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Author Topic: Lightning  (Read 664 times)
TANAKASAN
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« on: March 11, 2008, 02:47:50 PM »

In topic http://www.eham.net/forums/Elmers/176943 someone asks if he should unplug his equipment when not in use. I answered this post and my opinion was to disconnect the antenna and mains power. Some people (but NOT all) suggest grounding antennas when not in use so I'm hoping to stimulate some debate here about the correct technique, should I ground or not? Here's my two cents.

1) Research has shown that a lightning bolt originates at the ground, or an earthed point connected to ground. There are high speed photographs of this showing the genesis of a lightning bolt. If your antenna is floating above ground (unconnected) would it not prevent this?

2) A lightning bolt has travelled a few miles by the time it gets near your shack and it contains staggering amounts of voltage and current. A grounded antenna will probably not make much difference if you get struck, you're probably screwed anyway.

All opinions welcome, just go for it and have fun!!

Tanakasan
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N3OX
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« Reply #1 on: March 11, 2008, 03:23:57 PM »

Floating your antenna doesn't stop leaders from forming.   It may stop a lightning bolt starting to form from your antenna but unless you have a tall tower in an open field, there's not so much of an increased probability of the antenna getting hit, grounded or not (and it seems possible that a very tall tower doesn't care if it's grounded or not... the potential across the gap between the tower and ground is a small fraction of the total)

The trick is that if your property takes a direct hit, the coax provides a convenient lead into your home for the bolt.  This is where the "few miles" thing comes in.

You need the current to go into the earth outside your home WITHOUT FURTHER ARCING, whatever you do.

The WORST thing that could happen, and rather likely if you're a casual disconnector who hasn't thought it through is that the lightning hits your property, elevating the coax to 10,000 volts above ground... the coax that is just dangling mere inches behind your equipment with the curtains in between it and the grounded gear.  The arc through the curtains burns  your house down.

Most lightning bolts won't actually dump that much energy into your coax... sure, it will get warm, but your average lightning bolt does not melt or vaporize your average appropriately sized grounding hardware.  In fact, that's why it's appropriately sized.

Sure, there are 20,000 amps flowing through that wire but only for a few milliseconds and through a very small resistance.  It's all about whether or not you deposit enough energy in the wire to take it through melting, and if you use appropriate grounding hardware that won't *usually* happen.  And if you ground outside the house, when you do get a massive strike and it DOES vaporize your coax shield, it does it outdoors.

Most importantly, for all but the biggest bolts, no part of the grounding system itself turns into a white-hot plasma anywhere, and certainly doesn't do so inside your house.   A coax shield carrying 10 kiloamps for 20ms past your siding to your single point ground will not set your house on fire.  An arc carrying 1000A for the same amount of time probably will.

- - - - - -

If you disconnect the coax from the rig and drag it free and clear of the house so that it's straight underneath the antenna 50 feet away, you're as safe as you would be without the antenna or with the antenna properly grounded.  Lightning hits out there, it goes to ground out there.  Lighting hits the house, as you say, you're screwed anyway.  This is my lightning protection strategy for Field Day.  Set up all antennas at least 75 feet from the tent and drag the coax out there if there's a storm.

The folly of this approach always comes to light on 4th of July weekend when you've taken the kids to see Grandma and Grandpa and you have to spend the entire vacation wondering if your house is going to burn down because you forgot to drag the coax out of the house and you're watching your town get pummeled by severe thunderstorms on the Weather Channel.

- - - - - -

The most important thing to remember, I think, is that a lightning bolt is not, in fact, an infinite energy event on the scale of objects in your backyard.  Many hams have successfully spent the money on grounding and lightning protection hardware to allow them to actually OPERATE during a thunderstorm with little fear.  Commercial radio installations do it all the time too.  Radio and cellphone towers get hit all the time.  

Not everyone can do that, and I can understand not wanting (or being able to) to spend a thousand dollars on ground rods, 2AWG bare copper and CadWeld one shots to protect your backyard dipole between the trees, but a grounding panel and a few ground rods and wire to ground the coax shield where it enters the house and bond that to your service ground (OUTSIDE) are not so hard to do, and giving the lightning a direct metallic path to ground is a much better bet then disconnecting.

Home Depot has the hardware, and no, you won't have a quarter-ohm ground system that can pass 100kA giant strikes without damage to connected equipment, but you CAN keep your house from burning down, even if you forget to unplug and drag the coax a hundred feet away.

