For RF grounding, there are a few concepts you will want to learn.
- Skin effect. Basically the higher the frequency, the shallower a signal will penetrate a conductor. To this effect, at higher frequencies, a wide flat strap or metal foil is better at conducting an RF signal in its entire volume than a round DC current carrying conductor like a brake light wire. As an example, a 14-Gauge wire at 100MHz (approximately the center of the FM radio band), is only using about 4-5% of it volume (all at the surface) for conducting RF energy.
- RF Bonding. You want your vehicle to look to the antenna as a continuous ground plane - that is, your truck should appear as a monolithic piece of metal. By using only a few small and round conductors to ground your truck and radio(s), you are creating small tuned circuits in your body panels that react in unknown ways to RF energy (think of this as small body panel sized antennas everywhere). It's better to ground EVERYTHING (doors, hood, body, frame), and ground it frequently and with wide (and multiple if possible) ground straps. Remember that rubber isolators don't conduct much of anything, so in addition to a DC ground, you would want to RF bond the component as well (think engine, transmission, and transfer case here - nice big pieces of metal).
Spot welds (or screws) that are used to hold together body panels are cheap, but make terrible RF ground points - they focus energy down to a few tiny points of energy transfer, and the spacing of those welds can create a small resonant circuit in the skin of the vehicle. If you are ever going to do HAM or higher powered CB, you would want to get to each of the joints and bond them with a continuous weld or a copper foil bridging over the seam brazed to the metal.
As was said above, the antenna base gives your antenna a path to ground, any rust there changes the conductivity of the base, and makes it harder for controlled RF to get to your ground plane (this can de-tune your antenna). Even if you don't do high power CB or HAM, your truck would benefit from having the major components bonded/strapped with a 1" wide copper ground strap, either welded or using a sheet metal screw and a star washer to bite into the metal. The idea being you want as best DC and RF conductivity from all body panels to a common ground point. By making these connections as wide and low resistance as possible, you would reduce any ground loop threats to a lower level than the noise floor of the RF signal. With the ground at the same or nearly the same potential (voltage) at both ends of the antenna coax, there is no longer a significant energy returned over the antenna cable shield (and no more noise picked up as a result). Think "path of least resistance" here - if it's easier for noise to get to ground using your ground straps, it won't bother with your antenna wire.
This same thing is a problem if your radio only has a wire connecting to ground, and not a good solid RF ground to the chassis. If more RF energy gets returned to the ground of the truck by going through the antenna shield, then it will introduce noise into the RF receiver in the stereo. If you think the dash is a good ground point, then make sure the dash metal is actually a good ground point to the chassis first
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For you guys reading this with M1008, M1010, M1028, or M1031 trucks, it's helpfull to remember that your bed-box is not directly connected to the cab body, and that the cab is mounted on rubber isolators usually that need to be bridged with RF straps as well - but keep the straps away from the side of the frame rail with the fuel tanks, small sharp things like screw heads can puncture tanks in a crash.
I used to do car stereos, and with alternator whining problems I often found the RF ground lug included on the back of the stereo deck unconnected to anything, let alone the frame.
Thanks for the website link too tim! Good stuff!
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