TRANSMISSION LINES

Chapter 14 TRANSMISSION LINES

Lessons In Electric Circuits Volume II – AC Book
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Lessons In Electric Circuits Volume II – AC Book

  • Chapter 14TRANSMISSION LINESContents 482,14.1 482,A 482,50-ohm 482,cable?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 483,14.2 483,Circuits 483,and 483,the 483,speed 483,of 483,light . . . . . . . . . . . . . . . . . . . . . . . 474 485,14.3 485,Characteristic 485,impedance . . . . . . . . . . . . . . . . . . . . . . . . . . 476 491,14.4 491,Finite-length 491,transmission 491,lines. . . . . . . . . . . . . . . . . . . . . . 482 498,14.5 498,“Long” 498,and 498,“short” 498,transmission 498,lines . . . . . . . . . . . . . . . . . . 489 501,14.6 501,Standing 501,waves 501,and 501,resonance . . . . . . . . . . . . . . . . . . . . . . . 492 521,14.7 521,Impedance 521,transformation. . . . . . . . . . . . . . . . . . . . . . . . . 512 529,14.8 529,Waveguides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52014.1A 50-ohm cable?Early in my explorations of electricity, I came across a length of coaxial cable with the label “50ohms” printed along its outer sheath. (Figure 483,14.1) Now, coaxial cable is a two-conductor cablemade of a single conductor surrounded by a braided wire jacket, with a plastic insulating materialseparating the two. As such, the outer (braided) conductor completely surrounds the inner (singlewire) conductor, the two conductors insulated from each other for the entire length of the cable. Thistype of cabling is often used to conduct weak (low-amplitude) voltage signals, due to its excellentability to shield such signals from external interference.I was mystified by the “50 ohms” label on this coaxial cable. How could two conductors, insulatedfrom each other by a relatively thick layer of plastic, have 50 ohms of resistance between them?Measuring resistance between the outer and inner conductors with my ohmmeter, I found it to beinfinite (open-circuit), just as I would have expected from two insulated conductors. Measuringeach of the two conductors’ resistances from one end of the cable to the other indicated nearly zeroohms of resistance: again, exactly what I would have expected from continuous, unbroken lengthsof wire. Nowhere was I able to measure 50 Ω of resistance on this cable, regardless of which pointsI connected my ohmmeter between.473