COAXIAL CABLE CAPACITANCE AND CUT OFF FREQUENCY BASIC INFORMATION AND TUTORIALS

A coaxial transmission line possesses a certain capacitance per unit of length. This capacitance is defined by:


C = 24ε log pF/(D/d) Metre


where
C is the capacitance
D is the outside conductor diameter
d is the inside conductor diameter
ε is the dielectric constant of the insulator.

A long run of coaxial cable can build up a large capacitance. For example, a common type of coax is rated at 65 pF/metre. A 150 metre roll thus has a capacitance of (65 pF/m) (150 m), or 9750 pF.

When charged with a high voltage, as is done in performing breakdown voltage tests at the factory, the cable acts like a charged high voltage capacitor. Although rarely if ever lethal to humans, the stored voltage in new cable can deliver a nasty electrical shock and can irreparably damage electronic components.


The normal mode in which a coaxial cable propagates a signal is as a transverse electromagnetic (TEM) wave, but others are possible – and usually undesirable. There is a maximum frequency above which TEM propagation becomes a problem, and higher modes dominate.

Coaxial cable should not be used above a frequency of:


FCUT−OFF = 1/ 3.76(D + d) √ε


where
F is the TEM mode cut-off frequency
D is the diameter of the outer conductor in mm


d is the diameter of the inner conductor in mm ε is the dielectric constant When maximum operating frequencies for cable are listed it is the TEM mode that is cited. Beware of attenuation, however, when making selections for microwave frequencies.

A particular cable may have a sufficiently high TEM mode frequency, but still exhibit a high attenuation per unit length at X or Ku-bands.