Common Mode Coax Noise Filter, 4KW PEP, 500 KHz-5 MHz
Purpose. The CU-1-4000BCI is a 50Ω to 50Ω feed line choke/line isolator/noise filter, for the AM broadcast band frequencies, with “N” connector 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)
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 output of antenna tuner.
Typical common mode choking impedance for CB-1-4000BCI / CU-1-4000BCI is shown in the graph below:
AM broadcast transmits on a low frequency with large antennas. Consequently it takes a large antenna to receive the signals. Typical “antennas” are AC house wiring, phone lines, long satellite or cable TV cables, DSL/CAT5 lines running between rooms, etc. The device wires that connect to these “antennas” need to be choked with as many turns through the ferrite as possible. Some installation examples are shown below on this page.
For AC/ DC power cords and coax cables the F240-77 (2.4″ OD, 1.4″ ID) “Donut” toroid is the most popular. Also available are smaller toroid donuts and Mix 77 beads. Shown below is a graph with 8, 10, and 12 turns through the toroid – obviously more turns has higher choking impedance. Mix 77 toroid products are HERE and Mix 77 small beads (for small wires) are HERE
Shown below is a graph of frequency response verses choking magnitude for a F240-77 in the AM Broadcast band for a 12 turn coil around the toroid. (Same info as above but different frequency range).
Typical Installations for Donut/Ring Toroids
Extra Mix 75 Snap On Beads HERE – VERY effective for AM Broadcast RFI