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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Transmission Lines
Some basics should be briefly covered. Any two, long parallel conductors have an associated distributed inductance and capacitance, which imparts to them a characteristic impedance of √(L/C). That is why it is often said that the impedance of a typical coaxial cable used for radio-frequency purposes (RG-6) is 75 Ω. The unshielded twisted pair inside the Ethernet cable has an impedance typically 100 ± 15 Ω above 1 MHz.
This leads to the theory of transmission lines. When an AC signal is sent down this transmission line, it can travel great distances. If the transmission line is ideal, that is, if it has zero AC and DC resistances, there are absolutely no losses in the cable, because pure reactive elements (L and C) can store ...

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