After many requests we have created a compact, special balun for OCF antennas with vertical radiators. The CB-4-1500ODCV is a CUBE™ enclosure containing a dual core, bifilar wound, voltage balun to force equal voltages on the unequal sides of the off center fed antenna and force current down to the vertical radiator so it acts like a vertical antenna adding to the radiation pattern at low angles for DX.
The vertical radiator is typically 10 feet of 50 ohm coax for the 40 meter long (67 feet at the top) and 22 feet long for the 135 foot OCF.At the bottom of the vertical radiator you should have a line isolator or 1:1 balun (such as the Palomar BA-8E or BA-58E sleeve balun, or CB-1-1500 cube balun) to stop the current on the outside braid of the coax from using the rest of your feedline back to the radio as a long vertical radiator causing RFI and other problems as well as skewing the radiation pattern. The balun can be used with any size.
The balun is rated for full 1500 watts PEP from 1.8Mhz – 61 Mhz and usable to over 150 MHZ! (see graph) The balun can be used with either a 50 or 75 ohm coax vertical radiator and feedline.
The matching line isolator is the MC-1-3000 which should be used at the end of the vertical radiator.
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