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It's very much frequency dependent. For example, above about 5 MHz,
the "50 Ohm" standard starts to break down, but the exact details are
also dependent on the coax diameters, etc. Larger cables tend to
stick closer to the 50 Ohm standard to higher frequencies.
Particular field work a few years ago was made practically laughable
due to the need for some finely-tuned preamplifiers (tuned to
nearly-oscillating). The combination of that plus dealing with thin
coax cable resonances in the 4 - 10 MHz range made things nigh unto
impossible. A combination of damping resistors at either cable end,
plus inductive loading, made things workable, but it was a bad
oversight to have neglected the cables' effects on the experiment.
Stick any "50 Ohm" cable on a network analyzer and sweep through the
frequencies. Cable manufacturers are well aware of this, and
different cables are recommended when starting to push the boundaries
of what's feasible.