A radio frequency, RF, waveguide array. The array comprises a substrate and an electrical RF transmission line array. The substrate comprises a plurality of waveguides, each waveguide being elongate in a first direction. The electrical RF transmission line array is located on a face of the substrate and comprises a plurality of signal electrodes and at least two ground electrodes. Portions of the ground electrodes which are relatively distal from the signal electrodes have reduced height in the direction transverse to the substrate to reduce the amount of material required to produce them.