Define Velocity of propagation for co-axial cable?

Answer 1

The velocity factor is the speed at which an RF signal travels through a material compared to the speed the same signal travels through a vacuum. The velocity of propagation is inversely proportional to the dielectric constant. Lowering the constant increases the velocity.

Generally, the higher the velocity factor, the lower the loss through a coaxial cable.

General Rules for Coaxial Cable

D = diameter of insulation under the shield

d = diameter of inner conductor.

Velocity Factor, Velocity of Propagation, Vp

The higher the velocity factor, the lower the loss through the cable.

Raising the D/d has no effect on Vp

Raising the dielectric constant lowers Vp

Capacitance

Raising the D/d ratio lowers capacitance

Raising the dielectric constant raises capacitance

Impedance Raising the D/d radio raises impedance

Raising the dielectric constant lowers impedance

Attenuation or Loss

Raising the D/d ratio lowers attenuation

Raising the dielectric constant raises attenuation

So in simpler terms the larger amount of dielectric between the center conductor and sheath equals less loss in RF. Trunk cables have less loss than feeder legs.