One of the most misunderstood concepts is the difference between DC and RF ground (neither one can be considered a ground plane). A ground strap may work perfectly as a DC ground, but at some frequency that same ground strap will make a perfect antenna! We all know that an inductor can provide a good DC ground, but look like an open circuit to RF. And that a capacitor can provide a good RF path to ground, but not a DC path. Our ground strap, like any piece of wire, has both inductive and capacitive reactances. These reactances change as the frequency changes. For any given value of reactance, as the frequency goes up, inductive reactance also goes up, but capacitive reactance goes down. When inductive reactance and capacitive reactance in any given piece of wire are equal, that wire will become an antenna, and ceases to be an RF ground. There are a few things we can do to assure both a good RF and DC ground.
One of these is to use braided wire. Not just any braided wire mind you, but one which is flat and wide. RF flows at the surface rather than through the wire, and flat braid has more surface area for any given current carrying capacity. Thus it provides less resistance to RF than an equivalent round wire. It also has more capacitive reactance which increases the self resonant point. Flat braid is also much more flexible and less likely to fail due to repeated flexing.
The shield from RG8 works well if the length of the strap is short (under 10 inches or so). Just take care when you strip off the outer jacket that you don't cut through the shield itself. Discard any that is corroded or discolored. Flatten it out by pulling it over a rounded surface. A large, round screwdriver shaft works well for this purpose.
For longer lengths, one inch wide braid is a better choice. In any case, the requisite length shouldn't exceed 2 feet. If it has to be longer, then heavy copper flashing, like that used by roofing companies, is the material of choice. Remember, the ground strap must present a low impedance connection to effectively shunt RF to ground. This is especially true if you're using an auto-coupler, as the ground side connection must have a (much) lower impedance than the radiating element. I cover this in more detail in my Auto-Couplers article.
Good connections are also important to provide both a DC and RF ground path. Crimping and soldering are mandatory. Crimping provides a good mechanical connection, and soldering a good electrical one. Good quality lugs and connectors are a must too, as the cheap ones do not solder well. Where applicable, connections should have heat shrink applied over them. Although not strictly necessary, it gives a finished and professional look to your installation.
There are few caveats to be aware of before you start drilling, and screwing ground straps hither and yon.
Modern vehicles have dozens of wire looms placed throughout the superstructure. This includes the A, B, and C pillars, under both door sills, inside the doors, under the carpet, and behind almost every piece of trim. If in doubt, find out before you drill or screw.
When bonding the tail pipe for example, there are usually strengthening members, or existing mounting bolts which can be utilized. The bottom edge seam of the rocker panels is also a safe bet. Some vehicles have predrilled and tapped holes for accessories and/or for assembly purposes. They're usually metric which will necessitate a trip to the hardware store. What's more, they're safer than drilling into panels unless you know for certain, nothing is behind them.
While you're at the hardware store, pick up a supply of star (serrated) lock washers. The preferred type have both internal and external teeth, but are not always available in the smaller sizes. Standard lock washers (the split type) aren't worth the effort.
I typically use two sizes of lugs; ones with a 1/4 inch bolt hole, and ones with a #10 screw hole. Wire size depends on the braid size. Lugs for #12 are adequate for RG8 shield, and lugs for #4 work best for 1 inch braid material. I use the lugs with a 1/4 hole when I'm attaching to an existing bolt, and the #10 ones for everything else.
If you can find them, use lugs with built in star washers. Some versions have an embossed serration rather than an actual star washer design, and are easier to find. I purchased mine from Fastenal, but there are other sources such as Mouser Electronics and Ace hardware stores. Uninsulated ones are usually cheaper, but harder to find than the insulated ones.
The actual screw type is up to you. I use several different types. Number 10 self-tapping sheet metal screws work well if the material is 12 gauge or less. Self-drilling ones work better for thicker material. Phillips head and hex head both work well. In any case, I wouldn't buy any longer that 1/2 inch, and 3/8 is a better choice. Longer isn't going to hold better, and just might run into something you can't see.
I usually make up several 6” long straps for the doors, 10” long ones for both sides of the engine, both hinges of the hood and trunk, and at least three for the exhaust and tail pipes.
Remember to crimp and solder them as crimping alone allows moisture to seep into the connection with predictable results. If the strap is subject to abrasion, cover it with heat shrink tubing.
Heat shrink tubing requires a heat gun. Besides the tubing, all of the aforementioned companies carry heat guns with prices varying between $40 and $250 depending on both quality and duty cycle. Since we don't use one all day long, we don't need a high priced one, so here's a suggestion. Hobby Lobby sells (in store and on line) a heat embossing gun for just $19.95. It works perfectly as a light duty heat shrink gun, and its small size and light weight make it easy to store.
If you're not into making your own, you might want to look at the various styles of strapping braid made by Electric Motion. That's their EM2080 product in the photo.
The braid is preassembled with grommets every three inches or so. A 25 foot roll is about $50 which sounds expensive, but you don't have to solder it. Simply a sheet metal screw and star washer is all you need to make a good bond.
Caution is also needed when bonding the doors, hood and trunk. You don't want to end up drilling through the lid, or into a wiring harness underneath! Keep straps away from away from hinges, door stops, and weather seals. Hoods and trunks should be strapped across both hinges.
Almost without exception nowadays, exhaust systems are made of a good grade of stainless steel. Wire brushing a small area for the lug to bite into and using stainless steel hose clamps to secure the lug has always worked for me. I have never had one come loose or need retightening. The opposite end should be attached to the underframe or unibody strut work. If the car is undercoated you may have to clean a small area. A Dremel tool with a wire wheel works well for this operation.
If your vehicle is body on frame you'll need a bunch of straps to go between the body and frame. As an example, four separate straps (one on each corner) work well for a pickup bed. Incidentally, don't rely on any factory strap to provide a good RF ground. They're meant solely for DC grounding tail lights etc. and are just inadequate for RF grounding needs. The same can be said for factory engine strapping.
Engines are overlooked because most of them have visible ground straps (right photo). They're for DC of course, so adding wider ones can help curb ignition noise. For example, after the requisite copper foil shielding of the COP units, I still had S3 ignition noise on two bands. A short 4 inch strap attached to an unused, threaded boss in the rear head, to a firewall header bracket reduced the level to near zero.