[TenTec] A grounding question

[Dr. Gerald N. Johnson]

On Sun, 2007-01-07 at 23:27 -0500, Mike Bryce wrote:
> Guys...
> 
> I've got a 8 foot copper ground rod hammered into the basement where  
> the shack lives. From that ground rod I have 2" wide copper braid  
> going to 1/2 inch wide  by 1/8" copper strips.
> 
> The various radios and tuners, power supplies and what nots are tied  
> to these copper strips with short hunks of braided copper. (shield  
> from old RG8 cables) The equipment is NOT daisy chained together.
> 
>  From what I have been reading, all of what I have described above is  
> NO good. Great for DC grounding, but zip for RF.
> 
> While surfing the net, I came across an article on Ameriton's site  
> that says braided copper should be avoided at all costs as this makes  
> a really crappy RF ground.
> 
> They recommend at least 2" wide solid copper strips. Siting the "skin  
> effect"
> 
> I was always under the idea that skin effect only occurs on  
> frequencies higher than 30 MHz.

Sink effect occurs at 1 Hz and below. Just the skin depth is deeper than
most practical conductors. My fourth edition Reference Data for Radio
Engineers says skin depth at 100 MHz is 0.00035 inch. Since its
inversely proportional to the square root of frequency, dropping to 1
MHz should increase the depth a factor of 10 to 0.0035 inch. Skin depth
doesn't mean there's full current to that depth and none deeper, just
that to that depth 63% of the total current flows, more closer to the
surface, less deeper. Falls off exponentially (same function as the
current through a resistor charging a capacitor) as you go deeper.

So with 30 gauge strands in the braid, about 0.010 inch diameter at 1
MHz the strands are only 3 skin depths and the resistance is still
higher than a flat strap of the same width. And your coax braids aren't
as wide as their suggested 2" flat strap. So the flat strap has both
lower resistance and lower inductance, both handy when trying to get an
RF ground.

That rod through the basement floor isn't a great help at RF, but its
better than nothing. A wide strap to the service panel would help in
lightning protection (say 18" wide strap!).

The biggest argument in favor of the braid is that sometimes equipment
needs to move and the wide straps unless folded inventively tend to
prevent that equipment motion and, if too stiff, to break off the small
grounding screws sometimes provided on the equipment rear panel. A few
inches of braid may be a more practical compromise than breaking the
equipment or tying it down in an inconvenient position.

Two braids spaced a few inches apart will help lower the inductance
nearly to that of a wide strap. Just something to think about. An A
shaped pair spaced several inches apart on the main station ground strap
would have lower inductance than maybe a 2" strap, but wouldn't carry as
much lightning or DC power supply short circuit current in the steady
state.


> 
> So, what do the experts say??
> 
> mike
> 
> Mike, WB8VGE


-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer