Lightning and Antennas

Cstok

EF4
When I chase this year, I will be using a few mag mount antennas. One for the scanner, one for the data card and one for the phone. In years past, any time I get into a high lightning area I pull em down because I have a fear of a lightning strike travelling through the antenna into the car via the cable from the mag mounts.

What do you do during those types of situations. Are my fears unfounded, or is it best to pull em in when the CGs are firing?
 
I've been giving some thought to it theoretically. TTI it's pretty clear that few chasers take their antennas down during severe storms because it's when they need them most; most haven't been struck (yet); and the few that have weren't seriously injured, although the same couldn't be said for some of the attached equipment.

I would be concerned that the process of exiting the vehicle and taking the antennas down in a charged situation is potentially :rolleyes: more dangerous than staying in the vehicle. With that in mind I'd like to suggest something for discussion by more experienced hands.

What about installing cleats inside the vehicle, disconnecting the antennas, and winding the cords around them when you feel threatened? I think the coiled cords would present a big inductance to the massive unbalanced current of a strike and keep damaging voltage on the outside skin of the vehicle where it belongs.
 
I'm thinking that if a car takes a direct hit, the chances of the electronics in the vehicle surviving are slim regardless of whether the strike was to an antenna or the metal frame of the car. When lightning is a few feet away from a circuit board, all bets are off even if the bulk of the current flows along the outside frame. While the frame should provide protection to the vehicle occupants, any electronic device that is in such a close proximity to a lightning strike is not going to come out unscathed.

I would however worry about an antenna cable that was passing close to a person riding in the car, like a wire that came inside at the top of the passenger door.
 
I doubt that taking down the antennas would serve much purpose. You're more likely to be affected from nearby-strikes via the ground current anyways.
 
The reason why you're quite safe inside a vehicle isn't technically because the metal of the body is a Faraday cage. Instead it's due to what's known as the "skin effect".

Lightning appears to a conductor as humongous power half-cycle RF to ground. The charge follows the outside "skin" of a conductor to ground; thus even touching metal inside a vehicle isn't reported to result in serious injury (though I guess the high voltages are enough to generate a Van de Graff-like tingle through charge leakage into free air). Ground-loop shouldn't be an issue because a disconnected radio isn't in the loop.

My suggestion is that looping the disconnected antenna feed around a cleat may provide enough impedance to discourage any tendency of the charge to enter the vehicle and, if the cleat is fastened into metal connected to the vehicle body cause pretty much any charge to flash through the coax' outside insulation and back to the vehicle's skin.

KD7SMQ, FWIW
 
Lightning + Antenna = Target <_<

Lightning + Antenna + you grabbing them = TARGET! **BOOM!** Cease Fire :blink:
 
Lightning + Antenna = Target <_<

Lightning + Antenna + you grabbing them = TARGET! **BOOM!** Cease Fire :blink:
[/b]
Probably more safe to simply unplug the antenna from the cell phone or scanner while in the car. This shouldn't take more than a few seconds of work. I used to be fearless of lightning but after several close encounters of the CG kind I'm more apt to stay in the car when the shock and awe is close.
 
Last spring during a lightening storm I had my cell phone on my lap (antenna attached to my wilson mag mounted roof antenna) and low and behold I got one nasty currnet sent to my thigh (inches away from the family jewels)

Until then I always thought "ahhh not a threat" but now I unplug anything that is connected to the antenna during a CG event....imagine if that had been a super bolt, maybe wouldnt be here a chattin right now...

lesson: un plug during close cg's - but dont get out to retreve the antenna if its really crackin

ps: called Wilson and they said groung the antenna to the car / magnet which Ive done....that being said Id still unplug anything going to a laptop or your ear!
 
When I chase this year, I will be using a few mag mount antennas. One for the scanner, one for the data card and one for the phone. In years past, any time I get into a high lightning area I pull em down because I have a fear of a lightning strike travelling through the antenna into the car via the cable from the mag mounts.

What do you do during those types of situations. Are my fears unfounded, or is it best to pull em in when the CGs are firing?
[/b]

I actually did some extensive research on this. I ended up farming out to Usenet where I found via Google groups a guy that posted about 10 years ago who did grounding engineering and installations for large antennas (i.e., TV towers). I hunted down his email, then asked him just this question. His answer was long and involved, but the gist of it is this: yes, it's a risk. You're compromising the faraday cage/skin effect when you do this. Unless, of course, you ground the coax at the point of entry into the vehicle and install a gas-fired lightning protector just after the ground. Anything less than this and apparently you can get some pretty terrific potentials inside your car during a strike.

That said, it's unlear just how likely such a strike really is. Given that chasers have been chasing this long and none have been killed or hurt by in-car strikes, it's probably low. Hydroplaning seems to be much more of an issue.

