All You Want To Know About CATV Interference  By:  Chuck Manetta  N4YJC

Part #3:   Now for a little theory!
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There really are only three ways an unwanted signal can interfere 
with wanted television signal:

Radiated Interference  -  This type of interference is usually 
the fundamental frequency and is usually radiated directly out of 
your antenna!  But this method can also include signals radiated 
through your shielding, off of your shielding, off of your ground 
wire or anything else that could directly radiate your signal. 

Conducted Interference  -  This interference is propagated 
directly via a conductor such as A/C wiring or a common ground 
system.  There are two different types of conducted interference, 
differential and common mode.  I will explain these shortly.

And Induced Interference  -  Induced interference starts out as 
radiated interference and is picked up or received by some 
internal or external part of the affected television or its 
associated wiring. This type of interference is usually common 
mode.

Now to explain in more detail the distinct differences between 
differential mode and common mode interference.  In any 
electrical circuit there must be two paths, the forward path and 
the return path.

In the differential mode signal, it is conducted via a two wire 
pair, such as your A/C power cord or your antenna lead in.  In 
this case the circuit is created between the two wires in the 
system.  In one wire and out the other!  It is called 
differential because under normal circumstances these two wires 
are out of phase with each other.  This circuit occurs without 
the necessity of an earth ground.

With the common mode signal, the unwanted signal can either 
overpower the normal out of phase signal and make the multi-wire 
appear to be a single phased long wire antenna with the return 
path being through the internal circuitry (IE: Capacitors) of the 
television to the earth ground, thus completing the circuit.  Or, 
the signal can ride the outer coaxial shield and find the earth 
ground through the same internal circuitry to chassis ground.

It is important to make the distinction between the two, because 
leakage into or from the cable system, a differential-mode 
phenomenon, is clearly a cable company responsibility, but a 
common-mode signal conducted only via the coaxial shielding 
clearly is not.