[HD RECEPTION]
Antenna Grounding
Do I need to ground my outdoor antenna?
Tip by Don Landis: Offering advice to
NOT ground outside antennas is irresponsible, contrary to all known laws of physics on the
subject, and civil laws and codes pertaining to static charge buildup, and, down right
criminal!
There are three types of grounds. In order of difficulty to achieve, they are from most
to least: static; RF; and electrical. The methods and practices used to achieve each of
these is different. Static grounding is the most difficult to do with guaranteed results,
while electrical grounding is easy and can be considered to work perfectly in most cases.
RF and electrical grounding is not important nor considered in this discussion for static
discharge (lightning) safety.
The general rule on outside antennas is that they need to be grounded. There is no
known government agency that recommends antennas not be grounded for safety. Indeed, the
opposite is what is recommended and in most cases required by local codes. And, if you
don't do this and don't care, maybe knowing that violation of these codes may be grounds
for rendering your home owners insurance null and void in case of a fire caused by
lightning strike.
Direct and secondary static electricity (lightning) strike has strike probability
increased by a buildup of static charge at points of conductivity such as a metal mast or
pole of an outdoor antenna. Static electricity is built up during a thunderstorm with wind
blowing over the metal structures. This static charge builds and becomes an attractor to
the opposite charge of static build up in the storm clouds. By draining off the static
charge continuously, you reduce the probability of strike because the potential difference
is reduced. It was shown in the studies conducted on the Empire State building that
probability of a strike was a direct relation between the quantity of static buildup on
conductive structures. Conductive structures with no ground path were at the highest risk
while structures that were intensely grounded over several contact points were the least
risk. This is because it is actually rather difficult to completely eliminate all corona
point ( sharp pointed shaped geometry in the metal structure) static charge buildup even
with "good" grounding on multiple contacts of the metal structure.
In an antenna you can ground the mast, the boom, the dish, the director and reflectors
of the antenna by contact metal bonding to a ground wire run directly into the earth via a
deep ground rod, but you cannot directly ground the driven element or active element of
the antenna. All you can do is make a reasonable attempt at grounding it via a special
coax grounding block to reduce static charge buildup and reduce probability of a hit. In a
Log Periodic yagi most of the elements are driven so that most of the antenna structure
can not be grounded directly. In this case it is wise to use a coax grounding block. This
block is a device designed to bleed off high voltage spikes that reach dangerous levels
that would damage your receiving equipment. They don't directly short out the center
conductor to the ground because this would kill the signal but rather allow a small gap
that will on a continuous basis bleed off the building static charge before it reaches
dangerous damaging levels to your equipment. Using one of these grounding blocks located
just before the coax feed wire enters your building is what is recommended to effectively
reduce the probability of small static electricity damage to your receiver. It will NOT
protect against a direct lightning hit. Both the grounding block on the coax and a direct
ground wire to the mast of the antenna should be used.
What are the damage risks? In a simple static charge buildup the minor hits you will
get will be silent killers. These tiny hits will be damaging to your receivers RF front
end. It will most likely short out sensitive IC's and diodes in the receivers rendering
them useless. In the next worse case you take a secondary hit where the direct hit struck
a tree or utility pole near by. Now you may see some signs of obvious visible damage such
as the house wiring in your house catching on fire or your TV set getting fried right
before your eyes. This happens far less than the hidden damage hit. Finally we have the
rarest type of hit which is a primary direct lightning bolt strike to your antenna and or
house. In this case your antenna and house was struck with the main bolt. Usually this
will cause major fire damage to your dwelling and contents. Fortunately these hits are
rare except in places like the open farm lands where the house structure is the only
corona point sticking up out of the flat ground for miles around and it looks like a big
static attractor to the thunder cloud. In areas where you are surrounded by trees and
other structures your odds of a direct hit are much reduced but you are still at risk for
the secondary hit and the silent static killer.
One very important thing to know about grounding. Having it present will NOT protect
you if you take a direct or even a secondary hit. What it does, is reduce the probability
of getting hit in the first place and it provides much better protection against the
damaged caused by the silent hits which is the most common of all static electricity
damage. Many misunderstand this important point and I cannot repeat this point too often:
The grounding also helps eliminate the small silent killer hits. But if you have
conditions that are very severe and your grounding is less than ideal (most systems are)
then when you take that direct hit, having the antenna grounded or not won't matter much
at all. That little 12 Ga. copper or aluminum ground wire to your 10 ft. ground rod won't
drain a 50 billion volt lightning bolt where the spark diameter may be as big as a foot
across with multiple branches that encage your whole house for 2-3 seconds. Again, the
idea of the grounding is to reduce the probability of getting hit in the first place and
to continuously bleed off small static charges to prevent the silent killer to your
equipment.
This is a good timely discussion as many of us are returning to outdoor VHF and UHF
antennas this year to receive the local HDTV terrestrial broadcasts and we are beginning
the Thunderstorm season throughout the country. It is important to understand the hazards
of outside antenna structures, especially during a thunderstorm..
Don Landis, licensed broadcast TV
consultant, is the owner of a small TV production company, specializing in producing TV
commercials and 30 minute cable TV shows. Licensed amateur radio operator since 1964 and
Licensed FCC First Class in 1970.
mailto:dlandis@scubatech.com
http://www.scubatech.com
©2002 Scuba Tech Inc.
|
