This YouTube video, originally presented at the QSO Today Ham Expo, Fall of 2021, shows how to measure common mode current chokes using a NanoVNA.
In that video, Smitty describes two versions of a test rig he built to perform those measurements. This is v3.0 of that test rig.
tl,dr: Vector Network Analyzers (VNA) such as the NanoVNA, measure differential power, which travels on the inside of a coaxial cable. Common mode current, which travels on the outside of coax, is not normally seen or measured by a VNA. If we wish to measure the effectiveness of a common mode current choke (aka “1:1 Balun”), we have to get the signal from the VNA onto the outside shield of the coax. This test rig does just that: It routes the center conductor from the VNA to the shield of the device under test. Connect a common mode current choke (or any device you wish to measure the common mode properties of) to any of the available sockets: SO-239, N, BNC, or SMA, and measure the “S21 LOGMAG” to directly measure the effectiveness.
For a common mode current choke, you want as much loss in S21 LOGMAG as possible, across as much of the intended frequency range as possible (eg: the HF spectrum.) Ideally, it would be at least 25db of loss (an S21 LOGMAG of -25db) from 3MHz to 30MHz. More loss is better.