A large-area transformer coupled plasma (TCP) source has been designed and constructed. In our design, a plasma generation chamber and a radio-frequency (RF) antenna chamber have been separated with a dielectric material, and differentially pumped to accommodate large-area, relatively thin dielectric windows against mechanical pressures. With a large diameter (78 cm) chamber, a low frequency (4 MHz) RF source has been chosen. By calculating the plasma impedance from TCPRP codes based on a 2D heating theory [1], the diameter of a single turn copper coil antenna was optimized to provide high-density plasmas in the large area. Also the impedance matching circuit of this large-area TCP source has been designed from the calculated plasma impedance.