Common Mode Chokes by W1HIS - Yankee Clipper Contest Club

Common-Mode Chokes Confidential draft for YCCC review, by W1HIS 2006 April 6 Page 39 of 42Some of these chokes are packaged, “modular” common-mode chokes from K-COM. Although these devices are advertised as “filters,” theyare common-mode chokes, made by winding a bundle of four insulated magnet-wireconductors on a small ferrite toroidal core. They are available for four frequency ranges.The ones made for HF have impedances exceeding 1 kΩ from 1 through 30 MHz. Thetwo-wire version costs $17 and the two-wire version costs $23.The choke packaged in a black, nearly cubical box is from KI6KY’s firm K-Y FILTERS. This is also a common-mode choke comprising a multifilarwinding on a small ferrite toroid. However, this is a series string of two toroidal chokes;and its impedance is substantially higher than that of a K-COM choke. The two-wire, HFversion costs $25.The K-COM and K-Y chokes have RJ-11 modular plugs and jacks, and are quick andconvenient to install. The K-Y choke is more effective. One K-Y choke usually workswhere two K-COM chokes would be required, and costs less than two K-COM chokeswould. However, the same common-mode impedance can be gotten less expensively byrolling your own chokes.No amount of common-mode choking impedance in the telephone line cord is sufficientto eliminate RFI in some (fairly many) telephone sets. It is necessary also to insert acommon-mode choke in the handset cord (the “coil cord”) where it enters the base of thetelephone. K-COM makes a modular four-wire common-mode choke with the smallerconnectors used with a handset cord. Figure 16 (next page) shows one of these chokesinstalled on our kitchen telephone, which required it.