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Old 06-04-2010, 03:13 PM
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DaveInDenver DaveInDenver is offline
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Location: Larimer County
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Quote:
Originally Posted by rhyary View Post
Dave,
Thanks for your explanation but I need to convert this to specifics:
Bare with me:

1. my bumper is connected with bolts to the frame. Is this good enough RF grounding?
Without testing it you can't say absolutely, but probably not ideal. You have to picture that a capacitor is just two conductors with significant length parallel held electrically insulated by a dielectric. IOW, two painted or powdercoated chunks of steel in close proximity are pretty much exactly a capacitor.

When you put bolts (particularly smaller diameter ones) through, which are generally fairly poor RF conductors because of corrosion mostly, all you are doing is making one huge capacitor into a few just big ones wired in parallel, which is not different in magnitude than the one huge one (caps work electrically opposite to resistors and inductors, parallel is additive, series is subtractive).

What the braid does is provide a good path to alternating current (e.g. RF) that shorts the capacitor(s) and so no charge can build up, negating the bulk of the parasitic reactance. If you poked enough holes and filled them with bolts you would start to make the connection better for RF, but the ultimate goal would be to have a few tens of bolts, which would impact the connections mechanical strength and look bad to boot. You might also be able to make it work if you used unfinished stainless and aluminum with dielectric grease because the materials and bolts would make much better contact. But paint and rust and road grime all impact the path very negatively.
Quote:
2. In front, I have heavy duty grounding 2/0 and 1/0 from both of my batteries to the engine block and to the fender. These are DC grounding, but is it also good for RF grounding?
Despite their size and low DC resistance, these cables are generally not good RF grounds and so it is generally good practice to put a heavy braid parallel to the cables from the battery negative to the frame and block.
Quote:
3.I also have a 2/0 grounding in from the transfer case to the frame to assist the puny ground cable the 80 has between the body and the transmission/transfer case.
I would parallel that but for VHF/UHF and particularly for FM it's overkill to go crazy. But you really need to provide at least one solid RF tie for the battery negative, frame and block so that any grounds that tie into them have a good chance of making it home to the radio.
Quote:
4. The swing arm is on a bearings and I presume it may not be as well connected to the bumper as I would want. So I can see how a grounding strap can help.

5. Bracket I am thinking will be securely attached to the swing. Should I but a grounding strap between the bracket and the swing?
You want a good RF path from the antenna mount to the RF ground. The antenna wants to couple to the mount and anything close to the feedpoint is lower in impedance than things farther away. If you float it relative to it's desired return path you make the system more complicated electrically. That means that you might be able to get it to tune but the more stray reactance you have the more sensitive it will be to changes like rust and weather.
Quote:
6. up on the "shelf", see my red line in the picture, I presume I will use a grounding strap between the antenna and the shelf.
You want to use grounding braid between significant pieces of metal if there is any question of conductivity. What is significant changes the closer to the radiating element, too. An 8" plate under the antenna whip is very significant, while an 8" plate on your front bumper with the antenna on the back isn't critical.
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