High frequency mobile operation can be safe and fun if you take your time to install your equipment correctly. There is more to it than meets the eye, and I’ve done my best to cover them on my web site (http://www.k0bg.com). However, for some reason far too many amateurs leave out one of the most important aspects. That aspect is known as Bonding.
In simple terms, it is applying ground straps (left photo) to the various bolted on pieces of the vehicle to facilitate a contiguous RF ground potential between them. While most of the parts of a modern vehicle are securely fastened, and are more or less DC contiguous, they are not necessarily RF contiguous.
One very good example of this in the exhaust system. It is thermally and physically isolated from the body for obvious reasons. Typical of most of the bolted on parts of a vehicle, one end may indeed be DC grounded to the chassis, while the other end floats. This fact allows the exhaust system to act as an antenna, radiating all manner of ignition and electrical noise directly to your receiving antenna.
I’ve introduced a term here that needs further explanation. The word contiguous means sharing a common border. In this context, we want each of the bolted on pieces to act as if they were one continuous structure. This is very important for several reasons.
The most important reason should be obvious, but in case it isn’t; all manner of RFI and EMI, whether induced, radiated, coupled, or looped-in plague every single mobile operator. It makes little difference what vehicle you have, it’s size, its engine type, or construction (frame vs. unibody); they all exhibit some level of what we call noise. Adequate bonding will significantly reduce these maladies.
Bonding alone, however, will not magically fix every single vehicle-generated noise we hear in our radios. That takes a lot more work and may involve adding split beads, installing larger sized DC wiring to our transceivers, and a host of others. Again, I do my best to cover these in the various articles on my web site.
There is one thing bonding does that isn’t as evident as noise reduction, and that is its effect on antenna efficiency. It’s prudent to think of the antenna and the vehicle as a system. It’s analogous to a base station vertical and the ground plane under it, if any. The more radials we install (to a point) under a base vertical, the better it works. With respect to a vehicle vertical, the more bonding we do, the better it works.
Lets look at this another way. The body of any vehicle is a very poor ground plane. In fact, most of the effect is caused by the capacitive coupling between the various parts of the vehicle and the physical ground under the vehicle. The physical ground I’m referring to may be a driveway, roadway, dirt, gravel, or any other drivable surface. A surface, incidentally, we have no control over. Just like a base vertical with an inadequate number of radials, any vehicle has a lot of ground loss.
There are a number of technical publications which list the ground losses exhibited by vehicles. Apparently they all quote the same source as they are always listed as being between 10 ohms on 80 meters, to about 2 ohms on 10 meters. In reality, the figures are closer to 20 ohms to about 5 ohms, but may be more if there is excessive coupling between the vehicle’s body and the antenna (example: antenna too close to the body like it would be when mounted on the back bumper of a van).
I don’t want to offend the purists, so let me add this. Ground loss in this context is not just that induced between the body and the physical ground under it. I’m including coupling and stray losses because no matter what causes the loss, it reduces overall efficiency when the vehicle and the antenna are correctly viewed as a system.
My example about an antenna mounted on the back of a van points out one very important point; mounting position and/or height are not always ideal as often dictated by circumstances. Remotely tuned antennas are a good example, as their weight makes it very difficult to body mount them. In other words, we have to play the hand were dealt, and it isn’t always aces.
The only way we can ante up the best signal, is to do as much bonding as we can, and avoid poor mounting techniques. One poor mounting technique which comes to mind is installing the antenna atop a long stalk, and then using all manner of ground straps down to the frame. While this may DC ground the antenna base, it is not a substitute for an adequate ground plane, any more than a ground rod is adequate for a base vertical.
This article isn’t about proper mounting, per se, albeit an important attribute to efficiency. It is meant to point out how important bonding is to increasing efficiency and reducing noise.
Here is a little experiment you can try. You’ll need an MFJ 259B or similar antenna analyzer (owning one nowadays is as important as owning a good wattmeter and dummy load). Mount your antenna first, and then measure its input impedance on each of your favorite frequencies. Complete your bonding and measure it again. You’ll notice the resonant frequency has lowered slightly, and the input impedance has dropped. Depending on how good an antenna you have, it is not uncommon to see a 25 percent drop in impedance. This occurs because the net ground losses decrease.
Allow me to change tracks here for a moment. I don’t wish to get into a long-winded, technical discussion about mobile antenna input impedances, and what they consist of. If you really want to know about vertical antennas in general, http://www.cebik.com is a good site to peruse. A little closer to home, http://www.w8ji.com has several very good articles specifically about mobile antennas, and what makes them tick.
Both of the aforementioned sites contain antenna modeling data, which brings up another important point. Modeling a mobile antenna installation with EZNEC, or similar program, is an extremely complex operation which requires field measurements to correct for preprogrammed assumptions with respect to ground losses within the vehicle’s structure, and the vehicle’s capacitive coupling to the ground surface under it. Move the vehicle to another spot 25 feet away, and all of the parameters change.
There is one aspect of mobile operation a modeling program will help you understand, and that is what importance coil Q factors play when the ground losses are high. One of Tom Rauch’s, W8JI, articles addresses this very issue.
If you know how to use an antenna modeling program like EZNEC, here’s a little game you can play. Design yourself two 10 foot long, 80 meter, center loaded, mobile antennas. One with a coil Q of 50, the other one with a coil Q of 300. Now compare their efficiencies when mounted over a loss-less ground plane, one with 5 ohms of loss, and one with 20 ohms of loss. This little operation will exemplify why low loss mounting techniques, and proper bonding are so important.
The aforementioned exercise will also illustrate, albeit more subtly, why spending big bucks on a high Q antenna is a waste of resources if you’re unwilling to improve the ground plane under it, be that by proper bonding, or an adequate numbers of radials as the case may be.
While a cheap, short, low Q (inefficient), and poorly mounted HF mobile antenna will garner contacts with or without any bonding, playing Army and being all you can be takes more time and effort.
Alan Applegate, K0BG
PS: My thanks to L. B. and Tom for their comments.
One question which will undoubtedly arise from this article is what is the correct number bonding straps required? While the optimum number of radials for a base vertical is about 100 or so, the optimum number of bonding straps depends on the vehicle. While body on frame construction requires more than unibody construction, to suggest a specific number is a case of presumption as there are just too many variables.