[Elecraft] static buildup on antennas

[gary bartlett]

Can someone comment on this issue as it relates to balanced wire antennas
like dipoles fed with ladder line and matched to a K3 receiver through a
balun?  Is the problem with discharges from precipitation and other static
electricity sources with that feed configuration the same as for coax feed?
(Yes, I recognize, lightning would be a different issue.)   For that matter,
how does the K3 transmatch figure into the equation in terms of protecting
the receiver from unfriendly charge build-ups (both when the radio is
powered up and when powered off)?  

If the concerns are the same for both types of transmission line, what is
the protection configuration external to the radio for the ladderline
situation?

73,
Gary VE1RGB

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Don Latham
Sent: September 10, 2012 2:35 PM
To: Elecraft at mailman.qth.net
Subject: [Elecraft] static buildup on antennas

An antenna can get charged up in different ways. First, the earth has a
fair-weather electric field of about 100 v/m near the ground. This field
has a very high source impedance, so most things simply look like short
circuits. If, however, an antenna is left disconnected that is not
shorted, it will come to equilibrium with this static field and charge
up according to its capacitance. The charge will probably bleed off, but
not always. During high winds, especially with dust or snow, or even
rain, the charge on the antenna can become very much larger. The reason
for this is that not only can the particles, charged by friction,
transfer charge to the antenna, but also particles leaving a conductor
in the presence of a static field carry charge away from the conductor,
leaving the opposite charge on the conductor. This is why the exhaust of
a helicopter or aircraft can charge it up.
Because antennas are raised above the ground, the fair weather electric
fields can reach high values at the top of towers or other supports, and
the horizontal elements of wire antennas as well. For example, a 30 m
tower can have a field near its top of 3000 v/m. In addition, there is a
shape factor that can raise this value even further. Then, any sharp
point can go into corona discharge, so small currents will flow. This
means any wire ends not covered with tape or blobs of insulating goop
will generate small steady currents.
All of these static effects can easily be taken care of by simply
putting a 5k resistor to ground at the antenna terminations, either at
the antenna itself or at switching points, etc. A good friend and I have
K2's and suffered routine blowouts of the swr sensor diodes for these
reasons until we put 1/2 w 5k resistors on the inputs. no more problems.
During a thunderstorm, the static electric fields near the ground can
get as high as 5000 v/m or even more, depending on the arrangement of
the clouds with respect to your location. This means that the buildup of
charge on antennas as described above can reach really high values.
However, this static buildup will still be sufficiently taken care of by
the resistance trick as above.
Now we come to the other problem, induced pulses from lightning. As you
all know, the lightning discharge, whether in cloud or to ground is from
our point of view a huge antenna. The radiation peaks at about 1-5 MHz,
but there are components for various reasons well into the VHF region.
Proper antenna mast and antenna treatments can be found in the
literature so I won't go into that. Find some good articles and follow
their advice. One main thing to remember is to keep all mitigating
switches, arc points, and their grounding stuff outside the shack. In
case of a direct hit to any part of the antenna system, you don't want
it inside!

I hope this helps somewhat. More on the earth's fair weather field can
no doubt be found online.
73, Don


-- 
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
De Erroribus Medicorum, R. Bacon, 13th century.
"If you don't know what it is, don't poke it."
Ghost in the Shell


Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com


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