WØIVJ's Telephone RFI Filter Project
(Click on pictures to enlarge)
Email: w0ivj@arrl.net
Recently, with the proliferation of electronic telephones, radio interference into telephones has become an increasing problem.  Whereas there are several commercial filters available, my experience has shown that their performance is lacking.  Below you will find instructions for building a common mode telephone filter that performs well.  I have found this filter to be adequate in most cases.  RF generally gets into the telephone via the phone line where it is rectified into audio by the telephone circuitry.  The filter below gives good attenuation to these common mode HF, radio frequencies.  Photo A shows the completed filter assembled in a Radio Shack telephone module part number 279-355 or 279-455 depending on color.  The pigtail with the RJ-11 jack can be made by cutting a Radio Shack part number 279-346 cable in half.  The other end can be used for construction of a second filter.  The choke, which is shown in photo B, is made by winding 55 turns of twisted #28 enamel wire, bifilar style, onto an F-114-77 core.  These cores may be obtained from Palomar Engineering or Amidon.  Photo C shows the schematic of the filter.  Finally, a small piece of cardboard is glued to the back of the module to conceal the components.  This filter is best placed right at the telephone, although in some cases, it may serve better placed at the service entry point or both places.  If you have a two-line phone, stack another core and place it in series with the yellow and black leads.  Use another module as a back cover.

A...B...C