Last Modified: July 23, 2011
Contents: Basics; Your Average Vehicle; Ground Planes; Mounting Location Issues; Ground Straps; RF Grounds; DC Grounds; Conclusion;
There is probably more misunderstanding about grounds than there is any other aspect of amateur radio. Some of this misunderstanding comes from the fact we call a whole bunch of diverse things ground. The most basic one is the ground we walk on. It doesn't make much difference if it is asphalt, dirt (earth as it were), or concrete, we collectively refer to it as ground. Electronically, we add earth ground (earthing), DC ground, RF ground, ground plane, chassis ground, isolated ground, and a few others. While they may be similar in basic terms, one type of ground doesn't necessarily act as, or replace, another type of ground. Of all of the various grounds we deal with in mobile operation, the most important one is the ground plane!; essentially the missing half of a vertical antenna.
Antenna efficiency should be the goal for every mobile operator. Because ground losses dominate the efficiency equation, decreasing them by just one ohm, can make a significant difference in efficiency. This point should not be under estimated! Put another way, excessive ground losses can turn an otherwise efficient antenna, into an also-ran.
One very important point needs to be made here: A vehicle is an inadequate ground plane at any frequency under ≈100 MHz, no matter how large it is! Rather it acts like a capacitor placed between the antenna, and the surface under the vehicle which is the actual ground plane. Since the surface in question is a poor conductor of RF, ground losses occur. The term ground plane in the following text is therefore a bit of a misnomer, but is used to differentiate it from DC and RF grounds. One of the most misconstrued concepts about grounds in general, is the thought that an RF ground is the same thing as a ground plane. It is not! The only commonality is this; The coax shield should be connected directly to the antenna's ground plane. It doesn't matter whether the ground plane is RF grounded (earthed) or not. What's more, the ground plane should be directly under the antenna; within inches, not feet! Running a ground strap to the nearest hard point will not negate this premise.
For a better understanding of the need for a ground plane, read the Ground Plane Notes article. And please, don't use the term counterpoise. While similar to a ground plane, it is in fact a different animal altogether.
First of all, there isn't an average vehicle. In fact, between two, otherwise identical vehicles, there can be a great difference in the amount, type, and severity of RFI. For example, the egressed ignition RFI of one may be S9, an the other an S2. Even minor annoyances like fuel pump and AC fan hash vary greatly from model to model. The same can be said of ingress, especially to sound systems. Some of the more common noises are described in the Noise ID article.
Secondly, the suggestion that one specific model or brand is superior to another doesn't take the aforementioned facts into account. This begs the question, is one generally better than another? Well, maybe, but we have to be more specific. In this case we're speaking about ground planes, and to a lessor degree, RFI issues. With that in mind, we can make a few general statements.
As a rule, unibody vehicles exhibit less problems, both with ingress and egressed RFI. This is due mainly to the all-welded body construction. However, most still have sound insulated undercarriages for the suspension, engines, transaxles, exhaust systems, etc., and these need to be bonded to maximize RF continuity, and egresses RFI.
Body-on-frame vehicles tend to have more bolted on pieces, and this is especially true of pickup trucks. No matter where you mount an antenna on a pickup truck, the bed should be bonded to the chassis on all four corners, and to the cab on both sides. If you don't, you'll most likely be plagued with RFI problems, some of which you won't know you have.
One major point to remember, DC continuity is not the same as RF continuity! When you don't properly bond and/or wire your installation, it is possible to create a ground loop in the vehicle's wiring. When you create a ground loop, the resulting affect will often appear to be an RFI issue. Ground loops are the toughest of problems to find and correct.
The ground plane is one type of ground that needs a different name applied to it, because everyone seems to have a different opinion of what it is or isn't. It isn't a counterpoise, although many folks use the term synonymously. What it really is, is the missing half of a vertical (monopole) antenna. In an HF mobile scenario, the body of the vehicle, and the capacitive coupling to the surface under the vehicle, is acting like the missing half—a lossy one at that! On average, mobile ground plane losses vary between 2 and 10 ohms, 10 through 80 meters respectively. In most installations, the ground loss is somewhat higher due to improper mounting, bonding, and assumed ground conductivity.
The coupling between the superstructure of any vehicle, and the surface under it, is not consistent. As a result, there will always be standing waves between them. These standing waves are, in essence, the main cause of the ground losses in the first place. Please note, we're not talking about the standing wave ratio (SWR) of the antenna! It should also be noted that you can't measure these standing waves directly. It should also be noted that these standing waves are frequency dependent. However, they may indeed cause more ground loss on say 20 meters, than they do on 40 meters. This is in opposition to the normally excepted theory that ground losses always increase with a decrease in frequency. Looking at this in another way, you cannot assume any measurement you cannot directly measure!
The individual parts—the antenna, the vehicle, and the surface—should be viewed as a system! Change one, and you change them all. This fact is why proper bonding and, mounting are of prime importance. The issue is to make all of the bolted on pieces as RF congruent as we can. Here's another way of thinking about the system!
Let's assume our antenna represents a 50 Ω load (which is seldom does), and our transceiver outputs 100 watts. From Ohm's Law, flowing into the antenna will be 1.4 amps of RF current, at ≈70 volts. This same current, and voltage must flow in the missing half of the antenna, the ground plane as it were. Since both our antenna, and the ground plane are lossy (resistive), some of the current flow will be dissipated as heat, and will not be radiated. Quite obviously, we want to minimize these losses. Incidentally, excessive ground losses also contribute to common mode current, and other RFI issues. It should be obvious then, that minimizing system losses is a worthwhile endeavour.
