[RFI] HF Mobile Installation in 2001 Mazda

[Tom Rauch]

> > as long as the radio ground
> > point has low resistance continuity to the battery
negative
> > terminal ground point.
>
> Isn't that the reason why the recommendation to connect
directly to the
> negative battery terminal was made in the first place? To
ensure a low
> voltage drop at high currents?

What everyone seems to fail to consider is the vehicle
chassis itself is always less resistance than the long lead
from the radio ground to the battery post, and what happens
if the ground from the chassis to the battery opens up or
develops a high resistance, or if the battery positive
terminal faults to the vehicle chassis.

If a high resistance connection from the battery to ground
develops when the rig negative lead is connected to the
vehicle chassis, the rig simply sees less voltage.

If a ground connection fails and the radio is grounded to
the battery negative post at the battery, all the current
from the battery will flow through the radio's negative lead
through the radio to whatever the radio is grounded to. Now
I admit you can fuse the negative lead to prevent fires, but
it may not always protect things in that path from
electrical damage before the fuse opens.

On the other hand if you simply grounded the radio's
negative lead to the vehicle chassis (if you can't tell a
door from part of the unibody you could take the lead to the
inner fender near where the battery is grounded to the
chassis) no such problem exists, and things are much safer
for the radio.

I don't think there are very many people
> out there that measure the resistance between the negative
battery lead
> and the point on the chassis where they intend to ground
the radio.

Why would they need to? The goal is to not have potential
across the internals of the radio if a connection from the
battery clamp to  chassis opens. If you aren't sure what
plastic is and what a door or other non-chassis part is you
probably shouldn't be doing the install.

> > That's almost anywhere in a unibody
> > chassis.
>
> Yes, when the car is new. After a few years that can be
different,
> however. Been there, done that.

Oh come on now Al.  Are you actually trying to tell me the
chassis of your unibody vehicle develops a high resistance
path from major sheet metal components back to the inner
fenders?  The only case of that I can think of is if Amtrak
split the rest of the body in half at a RR crossing!

If your vehicle is so bad you can't depend on welded sheet
metal making a connection to the inner fenders or radiator
support (where ever the battery is grounded) it should be
scrapped.  If the argument is you can't maintain a solid
connection to major sheetmetal, then you sure as heck better
not be trying to make connections to a corrosive area like
the battery posts!!

Arguments for connecting directly to the battery post so far
have been extremely lame (at best). It certainly does not
improve RF characteristics, and it is a potential disaster
for DC. This nonsense all started because many mobile radios
used to have provisions for use in negative or positive
grounds, and those were very common radios and the most
popular radios installed in vehicles (Motorola and GE two
ways for example). In that case the negative lead floated,
and there were no harmful situations that would or could
arise from a fault in vehicle wiring.

When the negative lead is tied into the coax jack, mic
leads, key jack, speaker jack, and radio case you are just
asking for ground loop problems if the negative radio lead
goes anywhere but to the vehicle chassis. It certainly won't
make anything better running it to the battery terminal.

73 Tom