15 - RF power measurement

Abstract

Measuring radio frequency (RF) power is essentially the same as measuring low frequency AC power, but certain additional problems present themselves. For a continuous wave (CW) signal, the issue is relatively straightforward because the signal is a constant amplitude sine wave. For on/off telegraphy, the problem gets somewhat more difficult because the waves are not constant amplitude, and the average RF power depends on the ratio of on time to off time. In the case of the sine wave, a peak voltage reading instrument, such as a diode detector can be calibrated for root mean square power by the simple expedient of dividing the indication by the square root of two. If the meter measures power directly, then no waveform correction factor is needed. RF power meters use a number of different approaches to making the measurement. Some instruments measure the current or the voltage at a resistive load, and depend on the equations P = I2R or P = V2/R. Other methods are based on the fact that power dissipated in a resistive load is converted to heat, so the temperature change before and after the RF power is applied can be used as the indicator of RF power.