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If I connect the receiver and TV body to the earth, does it reduce or increase the chance of lightning strikes the dish antenna?

SamGibson
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Merlin
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3 Answers3

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It would increase the likelihood of the receiver getting struck, but the chance of getting hit would still be small. To understand this, we need to start with a description of how a lightning rod works.

A lightning rod offers a preferred path for lightning locally, rather than the building, so that instead of striking the building, it strikes the rod and travels to the ground. So it helps prevent the building itself from getting hit, but it is not attracting a lightning strike that otherwise would not have hit the building or somewhere quite close.

You might call this 'attracting' the lightning locally, but the details are a bit more nuanced. When the lightning strike is propagating downward from a charged cloud, shortly before striking the building, a weaker discharge begins at the lightning rod and travels upward to meat the larger downward discharge. From there, an ion path has been established, and the electrical current travels to the rod and then to ground, instead of hitting the building and then finding the best path to ground.

Here is a great high-speed video of the process happening. A photo from the article is below. You can see the upward discharge beginning at the lightning rods on the building.

enter image description here

So to get back to your question, I think that if there is no other lightning rod on the building, then it would increase the chance of it getting hit, but not drastically. It would get hit about as often as the building would have been hit without it, which isn't all that often. The lightning would probably hit the receiver, whereas without it, it would have hit the building. If there is a better lightning rod on the building, then probably very little impact.

Edit after looking into it a bit more: It appears that the theory of how a lightning rod works, whether by dissipating accumulating charge to prevent the strike (the 'dissipation theory') or by encouraging the lightning to hit the rod instead of the building (the 'diversion theory') is somewhat controversial. I found this source somewhat useful.

Most studies seem to show, though, that a more blunt surface is more effective at offering protection than a sharp one. There is an optimal zone for the radius of curvature of the surface. If the radius of curvature is increased too much, the protection decreases. A moderately blunt surface is described as offering a better conduction path for lightning to ground than either a sharp or very blunt surface. The source I gave offers this insight:

The results of this study suggest that moderately blunt metal rods (with tip height–to–tip radius of curvature ratios of about 680:1) are better lightning strike receptors than are sharper rods or very blunt ones

And a bullet point from an abstract this to scientific paper also seems quite pertinent:

(c) Elevated, blunt rods or horizontal conductors, suitably connected to earth, can provide better lightning paths to earth and therefore, better protection to structures in their vicinity than do sharpened rods.

I believe this supports the notion that the probability of a strike to an object such as a grounded dish antenna is increased rather than decreased, but again, there is controversy.

rpm2718
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  • I used this article and article a lot, thank you – Merlin May 06 '20 at 17:52
  • You are welcome. I think that in any case, there is agreement that the dish is not much more likely to be struck than the building would have been struck without the dish there. – rpm2718 May 06 '20 at 18:03
  • +1 for the historical context and contrasting both dissipation and diversion theories. Perhaps the argument about which theory is correct is a great example of [bikeshedding](https://en.wikipedia.org/wiki/Law_of_triviality), as really any kind of lightning rod is effective in practice at reducing structure damage and loss due to lightning. – Phil Frost May 06 '20 at 20:49
  • Yes,@PhilFrost, agreed. From the standpoint of designing lightning protection, what really matters is if they protect a building not, and if so, how far can you space them apart. For this question posed by the OP, though, the distinction actually matters. – rpm2718 May 06 '20 at 21:11
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If I connect the receiver and TV body to the earth, does it reduce or increase the chance of lightning strikes the dish antenna ?

Lightning will not see a length of cable connected to earth as a preferred path until it actually hits the installation. Think about a lightning conductor on a tall building; it's not fitted to attract lightning to the building (\$\color{red}{\text{because that would be silly and asking for trouble}}\$) but, if lightning were to actually hit the building, the current surge would tend to follow the path of highest conductivity.

Andy aka
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    Wouldn't the current surge tend to follow the path of *highest* conductivity? – Jost May 06 '20 at 08:59
  • @Jost my silly mistake - fixing!! – Andy aka May 06 '20 at 09:04
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    Doesn't adding a ground wire physically elevate a ground-reference potential, leading to an easier gas discharge across the slightly steeper gradient? – chrylis -cautiouslyoptimistic- May 06 '20 at 17:54
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    Given the inductance of free-space (or air) is circa 1 uH per metre and the massive di/dt of a strike I’d imagine a few metres of low conductivity wire having roughly the same inductance (1 uH per metre) doesn’t seem particularly attractive I mean, by now, after over a century of erecting lightning conductors you’d have thought that someone might have noticed that their tall buildings are getting hit more often than others that don’t have a conductor. – Andy aka May 06 '20 at 18:52
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The purpose of bonding metal objects to earth is primarily to reduce the chance of a lightning strike. This works by allowing charges from the earth to migrate up a metal conductor (grounding cable, antenna mast, or the classic pointed lightning rod), leak off into the atmosphere, and so help to dissipate the charge buildup between the earth and clouds that, if not reduced, could build up enough potential to allow a lightning strike.

Providing a conductive path to earth should a nearby strike occur is secondary.

Even large cell phone towers, while well bonded to the earth, usually have a series spikey rods near the top. The points at the tips of those rods help the charge to bleed off.

Go look at some pictures of farm houses and barns with an array of lightning rods along the ridge of the roof.

SteveSh
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  • Thanks for the article, I took a risk and connected the TV body and receiver together with a suitable wire and then connected it to the earth source to zero the noise and inductance of the body. I hope the lightning does not melt us. – Merlin May 06 '20 at 17:55
  • The chance or probability remains the same. What is reduced is the chance voltage surge travelling past the bonding point if it is properly designed. – skvery May 06 '20 at 18:24
  • What you are describing is the "lightning dissipation theory". Benjamin Franklin was a big proponent, but even in his time there was significant contention between this and the alternative "lightning diversion theory". The contention between the two theories is an [interesting historical and political study](https://www.nytimes.com/1983/06/14/science/lightning-rods-franklin-had-it-wrong.html) in in itself, but unfortunately (for Frankly) in modern times the data show the dissipation theory, while great in a small-scale lab, doesn't hold up in real-world data. – Phil Frost May 06 '20 at 20:43