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Short answer: 50 Ohms refers to the "characteristic impedance" of the coaxial cable design and is a necessary dimension to know in order to efficiently couple to inputs and outputs with minimal losses due to imput impedance mismatches.

Long Answer: Many people are familiar with the concept of using Ohms as a straightforward measure of DC resistance in a circuit that are familiar from HS science classes. There is a resistance component in every coaxial cable, but that is directly proportional to the cable length. That is not to what the 50 Ohm designation refers. In AC circuits there are capacitive reactive and inductive reactive effects in addition to purely resistive effects and they are all measured in Ohms and combined using complex math (using both real and imaginary numbers.) Inductive reactive effects are directly proportional to frequency and capacitive effects are inversely proportional to frequency. Coaxial wires are used as conductors whenever the frequency of the transmitted waveform cannot be transmitted by ordinary wire conductors without tremendous losses due to reactance. In fact, the enegy in a coaxial cable exists as waves in the insulator between the inner conductor and the outer conducting sheath. These waves have both traveling and standing wave components. The "characteristic impedance" refers to the design features of the coax cable as they relate to these standing waves and is independent of cable length.

Characteristic impedance is also used in stereo speakers. If you connect 4 Ohm speakers to a system design for 8 or 16 Ohm speakers, they will probably work, but not very well. Same with your walkman or I-pod headphones.

For a more detailed explanation of standing waves, take a graduate course in Microwave Circuit Engineering and then you'll know what's behind all the smoke and mirrors. But be prepared for really ugly math.

2007-03-13 03:00:02 · answer #1 · answered by Z-man 3 · 0 0

There are other coaxial cable impedances besides 50 Ohm

75 Ohm was already mentioned -- most commonly used to connect cable TV inside your house (and to route cable TV to households).

100 Ohm controlled impedance twinax cable is used in some high-end industrial instrumentation applications.

62 Ohm coax is sometimes used as RF feed to antennas.

50 Ohm cable is probably the most common impedance for a broad range of cable diameters and applicaions.

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2007-03-13 03:34:20 · answer #2 · answered by tlbs101 7 · 0 0

50 ohms is the impedance which is the effective resistance at the frequencies that the cable will be used at. 50 ohms is usually used for thin ethernet and satellite feeds and 75 ohms is also common for video and UHF receivers.

You need to terminate the ends with similar impedance otherwise you can get signal loss and distortion due to reflections.

2007-03-13 02:42:38 · answer #3 · answered by Anonymous · 0 0

Ohms is a measure of electrical resistance; how hard it is for current to go through the wire.
So the electrical resistance of the wire is 50Ohms.

2007-03-13 02:37:16 · answer #4 · answered by rgarf 2 · 0 0

I answered this question a few days ago, why not search for "50 ohm" using the search function at the top of the page?

2007-03-13 02:54:47 · answer #5 · answered by Anonymous · 0 0

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