An electronic device may include a rear housing wall, antenna resonating element, sensor board, and grounding ring. The grounding ring may couple antenna currents from the resonating element onto a ground trace on the sensor board. A switchable inductor may couple the grounding ring to the ground traces. The switchable inductor may be used to tune a frequency response of the antenna in a cellular low band. Additionally or alternatively, an inductor may couple the resonating element to the ground traces or other portions of an antenna ground. This inductor may serve to extend the effective length of the antenna resonating element in the cellular low band to compensate for dielectric loading by the rear housing wall, such as in examples where the rear housing wall is formed from zirconia.