Double female to female barrel connector to connect male UHF/PL259 cable to another male UHF/PL259 connector - very handy for connecting extra length RG-8/213/LMR400, etc. cable to another
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).