Friday, January 22, 2016

Common-Mode Choke

Today was a MAJOR snow storm.  12" of snow in 10 hours and it is still coming down!  So, with nothing better to do, I decided to get caught up on some old projects that have been hanging around. You can click on any photo to see a larger image.

Looking Out my Front Door

The first project I decided to tackle was to build a couple of Common-mode Chokes for Jimmy, W8JA.  The one I built was 3-1/2 turns through two stacks of four Mix-43 toroids.  These are available from Fair-Rite as Part No. 5943003801.  This choke's impedance should exceed 1-K Ohms from 1.8 to 18 MHz. and dropping to 550 ohms at 30 MHz.  The full description can be found in an April 2006 document called "Common-Mode Chokes" by Chuck Counselman, W1HIS.  You can find it on the Internet.  

It's a pretty simple job to build this choke.  It is just a 36-inch piece of RG8/U type coax passing 4 times through the upper group of four toroids and three times through the bottom four.  My first job was to build a 36-inch piece of coax with PL-259's on each end.  I used Belden 8267 cable.  Below is a photo of one of the connectors after it was soldered to the cable. 

PL-259 Soldered to Cable

Below is a photo of the completed choke.  You can wind the choke through the toroids with the PL-259's already connected.  So, instead of building a 36-inch piece of cable from scratch, you could just purchase one and wind it through the toroids.  I used a couple of Ty-Raps to secure the windings so they did not "un-wind."  An Amphenol PL-258 double-female connector with Teflon insulation is connected to one end to simplify the installation of this choke either at the antenna or at the rig.

Completed Common-Mode Choke
Building the cable from scratch took some time.  I need to improve my method of doing this if I plan to build many more!

Still, I got one project off my "To Do" list today while "snowed-in"!  On to the next!