# [TowerTalk] Back of desk grounding buss

Gary Schafer garyschafer at comcast.net
Sat Mar 22 12:04:22 EDT 2014

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> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
> Jim Lux
>
> Wide flat strips have low AC *resistance* because of skin effect, but
> the inductance isn't much different from a round wire.
>
> AC resistance is *very important* in applications like RF grounds for
> commercial broadcast antennas, because resistance = heat = lost power =
> lost money, so they use flat ribbon.

But lightning energy peaks around 1 MHz where low AC resistance is
important.

>
> Flat ribbon/strip has a high surface area to volume ratio, and AC
> resistance is all about surface area for RF because of skin effect, so
> if you're paying by the pound for the copper, it's the best deal.
>
> Inductance just isn't strongly affected by the shape of the conductor.
> The NBS monograph by Rosa (from 1907, it's one of the first ones
> published) has all the formulas.
>
> Flat strips don't have markedly lower inductance for a fairly simple
> reason..
>
> Consider your ribbon as a bunch of parallel wires.  Each of those wires
> has some inductance L, and you'd think that putting N inductors with
> inductance L in parallel would give you an inductance of L/N.  But the
> problem is that those wires are right next to each other, so they have a
> significant mutual inductance (the magnetic field of wire #4 is tightly
> coupled to wires #3 and #5 next to it, etc.).  That tight coupling means
> that the inductance of the parallel combination just isn't that much
> lower than of one wire.
>
> The inductance of two parallel inductors is:
> (L+M)/2
> where L is the inductance of a single inductor and M is the mutual
> inductance.

Yes flat strap has mutual inductance across its width but isn't mutual
inductance considerably lower with a flat strap than separate parallel
wires.

73
Gary  K4FMX

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