A time-domain reflectometer launches a step into a polyethylene coaxial cable ( , )

PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Electromagnetic waves and their propagation · Electromagnetic waves and their propagation

Modes, Packets and Media, Ch Transmission lines and electromagnetic waves in matter, plasmas and guides

Problem

A time-domain reflectometer launches a 1.0 V1.0\,\mathrm{V} step into a 50 Ω50\,\Omega polyethylene coaxial cable (εr=2.25\varepsilon_r=2.25, n=1.5n=1.5). Reading the oscilloscope trace: the incident step appears at t=0t=0; a reflected feature of height −0.15 V-0.15\,\mathrm{V} (measured relative to the incident step) appears at t=180 nst=180\,\mathrm{ns}; the trace then settles to a steady plateau and shows no further features out to 1 μs1\,\mu\mathrm{s}. ((a)) Translate the trace into a fault distance along the cable. ((b)) Translate the reflection height into the impedance of the fault, and state whether it is higher or lower than the cable's characteristic impedance.

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