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Velocity of Propagation

the actual speed at which the signal travels from one point to another along a conductor. The materials surrounding the conductor affect this speed. For example, in air, the velocity of propagation is 300 million meters per second and in FR-4 printed circuit boards it is about half that speed. It is usually expressed as a percentage of the speed of light. Air is the fastest medium (100%), Teflon is slower than air (70%), and FR-4 is slower than Teflon (47%). Typically connectors are faster than printed circuit boards; those connectors with the most air being fastest. Velocity of propagation affects skew. See dielectric constant.

VP (%) = 1/(√K) x 100

where VP = velocity of propagation and K = dielectric constant