Last Modified: April 14, 2014
Contents: Basics; A Different Slant; Your Average Vehicle; Mounting Location Issues; Odds & Ends;
It is the metal mass directly under the antenna, not what's along side, that counts. Remember too, DC continuity is not the same as RF continuity!
Many amateurs harbor the notion that DC grounding an antenna mount will magically act as, or replace, a ground Plane. It will not! While a ground strap may indeed DC ground the antenna's base, and it just might RF ground it too depending on the straps length and width versus the frequency of operation, it is by no means a replacement for an adequate ground plane under the antenna! Incidentally, the term ground plane is a bit of a misnomer, and here's why.
In an HF mobile scenario, the body of the vehicle and the capacitive coupling to the surface under the vehicle, is acting as a ground plane, and 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 losses are somewhat higher due to improper mounting, and bonding. It is also possible to have higher ground losses on 40 meters, than on 80 meters.
Since ground losses dominate the efficiency equation, decreasing them by just one ohm, can make a significant increase in ERP (effective radiated power). Further, excessive ground losses directly relate to the level of common mode currents. Common mode current causes all sorts of RFI issues, both ingress and egress, and a can drastically reduce the receiver's SNR. This point should not be underestimated! Put another way, excessive ground losses can turn an otherwise efficient antenna, into an also-ran.
Since amateurs deal with a variety of grounds (earth ground, DC ground, RF ground, ground plane, chassis ground, isolated ground, etc.), it's no wonder so many are confused about which ground is which. This said, in a mobile scenario, there is only one other (antenna) ground we need to deal with, and that's a proper RF return for the coax shield. Remember, RF must flow back to its source. It will do so in the shortest path it can find (the one with the least resistance). Ideally that's within the superstructure of the vehicle the antenna is mounted on. However, improper mounting (atop long posts, extended brackets, clamps, and luggage racks as illustrated in the drawing) causes an inordinate amount of RF to flow on the outside of the coax (common mode current), or down inadequately-choked motor control leads if so equipped.
As noted, the average recognized figure for ground loss, varies from 2 to 10 ohms (10 through 80 meters). It may be considerably higher than this value, but will never be lower! Further, ground losses are proportional to the square of the field intensity. So if you double the field intensity, the power losses increase by four times! This in the predominant reason that ground losses should be kept as low as possible, especially when using physically short antennas (≤1/8λ).
A Different Slant
If you still don't understand the importance of a ground plane, here is a short treatise from Dr. James D. Van Putten, Jr. He is the Physics professor emeritus of Hope College in Holland, Michigan.
I have been thinking about how to help persons who cannot seem to understand the problems associated with RF currents flowing on control or other lines. Here is a thought.
Electric fields begin and terminate on charges. By convention the field lines run from positive charges to negative charges. A positive charge does not have to be a positive electron (although they exist) but equally effective is the absence of a negative charge. Solid state physicists call these holes. An example of the absence of an electron is seen in capacitors. Think of a capacitor as two parallel plates separated by an insulator. When electrons flow onto a plate, they repel electrons on the other plate which flow out into the circuit. This creates an absence of electrons on the plate from which the electrons have been repelled resulting a positively charged plate. The electric field in the capacitor then runs from the absence of electrons on one plate to the electrons on the other.
A similar effect is present in antennas. When the transmitter drives electrons onto one wire of a dipole antenna (creating a surplus of charges), it draws electrons from the other wire (creating an absence of electrons). The electric field runs then from the absence of electrons on one wire to the excess of electrons on the other. This flow of electrons surges back and forth at the frequency of the signal being transmitted. The surging electrons are the current in the antenna wire.
When one uses an antenna that is essentially only one wire such as a mobile antenna or a vertical antenna, the transmitter drives electrons into that wire while drawing electrons from whatever other conductor is connected to the return line to the transmitter. This is often the outside of the shield of a coax or any other wires associated with the antenna or its tuner. In an automobile antenna, this can be the body of the car. Unless specific actions are taken to prevent the current from flowing on outside of the coax or on associated wires, the current will flow back and forth over the leads to the transmitter and any equipment connected to it. This RF can cause nips to the lips from microphones as well as malfunctioning in equipment connected to the transmitter. Prevention of the wayward current flow can be prevented by the judicious use of ferrite chokes.
The mental image of the electric field lines running from the absence of charge to real electrons can also help one to understand why radials or ground planes are necessary on any vertical antenna. There must be a conductor upon which the transmitter impresses electrons and another from which it draws electrons. This can be metal, sea water or conductive soil. As the power loss is proportional to the resistance of that conductor one can easily see why multiple radials or sea water is one reason that these increase the efficiency of a vertical antenna system.
Center fed, vertical half wave antennas require a more sophisticated analysis. In such an antenna there is a both a conductor on which to induce charge and a conductor from which to draw charge. However the electric fields associated with a vertical antenna must also obey all of Maxwell's Equations which require image currents to flow in any nearby conductor. These vertical image fields behave somewhat differently than horizontal image fields. The importance of this difference is that all vertical radiators require a conductive area in the near field around the antenna itself.
Your Average Vehicle
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. They are, after all, screwed together! 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.
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.
When you improperly bond and/or wire your installation, it is possible to create a ground loop within the vehicle's superstructure. When you create a ground loop, the resulting affect will often appear to be RFI. This is the reason ground loops are the toughest problems to find and correct. Thus, under no circumstances, should the body and/or frame be used as the DC power ground return. Doing so on a modern vehicle is a prescription for RFI, and operational problems with the various on-board electronics. Returns should always be directly to the battery or jump points as the case dictates.
Whether you use a monoband antenna, or 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 you'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 (and by how much), depends on what the conductivity under the vehicle was when the antenna was first resonated.
Mounting Location Issues
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!
To restate the above, 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, cap hat use, etc.). Remember the key to maximum efficiency is placing as much metal mass directly under the antenna as possible! That certainly isn't the case in the 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 in performance is very minimal. The reason is, the amount of capacitance between the bed wall, and the antenna's mast, seldom exceeds 2 or 3 pF. Even on 10 meters, this amount of capacitive loading is almost immeasurable. 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. Free and clear in other words.
The individual parts—the antenna, the vehicle and the surface it's atop—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—it makes little difference if it is DC congruent!
Odds & Ends
Some misguided mobile operators believe that adding a "counterpoise" to their mobile antenna will increase efficiency. This so-called counterpoise (a misnomer to be sure), is typically in the form of a spirally-wound, hamstick-type antenna. The truth is, it is a waste of resources, as the effectiveness is so slight, you couldn't measure it with laboratory-grade equipment! The same can be said of "V" configurations of two identical mobile antennas.