DC Grounded Antennas - The Myth,
The Legend, The Fantasy
This is a subject that we just had to write about. In the lightning protection business we
come into contact with many people who have had both dangerous and disastrous experiences
with Mother Nature. And one that has perplexed antenna users for decades is the
very common damage and destruction to radio equipment when connected to a so-called
"DC Grounded" antenna system.
For many years, antenna manufacturers have touted the positive advantages of owning
and operating a station with antennas whose feed systems are a direct DC short across
the input terminals, and hence both sides of the coaxial feeder cable are placed
at "ground" potential at the antenna site. In reality, there are no such
advantages to this kind of feed system, but it is singly the most dangerous ever
used from a lightning perspective.
The reason is pretty easy
to both explain and understand. Lightning bolts that streak from
clouds to ground frequently hit exposed metallic structures like towers
and high antennas. This is simply because the metallic nature
of the object electrically shortens the striking distance between ground
and sky. When a large voltage potential is reached between the two
during a storm the metal antenna acts like a prod, sticking up in the air
and drawing the first arc.
Lightning wants to reach
ground, and that's pretty much all it wants. And it will get what
it wants in the easiest and least resistive way possible. Just
about anything in the way can be easily vaporized out of the way by a good
sized lightning blast. If ten different paths to ground are
presented to a striking bolt (such as numerous transmission line conductors,
the tower frame, etc.) then the currents will divide quite nicely between
all of them, with the larger amount of current flowing in the path of least
resistance and so on.
"DC Grounded" type antennas provide a very neat dual path for those lightning
currents. Some of the blast will flow down the shield of the cable to ground level
earth terminal connections while the rest will simply flow down the center conductor
and ravage the radio connected at the other end. Keep in mind that at the point
of impact a bolt of lightning can easily deposit 50,000 volts or more respective
to ground. And for an instant the voltage at the radio equipment end will be the
same. By the time the balance of the surge comes to an end the equipment will have
long since been toasted, probably beyond repair.
The myth is that "DC Grounded" antennas offer good lightning protection.
The legend is that antenna manufacturers have been claiming it for decades. The
fantasy is that some of them still actually believe it. But it's not all hopeless.
Here's how you can tell if your present antenna is one of these and what you can
do about it. Disconnect the transmission line at the equipment end and measure across
the center and outer conductors with a VOM on the R X 1 scale. If only a few ohms
are measured then the antenna at the other end is a DC Grounded type. If you're
satisfied with the performance of the antenna otherwise and wish to continue using
it then you have two choices. First, disconnect the antenna whenever a storm approaches
and hope you'll always be there to do it on time. Or second, install a blocking-type
lightning arrestor that will shunt center conductor voltage to ground while blocking
voltage from passing through the arrestor. Be sure to install the arrestor at ground
level and ground the body of the device well.
If your in the market for
an antenna and wish to enjoy a bit of protection select the ones offered
that use capacitor or link feed systems. Capacitor feed systems such
as gamma matches are excellent feed systems and lightning protectors as
well. They isolate the center conductor and force lightning into
the shield.
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