On 10/12/11 9:36 PM, Tony wrote:
> On 10/11/2011 9:28 AM, Jim Lux wrote:
>
>> AWG10 solid is also used for a lot of
>> things. If you want bigger (AWG2 or 2/0), then stranded is where it's
>> at, although I don't know why you would need something that big for a
>> tower (you see it where it needs to bridge across something moving: like
>> a gate or a tilt fixture)
> Hi Jim,
>
> So there's no advantage in grounding a tower with AGW 2/0 vs #10?
>
Mechanical ruggedness?
Check your local codes on what's required for lightning grounding, but
something in the AWG4 to AWG10 is typical. Lightning has high currents,
but they only last a few tens of microseconds.
NEC requires, as a general thing, AWG6 for bonding various grounds
together. They're mostly concerned about something like a power line
short to your structure. AWG 6 carries enough current to trip the
breakers.
RF grounding is a whole other issue. Then you're really talking about
part of your antenna system, and you need to address that.
A couple things to remember:
inductance is mostly about length. Diameter and surface area and such
have a very, very weak effect.
AC resistance is dominated by skin effect: diameter and surface area
have a huge effect.
The lightning case is dominated by inductance, so resistance isn't a big
deal.
The power line safety case is dominated by DC resistance, so cross
sectional area is the important thing.
RF ground for HF is dominated by AC resistance (inductance gets tuned
out in your matching arrangement), so that's where you want flat strap, etc.
AM broadcast towers use flat strap because they need low AC resistance,
and if it happens to be a decent lightning ground, all the better.
1/4 wavelength vertical antenna performance is sensitive to losses in
grounding: use strap/ribbon. (not the case for 1/2 wavelength verticals)
Your internal shack ground is mostly about DC resistance and partly
inductance. Strap doesn't buy you much from an electrical performance
standpoint, but it is easier to hook up to than something round.
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