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What does it mean when the impedance of a cable is 75 or 110 ohms?

In a nutshell, OHMs is a measure of how much resistance a conductor poses to a certain amount of current when motivated by a certain amount of voltage. In a DC circuit (like from a battery) it's an easy thing, because the conductor characteristtics are the same for any combination of resistance, voltages, and current flow.

When you start talking about alternating current (AC) or variable DC, the characteristics of the conductor change with frequency.

Cables of certain type (twisted, untwisted, coaxial, twinaxial, etc) exhibit fairly consistant properties that can be adjusted to some degree by changing the composition or arrangement of the components.

So, if a cable says it's a certain impedence, the impedence is the nominal resistance of the cable. Matching the impedences of one system to another (like a cable connection to a transmitter) ensures that the maximum power is transferred between the two systems.

Most a/v stuff uses 75 ohm coax, most communication / radio systems use 50 ohm coax. UTP has a characteristic impedence of 10-120 ohms (again, depends on the construction of the cable).


There's lots of variables, and this is an extermely abreviated explanation.... anything specific you need to know?

FWIW

Scott

 
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