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[TowerTalk] Polyphaser inductance calculations & grounding

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Subject: [TowerTalk] Polyphaser inductance calculations & grounding
From: (Dan)
Date: Tue, 1 Jul 1997 21:08:39 -0400
Hi OM,s

While studying the Polyphaser "The Grounds for Lightning and EMP
Protection" (second edition, revised 1993), I have the following
observations & questions.

On page 3, there is an example of a 150 foot tower that has a coax line
with the shield bonded to the tower at the top and at the 15 ft point.  The
result is a 135 foot section of tower (36uH) in parallel with 135 feet of
coax (72uH).  Using the parallel inductance rule, the net inductance of
this section is 24uH.  The example goes on to calculate the peak voltage at
the top of the tower, etc.  Keep the use of the parallel inductance rule in
mind as the discussion continues.

On page 16, the case is made for using copper strap instead of wire for
connecting ground rods because the strap has lower inductance.  For
example: 10 feet of #6 AWG has an inductance of 4.4uH, while 10 feet of 3
inch copper strap has an inductance of 3.0uH.

Instead of using copper strap, why not use two #6 wires in parallel?
(Ignore the difficulty of making connections to the wires.)   I'm aware
that the strap has more surface area than the wire, and that "skin effect"
is a factor to be considered--but Polyphaser (if I'm reading it correctly)
suggests that the primary issue here is inductance.  Using the parallel
inductance rule, two #6 wires in parallel would have less inductance than 6
inch wide strap!  How much separation would be required between wires for
the parallel inductance rule to be valid?  (In the tower/coax example
above, there is very little separation.)

I'm not suggesting that anyone use parallel wires for grounding instead of
strap.  I'm asking for help in understanding this technical question and
how it relates to the practical use of grounding materials.

73 & thanks!

Dan Long (W4TQ)
8035 Tokyo Pt.
Dunnellon, FL 34433
phone: 352-563-0934

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