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Author Topic: That Pesky Ground In A Car  (Read 5964 times)
KE4DRN
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« Reply #15 on: September 20, 2011, 07:38:52 PM »

hi,

one of our club members says

'copper is better electrical conductor then the steel in your car'

73 james
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W9PMZ
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« Reply #16 on: September 21, 2011, 04:55:57 AM »

'copper is better electrical conductor then the steel in your car'

Exactly!!!

This is why when you add the negative lead back to either the ground point from the battery or the batteries negative post you have created a ground loop!

73,

Carl - W9PMZ
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K0BG
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« Reply #17 on: September 21, 2011, 06:46:16 AM »

That's just not true.
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K7RBW
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« Reply #18 on: September 21, 2011, 06:53:02 AM »

hi,

one of our club members says

'copper is better electrical conductor then the steel in your car'

73 james
All other things being equal, perhaps. But, in a car, you have a lot more steel running in parallel to the battery than you would in a 10ga or an 8ga (or even larger) wire giving steel the advantage in the automotive application.

The problem with using the car chassis as your return is that you really don't know what path the current is taking and how vulnerable it might be. Is it passing through places that could rust out or lose their conductivity causing the the current to prefer other, less desirable paths, for example? With a wire, the variables are reduced considerably.

In my new car, the car's chassis offers less resistance than a 10-ga wire (probably larger wires as well, but I haven't tested that). 10 years from now (if I still have the car), it's hard to say if that will still be true. I'm thinking it would, given all the internal parts of a modern car that rely on a good ground, but it's a lot harder to know for sure than it would be with just a wire.
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W9PMZ
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« Reply #19 on: September 21, 2011, 07:51:44 AM »

"That's just not true."

why?  if you have two points, one being the negative lead with lower resistance to the same point 10 feet away and the chassis has .1 ohm more resistance to where the radio is, it will be at a different potential, due to the resistance.  Isn't this a "ground loop"?
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K1CJS
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« Reply #20 on: September 21, 2011, 12:19:26 PM »

Avoidance of ground loops primarily has to do with RF noise being somehow injected into a coax cable.  They have nothing to do with a car electrical system.

If you were to include all ground electrical connections in a car into the catagory of ground loops, there would be so many ground loops in a car--even a 1960 vintage car, let alone a modern car--that you couldn't count them all.
« Last Edit: September 21, 2011, 12:22:29 PM by K1CJS » Logged
K7RBW
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« Reply #21 on: September 21, 2011, 07:14:48 PM »

why?  if you have two points, one being the negative lead with lower resistance to the same point 10 feet away and the chassis has .1 ohm more resistance to where the radio is, it will be at a different potential, due to the resistance.  Isn't this a "ground loop"?
You'd have a difference of potential until you connected them together. At that point they'd just be parallel paths.
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K0JEG
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« Reply #22 on: September 24, 2011, 11:37:09 AM »

I just installed a dual band mobile in a 2011 F-150. The battery terminals actually had connection points for a radio, both on the positive and negative terminals. I don't know if that's part of the towing kit or not, but there it is.

That fact, and the Ford installation guide I found on K0BG's site, tells me to use a negative return wire (fused) back to the battery.
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N6AJR
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« Reply #23 on: September 26, 2011, 11:51:25 AM »

Another though is that a car is not a stationary ground plane.  It is a vehicle, and moves and twists and torques while running down the road.  This can cause variation s in the path returning to the "other side " of the battery, as well as possibly even generating noise between different parts and metals.

I already mentioned my trunk lid mounted antenna picket fencing at speed. At rest it was in good contact with the hinges on the trunk lid and worked fine. At speed the trunk lid  floated up and down do to the air path over it and also the bumps in the road.  By running a ground strap from the  shield side of the antenna mount it gave the  trunk lid continuity with the chassis and the picket fencing went away.

  I also ground the radio to the chassis at the radio and the battery to the chassis at the neg post of the battery.  This seems to make the  negative potential the same across the antenna, car and radio, and helped the radio see the whole car as a ground plane.  At least that's the way I figure it.  I have done this on most all of my installs and  it works when I do, and not so well if I don't.  I am not an expert, but if it works, it works.
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