Measuring Power
How to measure RF power using a multimeter on dummy load kits and understanding how to apply the electronics theory behind it.
All K7RHY dummy load kits are designed with test pads that are used to measure the output power of your radio with a multimeter. The power is measured, while transmitting, as a voltage that is converted to watts using a formula derived from Ohm's Law.
Measurement Procedure
Configure your multimeter
Set your multimeter to measure DC voltage. If you keep your transmitted power below 20W, you will not exceed 35V. Verify that your multimeter is set to read in the correct voltage range.Attach the transmitter
Connect your transmitter output (antenna port) to the RF connector on the dummy load. For the most accurate measurement, keep the patch cable short.Connect the multimeter
Connect the multimeter probes to the test pads, TP1 and TP2, on the dummy load. They can be held in place by hand if you don't have alligator clips. The polarity of the probes does not matter. The mathematical conversion will always return a positive power value whether the measured voltage is positive or negative.Transmit a signal and measure the voltage
Transmit a signal from your radio and measure the voltage on the multimeter. The voltage may take a few seconds to stabilize.Do not exceed 20W
The dummy load can handle momentary loads above 20W, but the diode which allows you to measure power as a voltage cannot handle arbitrary large voltages. While there is a large buffer zone factored into the design, you will damage the diode if you go to far. The failure point varies based on the RF frequency. Keep the power below 20W and you won't risk damaging your equipment.Convert the voltage to power
Using thePower Calculator Tool, enter the voltage you measured and your transmission band, and the tool will calculate the power in watts.
Understanding the Power Calculation
Power is calculated according to Ohm's Law, which states that power is equal to the square of the voltage divided by the resistance.
| Frequency | Impedance |
|---|---|
| 1.8 MHz | 49.3 ohms |
| 14.2 MHz | 49.5 ohms |
| 28.8 MHz | 51.7 ohms |
| Frequency | Impedance |
|---|---|
| 52 MHz | 57.1 ohms |
| 146 MHz | 115 ohms |
| 435 MHz | 63.6 ohms |
The test pad on the dummy load that is used for measuring the voltage is connected to the center of the resistor network. Therefore, the impedance at that point is half the impedance measured at the RF connector. For HF frequencies, we can make a reasonably accurate calculation of power if we set R to 25 ohms because the reactance is low.
We're close to being able to rewrite the formula for this dummy load specifically, but we need to make one more adjustment. The voltage that we measure is on the other side of a diode from the voltage source, so we need to adjust for the voltage drop across the diode. Based on the diode specs, we approximate this to be 0.3 volts. Our modified formula for power is:
This is the equation used by the
Power Calculator Tool
for HF frequencies. When you select a different band, it adjusts the resistance variable based on lab-measured averages for this dummy load kit.About the use of approximations
As an engineer and a perfectionist, I'm not a fan of approximations. In fact, I feel physical discomfort from suggesting that you use them. However, the goal of this project is to provide a practical tool for hams to measure power. The approximations used in the tool are based on lab measurements of the exact hardware used in the kit you've been provided. The approximations are accurate enough for practical use. I could design a kit that would make computations based on real-time internal measurements, but that kit would be prohibitively expensive and provide no additional operational value.
The bottom line is, radio transmitters do not transmit at precise, stable power levels across their supported frequency range. The power output of a transmitter is a function of the input power and the component efficiency, which itself varies with frequency. Once leaving the transmitter, the power is further attenuated by the feed line (including insertion losses), the load, and the environment. For the purposes of ham radio, it's good to know that your transmitter is operating within a certain range of its rated power, but there's little value in knowing the exact, momentary power output.