Before revision: Its purpose is double: first , it transforms twin-lead's 300 Ω impedance to match the 75 Ω coaxial cable impedance; and second, it transforms the balanced, symmetric transmission line to the unbalanced coax input.
After revision: Its purpose is double: Firstnominally , it transforms twin-lead's 300 Ω impedance to match the 75 Ω coaxial cable impedance; and second, it transforms the balanced, symmetric transmission line to the unbalanced coax input.