Here is the relevant part of the conversation we had:

//Begin FWD

From: "Gary Coffman" <[email protected]>
To: "Ryan" <[email protected]>
References: <[email protected]>
Subject: Re: Lightning dangers from Faraday's cage compromise while storm chasing/spotting?
Date: Sun, 29 Jun 2003 16:06:23 -0400

> I've done a bit of back research on my question, and I think I have
> much of it answered. Mainly, I'm concerned that I may be introducing
> a lightning hazard into the passenger compartment of my car by using a
> magmount scanner antenna. The antenna is mounted to the trunk of my
> car and the coax is run through through the rear trunk gap, into the
> trunk, up through a gap at the edge of the rear seat upholstery, along
> the floorwell, and to a handheld scanner attached via velcro to the
> dash on the passenger side.
>
> From what I've read, it seems that this pretty much defeats the
> Faraday's cage, as I'd feared. Since I primarily use the scanner to
> find severe storms which usually contain awe-inspiring displays of CG
> lightning, this is a disconcerting thing to discover.

Yes, that certainly does destroy the integrity of the Faraday cage.
A hollow closed conductor will have all the current on the *outside*.
But if you provide a penetration via a unsuppressed cable to the
inside, it will bring a very high potential inside. Internal arcs then
become a very real concern.

> But I still wonder: how much of a risk is it to have this set up?
> Assuming for a moment that the mag mount antenna is directly struck,
> what is the most likely effect? Would the bulk of the current follow
> the coax into the passenger compartment, or would the current likely
> jump at some point from the coax to the car body? Seeing as my
> handheld scanner is not grounded, whereas my car body sort of is, I
> would hope so -- but part of me fears that by the time the actual
> discharge is occurring, the charge may simply go as far as it can along
> a conductive path (i.e., to my handheld scanner) and then find a new
> path -- even if that means arcing around in the inside of my car and
> blowing a hole through something not normally conductive.

That's right. Understand that a direct strike entails about 20,000 amperes
at millions of volts. Serious stuff. It wants to find a path to Earth, and
it will
explore every available conductor as a way to find one. The current will
divide in inverse proportion to the impedances of the various paths. In
other words, the largest part of the current will take the lowest impedance
path. But every path will be electrified, and until a low impedance path is
found, every path will be at a very high potential. So arcing is a virtual
certainty.

The eventual path to Earth will most likely involve arcing across the
tires from the wheel rim to Earth. If the car does take a direct strike,
it is likely the wheel bearings will be scored and will have to be replaced.
If having the vehicle struck is considered likely, it might be worthwhile
to install a light chain from the frame to Earth, ie a classic grounding
strap. This will offer a lower impedance path to Earth, and may save
the wheel bearings. OTOH, it may increase the likelyhood that the
vehicle will be struck in the first place.

> Honsetly, I could give a flip if the scanner was damaged or
> destroyed. What I'm worried about is getting zapped. Is internal
> arcing a reasonable possibility if my mag mount is directly struck? Is
> there any way to minimize the danger? Mostly you seem to recommend
> using polyphasers at the drilled coax entry point in order to give the
> lightning enough resistance to cause it to opt to follow the body of
> the car instead of the coax. But I'm not so keen on drilling holes in
> the car body; if there were a way to effectively use a polyphaser
> without drilling holes in the body, I'd do it in a heartbeat.

There's really no alternative to maintaining the integrity of the
Faraday cage. You don't have to use Polyphaser products, but you
do have to provide a very low impedance path for the current to take
to the car's outer skin. You can use a shunt fed grounded antenna
(note I don't mean Earth ground), or a slot radiator, but that's going to
entail drilling and bolting, or welding, to the car body too.

Note that the scanner, and you, aren't the only things at risk. Your
car's computers (modern cars have several) are also at extreme
hazard. They are very expensive to replace, and the car won't run
if one is damaged. So you have a large financial incentive to avoid
leading large potentials inside the vehicle even if you don't care
about your radio equipment.

> I thought about trying a glass mount antenna, but then I figured
> that that would be just as bad; I'm guessing lightning can easily jump
> through a fourth inch of glass to get to a highly conductive coax
> cable, and that's probably preferable to jumping a good foot through
> air to get to the car body.

Yeah, not to mention that glass mount antennas are poor antennas
anyway.

The big problem with any vertical rod antenna is that it is going to
break into corona and streamer formation as the field intensity
builds. This makes it the most likely target of a strike. In other words,
it behaves as a classic Franklin lightning rod. So if you can avoid
using a whip antenna, the probability of the car being struck by
lightning is reduced.

A shunt fed loop antenna, a slot antenna, or a patch antenna, all
directly body grounded, are alternatives to the vertical rod. A patch
antenna could be mounted to the inside of the glass. That would
eliminate streamer formation and a penetration of the Faraday cage
of the body. But a VHF patch antenna would be fairly large, about
19 inches on a side. That would obscure vision from that window.
A slot antenna would also be quite large at VHF. Both would also
be directional if mounted in a window. That would be a problem
as you changed directions.

