Vehicle Lightning Safety
Additional Lightning Safety Information from The National Weather Service
Lightning Video
Sam Barricklow K5KJ writes :
I can tell you from experience that a nearby, or even more so, a direct lightning strike can damage or destroy your
radios, and it can potentially damage you too even while you are inside your vehicle! But, you can take steps to
reduce the probability that your equipment will be damaged.
First, I've been actively chasing for over 20 years and my radios have been damaged by lightning only once. The
incident occurred near Happy, Texas during the late 1980s. My wife Patti and I were driving on the interstate north
of Lubbock, punching a core just behind (west of) a tornadic HP supercell updraft. Lightning struck a sign located
within 10 or so feet of the car at the side of the road. It instantly disabled my 2 meter XCVR's receiver front end
and the TX RF output final and driver amplifier transistors. Fortunately, there was no damage to the human
occupants, even though the microphone was in my lap. (We did experience temporary visual effects from the strike.
A pinkish-red ghost image in the shape of the windshield (the direction we happened to be looking when the
lightning occurred) was burnt into our retinas by the bright lightning flash. This image faded over a period of two
or three minutes as our vision slowly returned to normal.)
However, from past experience, if you handle an "ungrounded" radio or the antenna feedline while lightning is
striking close-by or when underneath a highly charged anvil, you can receive a strong shock. If lightning were to
directly strike your vehicle under these circumstances, the results could be serious for you.
Precautions, starting with a little background information
Use a D.C. grounded antenna. Some 5/8ths wave 2 meter antennas use a tapped loading coil with one side of the coil
connected to the shield and the other side connected to the antenna. The center conductor is connected to the coil
somewhere in between, at the 50 ohm impedance point. Antennas constructed in this way usually say so on the
packaging (e.g. D.C. grounded). Hy-Gain brand antennas were constructed this way in the past. Most of the Japanese
antennas only use a series impedance matching inductor to make the antenna look like an electrical 3/4 wave,
instead of the tapped coil that would provide a D.C. ground and a better 50 Ohm impedance match. The advantage with
a grounded antenna is that since the antenna and the car chassis are electrically connected, they will share the
same static charge, preventing (or at least minimizing) electrostatic discharge (ESD) through and damage to your
radios, under most circumstances. A grounded antenna will also prevent electrical shocks to you via the coaxial
feedline, since the car chassis and the antenna are already at the same voltage potential.
But, a *direct* strike to your D.C. grounded antenna would still likely destroy your radio and present a shock
hazard to you. The extremely rapid increase in electrical charge from a lightning strike will flow from the strike
point through *all* available conductors during its journey along "parallel paths" to ground (primarily on the
"electrical surface" of the car chassis). Even though the direct connection to ground (in this case ground is
considered to be the vehicle chassis) provided at the antenna may have a very low impedance (impedance = the
complex reactance to charge flow due to changing voltage potentials that includes resistive, capacitive and
inductive components) compared to the path through your radio, a small portion of the charge will flow through
your radio, but it will be much much less than it would be if you were to use an ungrounded antenna. Your radio
would provide only one of many paths to ground (in this case ground equals the Earth's surface underneath the
vehicle) with the majority of the charge flowing through your vehicle's sheet metal, to the frame underneath the
vehicle, then arcing to ground either through or across the surface of your tires (assuming your tires are wet or
have a film of mud or some other conductive medium on their surface). Or, it may arc from a piece of metal
(especially one with a point or sharp edge where voltage potentials naturally reach a maximum) that is closer to
the ground.
To effectively "drain" static electricity from your antenna to your vehicle chassis, your antenna must directly
connect the braid (shield) of your coax (through the antenna mount) to the metal shell or chassis of your vehicle
(as well as a D.C. connection to the antenna). (Mag mounts don't qualify!) As an added precaution, you should also
ground the metal chassis of your radio directly to the vehicle frame through a short wire (18 ga. or larger copper
wire or braid (braid is better) should suffice.
Gas Gap Lightning Protectors
For more protection, insert a gas gap type lightning discharge unit in series with your antenna feedline and ground
it to your car chassis through a short wire. The gas gap type protectors can handle much more current than the
solid state type, but they generally require a higher voltage threshold and more time (e.g. milliseconds) before
they break down. (These devices discharge across an enclosed "air gap" between two conductors, but apparently use
some type of gas to reduce the voltage required to initiate ionization.) You can buy gas gap lightning protectors
from:
Texas Towers
A Division of Texas RF Distributors, Inc.
1108 Summit Avenue, Suite #4
Plano, TX 75074
Hours (CST): Weekdays: 9AM-5PM, Saturdays: 9AM-1PM
Sales: (800) 272-3467
Local: (972) 422-7306
Fax: (972) 881-0776
E.mail: sales@texastowers.com
Tech: (972)422-7306
Spline Balls
I have not done this yet, but you could also reduce the chances of being struck by installing one or two spline
balls on your vehicle to dissipate static charge buildup through coronal discharge. The two-way radio and microwave
industries have used this technique for years and they swear that the spline balls reduce or eliminate lightning
strikes to their towers.
I recently found a metal chimney brush at the local hardware store (Lowe's) with what appeared to be stainless
steel "bristles" mounted on a metallic handle (the ends of the bristles would each touch the surface of an imaginary
cylinder about six or eight inches in diameter). You can fabricate a spline ball yourself using galvanized fence
wire and galvanized electrical conduit. A spherical surface is the preferred shape defined by the ends of the wires.
If you could find a way to strap and electrically connect one of these to your vehicle, you should be able to
effectively dissipate static electricity, at least theoretically. But, expect strange looks from other motorists
if you mount these on your vehicle. (The coronal discharge might be interesting to watch too!)
You could also install a ground strap, similar to those used to discharge aircraft, from your vehicle to ground.
A piece of oil soaked leather should be sufficient. A ground strap should also reduce or eliminate the shocks that
you sometimes get on dry days when exiting your vehicle.
The Crude Faraday Cage Theory
When spotting or chasing thunderstorms, I like to park underneath power lines. My idea is that the power lines act
as a crude Faraday cage and should receive any direct strikes that occur, instead of the vehicle. Of course if the
wind gets too strong, I move.
Some additional comments on lightning danger are located at:
http://www.k5kj.net
73,
Sam Barricklow K5KJ
