An Ohm is a unit of resistance — that is the resistance to the flow of electrical current through a circuit. In the most basic applications, where we are dealing with DC or Direct Current electricity, such as that from a typical 12-volt car battery, we are measuring the resistance in Ohms.
Alternating Current (AC) is more complex because it is not merely the magnitude (relative strength) of the signal that is being assessed, but the phase of the signal as well. An AC signal (waveform) is constantly switching its phase between negative and positive a certain number of times per second. For regular 120-volt house electricity, it changes phase 60 times per second. This is often abbreviated as 60 Hz.
Now, not all coaxial cable is created equal and that is where the coaxial cable impedance comes into play. It is the coaxial cable’s physical characteristics that will determine its impedance. According to Wikipedia, “The characteristic impedance of the coaxial cable (in Ohms) is determined by the dielectric constant of the inner insulator and the radii of the inner and outer conductors.” The cutaway drawing above is helpful in visualizing these characteristics. With these details in mind, over time, the industry settled on two characteristic coaxial cable impedances for the vast majority of applications (>90%): 50 Ohm and 75 Ohm.
50 Ohm Coaxial Cable was selected as the ideal compromise; offering high power handling AND low attenuation characteristics. Another interesting application for 75 Ohm Coaxial Cable isCoaxial Digital Audio. This is the orange or black colored RCA jack commonly seen on HDTV’s, BluRay Disc Players and other Home Theater Gear. It is sometimes labeled as S/PDIF Out. It transfers the 5.1 Channel Dolby Digital Surround Sound signal to the home theater system for decoding and playback into the various speakers. Digital signals generally look like a square wave instead of the typical sine wave seen with analog signals like AC power or analog radio/TV.
One last crucial point in regards to coaxial cables. The Impedance of the various devices being connected as well as the Coaxial Cable itself must match. So if you are, for instance, connecting a 75 Ohm video camera connection to a studio monitor, the coaxial cable must also be 75 Ohm AND the connectors on the coaxial cable (i.e. BNC connectors) must be 75 Ohm in Impedance. Every single time you have a mismatch in impedance, say between a 50 Ohm Coaxial Cable and a 75 Ohm Coaxial Connector (i.e. BNC), a standing wave develops.