Measuring High RF Power Accurately, Part 1

Power is perhaps the most frequently measured quantity in RF measurements, typically done using an RF power meter. What do you do when you are armed with a power meter (rated up to +30 dBm) and have to measure the output power of a 100 W transmitter? Intuitively, you might try to plug in a high power fixed attenuator to bring the power down to a safe level measurable by your sensitive power meter. Sounds sensible right? On the contrary, you may find yourself getting frustrated over unstable and inaccurate power readings.

Power Meter Ratings and Measurement

Before we go into the reason for this counter-intuitive phenomenon, let us take a look at some numbers and their implications.

The power meter is rated up to +30 dBm or 1 W. This implies that you should not subject it to anything higher than 1 W or it will be damaged. The transmitter is rated to give an output of 100 W or +50 dBm, which means the actual power could be slightly higher or lower. That would also mean that the transmitter power needs to be attenuated with a certain margin below the +30 dBm threshold to be on the safe side. Assuming that the margin of tolerance is 10 dB, a 30 dB or 1000x attenuation is needed.

Now, here comes the why. It is because of attenuator derating. The power rating for attenuators is typically specified at certain ambient temperature and derates linearly as temperature increases. An example of this specification for a 30 dB attenuator would be: “100 W full power at 25 oC and derates linearly to 10 W at 125 oC”. The majority of fixed attenuators usually employ a convection cooling method via heat sinks or cooling fins. As the input power increases, more RF energy will be dissipated as heat. This will raise the ambient temperature and the attenuator temperature. Once the temperature exceeds 25oC,  the attenuator will derate away from its initial calibration point and is no longer able to fully dissipate 100 W of RF power, resulting in unstable and inaccurate power measurement.

Surely you can employ more sophisticated cooling methods such as using fan or liquid cooling to keep the attenuator temperature in within desired range. But is there a more robust and advantageous method to perform high power RF measurements with your power meter?

You can read the answer in Part 2 of this series. And, you can check out the range of power meters offered by Measurement Assurance Technology, or contact us for help with finding a specific meter.