Common mode and differential signals in a coax line.
A simple picture below shows a piece of coax connected to voltage sources
and to a load. The definition is also simple. Sources and loads connected
between coax's inner wire and its shield are differential. Carrying such
signals is exactly what this cable is intended for, and cable's characteristic
impedance means a load you have to connect to the cable for a perfect VSWR=1.
Common mode sources and loads are connected between the coax shield and
a ground point. You can usually analyse these two groups independently:
looking for one group only you may think all voltage sources in other group
has zero value and can be replaced by short circuits.
Next picture shows a key idea of current balun.
If a cable forms a loop or a coil, then common mode source "sees" its
inductance. The inductance can be increased if a ferrite core is used.
Adding a capacitor one may construct a trap for common mode signal while
transferring differential signals at VSWR=1.
Finally you can use a piece of coax as a radiator:
The figure above shows a common mode source at the bottom side of the
coax, while in real life such a source is usually at its top side.