I do not think I have ever had a problem with common mode issues in an mobile before.
Make sure have common mode issues before you install a fix for common mode issues! It is kind of like giving yourself the vaccination against smallpox than walking into to a tent full of people dieing from Ebola that Smallpox vaccination did nothing for you against Ebola.
I have seen common mode issues in a mobile before. They come up more often when using a magnet mount antenna, or when using a mount that doesn't have enough metal under it, like mirror mounts. If you have a solid bolt on mount, you won't normally have these problems.
Remember the mostly dead (now) cb myth where people would tune their mobile antennas by cutting the length of coax? That only seemed to work because of common mode currents on the feed line. A whole myth's existence was dependent on common mode currents existing in a mobile environment...
Keep in mind that an ugly choke aka air choke still breaks the rule we where taught as kids about CB and not coiling up excess coax in a circle.
Air chokes don't really break those rules. If made properly, they will work well for the needed frequencies, what the rule in question was meant to warn against is randomly coiled wires.
Why? That choke will also cause some signal loss.
The air choke itself does not cause additional losses for the signals traveling inside the coax, except for the losses that the few feet of additional coax will naturally have (yea, you won't notice these losses as they are insignificant unless the coax is damaged). Think of it this way, the inside is a sealed system, and acts completely independently of the outside of the coax. The signal on the coax that is affected by the air choke is the signal traveling on the outside of the coax, aka, common mode currents.
An air choke is not the best way to get impedance match that is like spitting into the wind.
I agree with this statement. It takes more than a choke (if a choke is in fact needed) to get a match. The choke is, in and of itself, not part of the matching system, it simply prevents unwanted common mode currents. Tuning an antenna has requirements that go far beyond using a choke.
I think I should also add that a mobile environment is not the best place for an air choke. To often it will be right next to a metal surface, which will affect the choke, possibly making it less effective, and potentially useless. In these environments, if a choke is needed, I would highly recommend a ferrite based choke. Ferrite based chokes are not affected by the environment around them, like the sheet of metal they are inevitably next to. I would use them for all my RF choking needs as they are better in pretty much every way, shape, and form. They also don't cost any more than an air choke, and are often just as easy to make, although in many cases you do have to put a new end on the coax as well.
The DB
