----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Jim Brown" <jim@audiosystemsgroup.com>; "Tower Talk List"
<towertalk@contesting.com>; "Jim Lux" <jimlux@earthlink.net>
Sent: Friday, January 14, 2005 4:54 PM
Subject: Re: [TowerTalk] Station Ground
> > The classic example is a mobile rig, with the -12V wire
> going to the
> > battery, and the rig grounded to the car chassis at some
> point via the
> > antenna coax. The chassis bonding gets corroded, so now
> there's a
> > potential difference between parts of the chassis due to,
> for instance,
> > headlights or starter. The radio becomes a low impedance
> connection
> > between the two parts of the chassis.
>
> Headlights aren't that high in current.
130 W plus isn't unusual these days. That's 10A, a substantial current,
especially if it's being carried by a "sneak path".
>
> Starters and alternators ALWAYS ground to the engine block,
> as does the battery.
In theory... Interestingly, my old Datsun 280Z has a ground cable from
starter attachment bolt to battery (and that's the only way the block is
grounded, because the motor mounts are rubber). And, another separate ground
wire to the chassis.
My wife's Maxima had a similar arrangement.
We won't even get into wiring diagrams of British cars, progeny of Lucas,
the prince of darkness.
>
> Vehicle chassis are almost always unibody.
The main body might be unibody, but panels (i.e. fenders) aren't. And, of
course, there are cars made that have separate bodies on frames
(particularly trucks and truck based vehicles like some SUVs).
>
> You can avoid ANY issue with a ground by ground the radio to
> the chassis and NOT the battery. Grounding to the battery
> post is dumb.
Not entirely dumb. If you want to keep the supply and return wires side by
side for EMI/EMC, you might want to connect them at the battery. What's
dumb is having two ground paths.
>
> Houses generally don't have a battery, alternator,
> headlights, and starters. If they do, the real solution is
> proper wiring, not adding ground resistance.
The original question was about 12VDC powered equipment, and asked for an
example of where dual paths could create a problem.
>
>
> > There are also scenarios where the starter motor has an
> explicit ground
> > wire that comes loose. Likewise, the alternator.
>
> Not true. Alternators and starters NEVER have ground wires.
> They are commoned to the engine block via mounting brackets,
> and the battery high current lead is grounded to the block.
Not always. That may be the "intended" ground path, but, it's not unheard of
for that high current lead from battery to block to fail, and the ground
path to be: battery, frame ground, some mystery path, block, starter. If
your radio is the "mystery path" you're in bad shape.
With modern engine control units and coil per plug ignitions, fuel
injection, etc., I wouldn't make any particular assumptions about the wiring
diagram. My old 280Z has an entirely separate harness for the EFI, grounded
I know not where, but probably somewhere troublesome.
>
> Bad wiring techniques are not corrected by installing an
> intentionally bad ground. After all, radios require
> feedlines and the shield is BIG and it should be well
> grounded.
Certainly I agree, and I'd never advocate a deliberately bad ground. My
contention is that there can be unintended side effects from "install a
really good chassis ground on that box", and you shouldn't just wire it up
any old way.
Jim, W6RMK
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