>What does it mean that a cable (for networking, or TV, for instance) has an "impedance of 50 ohms", for instance?
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Television sets typically have two antenna connectors. One can be used to directly connect to a folded dipole antenna and has an input impedance of 300 ohms. A 300 ohm twin lead cable connects this terminal to the antenna.

The other connector is a coaxial connector that has an input impedance of 75 ohms. It is commonly used to connect to a VCR output or Cable TV (can also be connected to an antenna that has an output impedance of 75 ohms.

An ethernet card can have a coaxial connector. The circuit impedance the connector is connected to is 50 ohms so the cable impedance should also be 50 ohms.

Basically, if you choose the wrong wire (primarily at high frequencies) you will have increased reflections in the wire. The effects on a TV are easily seen in the picture (ghosting).

Manufacturers design input/output circuits to have a particular impedance. In order to assure best operation, the cable should match that impedance.


>I understand that impedance (Z) is calculated as sqrt(R^2 + X^2). Resistance (R) is dependent on the length of the cable, and the cable might be cut to different lengths. Reactance (X), on the other hand, is frequency dependent. So what does the "50 ohms" (or whatever the value stated) actually mean?
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>TIA, Hilmar.