Measuring High RF Power Accurately, Part 2

In the first part of this series, we saw that using an attenuator in a high RF power measurement scenario posed several disadvantages and may require quite a bit of effort to minimize those problems. So, is there a more robust and advantageous method to perform high power RF measurements with your power meter? The answer is yes.

The issue we had with the attenuator was RF energy being dissipated in the form of heat, which in turn caused the attenuator to derate. The key to overcoming this issue is to minimize the RF energy from being dissipated as heat. This then implies minimal attenuation. How then is that possible to measure such high power with just minimal attenuation? That is the beauty of using a directional coupler.

A directional coupler conveniently gives you the ability to sample incident and reflected power accurately with minimal disturbance to the transmission line. Since it is only sampling the power via coupling, most of the RF energy is actually channeled to the load. Therefore, very little is being dissipated at the sampling port and this keeps the sampling port at a relatively low temperature. This, along with high directivity and low insertion loss, means the power at the sampling port becomes extremely stable and isolated from changes in reflected energy and temperature even as the power levels increase. To understand how this is possible, let us take a look at how a directional coupler works.

Figure 1: Directional Coupler Signal Flow Diagram

A directional coupler is essentially a four-port passive device consisting of two transmission lines. These lines are the main line which is a through path from input port to output port and the coupled line which receives a fraction of power from the main line via coupling mechanism. Referring to Figure 1, the input signal travels from Port-1 toward Port-4 (forward direction) and a portion of the power is coupled into Port-3 only. When that signal arrives at Port-4, some of that signal is reflected back into the main line and a portion of that reflected signal is coupled into Port-2 only.

Figure 2: High RF Power Measurement Setup Diagram

Figure 2 shows how a directional coupler fits into a power measurement setup. The high power transmitter (incident power, Pinc) is connected to Port-1, forward coupled power, Pcp (Port-3) is connected to the power meter and output power (Port-4) is connected to an antenna or a load. Port-3 is not used and should be terminated. From this setup, you will realize that you are able to make measurements without needing to disrupt the transmission unlike the attenuator method. You can measure the power even while the entire system is still in operation and you will be able to continuously monitor power flow or fluctuations. This is the other advantage of the directional coupler method over using attenuator.

What has been your experience when measuring high RF power? Have you used the directional coupler method? We’d like your feedback in the comments below. And contact us if you'd like to discuss your RF power measurement needs - MAT sells and services power meters, sensors, and accessories.