Introduction

In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts – the more wattage, the more power, or equivalently the more electrical energy is used per unit time [1]. Energy transfer can be used to do work, so power is also the rate at which this work is performed [2].

For systems or circuits that operate at microwave frequencies, the output power is usually the critical factor in the design and performance of that circuit or system. Measurement of the power (signal level) is critical in understanding everything from the basic circuit element up to the overall system performance. The large number of signal measurements that can be made and their importance to system performance means that the power-measurement equipment and techniques must be accurate, repeatable, traceable, and convenient.

In a system, each component in a signal chain must receive the proper signal level from the previous component and pass the proper signal level on to the succeeding component. If the output signal level becomes too low, the signal becomes obscured in noise. If the signal level becomes too high, though, the performance becomes nonlinear and distortion can result. The uncertainties associated with the measurement of power also play a very important role in the development and application of microwave circuits. For example, a 10 W transmitter costs more than a 5 W transmitter.