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A direct hit on my home by lightning is very, very unlikely. However, many lightning strikes that hit Earth nearby are by far the main hazard for ham radio gear (in my opinion, after 60 years in the hobby). The huge electromagnetic pulse generated by such nearby strikes spreads in all directions at the speed of light.

I have been taught that such EMP will induce voltage on antenna wires, power lines, guy lines, etc—anything that can serve as an "antenna" for the pulse will have a voltage induced, sometimes very substantial. Here's the question: Doesn't such an EMP impose a voltage spike on both the braid of a coax feed line as well as on its center conductor? And, if so, how does a gas-discharge tube protection gadget on the center conductor of coax provide any safety at all? I am thinking that the old-fashioned idea of just "disconnect the coax and throw it out the window" still has appeal.

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  • $\begingroup$ Just throwing the coax out the window wouldn't do much good when lightning strikes nearby if the equipment were still connected to the electric mains. $\endgroup$ – rclocher3 Apr 9 '18 at 15:50
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A gas discharge lightning arrestor should be installed directly on the grounding system that consists of multiple ground rods properly spaced and bonded together. The case of the lightning protector directly bonds the shield of the coax to the ground rods to dissipate (bring to near earth potential) any shield induced currents. The gas discharge tube conducts when high voltage is present to dissipate any center conductor currents to the ground system. Ideally the low impedance ground rod system takes most of the induced current thereby saving the station equipment from damage. Professional installations such as cell (mobile phone) sites are successfully protected by such well engineered systems.

Antenna systems can also build up dangerous or damaging voltages from wind or rain in the absence of lightning. A good lightning protection system can help protect equipment in this case as well when a manual disconnect procedure might be overlooked.

Disconnecting the antenna from the station is an approach that is commonly mentioned although this requires a realtime awareness of conditions that warrant the disconnect. Keep in mind that this techique may not protect the antenna and feedline from damage. Also remember to carefully discharge any built up charge prior to reconnecting the antenna system to the station. Use low power to check the antenna system SWR upon reconnecting.

The subject of lightning protection can take many pages to convey the best practices and nuances. The ARRL has recently published a book by Ward Silver on the subject that I recommend as a good reference. The ARRL also has linked information on grounding available on their website.

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  • $\begingroup$ I use static drain resistors across GDTs and the feedpoints of my antennas. That not only drains static away but it greatly prolongs the life of the GDTs. My 160m inverted-L had no GDTs, but a spark gap in parallel with the resistor. $\endgroup$ – Mike Waters Apr 9 '18 at 11:37
  • $\begingroup$ "Disconnecting the antenna from the station is an approach that is commonly mentioned although this requires a realtime awareness of conditions that warrant the disconnect." I reverse this statement: cables need to be connected to the equipment only when it is to be operated. According to maps of ground conductivity, I live on the lowest-conductivity earth in the US, which would make a reliably effective ground protection system prohibitively expensive. I have had closely-space near-direct hits but never suffered antenna, cable or equipment loss. This may be the ultimate case of "YMMV". $\endgroup$ – Brian K1LI Apr 12 '18 at 0:48
  • $\begingroup$ @BrianK1LI I also disconnect mine. However, W8JI has the expertise and the means to install a ground system so that he doesn't have to disconnect his coax during close thunderstorms. Lots of copper is required for that! :-) Also, look at all the VHF repeaters that are available 24/7/365. It's about design with lightning in mind. $\endgroup$ – Mike Waters Apr 12 '18 at 23:13
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Most lightning damage does not come from EMP, that is, energy received by electromagnetic radiation. Much more significant is the electrical potential difference between two points in the soil created as 15 coulombs of charge are dumped in an extremely short span of time in a small area, with current on the order of 30 kA.

For this reason, an effective protection system must ground the station at only one point. With a single point ground the protected zone may rise to a high electric potential relative to some distant point, but as any two points within the protected zone are at the same potential, the equipment just "floats over" the surge.

Where possible, the potential difference within the protected zone is minimized by maximizing the conductivity between components. For grounded components this is easy: simply connect them with a lot of copper.

For signal lines this is more difficult, because signals can't simply be shorted to ground.

In the case of coax, the shield already affords a high degree of protection to the center conductor within. For the same reason signals don't "leak out" of the coax, most of the surge energy can't get in.

The gas discharge tube (GDT) is meant to deal with the residual. If the potential difference between the center and shield is sufficient to ionize the gas the tube acts effectively as a short, clamping the difference between them to some value which is hopefully below the damage threshold of the connected equipment.

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Actually, the shield of the coax does not protect the center conductor at all from lightning. Lightning can travel up the coax as common mode current on both the shield and center conductor at the same time.

Also, the coax can act as a capacitor, storing the charge of the lightning.

Gas discharge tubes need to not only short the shield to ground, but also the shield to the center conductor to bleed away this stored voltage.

Typical coax is rated for 600v before it internally arcs, so you could easily have up to 600v between shield and ground. I've seen reputable commercial RF installers recommend surge protection not only at both ends of the coax, but also in multiple places along the run for runs longer than 75ft just to bleed off this capacitive voltage.

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