What is the purpose of adding an inductor to a Pi-network to create a Pi-L-network?

A standard Pi-network is often used at the output of radio frequency amplifiers to match the impedance of the active device to the antenna system while simultaneously acting as a low-pass filter. This filtering is crucial for preventing unwanted high-frequency harmonics from being transmitted over the air. By adding an additional series inductor to the output of a standard Pi-network, it becomes a Pi-L network. This extra inductor adds another filtering 'pole' to the circuit, which steepens the frequency roll-off curve above the desired operating frequency. The result of this steeper roll-off is significantly greater attenuation of harmonic frequencies compared to a standard Pi-network alone, ensuring a much cleaner transmitted signal.