A DDRR loop could be mounted on the roof. It is very low profile,
similar to a luggage rack in appearance, but it needs a very good
electrical connection to the roof to form a vertically polarized omni
pattern. So you're back to drilling and bolting, or welding, to the car
body.

Now I should say that lightning strikes on cars are fairly rare. I
should also say that the car windows make the body less than a
perfect Faraday cage anyway. But fixing a vertical whip antenna
on the car does increase the odds of it being struck, and leading
the signal inside the car with an unsuppressed coax does increase
the likelyhood of an internal arc which could harm you, or the car's
expensive electronics.

If I were storm chasing, I'd give this serious thought. I'd want to
use an antenna form that isn't as apt to break into streamer
formation as a vertical whip, and I'd suppress the coax where it
enters the vehicle. That means some holes in the car, but I consider
that a minor issue compared to what could happen to the car, or
you, if lightning were to strike the setup you now have.

Gary

//End FWD
 
Guys,

For over 10 years in railroad telecommunications I've been in installing, repairing and replacing radios, cellphones and vehicle location devices for scores of vehicles, including hyrail, as well as thousands of locomotives that run through the maintenance terminal where my shop's assigned. In all that time, for all those vehicles I've never seen one in my shop or any other one for lightning damage.

Lightning does strange things and takes weird paths to discharge. About 15 years ago a volunteer firefighter North of Topeka was severely injured when lightning struck him while sleeping on a couch in his bermed home built into the bottom of a hill. The lightning came in via a small scanner antenna, blew a hole through his concrete wall you could stick your head through and nailed him. His neighbor, a ham radio operator, lived at the top of the hill and had an impressive antenna farm. While he'd had his share of lightning strikes previously he wasn't touched during that storm.

If lightning -did- strike your vehicle, don't count on having devices turned off and packed up being safe. If you're really worried about lightning blowing up your stuff add a communications equipment rider to your auto insurance. Lightning doesn't take bends well at all so coiling or looping coaxes will only make a strike simply jump to another path.


JH
 
Wouldn't a strike to your vehicle probably leave you deaf? I thought I've heard reports of people going deaf or blind after having extremely close calls with lightning (or being struck directly). I would think you'd at least lose conciousness for a bit, just from what I've heard or seen from folks who have either been hit or been within 10 or so feet or a strike.
 
Wouldn't a strike to your vehicle probably leave you deaf? I thought I've heard reports of people going deaf or blind after having extremely close calls with lightning (or being struck directly). I would think you'd at least lose conciousness for a bit, just from what I've heard or seen from folks who have either been hit or been within 10 or so feet or a strike.
[/b]


Correct Jeff. My brother in law was knocked unconcious as he ran across a field during a storm (he admits it was stupid now). His friends found him face down and thought he was dead. He woke up and can't remember a thing for that moment in time..but no perminant damage as of yet (several years ago).

One pant leg had a very nice burn mark where the strike either went up his leg or down his leg.

I don't care how many antennas you have on the car. For heaven sake..stay in the car.

-Tyler
 
<_< I started doing this in 1989 and each storm unit I have operated has always been a antenna farm. While we have some very close calls and have lost a radio or two we have never had any serious problems. We chase lightning and the rest of the storm is iceing. I would agree you are at more of a risk by getting out to remove them than you are at getting hit. And unhooking antenns will not help if you take a direct hit. Turning your neg. ground into a lightning rod. :(
 
When I chase this year, I will be using a few mag mount antennas. One for the scanner, one for the data card and one for the phone. In years past, any time I get into a high lightning area I pull em down because I have a fear of a lightning strike travelling through the antenna into the car via the cable from the mag mounts.

What do you do during those types of situations. Are my fears unfounded, or is it best to pull em in when the CGs are firing?
[/b]

I've been "bit" very hard using scanners and other devices attached to outside antennas. On some of them I rigged up a grounded alligator clamp that will attach around the outside ground of the coax end (B&C, PL259 or what ever). This will lead the current to frame ground of the vehicle. Best ground may be a bolt from the steel seat where it attaches to the floor and frame. If possible use a wide copper strap not a wire. Don't lead it to anything under the dash, or you may wind up walking after your instruments go up in smoke. Other than a totally numb hand the biggest problem is burning up equipment. I've lost a bunch of scanners over the years to corona, in fact I had to buy a new one for this spring; lost the last one on June 12th at Jayton, TX. One of those 1-2 inch sparks is more than enough to fry the average Bearcat. If a chaser loses a scanner no doubt that's the reason it just quit working and I doubt they knew when it happened. Finally, if you're already in the storm and decide now is the time to unscrew things, don't, this is about the best way to get a nasty shock...especially under an active anvil canopy where the potential is very high.

gene moore
 
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