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This paper examines transmitter optimization for wirelessly powering a small implant embedded in tissue. The wireless link between the transmitter and receiver is first modeled as a two-port network and an expression for the power transfer efficiency derived. For a given small receiver in a multilayer tissue model, the transmitter is abstracted as a sheet of magnetic current density for which the optimal distribution is analytically found. The optimal transmitter is compared to the point and uniform source across a range of frequencies. At higher frequencies, the optimal current distribution is shown to induce fields that exhibit focusing. The effects of constructive and destructive interference substantially improves the power transfer efficiency and reinforces operation in the low GHz-range. The optimal transmitter establishes an upper bound on the power transfer efficiency for a given implant and provides insight on the design of the optimal transmit antenna.