Keith,
Here's a paragraph from the Times Microwave systems article/brochure on
"Grounding and Lightning Protection for Wireless Networks", pg 10.
"Most of the lightning energy goes down the tower to earth with current divided
between the entry
panel ground system and the earth ground connection. If coaxial cables on the
tower were turned
towards the entry panel at a lower point on the tower and shields were bonded
to the tower there, less
potential and current flow would be applied to the entry panel and master
ground bar. Always
direct cables to the entry panel at the lowest practical location on the
tower." There's a drawing of the potential curve along the height of the tower
for illustration.
I plan to bury my cable in conduit, metal is better than plastic, and have poly
phasers at the entrance using a copper panel like you have. A buried metal
sheath conduit for the coax run may act like a distributed capacitance that can
offer some shunting to the high frequency lightning impulse on the coax shield.
Reading the article indicates the lightning pulse is higher on the shield,
velocity factor difference, therefore "grounding kits" should be placed on each
coax run at the top and bottom of the tower (for 100 foot tower), recommended
tower/shield grounding is on page 8 of that article.
Page 7 seems to warn against placing coax feeds above the tower base here:
"If the bottom coaxial cable ground kit (where the coaxial cable leaves the
tower) is at any elevation
above the earth, the overall inductance to earth of the tower below the ground
kit, the series
inductance of coaxial cables to the building, and the paralleled entry panel
ground conductors, is
sufficient to cause a substantial peak voltage. The resulting voltage on the
coax shield will drive current
to the equipment where the electrical safety ground provides a path to ground
through the
equipment chassis.", page 7.
My plan, in mid execution now, is to have an 8 foot ground rod for each leg of
the tower, these ground rods are tied together with #4 copper and tied to a
UFER ground built inside the tower foundation when the base was poured and
three extended 8 foot ground rods off each base ground rod, at least 16 feet
away from the base ground rod. These outside ground rods are tied together in a
star/ring configuration. All coax will be grounded at the top of the tower and
to this grounding system at the base before it enters the conduit to the
entrance panel, ~100 feet away. By burying the cable and it, in my case, being
100 feet away from the tower the lightning impulse should be diminished some by
the time the wave has traveled to the entrance panel.
Here's a link to the article/brochure:
http://www.timesmicrowave.com/documents/resources/protectbrochure.pdf
There's also an article by the author here with some of the same data and more
discussion.
http://www.timesmicrowave.com/products/protect/downloads/aglarticle.pdf
HTHs,
73,KenKD2KW_____All tyranny needs to gain a foothold is for people of good
conscience to remain silent.
Thomas Jefferson
He who sacrifices freedom for security deserves neither. Ben Franklin
"You do well to wish to learn our arts and ways of life, and above all, the
religion of Jesus Christ. These will make you a greater and happier people than
you are. Congress will do every thing they can to assist you in this wise
intention; and to tie the knot of friendship and union so fast, that nothing
shall ever be able to loose it."[28]
George Washington
“But those who hope in the LORD will renew their strength. They will soar on
wings like eagles; they will run and not grow weary, they will walk and not be
faint.” Isaiah 40:31 (NIV)
C. S. Lewis: "If you look for truth, you may find comfort in the end; if you
look for comfort you will not get either comfort or truth, only soft soap and
wishful thinking to begin, and in the end, despair." Jesus promised that "the
Spirit of truth" will "guide you into all truth" (John 16:13). Have you sought
his guidance yet today?
"Clothe yourselves, all of you, with humility toward one another, for God
opposes the proud but gives grace to the humble." 1 Peter 5:5
"O Lord, thou art my God; I will exalt thee, I will praise thy name; for thou
hast done wonderful things; thy counsels of old are faithfulness and truth.”
Isaiah 25:1
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