Having been in the hobby for more than a few decades, various issues with RFI in the shack are often attributed to “common mode current” – that is a signal that is injected into our coax in such a way that it impacts both the center conductor as well as the shield together (hence “common”). This can happen when a feedline is in the radiation pattern of an antenna. Other noise signals can be coupled into our coax when it runs along the ground.
Its pretty easy to determine if a common mode choke is effective – just put it into the feedline and see how things respond. But that always left me wondering which common mode choke performed the best.
Halibut Electronics has developed an inexpensive test rig that allows you to test various common mode chokes. Essentially it is a set of different connectors (SMA, BNC, SO-239, N, and Banana Jacks) that allow pretty much any choke configuration to be tested. It is used in conjunction with a VNA to measure the actual common mode reduction. Thoughtfully it even includes a switch that allows you to calibrate the VNA (open/Short/50 ohm).
