Custom antenna structures may be used to compensate for manufacturing variations in electronic device antennas. An electronic device antenna may have an antenna feed and conductive structures such as portions of a peripheral conductive electronic device housing member and other conductive antenna structures. The custom antenna structures compensate for manufacturing variations in the conductive antenna structures that could potentially lead to undesired variations in antenna performance. The custom antenna structures may make customized alterations to antenna feed structures or conductive paths within an antenna. An antenna may be formed from a conductive housing member that surrounds an electronic device. Custom antenna structures may be interposed between an antenna feed terminal and the conductive housing member to adjust the effective location of the antenna feed. Custom antenna structures may include springs and custom paths on dielectric supports.