> If you look at the ARRL RFI Handbook, it excerpts 2-way
radio
> install recommendations from the big 3 automakers. They
all
> suggest running a negative lead back to the battery, but
they
> disagree about how to terminate the lead. One suggests
> connecting it to the battery with a fuse, while another
suggests
> that this is a bad idea because if the fuse blows the
radio DC
> return current will be forced through the antenna coax
braid.
> Instead they suggest connecting the negative lead to the
battery
> to chassis connection point that sits adjacent to the
battery.
The best configuration, assuming the installer is somehow
not able to figure out what is a good connection to the
unibody, is to run a lead to the sheet metal right next to
where the negative battery lead is connected to the vehicle
chassis. The reason for this is if something opens up, you
won't subject the radio's internals AND the coax to full
negative battery line current that runs ALL the accessories
PLUS your radio. Anyone who understands basic electricity
can draw the circuit out on paper and see why that is true.
The second best thing to do would be to connect to the
battery minus and use a proper fuse. As for warnings that
full radio current might flow through the coax if the fuse
opens, that's pretty much a "nonsense worry" unless you are
using RG174 to feed the antenna. First, our radios don't
draw enough current to be a worry with even something as
small as RG58. Second, the coax and other multiple grounds
are in PARALLEL with the negative fuse. The positive fuse
handles the entire current. If they are the same size and
you blow the negative fuse, you should be very happy. That
means the negative fuse probably protected the radio
internals from a ground lead over-current caused by a bad
battery connection.
I know it might be tough to swallow, but we have so many
ground loops and paths from the radio to the body and the
body has such low resistance.... connecting to the battery
actually aggravates ground loops that often inject low
frequency noise into the radio. If the radio floated, say
something like a FM radio on a mag mount, or if the ground
connections were all poor, or if the radio had a floating
power return then that's another issue. In that case a
battery connection causes no harm, and can reliably do some
good.
This stuff isn't that hard to look at on paper.
As far as automotive manufacturers knowing about noise
suppression and how our radios work and interface with
automobiles, I don't have a lot of faith in that concept. If
they actually knew what they were doing and if the problem
was conducted noise, I wouldn't measure radiated emissions
dozens of feet from the vehicle and our rigs would still be
noisy on a dummy load instead of just on antennas.
73 Tom
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