Characterizing 3-Port Devices with a 2-Port VNA

VNA's come in all shapes and sizes. Some have one port, some have two ports, and some have 4-ports. So, what do you do when you have a 2-Port VNA and you want to measure a 3-Port device? Your traditional 2-Port VNA is only capable of gathering 4 S-parameters (S11, S21, S12, S22), but nine are needed to fully characterize the device.

Short answer - you don't need to spend the capital to get a 4-port VNA. Keep the 2-port VNA. All you need is an RF termination.

To illustrate, let's take a traditional Wilkinson Power Divider with one input and two output ports. The characteristic impedance of the circuit is 50 ohms, the operating frequency is 2.4GHz, and we want to measure the circuit response from 2GHz - 2.8GHz. After calibrating the 2-port VNA across the measurement frequency range, follow these steps.

Step 1 - Connect VNA Port 1 to the input port of the power divider and VNA Port 2 to the first output. Place a RF termination on the unmeasured output port. Take a measurement.

Step 2 - Keep VNA Port 1 to the input port of the power divider. Move VNA Port 2 to the second output and place the termination on the first output. Take another measurement.

Step 3 - Place the RF termination on the input port of the power divider. Connect VNA Port 1 and Port 2 to the output ports. Take the final measurement.

Alright, so what's happening here? If you measure two out of the three ports on this divider, the unterminated port introduces reflection/VSWR due to an impedance mismatch. The unused output port is terminated with a matched load to minimize signal reflections and maintain proper impedance matching during the measurement process.

From these three measurements, you now have accurate reflection coefficients on each port, forward/reverse transmission coefficients from the input to both outputs, and output port to output port isolation.