Lightning precautions on rooftop antenna?

Are there any devices or precautions I should bear in mind if asking a professional installer to fit a roof mounted pole for a 4G antenna?

I am thinking of a 2-3m pole with a pair of yagi-type antennas, 1m coax cables terminated in Type-N connectors to an outdoor modem/router that then runs down the side of the house over Cat5 to my PoE switch.

The building is a bungalow with much taller trees nearby and one taller property within 50m

Almost impossible to protect against a direct strike. All you can do is make sure the installation is well and truly connected to an earthing point. A substancial earth rod and very thick cable.

Don't ever recall seeing lightning protection on tv aerial installs which are usually above roofline

We once had a direct hit on our TV arial - just blobs of aluminium alloy all over the garden and a bare pole on the chimney stack, coax cable burnt through in several places, and a dead computer that was on the other side of the wall where the coax ran down.

Surely a suitable lightning strike system offering very low resistance to earth is the way to, which means the lightning is more likely to hit that than your antenna

You can also end up with your house on fire, holes in the roof, plaster blown off the wall where the lightning strike used the house wiring as a path to earth, the list goes on.

You would need proper lightning protection, aka a thick piece of copper to the ground and proper grounding. This will be expensive. Even with all this a nearby strike can still take everything out with raised ground potentials. The lightning strike for a brief moment raises the ground potential near the strike many thousands of volts, which can raise the earth potential in the house and electricity from the strike starts flowing in the wrong direction...

Frankly in the UK the main solution is insurance as we don't get sufficient lightning to make it an issue unless you have a tall building.

In reply to a post by jabuzzard:

Frankly in the UK the main solution is insurance as we don't get sufficient lightning to make it an issue unless you have a tall building.

+1 as seen on church spire's.

As the Vogons say "Resistance is useless".

I thought that was the Borg.

In reply to a post by jabuzzard:

I thought that was the Borg.

Hi,

The Borg was "Resistance is futile" not useless......Direct quote is "Your biological & technological distinctiveness will be added to our own. Resistance is futile."

Yours,

HGTTG Vogons were "... is useless." Borg are the ones who have "... is futile."

And the Vogon's came first - suggests a little plagiarism by the Borg?

In reply to a post by ian72:

And the Vogon's came first - suggests a little plagiarism by the Borg?

Douglas probably knew Roddenberry well.

In reply to a post by DanielCoffey67:

Are there any devices or precautions I should bear in mind if asking a professional installer to fit a roof mounted pole for a 4G antenna?

Yes - if my experience is anything to go by...

In mid 2000, I paid £40 for the "Ondigital Aerial upgrade". This comprised an enormous Yagi, on the top of tall pole. It joined the gaggle of three other aerials up there.

At 18:01, on the 2nd July 2006 (The Indianapolis F1 race had just started!), said aerial was struck by lightning

Immediately prior to this, I'd been messing with one of these new-fangled Freeview box things, using one of the other aerials. When it started lightning , I pulled the aerial wire out and dangled it over the radiator ... just in case!

The lightning probably hit all 4 aerials; certainly getting the one I had been handling seconds earlier, as a huge flash and a bang filled the room. There was only a little scorch mark on the radiator to show anything had happened, but everything was suddenly very quiet...

It turned out that the lightning had entered the old Samsung NTL/VM box (via the RF input), and had then exited the box, via its ethernet connection and the mains (blowing the 30A trip on its way out of the building).

The aerial itself was remarkably unscathed, though its wiring was no longer intact. Virtually every other piece of electrical equipment in the house was dead though. The insurance claim went on for months!!

(My tv aerial now lives in the loft )

Edited by deleted (Fri 27-Sep-19 00:11:36)

So if it happens again you lose the roof? (At best!)

Sounds like a very unfortunate incident.

As I wrote, we have some mature trees surrounding us that are greatly taller than our building and an immediate neighbour that is a full storey taller than us.

I remember reading that considered across a broad area, lightning strikes on a 50m grid, generally choosing the highest point in that grid. Hopefully we are concealed among much better targets.

There was a Channel 4 documentary, way back in the 80's, where they did some original research into lightning. They found that as the air starts to ionize, 'tendrils' from objects on the ground start to grow upwards, towards tendrils coming down from the clouds. When two of them connect, the current flows...

