[Elecraft] OT: Grounding Question

[Jef Allbright]

At very low frequencies, where the ratio of skin depth to conductor
thickness is significant, then the inductance from the flux within the flat
conductor will tend to confine current to the edges.  In such a case, a
circular cross-section conductor will present less impedance.

The vast majority of the energy in a lightning strike is well below 1 MHz
and at 125 Hz the 100 effective skin depth is nearly a half inch.

As a practical example, following the R56 industry standard for grounding
and lightning protection for radio towers and their equipment shelters, you
will find a round, usually 2-gauge ground "halo" circling the walls at a
short distance from the ceiling.  Strap is NOT used.

On the other hand, grounding at HF frequencies, where several skin depths
can be easily achieved with copper strap (0.005 inch at about 1 MHz) then
thin flat strap or hollow tubing is the way to go.

- Jef  N5JEF



On Wed, Apr 25, 2018 at 12:27 PM, hawley, charles j jr <
c-hawley at illinois.edu> wrote:

> I gave the reference. 4th edition of Terman, Electronic and Radio
> Engineering. Page 22. The chapter on skin effect. The rf current is pushed
> to the outside edge of any conductor because the rf current is distributed
> in such a way as to make most of it flow where it is encircled by the
> smallest number of flux lines. And that’s the outside edge of any
> conductor. The rf current in the center obviously would be encircled the
> most. Terman gives a very clear description if you can source his text. He
> states that it is not the amount of conductor surface, but rather the way
> in which the conductor material is arranged. And I do realize that this is
> a very misunderstood subject. I’m not saying that flat ribbon doesn’t have
> skin effect, but just where the rf flows. Much of the outside surface in
> the middle of the ribbon has a high resistance to rf. So if you are using a
> ribbon of wire, it would have to be much larger than a round wire to have
> any advantage.
> I have the Radio Engineers Handbook also and it does not have the same
> chapter on skin effect.
>
> Chuck KE9UW
> c-hawley at illinois.edu
>
> Sent from my iPad
>
> > On Apr 25, 2018, at 1:34 PM, Josh <josh at voodoolab.com> wrote:
> >
> > You're going to have to explain that to a whole lot of high current flat
> wound inductors & transformers.
> >
> > What are you referencing? The only Terman I have handy is Radio
> Engineer's Handbook.
> >
> > 73
> > Josh W6XU
> >
> > Sent from my mobile device
> >
> >> On Apr 25, 2018, at 11:17 AM, hawley, charles j jr <
> c-hawley at illinois.edu> wrote:
> >>
> >> RF does not flow on the entire surface of flat copper tape (Electronic
> and Radio Engineering by Frederick E. Terman 4th Edition, p 22). The RF
> current only flows on the outside edges of the strip, not on the middle
> outside surfaces. Think of looking at the end of a longitudinal slice out
> of a solid copper rod.
> >>
> >> Chuck KE9UW
> >> c-hawley at illinois.edu
> >>
> >> Sent from my iPad
> >>
> >>> On Apr 25, 2018, at 12:32 PM, Fred Jensen <k6dgw at foothill.net> wrote:
> >>>
> >>> Sounds like you're following the rules.  You might consider flat
> copper tape instead of wire for bonding. Lightning is an RF event and
> currents will be confined to the outside surfaces of the conductors.
> Consequently, the surface area rather than volume of the conductor is what
> matters most. The conductors on one of the original transmission lines from
> Hoover Dam to Los Angeles were hollow.
> >>>
> >>>
> >
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