Why is thick braided ground needed on some car radios?

Question

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.

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Caesar Valenti · Accepted Answer

The explanation you previously received seems pretty much  dead on.....go with it.cvmm wrote:

Arfa Daily · Answer

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

Michael A. Terrell · Answer

The dash is mostly plastic, so the only ground is through the coax to the antenna, or through the speaker wiring. the varying current across the higher resistance ground changes the voltage drop between the radio and the car's chassis, which affects the radio's performance. I have seen several volts between the radio case and the chassis on some radios. Some car makers used a heavy steel bracket from the rear of the radio's case to the firewall to insure proper grounding.

Why is thick braided ground needed on some car radios?

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