Much has been written about Baluns and Ununs of all types. Jerry Sevick (W2FMI, SK), W8JI, G3TXQ, VK5AJL and others have provided detailed analyses regarding design and testing. Sevick has written a very extensive book about the subject and this is available from multiple sources for a small cost.
Here is an extensive Sevick article regarding Baluns. The article’s picture “H” and its associated text show how to build a very simple 1:1 current balun to interface ladder line (a.k.a. “Twinlead”) to the tuner.
The Current Balun is sometimes known as a “Guanella” balun or a “Choke” balun. All these names describe the same device.
Jerry Sevick’s Transmission Line Transformers Handbook (48 Improved Designs) is found at the indicated link.
W8JI has written an extensive article regarding the testing of baluns and it is worth reading for background and an understanding of what works and what only seems to work, regarding these.
This analysis by G3TXQ shows that a 1:1 balun ahead of the twinlead-fed tuner is preferred over a 4:1 balun. Many, including me, thought that a 4:1 was the proper device but now it seems the 1:1 might have the advantage. Both will likely work and each has its advantages and disadvantages. If your 4:1 is working for you (no RF in the shack, seems to radiate OK), just leave it alone.
Here is VK6YSF’s web page showing how to make a simple 1:1 current (Guanella) balun.
Here is an interesting web page concerning the use of “zip cord” (a.k.a. lamp cord) to make a current balun.
VK5AJL provides a lot of information and instruction regarding voltage and current baluns and ununs. Go also to his home page and see much more of interest.
Palomar Engineers sells a range of FERRITE Cores useful for baluns and ununs. Click here for their website. Explore it, there’s a lot there. Mixes 31 and 43 are best for HF use (31 is better for the low bands, with mix 43 having a slight advantage from 14 to 30 MHz).
Amidon sells a range of IRON POWDER cores for baluns and ununs. They also sell FERRITE cores. Click here for their website and explore it. The Ferrite Red type 2 mix is said to cover 2 – 30 mhz.