[TowerTalk] Routing Coax and Control Lines

[Ken Mitchell]

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.
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