Ah, an ancient topic still worthy of discussion. I'll repost something I posted to Storm Track (the email list) a long, long time ago -- it's a reply I got from a guru on this topic from Usenet, Gary Coffman.
This is a forwarded message
From: Ryan <
[email protected]>
To:
[email protected]
Date: Monday, July 14, 2003, 9:00:29 PM
Subject: Followup post on question of mobile HAM/scnanner lightning hazards
===8<==============Original message text===============
Hello! A while back I posed the question to this list: "Do mobile
HAM/Scanner/TV/whatever antennas compromise the faraday cage of your
car, thus making them a lightning hazard for those inside the car?"
Someone here pointed me in the direction of a guru named Gary Coffman
on Usenet. He doesn't hang around on Usenet any more, but he was nice
enough to respond to my personal email. His comments are interesting
and of value to chasers, so I'm reposting them here. Turns out it's
possible to create a very lighting-safe car antenna setup, but that
it's a lot more work than most chasers would go through.
--Begin FWD--
From: "Gary Coffman" <(Ask Me If You Need To Know - Ryan)@bellsouth.net>
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--
