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| Difference between coax cable and shielded cable |
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| tkamiya:
What exactly is a difference between a shielded cable and a coax cable? They are both coaxial in nature. Coax is made for a certain impedance and RF attenuation and mode. I know that.... but what really determines if a cable is a coax or a shielded cable? (yes, I know coax is a form of shielded cable and vise versa) I always wanted to know this. |
| AG6QR:
Shielded cable is any cable with an outer shield. Inside the shield may be multiple conductors, and they generally won't be coaxial with each other. In other words, if you slice a three-conductor stereo headphone cable apart, you won't find three concetric circles of three different diameters. Instead, you'll find two ordinary (non-coaxial) wires surrounded by a conductive shield. |
| tkamiya:
I see.... that's very true. OK, then what's the difference between single center conductor shielded cable and a coax? |
| Berni:
The coax cable is as the name suggests coaxialy symmetrical. This makes the center conductor evenly spaced to the shield at all points and directions in a cable. Combined with the dielectric material between the two this gives the cable a so called characteristic impedance. That's the 50 Ohm or 75 Ohm or 120 Ohm that you often hear. In a normal multi conductor shielded cable there is no such even geometry and as such the characteristic impedance of the wires are all different and random. This impedance is important in RF applications as driving and loading the cable with the same impedance as the cable causes the cables parasitic capacitance and inductance to "disappear" and allows signals from 0 to >1 GHz to be sent for miles along such a cable and still make it out the other end without significant distortion. How and why this works is a topic for a whole new very long thread. |
| hagster:
Any cable where the conductor and shield are centered on the same axis is coax. It may of course have a different characteristic impedance depending on the ratio of the diameters and permittivity of the insulator. However coax is only one way of having a controlled impedance. Many digital systems use twisted pair type wires to achieve differential signaling with a relatively controlled impedance(often 100ohms). |
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