Cable shortening coefficient

The wavelength shortening factor in a feeder line (cable) indicates how much shorter the wavelength in the line, filled with a dielectric lk, is compared to the wavelength l in free space:

n=l/lk

For shielded lines fully filled with insulation,

n=ve

where e is the dielectric constant of the insulation material. For air lines, n=1. With a known shortening factor n, the wavelength in the cable is:

lk=l /n

In foreign reference literature, instead of the wavelength shortening factor, the transmission line velocity factor (K=1/n) is given.

Radio waves in free space propagate at the speed of light (c=3*10⁸ m/s). In a transmission line, their speed is reduced by a factor of K. The value of the transmission line velocity factor depends on the feeder’s design.

The cable shortening coefficient (SC) is a number by which the wavelength in a vacuum must be multiplied to obtain its length when propagating oscillations of the same frequency in a transmission line. The shortening coefficient depends on the dielectric used to fill the space between the conductors of the transmission line and on its construction. For coaxial cables with a polyethylene dielectric, the shortening coefficient is 0.66; for coaxial cables with a fluoroplastic dielectric – 0.86. The shortening coefficient of a coaxial cable with air filling and of an open line with air dielectric is equal to 1 (for more precise calculations, the shortening coefficient is approximately 0.96–0.99). The shortening coefficient of open lines with a solid dielectric is usually 0.82.

Table of Shortening Coefficients

Standing Wave Ratio