Last Modified: January 17, 2013
Contents: Basics; Ground Straps; How to Make & Attach Them; Where to Put Them; Odds & Ends;
Bonding, while important, is just one of many steps towards an effective, and efficient mobile installation.
Bonding, sometimes referred to as strapping, is one most important aspects of mobile radio. Antennas, antenna mounting, and Wiring are some of the others. However, one very important point needs to be made before we go further.
A vehicle is not a ground plane, but rather acts like a capacitor between the antenna and the surface under the vehicle which acts as the ground plane. Since the surface in question is a poor conductor of RF, ground losses occur. If we wish to maximize the system efficiency (the mobile station as a whole), we need to maximize the RF continuity of the vehicle; hence proper bonding. Obviously, proper antenna mounting, and placement are important too.
Bonding also minimizes the leakage of RFI into (ingress) and out of (egress) the various bolted on parts of the vehicle. The exhaust and tail pipes are good examples of RFI egress. It is not uncommon to see a 20 to 30 dB drop in received noise levels once they're properly grounded.
If you're interested in seeing what affect bonding has, here's a little experiment you can easily do. Install your antenna first and use an MFJ 259B or similar antenna analyzer to measure the on-resonance (X=Ø) input impedance of your antenna. Then follow the suggestions below and once you're done, measure it again. The resonant point will drop slightly and the input impedance will drop as well. This occurs because bonding lessens the ground losses which are reflected in the input impedance. The better the quality of the antenna, the more noticeable the change will be. How much change occurs in the input impedance is reliant on several factors, not the least of which is where the antenna is mounted. The higher the (proper) mounting position, the more evident the change will be.
Bonding is especially important for body-on-frame vehicles, like pickup trucks. In these cases, the bed and main body should be bonded to the main frame as well as between then. Remember, bonding horizontal surfaces (trunk lid, hood, etc.) will have a greater effect than bonding vertical ones like doors, and hatches.
It is important to remember, that the transmitted RF must return back to its source. Low mounting forces a larger portion of this return current to flow through the lossy surface under the vehicle, rather than through the (less lossy) vehicle's superstructure. Therefore, it is easy to see why proper mounting, and proper bonding are requirements to maximize radiation efficiency.
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 then, 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 (in ohms) also goes up, but capacitive reactance (in ohms) 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.
The truth of the matter is, a short, wide piece of copper, is superior to braided copper strapping. Although quite flexible at first, once it bends a few times, it work hardens, and becomes rather brittle. Meaning of course, it soon breaks. There are flexible alloys like Monel®, which don't work harden nearly as readily, but the cost is almost prohibitive. Hence, braided strapping is an affordable alternative.
How To Make & Attach Them
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 what's behind the panel 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 cannot 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 making 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.
The aforementioned fact is the reason to use star washers. When properly installed, they bite through the various finish layers, and into the base metal below. Once exposed, the zinc compound seals the connection. The use on NoOx® and similar compounds really isn't necessary, but they do retard rust and oxidation of the mounting hardware. In any case, do not use these compounds under star washers, and between the mount and the vehicle's structure. In other words, let the zinc oxide do its job of preventing rust.
When making connections to the bodies and frames of vehicles, do not sand the finish to bare metal! The reason is, bodies and frames of modern vehicles are dipped in a zinc compound. This zinc compound oxidizes in the presents of air (oxygen) and seals scratches in its surface.
It is always wise to purchase a repair manual for any vehicle you buy. They included photos and/or drawings of the various sub assemblies, along with schematic drawings and/or pictorials of the various on-board control systems. Connector locations, wiring colors, wiring sized, fuse ratings and fuse block locations, and a myriad of other informational data every mobile operator might need to complete their installation. Prices vary a lot, but most are around $75 to $100. Sometimes, if you're insistent, dealers will provide you one with your purchase.
Where to Put Them
As noted above, horizontal surfaces are more important than vertical ones. So if you're not into spending hours bonding everything in sight, pay attention to the hood, trunk lid (if any), and the exhaust system. If you've resorted to using (less than desirable) hatch mounts like the K400, make sure the hatch is very well bonded. If you end up with common mode problems, lack of bonding in these areas is the first place to look!
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. Just don't brush past the zinc layer to bare metal!
The exhaust system should be bonded before and after the catalytic converter, and at least once after the muffler. If tail pipe is extra long, a fourth strap might be worth the effort. By the way, DX Engineering sells exhaust system grounding kits, shown left, for those who don't wish to go to the trouble to home brew them.
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. And, a bond between the bed and the cabin is also advisable. 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 inadequate for RF grounding needs. The same can be said for factory engine strapping.
Depending on the vehicle, there can be several dozen other places where ground straps will provide a benefit. These include, but are not limited to, bumpers, bumper backing plates, suspension parts, rear axles, tailgates, or virtually any bolted-on piece of hardware.
There is an old cliche that says an ounce of prevention is worth a pound of cure. If you'll just take the time to do your bonding correctly, you'll be amply rewarded. Short cut it, and the results won't be worth the effort. I spent some eight hours making, and installing the straps on my Ridgeline. If it takes you much less, you probably didn't do it right. By the way, there are hi-res versions of these photos in the Photo Gallery.
Caution should be exercised when bonding doors, hoods and trunks. You don't want to end up drilling through the lid, or into a wiring harness underneath! (Another good reason to buy a service manual.) Keep straps away from away from hinges, door stops, and weather seals. Hoods and trunks should be strapped across both hinges. If you mount an HF antenna on a trunk lid, you will indeed have excessive common mode current, no matter how well you bond the lid and/or mount. Making matters worse, common mode choke have to be placed outside the vehicle to minimize RFI ingress. This goes for control leads chokes too. This is another good reason to avoid trunk mounting on any band below 2 meters.
Odds & Ends
There is one more aspect of bonding that merits mentioning. As stated above, even seasoned amateurs confuse DC and RF grounds with ground planes. As a result, too many amateurs believe a DC ground strap from the mount to a hard point on the body or chassis is a substitute for a ground plane. It is not.
Lastly, a 1/4 wave vertical antenna (loaded or not) is one half of a dipole. When used as a base station antenna, a vertical must have a number of radials under it. These radials act as the missing half of the dipole. In the case of a mobile antenna, the missing half is supplied by the vehicle, and its apparent capacitance to the ground underneath the vehicle. Since ground loss is the single biggest factor with respect to efficiency for any vertical (especially a mobile one), maximizing what ground plane a vehicle represent, is essential. As with the number of radials, the more you have, the better. This is true of bonding as well.