every Conductive Path Crosses the Boundary
A station seldom has only coax. Mains power, protective earth, Ethernet, telephone or fibre equipment with metallic power leads, rotor cable, relay lines, USB, alarm wiring and metal plumbing can connect equipment or buildings at different potentials. During a lightning event, the damaging quantity is often the voltage difference created between those paths while current is shared through the installation.
The design objective is not simply to make one path open. It is to control where lines enter, bond the required conductive services into a coordinated arrangement, use SPDs or isolating interfaces suited to those lines, and keep connections short and routed as the design requires. “Single-point entry” is useful workshop shorthand, but the real system may use a bonding network and lightning-protection zones rather than one magical stud.
Floating Antennas and Static Bleeders
Some antennas are intentionally DC-grounded; others are DC-isolated by their feed or matching arrangement. Neither condition alone determines lightning safety. A DC-grounded antenna can receive a direct strike, and a DC-open antenna can be part of a compliant system when its bonding, clearances, SPDs and charge control are correctly designed.
A high-value resistor, RF choke or symmetrical bleeder network can drain slowly accumulated charge. It must be rated for the RF voltage, environmental exposure and expected energy. It is not an equipotential-bonding conductor, PE conductor, surge arrester, down conductor or substitute for an LPS. A charge drain and a lightning-current path solve radically different problems.
Open-wire feedlines deserve the same discipline. Bleeders may be useful, but their topology must preserve balance and withstand the differential RF voltage. Whether the line is bonded, isolated or switched at the entry is a design decision governed by the complete antenna, tuner, building and protection-zone arrangement—not a universal rule that every open-wire line must be parked to earth.
A Better Evaluation Checklist
- Begin with the building: determine the applicable national rules, risk assessment and whether an external LPS exists or is required.
- Map every entering service: coax, mains, PE, network, control, rotor and any conductive path between structures.
- Define protection zones and entry treatment: identify bonding points, SPDs, isolating interfaces, cable routes and separation distances as one design.
- Demand relevant product data: open-gap impulse withstand, surge-current ratings where applicable, environmental limits, failure state, maintenance interval and installation constraints.
- Separate operating automation from safety: lightning alerts, PTT inhibit and automatic reconnection are useful controls, but they do not certify the protective system.
- Verify and maintain: inspect bonds, connectors, electrodes, SPDs, switch operation and remote alarms at intervals appropriate to the installation and after suspected events.
The Engineering Verdict
An automatic coax disconnect can be a sensible final layer for equipment isolation. At a remote station, automatic PTT inhibition and line separation may be valuable. The benefit is conditional, not magical: it depends on tested withstand, physical layout and integration with the rest of the protection system.
The correct hierarchy is risk assessment first; external and internal lightning-protection measures next; coordinated treatment of every incoming service; and only then optional operational layers such as detection, switching and delayed reconnection. Opening four coax connectors changes four conductors. Lightning protection manages the structure, its occupants and the complete current-sharing network.
Mini-FAQ
- Can an automatic coax disconnect help? Yes. It can isolate selected equipment paths and inhibit transmission, especially at remote stations.
- Does a visible air gap prove direct-strike protection? No. That requires relevant impulse-withstand data, installation constraints and integration into a designed protection system.
- Is a power-off grounded state fully fail-safe? No. It addresses one failure mode; mechanics, wiring, impulse stress, bonding, control paths and maintenance still matter.
- Should every antenna be DC-grounded? No universal rule follows from DC continuity alone. Static control, PE, bonding, SPDs and lightning-current paths are separate functions.
- Can a static bleeder replace a surge protector? No. A bleeder drains slow charge; it is not designed to carry lightning impulse current or clamp a fast surge.