High frequency characteristics of the cable itself usually mean transmission characteristics, that is, frequency characteristics of attenuation and phase delay of sinusoidal waves. This is based on the assumption that the frequency of sinusoidal waves do not change on the transmission line.
This assumption is also used in most electric/electronic circuits other than cables. If the frequency of the input sinusoidal wave changes in the circuit, concepts like the transfer function (system function) becomes meaningless, and basic theories on electricity, such as alternating current theory and transient phenomena analysis, cannot be used.
Therefore, invariance of frequency is very important. Why is the frequency of a sinusoidal wave invariant between the input and output?
This is the question.