AM Broadcast RFI Kits

Off Center Fed (OCF) antennas are very convenient as they will allow multiple bands of operation with a simple wire antenna which is fed off center. For example, the Palomar Engineers 4010 OCF is a very popular antenna and consists of an off center fed dipole with sides of 55 feet and 11 feet and it will work all bands from 40-6 meters.The End Fed OCF uses the same dimensions but the wire section is 55 feet as in the OCF and the coax outer braid is used for the “other” short side of the OCF dipole. We place a choke on the coax at the 12 foot distance from the matching unit and we essentially have an OCF antenna with its great frequency range, but with only a 55 foot wire length instead of the 66 feet required by the regular OCF. You feed the antenna from one end which may be more convenient for some installations. This antenna will work on 40-30-20-17-15-12-10 and 6 meters. Most bands are under 3:1 SWR and easily tuned by your transceiver’s internal antenna tuner or external tuner used with an amplifier. (see BULLET-4006 for specific details and SWR curves).

These antennas are MUCH BETTER PERFORMERS than End Fed Half Wave antennas because they work the WARC bands, DO NOT HAVE A DANGEROUS HIGH VOLTAGE at the antenna connection and are SHORTER to fit in smaller spaces yet provide more band coverage up to 6 meters!

When considering solutions for AM broadcast (550 Khz – 1.8 Mhz) you should consider Mix 77 ferrites available in a variety of shapes and sizes.  Choose a size to give the maximum number of turns of the affected wire or cable.

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 1024x648 - AM Broadcast RFI Kits

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).

AM Band Choking Graph F240 77 1024x648 - AM Broadcast RFI Kits

F240-77 Choking Impedance in AM Broadcast Band

 

 

Typical Installations for Donut/Ring Toroids

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AC Cable on F-240 Toroid – 6 turns

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Ethernet Cable on F-240 Toroid – 7 turns

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Audio or Mic cable on F240- 12 turns

 

 Extra Mix 75 Snap On Beads  HERE – VERY effective for AM Broadcast RFI