Electronic devices may include radio-frequency transceiver circuitry and antenna structures. The antenna structures may include a dual arm inverted-F antenna resonating element and an antenna ground. An antenna feed may be coupled between the inverted-F antenna resonating element and the antenna ground. An adjustable component such as an adjustable inductor may be coupled between the inverted-F antenna resonating element and the antenna ground in parallel with the antenna feed. The adjustable component may be operable in multiple states such as an open circuit state, a short circuit state, and a state in which the adjustable component exhibits a non-zero inductance. Antenna bandwidth can be broadened by coupling a loop antenna resonating element across the antenna feed. A portion of the antenna ground may overlap the loop antenna resonating element to further enhance antenna bandwidth.