Whether you use a monoband antenna, of a remotely controlled multiband one, once properly matched it will have a relatively low SWR. Once tuned, few mobile operators keep an eye on the SWR while underway (and for good reason!). If you did you would notice that the SWR changes over a rather wide range depending on the surface we're driving on. One would correctly assume that changes in ground conductivity under the vehicle affect the ground loss figure, whatever that may be. However, there is another factor at play. As mentioned above, there is capacitance coupling between the surface, and the body of the vehicle. Changes in ground conductivity also chances the amount of this capacitance. It has the same effect a cap hat does, but in this case, it is at the base of the antenna, not the very top. As a result, the resonant frequency of the antenna changes. Whether the resonant frequency increases or decreases, depends on what the conductivity under the vehicle was when the antenna was first resonated. It is this capacitance change which causes most of the SWR variations we see, rather than the actual ground loss figure, whatever it is.
The best place to mount an HF mobile antenna, is in the center of the roof. This places it as far away from the surface the vehicle is sitting on, and as far away from the vertical surfaces of the vehicle as possible. With respect to system losses, any other position on the vehicle will exhibit more loss. And contrary to popular belief, DC or RF round straps will not negate this premise!
As stated, low mounting heights increase ground losses. The reason is, a goodly portion of the return current is forced to flow in the lossy surface under the vehicle, rather than through the vehicle's less lossy superstructure. How much affect this has on efficiency depends on a lot of factors, especially the quality of the antenna itself (coil Q, overall length, etc.). Since ground losses dominate the efficiency equation, a ground loss change as low as one ohm can make a significant change in efficiency; in some cases, as much as 15 dB or more. One key to increased efficiency, read that as low ground losses, is placing as much metal mass under the antenna as possible, as depicted in the pictorial below right. That certainly isn't the case in the upper right photo!
This is a good point to bring up a hotly debated issue about mounting mobile antennas down inside the bed of pickup trucks. Unless the mast of the antenna is very close to the pickup bed wall (well less than an inch), the reduction is performance is very minimal. The reason is, the amount of capacitance between the bed wall, and the antenna's mast, seldom exceeds a few pF. Even on 10 meters, this amount of capacitive loading is almost immeasurable without sophisticated lab equipment. It certainly can't be measured with an inexpensive antenna analyzer. Where you can get into trouble is mounting the antenna's coil too close to sheet metal. In this case, the reduction in performance is easily measured, even with an SWR meter in some cases. It is always best to keep the coil as far from sheet metal as you can.
Let's dispel another notion about pickup truck beds. Some would have you believe, that mounting an antenna above the bed opening on outriggers actually reduces ground losses. These pundits argue that the bed becomes a resonant cavity, with a very low impedance. This is pure junk science at its best! Remember, it is the metal mass directly under the antenna, not along side, that counts!
The next two sections talk about ground straps, bonding as it were. In this context we're taking about braided strapping. It can be commercial braided strap like the Electric Motion material shown at left, one inch wide braided battery cable, or the shield off discarded RG8. What ever it is made out of, there are a few things to remember.
First, straps much be kept short! That is to say, inches not feet! You often see ground strap running from antenna mounts down to the frame of the vehicle, and sometimes they're several feed long. From an RF standpoint, they do nothing. Yes, they provide as DC ground, but they're no substitute for a properly installed antenna mount. Think about this. A 4.5 foot long ground strap is a perfect 6 meter antenna!
The width of the strap is important too. Remembering that RF flows on the outside of the conductors, the total surface area is important. Obviously, braided cabling has much more surface area than a round wire. The larger the surface area, the lower the impedance, which is exactly what we're striving for. Here's a good rule of thumb to follow is. For each foot in length, the width should be one inch wide. In other words, the longer the strap, the wider it had be in order to keep the impedance low.
Speaking of low impedance; wide, solid copper strips have the lowest impedance of all, however, they're not very flexible to begin with, and they tend to work harden. As a result, solid copper strips shouldn't be used if the inter-connection needs any flexibility; door straps for example.
If your antenna mount is securely fastened to the frame or body work of the vehicle, and the coax is securely fastened to the mount, no additional strapping is needed. If you added a strap, and it cured an RFI or SWR problem, then something else in your installation was amiss.
There is only one RF ground (if we can call it that) we need to deal with, and that's a proper return for the coax shield. It should be very close to the base of the antenna, coincident to any matching device used (coil, UNUN, etc.), and most importantly, as close to the metal mass of the vehicle as possible. This negates antenna mounting atop long posts, extended brackets, clamps, and luggage racks.
There is one more thing to consider when the coax shield connection is raised above the ground plane. Not only are ground losses increased, the amount of RF flowing on the motor control leads also increases, as do common mode currents flowing on the coax. Therefore, the requisite RF chokes may need to have an impedance in excess of 10 k ohms. There is more information on this in the Antenna Controller article.
One of the most misunderstood aspects is the notion that DC grounding an antenna mount will magically act as, or replace, a ground Plane. It will not! The only way a ground strap could act as, or replace, a ground Plane is to make it long enough to be radial; a ridiculous notion! While the strap may indeed DC ground the antenna's base, and it just might RF ground it too depending on its length and width versus the frequency of operation, it is by no means a replacement for a ground Plane. Nor is it a substitute for proper bonding.
Another misunderstood aspect is adding DC grounds to the various transceiver parts in an effort to control RFI and/or high SWR. If a DC ground (in this case RF ground as well) addressed your problem, then something else in your installation is incorrectly installed or mounted.
As mentioned above, proper bypassing of the motor leads of remotely tuned antennas is an absolute necessity. This is covered very well in my controller article. If you don't properly bypass (choke off) the RF from the motor leads, all sorts of maladies will occur. This includes erratic operation, RFI ingress, and a whole lot of frustration. By the way, these chokes much be mounted outside the vehicle, as close to the antenna base as possible. Remember, all the cabling before the choke is part of the antenna, and will radiate.