Re: [TowerTalk] Station Ground

[Jim Lux <jimlux@earthlink.net>]

At 02:55 PM 1/14/2005, Jim Brown wrote:
> On Fri, 14 Jan 2005 14:37:52 -0800 (PST), N6KJ wrote:
>
> >Why do you need "beefy" connections from bus-bar
> >to ground when you use much smaller wire
> >connecting the equipment to the bus-bar?
>
> It's all part of fooling ourselves about the basic physics. The
> impedance of a conductor at RF is FAR more dependent on its
> INDUCTANCE than upon its resistance.  So the only value of that
> "beefy" wire is that it might take a microsecond longer to melt in
> the case of a direct lightning hit. The smart money is on wide
> copper strap, not braid, for lower inductance and longer life.
>
> There's a discussion of some of this in a power and grounding white
> paper on my business's website.
>
> http://audiosystemsgroup.com/SurgeXPowerGround.pdf
>
> Jim Brown  K9YC

One hopes that you're not depending on the chassis ground connection of 
your radio to carry the lightning surge.  Presumably, you've got some sort 
of transient suppression/lightning arrestor/other path for the lightning 
transient to take to ground, which DOES need a short low inductance path.


However, I think that the "ground lug" on amateur equipment is more in the 
nature of a "equipment ground"  which is not expected to carry any 
significant RF power.  It's sort of an open question whether it could be 
considered a "technical ground" (as described in your white paper).  Since 
grounded, exposed metal connectors are the norm in the RF world, and I 
suspect that not much ham gear uses balanced audio with galvanic isolation 
from chassis, separate chassis grounds might cause more problems than they 
solve.

(e.g. the "starter motor current through the radio" problem in mobile 
installations)

However, I'm open to discussion.  If someone has a credible scenario where 
the chassis ground, in ham equipment, is going to be asked to carry 
significant RF power (where inductance would make a difference), let's roll 
it out and take a look.

I'll be the first to admit that effective grounding system design is a 
challenge.

Jim, W6RMK 


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