An apparatus includes an antenna element. The antenna element includes a first portion of a multi-layer printed circuit board (PCB) and a cap covering at least part of the first portion of the multi-layer PCB. The multi-layer PCB includes multiple substrates, and the first portion of the multi-layer PCB includes a first slot through the multiple substrates. The cap includes a second slot and defines a space between the first portion of the multi-layer PCB and the cap. The cap and a conductive layer of the multi-layer PCB form a waveguide structure through which wireless signals radiate from the antenna element.