2:1 (100:50) Baluns are useful between 50 ohm coax and 100 ohm balanced lines for loops, cubical quads, log periodics and other antennas where the impedance at the end of the feedline is 100 ohms (1/2 wavelength feedline).
RFI/EMI Solutions from KHz to GHz
Ferrite Snap On, ID = .40 in/10.15mm, Mix 75, 10 pack - very useful for low frequency 150 KHz - 10 MHz. Better than Mix 31 below 4 MHz.