Where does impedance matching need to be done to minimize transmission line losses in an antenna system?

To minimize power lost as heat within a transmission line, the standing wave ratio (SWR) on that line must be kept as low as possible. This requires matching the impedance right at the junction where the transmission line meets the antenna feedpoint. If you place an impedance matching device (like an antenna tuner) at the transmitter instead, the transmitter will be protected and deliver full power, but the SWR on the transmission line itself remains high. This means the reflected energy will still bounce back and forth between the tuner and the antenna, incurring additional line losses with every trip.