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
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« Reply #2 on: March 11, 2008, 03:27:37 PM »

The idea is to provide as low an impedance path to ground as possible for the lightning currents. You want that path to be outside. You want to eliminate or minimize paths to ground via your inside wiring, equipment, etc. A good single-point ground just outside the cable entrance point provides a low impedance path where most of the current will flow, minimizing current via inside paths.

Most lightning damage is caused by currents induced into cables when lightning hits some object other than the antenna itself. This damage can easily be prevented by proper grounding.

I seriously doubt that leaving an antenna ungrounded will eliminate the step leader and prevent a strike. The insulation on your cables is generally only good for a few hundred to a thousand volts. In addition, your cables generally have a good deal of capacitance to ground and lightning contains a lot of high frequency component that can easily flow through that capacitive reactance.
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KB9CRY
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« Reply #3 on: March 11, 2008, 03:27:54 PM »

A grounded antenna will probably not make much difference if you get struck, you're probably screwed anyway.


And this is why every time there's a lightning storm and the commercial AM and FM and the Fire Dept and Police Dept antennas get hit, they go off the air because their antennas and equipment get zapped and they have to replace everything!!!!


>>>>>>NOT...........


Totally false logic.  A properly designed and installed grounding system will SAFELY shunt all that power to ground and protect your antennas, your equipment, your house, and your life.  The radio stations mentioned above NEVER EVER disconnect and they get hit all the time and everything keeps on ticking.


YOU TOO can do the same.  

Tanka and others, you really really should read up on the subject before you spew these falsehoods.


Do a Search on Grounding on the main forums page.
Go to the Polyphasher and ICE websites, plus the one KJM has or will list, read up on the info, and then report back here on what you've learned.
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WB2WIK
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« Reply #4 on: March 11, 2008, 03:44:40 PM »

43 years in ham radio with large outdoor antennas and I've never had any sort of lightning damage.  Never disconnect anything at all.

I have a multi-step program to avoid all lightning risks:

1.  Live someplace where there isn't any lightning.

The other steps aren't necessary.

WB2WIK/6
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W5FYI
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« Reply #5 on: March 11, 2008, 05:23:13 PM »

The idea behind grounding the antenna is not so much to protect the equipment from a lightning strike, but to prevent the strike in the first place. Somewhere floating around the Internet is an AP news story from six years ago titled "Researchers find that blunt lightning rods work best," which states that lightning is drawn to blunt rods more frequently than to Franklin's sharp rods. The fact is, Franklin knew what he was doing; he used sharp-tipped rods to prevent lightning strikes, not attract them.

Assuming you have a wire antenna, or an antenna with reasonably sharp edges on it, grounding will enable the antenna to slowly dissipate ground charges into the atmosphere, reducing the overall likelihood of a lightning strike. Leaving it ungrounded may encourage a feeder strike to occur, and take the shortcut through the wiring, resulting in a full-fledged high-current strike. And like a transformer, a high-current surge outside the house will induce currents inside, wherever there are unshielded conductors, i.e. on the power lines to your radio, phone, TV, etc.

I hope this helps. FYI, Stew
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KB9CRY
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« Reply #6 on: March 11, 2008, 05:28:37 PM »

The idea behind grounding the antenna is not so much to protect the equipment from a lightning strike, but to prevent the strike in the first place.


There is an element of truth to this however a properly designed and installed (it doesn't cost thousands of dollars also) grounding system is designed to safely and properly shunt induced charge and direct strike charge safely to ground.  The secondary benefit is that it indeed is constantly bleeding off static charge which in turn reduces the chances of the system to build up static charge and in turn initiate the ground leader others have talked about.  The Polyphaser website has an excellent article about what is lightning and how it works.
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W8JI
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« Reply #7 on: March 11, 2008, 05:37:06 PM »

Whether you ground the antenna or not the chance of a hit is exactly the same.

This is because the cloud accumulates the charge, and the charge in the cloud causes earth charges to pile up at the highest points below the cloud. The sharper and taller the point the more likely the hit. Grounding is meaningless as is floating because the charges will migrate into the high areas no matter what.

Think about this. The voltage gradient is millions of volts. What the heck difference does 1000 or even 5000 volts matter?

The fact is the only thing we can do is NOT make the antenna the tallest thing around or make it significantly blunter. All of the rest only serves to route the lightning around sensitive things a bit safer.

There is a whole lot of voodoo and fairy tales about lightning but the reason is understandable. It is random luck if something gets hit unless it is terribly tall compared to surroundings. Like those deer whistles on the front of a car or a good luck charm, the people who have good luck will want to blame it on something they think is science.

