blckhole at aol.com
blckhole at aol.com
Mon Apr 18 12:21:31 EDT 1994
I quote from a recent PolyPhaser article on "Ham Station Grounding".
"Since the tower is a conductor and is well grounded, all of the coax lines
should be grounded (using a grounding kit) at the top of the tower close to
the antenna and at the base of the tower before they come toward your
equipment. For taller towers, groundign should occur at 150 foot intervals
from the top, plus at the bottom.
During the strike event, the tower and the coax lines will mutually share the
strike energy. If the coax lines are not grounded as they leave the tower or
are completely isolated from the tower, more energy could traverse the coax
toward your equipment then is conducted to the ground system by the tower.
Such a large inductive voltage drop may cause arcing between the coax lines
and the tower which in turn could cause deterioration (pin holes in the coax
for moisture to enter) or destruction of the coax lines.
Notice the 'bottom' in this section. Since all towers have some inductance,
leaving the tower at some point above ground will allow some of the strike
current to continue on the coax line (both the center conductor and sheild)
toward your equipment. Once at the equipment, the current will follow the
chassis to the safty ground. This will elevate the equipment cabinets to
deadly voltages. Deadly for both people and components.
Even though inductive properties of the coax cable appear to be beneficial,
and some extra inductance can be created by adding a few turns to the coax:
don't do it. The added turns can also act like an air wound transformer
which can couple more energy into the line. This is just the opposite of the
desired effect. Instead make sure that the coax lines leving the tower
remain at right angles to the magnetic field surrounding the tower."
(PolyPhaser Corporation STRIKING NEWS, Feb 1994)
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