A computing device includes a metal frame forming an exterior surface of the computing device and including an array of resonant cavities. Each resonant cavity has a center axis and defining a volume within the metal frame. Each volume contains a corresponding metal plate positioned within the volume on the center axis of the resonant cavity and a corresponding metal feed line positioned to capacitively drive the corresponding metal plate and the resonant cavity. A least a portion of the corresponding metal feed line is positioned within the volume on the center axis of the resonant cavity.