What Use Is An F-call?

What use is an F-call? During the week I was spending a little time with an experienced amateur and I lamented that getting ferrite cores locally was a trial in futility. I'd spent hours attempting to find a reliable supplier and as of yet have not actually succeeded. I want to experiment with ferrite cores to build some antennas and to test different balun designs, seeing that the Internet is full of them, each with different opinions on what is good and bad, what works best and how to magically make something work with a balun. Of course I can read all I want, but actually getting your hands dirty is generally a better way to get a feel for what happens and what's needed. It was suggested that I could make an equivalent solution, creating a choke using coax cable, 6 inches, 6 windings was the suggestion. Since then I've also found 4 and 4 and 8 and 8 and other variations. I should note that I've not yet built one with all the other distractions in my life, but I'm feeling much less worried about locati