This RFi kit was created to eliminate RFI issues from the excellent Heil Parametric Receive Audio System which can highly tailor audio signal from your radio to a a powered speaker or multiple headphones. However when installed near a high powered HF antenna it can ave RFi issues. This kit was created out of necessity from a ham using high power antennas near the radio. The use of the ferrite filters completely eliminated the RFI issue allowing the ham to enjoy the use of the PRAS.
The ferrite filters are selected to suppress RFI from 1 to 300 MHz. The filters are easy to install, don’t require modification of the protected equipment and work in almost all cases. See product picture for suggested filter use.
What is included in the kit?
The RFI-PRAS kit contains a a ring filter for the AC power cord used on the DC supply cables to the PRAS and high powered speakerr and 3 additional snap-on filters for the both ends of the speaker cable and for the audio input (from the transceiver/audio source) to the PRAS..
You get 1 ring filter and 5 1/2" ID snap on filters and an installation manual.
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