Multimode Coupled Slots for Wideband Circularly-Polarized mm-Wave All-Metal Antennas

All-metal compound radiating elements able to radiate a circularly-polarized field over a wide bandwidth are presented. Such elements comprise an active waveguide-fed slot which couples to a backshorted slot. When this coupled slot supports the propagation of several modes by bending its shape, a remarkable wide polarization bandwidth can be achieved. As an illustration, an optimum H-shaped all-metal radiating element reaches a 1-dB axial ratio bandwidth around 25%.