This probably explains why standing under a tree, to use it as a personal lightning conductor is not recommended...as in, the small tendril from the top of your head, may connect up in addition to the big one from the top of the tree!

(I'm no expert - just fascinated by the subject!)

Edited by deleted (Fri 27-Sep-19 11:46:28)

It should be noted that the purpose of a lighting conductor is NOT to carry the load of a lightning strike to ground but rather to dissipate the build up of charge and thus make a lightning strike less likely. If you still get a strike then it can still be pretty messy.

That's not to say you shouldn't consider putting one in, just that the load rating of the cabling is a moot point if you still get hit. The main function should be to provide a good path from ground up into the sky (reducing the strike risk) and less so a better path to ground in the event of a strike (when damage is likely in any case).

In reply to a post by andyhurley:

It should be noted that the purpose of a lighting conductor is NOT to carry the load of a lightning strike to ground but rather to dissipate the build up of charge and thus make a lightning strike less likely

But isn't the dissipation of the charge, the very definition of Lightning ?

I can see that something tall and spiky might set it off sooner, so it might be a smaller event - but it's still a spark gap, rather than a 'bleed resistor'.

On the subject of what do with the current from a strike: I remember reading that aerial down-leads should be earthed where they enter the building. It didn't seem sensible to me to introduce Lightning into the building (whether via its existing earth wires, or even water pipes). I thought an external length of 45A cooker wire, with all three conductors connected to an earthing rod might be the way to go. I acquired an old GPO rod, but never got as far as implementing it.

In reply to a post by PhilHornby:

... They found that as the air starts to ionize, 'tendrils' from objects on the ground start to grow upwards, towards tendrils coming down from the clouds. When two of them connect, the current flows...

I found a site that backs up what I half remembered from 30 years ago

In reference to www.wunderground.com/resources/education/lightning.asp:

If the negatively charged area at the bottom of the storm gets large enough, sends out a channel toward the ground called a step leader. It is invisible to the human eye and moves in steps toward the ground. When the step leader nears the ground, or a target like a radio tower, it repels all the negatively charged in the surrounding area, and attracts all the positive charge. As the positive charges collect in high enough concentration, they send out small bolts of ground to air lightning called streamers. If the streamers can make contact with the step leader, an electric current wave propagates up the channel as a bright pulse -- lightning!

Also: https://science.howstuffworks.com/nature/natural-dis...
and
https://www.youtube.com/watch?v=P8vqm_ABfqg

Edited by deleted (Wed 02-Oct-19 00:16:09)

Is 45A cooker wire going to do much as a quick google suggests the current in a lightning strike is orders of magnitude higher than that, any "normal" cable is going to get burnt out if it takes the current from a strike.

I don't know. I know the currents involved might be thousands of Amps, but it's only for a very brief duration.

Interestingly, when my aerial(s) were struck, only one of them was slightly fried - you might have expected all the down-leads to have caught fire, or vapourised - but they didn't.

No doubt there's a standard somewhere, that says what a Lightning Conductor should be connected to.

Normally lightning conductor "wire" is a strip of rather thick copper attached to the outside of the building. It tends to go green.

The idea being that insulations is unnecessary for a ground and if it does get hit it won't need replacing, especially as lightning does strike twice, usually a couple ms after the first one.

Er that's wrong physics and known not to be true for hundreds of years now. Benjamin Franklin the originator of this incorrect theory retracted it himself.

Basically a lightning conductor puts the top of the conductor at the same potential as the ground. The lightning is looking for the lowest resistance route to the ground (well actually it's the other way around but still the same). Being higher up makes the route from the cloud to the ground via your lightning conductor the lowest resistance (or more correctly impedance) route to ground and electricity (Kirchhoff's circuit laws) always takes the lowest impedance route to ground.

https://en.wikipedia.org/wiki/Lightning_rod

The fundamental principle used in lightning protections systems is to provide a sufficiently low impedance path for the lightning to travel through to reach ground without damaging the building.

Edited by jabuzzard (Thu 03-Oct-19 13:44:42)

.... and that�s the issue, the enormous unpredictability of lightning.

Went to a property where the strike had been to the �swan neck� bracket holding the drop wire on the eaves. Everything electrical was borked in the house. About every two metres along the span the copper pair had �blown� out of the drop wire. The DP block at the top of the pole had vaporised, and every other house attached to the DP had at least some form of damage, phones and routers gone, most needed a new NTE.