Digital Circuit Boards: Mach 1 GHz

3.6 The Radiation Pattern Along a Transmission Line

The energy that is transported along a transmission line moves in the electromagnetic field between conductors. The velocity of propagation is c/, where c is the velocity of light and ε _R is the relative dielectric constant. The only area along the transmission line that radiates or cross couples is where the wave is changing amplitude. For a step function, this area is the leading edge, as it moves down the line. As the rise time increases, the radiating area also increases. As an example, a wave travels 1.5 cm during a rise time of 100 ps. If the line is series terminated at the logic source and the line is 6 cm long, the radiation pattern lasts for one round trip of the wave or for 800 ps. The radiating pattern is the same for the outgoing and reflected wave. The rise-time frequency for a radiated signal is 3.2 GHz. In the case of a fast leading edge, the radiation at a distance from a transmission line appears to emanate from a moving current element, that is, a moving dipole.