Radiation from transistor in microstrip line

Closed form expressions for the field and total power radiated from a transistor in a microstrip line are given in terms of the three-port S parameters of the device. The expressions are confirmed experimentally. Power radiated can be substantially higher than that from an equivalent open circuit line with equal power incident with actual values dependent on the transistor gain. Such calculations are important in assessing the likely radiation pattern perturbation in planar active antennas.