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Re: Then what is the frequency response of wire?

The R's L's and C's of transmission lines aren't actual lumped elements, and treating them as such turns out to be a poor computational technique. They are actually differential quantities whose units are Ohms, Henries and Farads per unit distance respectively. The mathematical treatment of these quantities leads to the so-called telegrapher's equations which are partial differential equations that predict the voltage and current on the line as a function of two variables - position and time. There's a decent treatment of this topic here , and probably better ones to be found by googling.

These equations assume the so-called TEM mode of propagation. TEM mode is most simply described in the following way. Assume an infinitely long, straight coax, and a cartesian coordinate system with the z axis lined up with the center conductor. With TEM propagation, at any value of the z coordinate, say z0, the E and H (electric and magnetic) fields will be in a plane perpendicular to the z axis, passing through z0. That is, they won't be "sticking into" or "sticking out of" that plane. This holds true for coaxial structures provided the losses in the conductor aren't very large. But if you have something like a twisted pair, that's a pretty complex geometry for which this model is not a good fit. The coax case is nice because its cross-section does not change as you move along the z axis.

To make a long story short, the solution of the telegrapher's equations for simple geometries like coax leads to a closed-form solution for the two-port circuit matrix. If you have a SPICE simulator, such as the nifty freeware LTSpice (AKA SwitcherCAD) from Linear Technology, and the parameters of the coax from its data sheet, you can compute the cable's frequency and transient response for a given source and load impedance. You'll find that these behaviors are not an intrinsic property of the cable itself, but are a very strong function of the source and load impedances.

If you want to download a copy of that freeware, it can be found here. I can provide you with a simple project that simulates a 30 ft interconnect using Belden 89259, which allows for varying the source and load impedances and computing the frequency and transient responses (square wave).

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