BALUN. balanced to unbalanced. ie, coax - unbalanced, dipole antenna- balanced.
the number of turns on the torroid to the material type vary slightly. 8 to 10 turns on the torroid for upper HF and 12 turns for lower HF bands.
for 7 to 30MHZ you can use 43 material and for 7 to 1.8MHZ you can use 31 material.
I have made the following using FT240-31 torroids purchased from ebay.
BELOW. this I made using a FT240-31 core and wound #14 enamelled wire and terminated with 2 x 6mm stainless bolts to attach to the feed point of a beam antenna.
BELOW. above balun under test. 50ohm resistor across the terminals and the other end attached the output of the MFJ 259 analyzer, 1:1.1 from 14mhz to 29.5mhz.
BELOW. this one I made from a FT240-31 core and wound Aircell5 coax direct on the core, 10 turns. this was boxed to use on a 80mtr elevated 1/4 wave vertical with elevated radials. so-239 is the input and the top of the box is where the antenna connects.
BELOW. this is a FT240-31 core with Aircell5 coax (40 meters) wound directly onto the core at the feed point end. easy to attach different dipoles etc.
These are all made by me, for use by me. they all worked as described.
73, si vky.