How to measure the output power of a radio transmitter

When setting up homemade radio transmitters, you need to know their output power. There are special devices on sale that will measure the RF output power, but I also use a method to measure the output power using an oscilloscope. Here's how I do it. This method can measure the output power of radio transmitters.

You will need a good 45 cm long coaxial cable with a 50 Ohm characteristic impedance. For the load, you will need a non-inductive 50 Ohm resistor, I made it from two 100 Ohm resistors connected in parallel. I got a resistor with a dissipated power of 4 W. This resistor simulates an antenna that should be connected to the output stage of the radio transmitter. Of course, you will also need an oscilloscope that should measure well and work at the frequency of the radio transmitter.

My homemade radio transmitter operates at a frequency of approximately 7 MHz with a power amplifier on a BS170 transistor. The output power of such a transmitter will not exceed 2 W, this is a low-power radio transmitter. Connect the cable to the output of the radio transmitter where the antenna or cable should be connected.

Connect a 50 ohm resistor to the other end of the cable.

This is what it looks like

There is a divider on the oscilloscope probe. Set the divider to 10, this is what you need to do when you measure frequencies above 5 MHz, this way the readings will be better.

We supply power to the radio transmitter. My transmitter, when powered by 9 volts, consumes a current of 60 mA. This means that the power consumption of the radio transmitter from the power source is 540 mW. Here is the formula: W = V * I or 9 * 0.06 = 0.54. Why do you need to know this? This is necessary in order to exclude gross errors in measurement. If after calculations it becomes known that the output power will be 700 mW with a power consumption from the power source of 540 mW, then there is clearly an error here.

So, the oscilloscope shows the range of the double amplitude of the Peak-Peak signal 1.28V. This is the range of two amplitudes of the positive and negative half-waves, we only need to know the amplitude of one half-wave. To do this, divide 1.28 by 2 and get 0.64 and multiply by 10. Ten is the division of the divider that I installed on the probe. It turned out to be 6.4 Volts. This is the amplitude of one half-wave or Peak.

So, the oscilloscope shows the range of the double amplitude of the Peak-Peak signal 1.28V. This is the range of two amplitudes of the positive and negative half-waves, we only need to know the amplitude of one half-wave. To do this, divide 1.28 by 2 and get 0.64 and multiply by 10. Ten is the division of the divider that I installed on the probe. It turned out to be 6.4 Volts. This is the amplitude of one half-wave or Peak.

The first formula for calculating the output power of a radio transmitter is quite simple. We take 6.4, multiply it by 6.4 and divide the product by 100. We get 0.4096 or round it up to 410. 410 mW is the output power of the transmitter.

The second formula. 6.4 Volts (Peak) divided by the square root of two .The result is 4.52548339959. This is RMS. Then we square it and divide by 50, which results in 0.4096. The output power, if rounded, is 410 mW.