Radio Frequency Power Measurements

This chapter presents a discussion on radio frequency (RF) power measurements. The chapter discusses several methods of measuring power, error sources in RF power measurement, and typical commercial instruments that are used for RF power measurements. The assumption in this chapter is that the RF power is being measured to determine the output level produced by some circuit or device. In DC circuits involving only resistance, RF measurement is straightforward: measuring the voltage and current and calculating their product. In AC sine wave, over one cycle, the voltage rises from zero to a positive peak value and then falls back to zero. The current follows a similar path. The current then reverses its direction, rises to a negative peak, and then falls back to zero. If peak power is required (rarely), then the peak voltage and peak current should be used. The real power in an AC circuit is the equivalent DC power that would produce the same amount of heating in a resistive load as the applied AC waveform. If the load is resistive—that is, there are no capacitive reactances or inductive reactance elements present—the root mean square (RMS) values can be used to find the equivalent or RMS power. The chapter concludes with a discussion of micropower measurements and some error and uncertainty sources.