Grounds, RF & DC
Last Modified:
Contents: Basics; Your Average Vehicle; Ground Plane; Mounting Location Issues; RF Grounds; DC Grounds; Conclusion;
There is probably more misunderstanding about grounds than there is any other aspect of amateur radio. I think 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's asphalt, dirt (earth as it were), or concrete, we collectively refer to it as ground.
Electronically we add earth ground, DC ground, RF ground, ground plane, chassis ground, isolated ground, and a few others. No wonder why so many are confused.
Each one of these has special properties, and it's unfortunate we use the same word for all of them. This motivates some amateurs to use alternative words. For example, using the term counterpoise in place of the term ground plane. While they are similar, that are in fact, not the same thing.
It is very important to remember, that one type of ground doesn't necessarily act as, or replace, another type of ground. Hopefully, what you read here will clarify some of the misunderstanding.
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 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.
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, and in this case we're speaking about image 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.
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. If it is a long bed model, perhaps in the center as well. 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 effect will appear to be an RFI issue. Trust me on this, ground loops are one of the toughest problems to find and correct.
This 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. In fact, there is an alternate name which seemingly has disappeared from the amateur lexicon, and that is Image Plane.
A loaded, quarter wave, mobile HF vertical antenna, is nothing more than half a dipole. This's why it is sometimes referred to as a monopole. We wouldn't erect a dipole with just one leg, yet amateurs will erect a quarter wave monopole (vertical) without an adequate Image Plane (ground plane) under it.
While it is possible to add enough radials to make a base station vertical rather efficient, it is not a luxury we have when operating mobile. In fact, it is difficult to achieve efficiencies greater than 80% even on 10 meters.
Instead of an adequate Image Plane, the average vehicle represents a capacitance to the surface under it. Depending on the vehicle, how well it is bonded and/or welded together, the capacitance varies between .002 uF and .004 uF. This represents a frequency-dependant ground loss of between 2 and 20 ohms. In most installations, the ground loss is somewhat higher due to improper mounting, bonding, and assumed ground conductivity.
Digressing for just a moment. Almost without exception, ground loss is always greater than the value typically accepted. Recently, new technologies have supported this conclusion, as readers of the ARRL publication QEX can attest to. Thus, the aforementioned stated losses are only a starting point, and admittedly on the low side.
We can't improve the ground conductivity under the vehicle with radials for obvious reasons. We can increase the size and Q of the antenna, but there is a practical limit. We can select the type of mount, the mounting location, and mounting height which will reduce, but not eliminate, the over all system losses. It is a real conundrum we all deal with. To quote L. B. Cebik, W4RNL, "It is a fascinating arena of trying to squeeze the last ounce of available efficiency from largely undersized antennas." I might add, and from largely undersized image planes!
It's important to emphasize, that a mobile antenna and the vehicle it is mounted on, should be viewed as a system. In other words, the body of the vehicle (and its capacitive coupling to what's under it) is the missing half of our antenna much as radials act as the missing half of a base station vertical; the Image Plane, as it were.
Since the body of the vehicle (and its capacitive coupling to the surface under it) is one half of the antenna, properly bonding the various bolted on pieces of the vehicle is of prime importance. What's more, you almost can't have too many.
If I haven't made the image plane issue clear, I suggest you read this article.
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 Image Plane 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!
I don't want to over simplify the point I'm trying to make here, but hopefully the following statements will exemplify the understanding:
1). If there isn't a large mass of metal under the antenna the ground losses increase dramatically! For example, mounting an antenna on the bumper of a van. Installing a ground strap to the nearest hard point on the body is not going to negate this fact.
2). The major reasons for the additional losses in Statement 1 are capacitive coupling of the antenna to the body of the vehicle, and capacitive coupling of the antenna to the surface the vehicle in resting on (the drawing at left is a prime example). Even the antenna mount causes some loss. These types of losses are commonly referred to as stray coupling (capacitance) losses. It should be noted that stray capacitance losses raise the input impedance, but for all the wrong reasons.
3). An average vehicle exhibits a capacitance to the surface under it of about .002 uF to .004 uF. It varies depending on the vehicle's ground clearance (there's that word ground again), the size of the vehicle, the construction of the vehicle (i.e.: frame or unibody), and whether it is well bonded or not.
4). As stated above, the antenna and the vehicle it is mounted on should be viewed as a system. In other words, coupling losses (stray capacitance), Image Plane losses, and/or mounting losses need to be considered collectively. Incidentally, it is not uncommon for these system losses to be as high as 25 to 35 ohms.
When you factor in the reduced radiation resistance short antennas have, and the large resistive losses (primarily in the loading coil) inexpensive antennas add to the mix, it is easy to see why efficiencies range in the .5% to 10% range (80 through 10 meters respectfully) for the average mobile installation. In other words, reducing Image Plane losses is the single most important factor we have to consider.
Further consider this; the average mobile operator can increase his/her effective radiated power by as much as 20 dB (twice what an amplifier will give you) by properly installing his/her mobile station. The drawing at right is an ideal, but it requires drilling holes; a misinformed taboo to many mobile operators.
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. If the mount is securely fastened to the frame work or body 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 problem, then something else in your installation was amiss.
Speaking of the shield connection, it should be very close to the base of the antenna, coincident to any matching device used (coil, UNUN, etc.). If it isn't, you're going to have RFI issues, high SWR, and most certainly, low efficiency. This is especially true of any mount which utilizes set screws and other forms of clamps. Sooner or later, the connections will loss their continuity, with predictable results.
One of the most misunderstood aspects is the notion that DC grounding an antenna mount will magically act as, or replace, an Image Plane. It will not! The only way ground strap could act as, or replace, an Image 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 an Image 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. 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. This goes for coax mounted chokes (not baluns or isolators as they are often called). If one is needed to choke RF off the feed line (coax), it is a sure sign you have an improperly installed and/or impedance matched antenna.
There is something missing here which needs to be mentioned, and that's proper bypassing of the motor leads of remotely tuned antennas. 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. If you're using a remotely controlled antenna, it behooves you to read the article.
There is one more important point which needs repeating. The better the quality of the antenna, the better it must be mounted, and the better the vehicle must be bonded, if the maximum efficiency is to be achieved. The reason for this premise is this; the better the antenna, the less RF energy is wasted as heat within the antenna itself. Since the other half of the antenna system is the vehicle, more current will flow through its structure, and the surface under it.