# Cut-off frequency in coaxial cable

The cut-off frequency of an RL circuit is described by the following equation: $$f = \frac{R}{2\pi L}$$ where R is the resistance of the circuit and L is the inductance. Applying this equation to a straight length of RG-58/U coax cable (no resistors or inductors in the circuit) with the following specs: $$Length = 5m$$ $$OD = 5mm$$ $$ID = 0.81mm$$ $$L = 0.00000159H$$ $$R = 0.163\Omega$$ yields a cut-off frequency of 16.32KHz. I know the cable is good for at least a few GHz. What's going on here? Does the formula not work for distributed inductances?

• That formula does not work for distributed inductances, no. And I'm suspicious of your resistance measurement -- it seems too high. – TimWescott Jan 8 '19 at 20:49
• That might apply to short circuit current BW but if you use matched R source and load, the RLC Telegrapher's equation prevails for distributed RLC – Tony Stewart Sunnyskyguy EE75 Jan 8 '19 at 20:50