It’s no secret that common mode RF riding on the outside shield of a feedline is a super-villain to Hams, causing interference, tuning problems, erroneous meter readings, and distortion or diminishing of the antenna’s radiation pattern caused by RF affecting the pattern’s nulls. Operators have used a number of strategies to thwart this ubiquitous nemesis with varying degrees of success.
Though widely used, both coiled coax chokes, also known as “ugly baluns,” and inline-type chokes have issues that limit their ability to mitigate RF, such as adding electrical length and narrow-band resonance. The use of properly placed ferrite toroid beads has shown to be a more reliable and effective method for stopping stray RF on the outside of feedlines. This is particularly true with the feedlines of phased array antenna elements, which suffer from distorted patterns when devices other than ferrite chokes (e.g., matching coils, inline chokes, baluns, or coiled coax chokes) are used.
But even when ferrite beads are your chosen method, be warned: Without the right number of beads, the correct placement, and use of the ferrite mix that works best over the desired frequency range, your airtime could still be plagued by annoying common mode RF.
DX Engineering kits provide good options. Recommended for use on all feedlines, DX Engineering High Impedance Common Mode RF Choke Kits make it easy to prevent RF from ruining your contesting, rag-chewing, or DXing endeavors. The kits contain 0.500 inch I.D. Mix 31 ferrite toroid beads which slide over 0.405 inch coaxial cable. They should be installed at the antenna feedpoint end of the coaxial cable or phasing feedline before installing a coaxial connector. Provided heat-shrink tubing keeps the beads in place and shields them from the elements.
These kits can also be used to…
- Make your own RF choked feedline cables for four squares, phased array, or single antennas using either 75-ohm RG-11U or 50-ohm RG-213/RG-8U size cables
- Build excellent chokes for single feedline antennas
- Suppress RFI on almost any wire or cable
Choose from two kits:
DXE-RFC-160-20—40 beads for antennas from 1.8-30 MHz
DXE-RFC-20-10—30 beads for antennas from 14-30 MHz
For more about preventing common mode RF from affecting your station, read this OAB article on DX Engineering’s Maxi-Core 20 Baluns and Feedline Chokes.
