[TowerTalk] Grounding mast?
RadioIR at charter.net
Wed Jul 30 09:36:46 EDT 2008
N2EA wrote: ...Points out lack of conductivity across rotors, and
asserts 300-500V per meter above ground as the possible potential
Did you mean across the balun, or between floating elements and the
boom? I can't imagine anyone getting static build up across motor
bearings (and thrust bearings). Who says that there is lack of
conductivity across rotator bearings? Reference please.
N2EA wrote: ...."The discussion recommends a positive ground between the
mast, and the tower, as well as a good dissipative ground for the tower
itself, as lightning protection would dictate."....
Logic should tell you that the impedance of any strap long enough the
handle rotation placed across a rotator will be orders of magnitude
greater than the impedance thru the bearing and won't change the current
thru the rotator significantly.
I always thought that the connection at a thrust bearing at the top of a
tower should shunt off most of the current due to a lightning strike.
That is, until I did a simulation to prove it. The results of the
simulation was that the legs of the tower and the mast operate much like
a large leaky transmission line. The tower is open enough to NOT keep
the fields on the outer surface of the tower legs. The currents flow
down the legs on both inner and outer surfaces (sides next to the inside
of the tower). This causes large currents to be induced into the mast
flowing in the opposite direction from the currents in the tower legs.
There was more current in the mast than current in two legs of the
tower. Of course the magnitudes vary with frequency.
Simulation of a strap placed across a rotator with good bearing
connections showed insignificant current in the strap. Also a
simulation using a rotator with a gap across its bearings, showed that a
strap would not keep the voltage across the bearings low enough to
prevent them from arcing (and welding). In short, I don't think there
is ANY way of keeping lightning currents out of a rotator (short of
making the top of the tower a solid enclosure).
jim Jarvis wrote:
>Been following the SteppIR v. Stepper.IT thread while travelling.
>Had a few minutes to browse both websites, this morning,
>and came across a short commentary about lightning and ESD protection
>for the steppIR.
>It raises the question of static charge buildup due to wind motion,
>in (one assumes)
>drier climates. Points out lack of conductivity across rotors, and
>asserts 300-500V per meter
>above ground as the possible potential buildup. Routed into the
>shack, if the charge isn't
>The discussion recommends a positive ground between the mast, and the
>tower, as well
>as a good dissipative ground for the tower itself, as lightning
>protection would dictate.
>The physics of this are real. What amazes me is that this is the
>first time I'm seeing the
>discussion, in these terms. Does anyone actively ground their mast,
>across the rotor?
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