J. Mc Laughlin wrote:
Dear Jim:

Now I begin to see light. While type 31 must be at least three years old,
the world has not yet caught up to its existence.


the *amateur* world hasn't caught up.. I haven't looked, but I'd venture
that the ARRL handbook still doesn't mention it (if only because it
would be a expensive and herculean task to revise the entire handbook
every year). And, folks writing in QST tend not to be in the business,
so they're using what they learned in the handbook.

For instance, the RFI/EMC page on the arrl web site says:
"Original text reprinted from February and March 1992 QST "Lab Notes"
columns Copyright © 1992 by the American Radio Relay League, Inc. All
rights reserved."

"Or, you can make a common-mode choke by wrapping 10 to 20 turns of the
antenna feed line or CATV cable through a ferrite toroid. Follow the
same procedure with the ac line. Use #75 (also known as "J"), #73 or #77
material if the interference is mainly from signals below 10 MHz. Use
#43 ferrite material for the higher bands or low VHF."

"To make a ferrite common-mode choke, wrap 5-10 turns of a conductor
onto an FT-240-43 ferrite core. (The "240" indicates that the outer
diameter of the core is 2.4"; the "43" designates the material. Other
materials may be useful, but 43 is a good all-around material.)"





(to be fair, they do link to K9YC's writeup)