There’s a handful of electronic components to choose from to help get rid of a surge from lightning. Diode rectifiers, zener diodes and special transient suppressor diodes are all widely used. One of the most widely used devices is an MOV (pronounced “em oh vee” not mauve) short for Metal Oxide Varistor. Varistor is a term for a variable resistor of any kind, and chances are you own some MOVs right now, built into something else you own, perhaps as a surge protected AC power strip. What makes MOVs variable is that as the voltage goes up, their resistance goes down. Because of that, they can dissipate that excess voltage as heat. That means they will eventually wear out and fail, so it’s best to construct the protector in a way that the MOVs can be removed and replaced (if it's not expected to be thrown out).
Here’s a “do what I say not what I do” moment. MOVs should be chosen based on the operating conditions and not because you have some – which is what I did. This website, from industry giant Littlefuse, can help you choose an MOV, (that's a specific page in multi-page document) as can
this article from industry publication Electronic Design.
If you’re doing design for a living, you'll be paying attention to those selection criteria. For hams, the simplest Rule of Thumb (I like those) is to choose an MOV rated at 20% higher voltage than the normally expected voltage. In my case that would be above 30V. It so happens I have about hundred surplus 75V MOVs, which is three times my expected voltage and still very far below the anticipated lightning surge. The ones I have are rated at 2500 Amps on an 8/20 microsecond pulse (reaches peak in 8, declines to 50% at 20) or 12W for 2msec.
I simply need a way to break my cable and connect an MOV from all six wires to ground. That should be a robust ground connection.
Those two big holes on the right are how the metal plate local ground gets connected to the system ground: a stainless U-bolt with anti-seize compound.
Whenever you’re designing something like this, it seems to be an encouraging thing to discover that there’s a commercial product that does almost exactly the same thing. I found out midway through the design that amateur radio supplier DX Engineering has a product that’s conceptually identical to this, their Rotator Control Line Protectors, DXE-IS-RCT.
The major difference between theirs and mine is that they have two banks of four wire connections because some rotators have eight wires and theirs is a partially sealed box. Other differences are that I used stainless steel washers and cap screws for the MOV ground connections, although the terminal strip hardware isn’t stainless, and DX Engineering uses a more “professional” ground connector. Since I have an aluminum tower, I’m more concerned about dissimilar metals in contact than someone with a galvanized steel tower.
It's mounted to the tower with the stainless and aluminum interfaces smeared with an anti-seize compound that is also somewhat conductive. The ends of the cable I cut to expose those 12 wires, though completely insulated wires, and pretty much everything was coated with an outdoor, non-corrosive, RTV silicone rubber.
I will eventually have to replace this, and it's going to be a massive pain. It might be good to have some extras built and ready to swap out.
EDIT 3/14/20 2045 EDT: to lengthen the title. I'm going to search for this some day and the original title just isn't useful enough