What device should you use to connect a coaxial cable to window line?

Coaxial cable is an unbalanced transmission line, while window line (a type of parallel-wire line) is balanced. Whenever you transition between unbalanced and balanced systems, you must use a balun (BALanced to UNbalanced transformer) to ensure proper RF current flow. Connecting these lines directly without a balun will cause common-mode currents to travel on the outside of the coax shield, leading to unwanted radiation and potential RF interference. Tuning stubs and phasing harnesses are used for impedance matching and array combining, not for converting line symmetry.