The formula to be used to calculate the power output of a transmitter into a resistor load using a voltmeter is:

When testing a transmitter's output power using a dummy load, you often use a known load resistance (R) and measure the voltage drop across it using an RF voltmeter (E). According to Ohm's Law, current (I) is equal to voltage (E) divided by resistance (R). Power is calculated using Watt's Law: P = E x I. By substituting the Ohm's Law equivalent for current into the power equation, we get P = E x (E / R). This simplifies to P = E^2 / R (or E squared divided by R). Using this formula allows you to easily calculate power without needing to directly measure the current, which is practically difficult at high radio frequencies.