Your rotor cable and control box can cause RFI into your radio station and you need to suppress common mode RFI induced into the cable from your antennas. In many cases the rotor cable acts as an unintended counterpoise for your rotated antennas and can cause weird SWR issues since the control cables become part of the antenna,
The SOLUTION is simple: isolate the cable from RFI by adding the Palomar RFI kit for antenna rotors. Add 3 snap on filters near the rotor and another 3 snap on filters at the rotor control box and a ring filter for AC powered rotor control boxes to suppress RFI common mode currents that can affect the antenna SWR and radiation patterns. In addition, these filters may also help reduce station receiver noise since many rotors are connected to the station ground and the rotor cable acts as an additional receiving antenna.
Isolating the rotor cable works on the same principal as isolating coax cable - keep the TRANSMIT signal on the antenna by proving a feed line choke at the rotor itself and keep noise out of the station on RECEIVE by choking the cable at the rotor control box ( by suppressing noise pickup on the cable).
Kit includes 7 filters -six 1/2" snap on filters for the cable and a ring filter for the AC/DC power supply for the control box. Simple to install and cheap insurance to solve both the TRANSMIT and RECEIVE issues caused by rotor control systems.
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
F240-77 Turns Comparison from .1 – 8.1 Mhz
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).
F240-77 Choking Impedance in AM Broadcast Band
Typical Installations for Donut/Ring Toroids
 AC Cable on F-240 Toroid – 6 turns |
 Ethernet Cable on F-240 Toroid – 7 turns |
 Audio or Mic cable on F240- 12 turns |
Extra Mix 75 Snap On Beads HERE – VERY effective for AM Broadcast RFI