Actually, the length of a cable has little effect on its impedance characteristics. Impedance characteristics are an intrinsic property of the cable and are determined by its physical dimensions (the diameter of the inner and outer conductors) and the dielectric material between them.
The impedance formula is:
where,
R: resistance per unit length, L: inductance per unit length,
G: conductance per unit length, C: capacitance per unit length,
ω: signal angular frequency
Not all formulas include a cable length parameter, which means that the impedance characteristics are determined by the geometry of the cable (e.g. cable width, spacing) and the dielectric material, and are independent of the total length of the cable.
However, cable length can affect signal transmission characteristics such as attenuation and phase shift, especially at high frequencies. Longer cables result in greater attenuation and can also cause signal reflections if the cable length is not a multiple of the signal wavelength, but these factors do not change the intrinsic impedance of the cable itself.
To summarize, when the impedance characteristics remain constant regardless of cable length (e.g., the most common impedances for coaxial cables are 50Ω and 75Ω), the length affects the performance of the cable in the transmission environment.
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