TABLE OF CONTENTS Why not just measure voltage and current? Ohm's Law is the linear estimate of Maxwell's electromagnetic field equations. Maxwell's equations show that voltage and current are actually waves dependent on frequency. Think about measuring a voltage sine wave with an oscilloscope. The location of the observation point determines the actual voltage that is measured. If the time base of the oscilloscope is continually reduced, eventually a straight line will be seen on the display because only a very tiny piece of the wavelength is being examined. This analogy can be applied to an RF signal that is being measured on the same oscilloscope. Instead of continually being reduced, the time base of the oscilloscope is held constant, and the frequency of the signal is continually reduced. The same thing happens on the display. At first, we can see a sine wave with multiple periods, but eventually we see only a straight line across the display. If we see a straight line on the display and ask the question, What is the voltage? we can simply use Ohm's Law, because at very low frequencies, the wavelength is immaterial. The higher the frequency is, the less meaning Ohm's Law has. So, for frequencies starting at about 30 MHz and higher, power measurements become more important.
