"mm" wrote in message
...
On a mail list, I asked why the ground on a Chrysler car radio was so
big. They use a woven, uninsulated, strap, that is more than a
quarter inch wide and 2 or 3 mm. thick, with heavy ring connectors at
each end, going from a post on the back of the radio's case to part of
the dash frame. This strap is far bigger than the sum of the 3 power
wires, for the built-in clock, the radio, and the dashboard
illumination.

But it must be important. When a friend came to me once with a
factory AM radio in an '84 Chrysler K-car with no ground wire other
than the one in the wiring harness, it gave a garbled sound, almost
impossible to listen to. After adding a 14 gauge stranded wire where
the ground strap went, the sound was fine.

This is the explanation someone replied with:
"It's an RF ground for the radio signal. If it were just DC ground
than it could be much smaller. But to get good radio signals when
there's only mico-volts of signal available in the first place you
want to have a very low impedance ground.

"And at RF frequencies the impedance of a conductor is more related to
the surface area of the wire than to the cross sectional area of the
copper, unlike DC current. Thus RF grounds are frequently braid or
thin/wide copper straps or some such."

I know that RF frequencesi travel on the surface, but other than that,
is there any truth to this?

And if so, why do none of the after-market radios that I've seen, or
other brands of original equipment radios afaicr, have a ground strap,
like Chryslers have had for decades.

Thanks.


The basic explanation is good. As to why the Chryslers particularly needed
it, is anyone's guess. There could be many reasons, such as where the
antenna is located with respect to the radio, the type of cable used for the
antenna, where the ' normal ' power ground is picked up, how far away from a
low impedance ( RF-wise ) point, the main and auxilliary power feeds are
picked up, how well the dash as a whole is grounded to the chassis of the
vehicle and so on.

RF is funny stuff, particularly in cars. I have had car radio installations,
particularly older AM ones, that have suffered from ignition pickup or
alternator pickup or odd noises, presumably from EMU electronics or similar,
that have been cured completely, by adding an earth strap like you are
describing. Sometimes, even moving the grounding point a foot from one side
of the radio, to a different grounding point on the other, can make the
difference. Doesn't seem to be such a problem with modern cars, though.
Perhaps they've designed the radios to be more immune to this sort of thing
now, with such large pieces of the dash being made of plastic.

Arfa