A plasma electrodeless lamp comprises a substantially hollow metallic body, closely receiving two coupling elements, the first coupling element connected to the output of an RF amplifier, and the second coupling element connected to the input of an RF amplifier. The first coupling element is conductively connected (grounded) to metallic lamp body at its top surface, while the second coupling element is not. The lamp further comprises a vertical metallic post, the post being grounded to the metallic lamp body at the post's bottom surface. The lamp further comprises a dielectric sleeve which closely receives the metallic post, and which is in turn closely supported by the lamp body or alternatively or in combination a tuning stub. The lamp further comprises a bulb that is closely received by the metallic post, and that encloses a gas-fill which forms a radiant plasma when excited.