Vector network analyzers (VNAs) are something of an "expensive voltmeter" for the RF/microwave industry. They are invaluable test tools that are used to measure all four S-parameters on active and passive devices, and can use those test results to create software models for the purpose of simulating a device's or circuit's performance behavior before committing to building a prototype. VNAs are available from a wide range of manufacturers in many configurations and frequency ranges, from lower-microwave frequencies through millimeter-wave frequencies. To learn more about which companies sell them and what to look for, click here.