A small ferrite toroid for use in the G3TXQ 1:4 CobWeb balun.
If you are building a general-purpose (7-30MHz) 1:1 balun/choke or line-isolator, use the FT140-43 for QRP-100w) or FT240-43 which is more than adequate for UK 400w limit. We do NOT recommend using the 61-mix for normal choke/baluns.
Toroid sizes/types are easy to ID, the "140" translates to 1.4" (size) and the "61" is the ferrite mixture. What type do you need? 61 is used for the G3TXQ CobWebb balun, 43 is good from 7MHz and up (and a good general-purpose 7-30MHz solution) while 31 is good for lower HF bands. Remember, you can stack the cores for increased choking and power-handling. As always, ratings need to factor in the aerial (SWR) match and duty-cycle.