The only thing common about common mode current, is its commonality in mobile installations.
Common mode currents can flow in all sorts of systems, not just antenna feed lines. Since we're dealing primarily with mobile HF antennas which are fed with coaxial cable, we'll limit our discussion to them.
In an ideal world, RF flows down the outer surface of the center conductor of the coax cable cable, and returns on inner surface of the coax shield. When there is an imbalance in the antenna (for what ever reason), current will flow on the outside of the coax shield. This may not seem possible, but it's important to remember, unlike DC, RF current doesn't flow through the conductors, it flows on the surface of the conductors. The current which flows on the outer surface of the shield is called common mode current. In other words, it is the unbalanced current not returned within the coaxial cable.
This leads to a very important question. If the current isn't returned in the cable, where does it go? The answer is, it radiates! In fact, the amount of radiation from the coax cable is directly proportional to the common-mode current on that cable. This leads to another question. What causes common mode currents? Well, hopefully this article will answer that question sufficiently.
It is important to remember that a vertical antenna is one half of a dipole. That's why they're sometimes referred to as a monopole. The missing half is replaced by a ground plane typically made up of radials. Unfortunately, we can't use radials to provide a ground plane in a mobile scenario, so we rely on the mass of the vehicle, and the capacitive coupling the vehicle has to the surface under it. From an RF standpoint, the resulting ground plane is wholly inadequate, and as a result the ground plane losses are high.
Standard text sources state that mobile HF ground plane losses average between 2 and 10 ohms (10 through 80 meters). The truth is, the average is closer to 5 to 20 ohms (respectively), and may indeed be higher. The more ground plane loss, the higher the common mode currents, and the more severe the RFI problems. It should be evident that minimizing ground plane losses are important, both from an efficiency standpoint, as well as curbing both ingress and egress RFI. Bonding is one way to do this.
If you're looking for a more technical explanation of what ground planes are, read the Ground Plane Notes article.
In all honesty, there is always some common mode currents in any antenna system. The question is, how much, and it's a question without a definite answer. Suffice to say, in a mobile scenario, improper antenna mounting is the number one cause! The question you need to ask yourself is this; why spend big bucks on a decent HF mobile antenna, and then mount it on a dime store mount? Read the highlighted article, and at least you'll know the consequences of poor mounting techniques.
The best way to control common mode currents is with a choke. You can use the same type of choke that you use for the motor control leads. That is, mix 31 split beads, and preferably the 3/4 ID units. They can be purchased from DX Engineering and others. These will allow 6 to 7 turns of RG58, and 5 to 6 turns of RG8X (as shown in photo). Note that the coax is not tightly wound around the choke. In this case, the diameter is about 3 inches. Any tighter, and the core could migrate and cause a short.
The choke should be installed as close to the base of the antenna as possible. The last place to install them is inside the vehicle. After all, we want the RF on the outside, not the inside of the vehicle.
How many beads you'll need is not a cut and dried scenario, but there are factors which need to be considered. Short, stubby antennas will always exhibit more common mode current problems than full-sized antennas. The main reason is, the amount of RF flowing in the portion of the antenna below the coil is concentrated is a much smaller physical area, than it is in a larger antenna.
Those nifty appearing, clamp type mounts seemingly are the rage, but the fact remains, they too add to the level of common mode current. Remember, the further away from the ground plane the base of the antenna is mounted, the greater the common mode currents.
There is one more major consideration. If the impedance of the choke(s) is too small, and the level of common mode current is high, it is possible to over heat the choke which could destroy the ferrite material. There is a hidden factor here. If indeed the level is high enough to destroy a choke, you best revisit your mounting technique.
If you want to compare a few common solutions to the common mode problem, take a look at this page on Steve Hunt's, G3TXQ, web site. It is very interesting data.
Certainly there will be RFI issues especially if you're using an automatic antenna controller. And, the resulting RFI might be doing funny things to the electronics. However, the biggest factor is what we're doing to our receive capabilities. If common mode current can flow out as a radiated signal, then other signals (common mode noise) can get in the same way!
What really matters is the S+N/N ratio (signal plus noise to noise) said antenna system can induce in our receivers front end. While receiver dynamics are important, common mode noise exacerbates the problem. Fact is, in a mobile scenario, the coax cable makes a better noise antenna than it does a signal antenna! It is, after all, inside the vehicle where most of the electrical noise is generated in the first place.
This brings up one more point. If you wish to minimize both common mode currents, and common mode noise, the requisite chokes must be mounted outside the vehicle! Remember this; Any part of the coax or control cable before the choke, it part of the antenna, and should be kept outside the vehicle.
What ever RF is dissipated in the requisite choke(s), and as ground plane loss, goes up in heat, and is not radiated. Quantifying the amount of loss in any given installation isn't all that difficult. At the bottom of this page on efficiency is one methodology, but that only tells you what your approximate efficiency is. It doesn't tell you how good it could be if the antenna in question was better mounted, or how much better a full-sized antenna would be. Until you go through the exercise yourself, you'll have to believe me when I say, the difference can be as much as 25 to 30 dB, although the average is closer to 15 dB.
In my Antennas, Commercial article, is a section on stubby antennas. I mention them again here, because they have become the leading cause of common mode current RFI. They're popular for a number of reasons, not the least of which is their ease of mounting. The hidden factor here is, the usual mounting methodology greatly increases common mode currents. If you're using one of these stubby antennas, it behooves you to read the article.