So I may be building a tall antenna tower and kiln chimney on land we are developing. Taller they are, the more efficient. But there’s one thing that will make those structures horrifically dangerous if unprotected, lightning strikes. An average lightning hit can supply heat energy of 1-10B joules, or up to 10 billion watt seconds. If the energy is significantly absorbed into the structure or directed to the attachments, massive property damage and safety risks can happen. Lightning strikes can turn tall trees into splinters if the energy somehow passes through the core, not the surface. Fortunately, high voltage and current pulses of short durations greatly prefer to travel on the surface of a conductor and avoid the core. That makes lightning rods and sharp tips very effective.
Lightning arresting is very effective, too. I currently work with very static sensitive electronic components that are designed to operate with repeated lightning strikes. A microwave LNA in the X-band may have an absolute maximum rating of 60V static discharge, or about 5V normal operation. After the IF and ADC’s, many FPGA’s processing the data use a highly regulated core voltage of 1.000 volts. I’ve seen what happens when higher voltages breach the very expensive FPGA, the countless LUTs within are instantly destroyed.
So how can sensitive electronics be designed to withstand repeated direct full strength lightning hits? Very easy, shielding and use of high pass quarter wavelength strips that safely attenuate nearly all of that energy. Most of the power from lightning is at lower frequencies and very little ever reaches into the GHz area. I learned many years ago a neat trick at antenna towers, place on large loop of the coax after the grounding rod attachment and before entering the building. The high voltage pulse sees that loop as near infinite resistance and will take the ground pad instead. Lightning and AC waveforms greatly prefer straight paths and outside of conductors.
Since the structures are likely to be around 30 feet tall, the chances of absorbing energy from at least nearby lightning strikes may be very significant. It might even be only kilojoules, or watt seconds, but such an indirect hit could crater semiconductor junctions within our electronic devices. We might not even notice immediate failure as static electricity is known as a “time bomb” with semiconductors. It only needs to weaken the junction with a tiny microscopic crater, that many months later will degrade into a full short and lead to even circuit board damage in a secondary failure event.
Over the course of my career, I’ve always installed outdoor wiring in metal conduit and considered lightning hits as they do happen. The properly designed level sensors on top of tall silos were all well grounded and protected, with zero failures. The electronics on the other side of the current loop in the facility wouldn’t even be affected by direct lightning hits. Every step taken to attenuate the energy multiplies the effectiveness and it shows.
Why am I writing about this? Because I have seen many people erect structures without much consideration of lightning strikes and the damages are widespread. Even small structural changes, such as metal siding leading to a ground pipe may incur minimal cost, yet profound protection.
Since we are developing off grid property in the middle of the country, a tall tower for communications can be a wonderful asset. And since it will be the preferred target for lightning strikes, it may even help protect our property (if it’s properly designed.)
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