When you bring balanced feeder into the Shack, it's good practice to use a Choke/Balun to interface it to your ATU. This Choke allows you to connect 300 or 450-ohm feeder to your antenna tuner and is best suited for "indoor" use, ie: under the desk, right behind the tuner.
It's the same type of Choke found in our boxed G5RV offering and has been developed as it removes the need for a patch cable, can be used (with care) /P, and is cost-effective. Using a 1:1 Current Balun at the junction of balanced-feeder+coax is preferable, in most cases, to a 4:1 Balun - and certainly better than the internal 4:1 (voltage) balun found in many tuners!
31 or 75 mix? 75 has better performance below 10MHz (eg: Doublet for 160-80-60-40m), but for a general-purpose HF Choke, 8-turns on a 31-mix is best.
Length : Shorter is better, but if you need more cable to route into your ATU/Radio, that's fine - the ferrite is always close to the balanced terminals!
If you'd prefer a patch-lead version (PL259 to PL259), we have those, too!
Using a Balun on a G5RV - Really?
Some articles say you should, others say you shouldn't. G3TXQ's website explained why you should and included some pretty diagrams to prove his point. Our own experience (and the lack of units sent back to us!) tells us that using a Choke/Balun on a G5RV is indeed a wise move. In his original description of the G5RV, the inventor initially suggested the use of a Balun but had changed his mind by the early 1980s. Modelling software now allows us to delve deeper into the science without climbing a ladder or walking down the garden in the pouring rain. It doesn't stop the disagreements on the QRZ Forums, unfortunately :)