As the Internet of Things (IoT) requires communication technologies to interconnect many devices, a low-power wide-Area network (LPWAN), especially LoRa, has emerged due to its power and range benefits. Since the physical layer of LoRa remains susceptible to deliberate jamming attacks, we propose a friendly jamming scheme that protects the legitimate communication against attacks by using the imperfect orthogonality of the spreading factor (SF). SF imperfect orthogonality allows a stronger signal to disrupt weaker signal with different SF, therefore we use the strong friendly jamming signal to hide the legitimate signal from attackers. The legitimate receiver can cancel the friendly jamming signal and retrieve the initial data. We implement the scheme with software-defined radios, showing that our friendly jamming scheme is efficacious to impede the jamming attack.