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I recently visited this tower on top of a rock. There is a wire loop around the tower, with extensions going down the rock. This picture provides a good view of the tower and wires.

The wires that extend down the rock pass by the guy-wires, and they are connected to them. But the connection is peculiar:

Wire loop around a guy-wire

My guess is that the wires extending down the rock are part of a lightning protection system that conduct lightning from the tower down the rock to better conducting soil. But what is the purpose of the loop around the guy-wire? It seems too purposefully circular to merely be an elaborate way to attach the wire.

One thing I considered, is that a lightning strike will cause a sudden change in current through the guy-wire, thereby creating a magnetic field around the guy-wire that in turn induces eddy currents in the wire. These will run in cross-sections of the wire, not around the loop. Even if this effect is significant, I don’t understand why it would be desirable.

I tried to do some research, but it is difficult without knowing the terms to search for. One device I stumbled upon is a counterpoise. It might be that the wires are part of a counterpoise, although there are few of them, and they are not suspended above the ground. The tower is a radio mast according to the Wikipedia article about the place. But then still, why would one need such a loop to connect the counterpoise to the guy-wire?

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  • $\begingroup$ By any chance were you able to see what this wire connects to? For example, does the gray wire that loops around the guy wire go directly into the ground after it goes off the edge of the rock formation? Is the wire attached to the antenna mast (or whatever the guy wire is holding up)? $\endgroup$ – the_photon Jun 25 at 20:36
  • $\begingroup$ I'm not sure that we can answer on the electrical code requirements of Sweden - that isn't a physics question per se. $\endgroup$ – Jon Custer Jun 25 at 20:41
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    $\begingroup$ I can't see any reason why the loop would improve the performance of a lightning protection system. Suspect that the reason for the loop is something more mundane such as providing stress relief against thermal contraction/expansion or to make it easier to extend the lightning protection system wiring to additional towers at a later time, etc.. $\endgroup$ – Samuel Weir Jun 25 at 21:11
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    $\begingroup$ I presume it's antenna and since dirt is poor conductor of current - rock is even worse, so my guess is it's a ground wire. $\endgroup$ – Cinaed Simson Jun 25 at 21:23
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    $\begingroup$ If you don't get a good answer here and it is an electrical reason, you might ask on electronics.stackexchange.com. $\endgroup$ – BowlOfRed Jun 26 at 16:37
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I'm only guessing, but this is almost certainly a ground ring, also confusingly called a counterpoise (different from the above-ground one, providing a signal return ground for an AM range monopole, that you are referring to), a protective device to limit the ground potential difference in case of a power fault or a lightning strike, or to reduce interferece from ground leakage currents. This blog post gives a good explanation:

There are quite possibly several dozen codes and standards that govern grounding for towers in some form or another (IEEE, NFPA, MILSPEC, Motorola R56, IEC, ANSI, TIA, EIA, etc.), so we will not try to list them all here; additionally most companies have internal standards that have minimum grounding requirements for towers that are above and beyond those in the standards. But that said, you can meet the vast majority of grounding requirements by simply installing a ground ring (sometimes called a loop or counterpoise) around the base of the tower. Typically, the ground ring runs at least 1.5-ft below grade and 3-ft outside the perimeter of the tower, encompassing all of the legs of the tower. This conductor will be bonded to each tower leg via exothermic welding, irreversible compression fitting, and/or a double-bolt connection. The conductor itself will be bare copper placed in direct contact with the earth and typically ranges in caliber from a #2 gage to 4/0 AWG (or higher), depending on the current levels that are anticipated on the conductor (i.e. electrical utility fault and/or lighting strike). This ground ring must be supplemented with 10-ft ground rods (electrodes) at regular intervals (such as every 20-ft), with ground rods always placed immediately next to the footing. And of course, the ground ring around the tower must be bonded back to wherever the equipment is located.

Think what happens in case of a lightning strike. The current would flow through the tower and its guys, and dissipate into the ground. The ground is solid rock, and has quite a high resistance. This means a potential field (in the sense $U(\mathbf{r})$, where $\mathbf{r}$ refers to the point on the ground) would develop, with the highest gradient near the point of the best ground contact, where the most current "enters" the ground. When somebody is standing on both legs, the potential difference between the person's feet may be unsafe, or even lethal--the so called step voltage. The close you are to the point where current enters the ground, the larger is the potential difference. The loop in contact with the ground provides a return path for this current, so that the GPD fields outside of the ring is reduced substantially.

But yeah, the ring on the ground looks quite feeble to me, not in a good contact with it at all. The loop alone does not seem to reduce much of the resistance of the whole guy-and-stake system w.r.t. the ground. But then, it's nearly impossible to bury a ring into the solid granite, certainly not at the depth of 0.5m as the blog post suggests, and this ring is still better than no ring at all. You certainly do not want to be inside this circle during a lighting strike!

Another possibility is the loop is designed to reduce the RF noise from the normal functioning of the tower, to absorb the high-frequency leakage currents flowing through the surface, especially when the ground is wet--so your guess might be correct, too, it might be there to reduce interference. But again, I do not see any equipment shed or anything at all around the tower that should be protected from such an interference. FWIW, the relay dishes on top of the tower work in the range of low tens GHz. These waves are absorbed by nearly everything, including dust in the air (and even more by rain) and won't affect any device not in the closest vicinity, and out of direct line of sight. As an analogy, if they were light, almost any surface would look coal-black.


And the view from the top at sunset is indeed stunning! Look what we've got here. :)

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