An electronic device may have a cover layer and an antenna. A dielectric adapter may have a first surface coupled to the antenna and a second surface pressed against the cover layer. The cover layer may have a three-dimensional curvature. The second surface may have a curvature that matches the curvature of the cover layer. Biasing structures may exert a biasing force that presses the antenna against the dielectric adapter and that presses the dielectric adapter against the cover layer. The biasing force may be oriented in a direction normal to the cover layer at each point across dielectric adapter. This may serve to ensure that a uniform and reliable impedance transition is provided between the antenna and free space through the cover layer over time, thereby maximizing the efficiency of the antenna.