An RF electrodeless plasma lamp with improved efficiency in higher lumens per watt includes a waveguide body, in which an RF signal drives the entire structure at the resonant frequency of the structure. The resonant frequency of the structure is lowered by increasing the overall capacitance of the waveguide body by adding at least two layers of dielectric material between the input feed and the bulb of the lamp. The layered structure can include an air cavity disposed between a dielectric layer and the input feed. In lowering the resonant frequency of the lamp, the device is capable of using RF amplifiers that have higher efficiency, and thus has a higher lumens per watt ratio.