Sad, but that's really how it works.

73 Tom


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N3OX
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« Reply #8 on: March 11, 2008, 08:32:55 PM »

"The voltage gradient is millions of volts. What the heck difference does 1000 or even 5000 volts matter?"

It makes a vanishingly slight difference in the probability of the lightning striking that object when considered over tens of thousands of strikes?  Hmm, guess we can define that as "doesn't matter"

". It is random luck if something gets hit unless it is terribly tall compared to surroundings"

Yup.  Lightning to the top of a 1500 foot broadcasting tower probably has a 50,000 volt lower breakover voltage... still chump change compared to the overall potential difference, but at least 5% of a million.

Just gives it a larger probability of taking a hit... and guess what?  It just keeps broadcasting ;-)

73
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K9KJM
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« Reply #9 on: March 12, 2008, 02:49:47 AM »

CRY makes the good points here.
JI seems to miss the point.   It DOES make a major difference if the antenna coax is grounded or not.
(I agree it makes no difference in the potential to take the direct strike or not) It makes the big difference in that if your coax is disconnected and NOT grounded, You have the makings of a major arc inside your house to burn it down!

For some good info see: http://members.cox.net/pc-usa/station/ground0.htm

Be sure to go to the the provided links on the last page, Polyphaser and others. This is a very long read by the time you go through it all. Well worth your time if you want to know how lightning protection really works.
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WA3SKN
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« Reply #10 on: March 12, 2008, 05:23:53 AM »

"Floating" the antenna will allow a static charge to build up, actually increasing the possibility of a lightning strike.  Grounding the antenna will keep the potential low and eliminate this attraction.
IT WILL NOT PREVENT A STRIKE!
Best to ground the antenna outside and disconnect the eqpt inside.  (And hope for no strikes!)
73s.

-Mike.
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AD4U
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« Reply #11 on: March 12, 2008, 07:10:09 AM »

I am not qualified to add anything to this post, but I sure enjoyed reading it.  I used the Polyphaser website and their excellent book and video on lightning and grounding as a basis for the ground system I installed in 1994 after the devastating lightning strike my shack received in 1992.  So far, so good.  And I live in a high lightning area.  

All I can add is this, and this is just my opinion:

In spite of all our efforts and expense installing a great ground system, every now and then there comes a lightning strike that seems to defy the laws of physics.
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TANAKASAN
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« Reply #12 on: March 12, 2008, 12:37:40 PM »

"I am not qualified to add anything to this post, but I sure enjoyed reading it."

Me too. My knowledge of this particular branch of physics is limited and in the last five minutes I've learned a lot by reading through the posts. I must admit that the thought of a lightning arc inside the shack (because the antenna wasn't grounded) hadn't crossed my mind.

In my particular case lightning protection will require some thought as I live in a 4th floor apartment.

Tanakasan
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N3OX
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« Reply #13 on: March 12, 2008, 03:38:49 PM »

"
In spite of all our efforts and expense installing a great ground system, every now and then there comes a lightning strike that seems to defy the laws of physics."

Probably doesn't defy anything.

The probability distribution of most weather-related events has long tails.... that is, there's a higher probability of extremely intense events than if they were taken from a normal (gaussian) random distribution.

The tails are rather long and flat compared to the rest of the distribution...  I know wind gusts and wave height both have this sort of distribution, perhaps lightning does as well, but I'd have to check.

http://en.wikipedia.org/wiki/The_Long_Tail

If lightning strikes have this distribution, what this means is that if you look at the probability that a strike has the MEAN amplitude vs. the probability that it has double the mean amplitude, you get a big difference.

But if you look at the probability of a strike with 10x the mean amplitude vs 100x the mean amplitude, while both of them are very unlikely, they're not so *different* in likelihood.

So I haven't found info yet that lightning strikes follow a power law distribution, but if they do, most strikes will be small, but when you go out on the tail you could get really massive ones with not so different likelyhood as just plain big ones.

73,
Dan


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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AD4U
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« Reply #14 on: March 12, 2008, 03:51:44 PM »

In my post I said, "Every now and then there is a lightning strike that SEEMS to defy the laws of physics".  I did not say the lightning strike actually defied the laws of physics.  The key word here is SEEMS.

What I was trying to get across is that in spite having the very best ground system a typical HAM can afford to install, once in a while lightning strikes and something in the shack gets zapped.

Regardless of what you do, sometimes ZAP HAPPENS!

Dick  AD4U
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