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Propagation Delay

the speed or rate with which a signal travels between the input and output of a transmission line, usually expressed in nanoseconds per foot. The materials surrounding the conductor affect this speed. Space or vacuum is the fastest medium (1 nsec/ft), whereas air is 1.0167 nsec/ft. Teflon is slower than air (1.45 nsec/ft), and FR-4 is slower than Teflon (2.12 nsec/ft). See also electrical length.

PD (nsec/ft) = (√K)/Co , where PD = propagation delay

K = dielectric constant and Co = speed of light » 1 ft/nsec