Purpose. The CU-1-4000BCI is a 50Ω to 50Ω feed line choke/line isolator/noise filter, for the AM broadcast band frequencies, with SO-239 coax input and output connectors. The choke is rated at 4 KW watts PEP into a matched load for .5-5 MHz. The balun/unun is designed to have a high RF impedance to common mode currents on the outside of the coax braid while passing RF currents on the inside of the coax braid to the antenna (see graph below). Effective choking impedance > 3K to 5 MHz, over 10K in AM broadcast band. Excellent choke for AM broadcast and 160 meter band common mode noise reduction.
For maximum common mode current protection (and receiver noise floor reduction due to common mode current pickup by the coaxial braid), use one common CU-1-4000BCI at the antenna and another at the transmitter equipment connection or between antenna tuner and amplifier.
Typical common mode choking impedance for CB-1-4000BCI / CU-1-4000BCI is shown in the graph below: