Various embodiments are directed to antenna designs that may improve the performance of a mobile computing device. Some embodiments are directed to a mobile computing device assembly comprising accessory incorporating a supplemental antenna designed to be adjacent to at least one internal antenna of a mobile computing device when the accessory is attached to the mobile computing device. The supplemental antenna and the internal antenna may cooperatively form an antenna system for the mobile computing device resulting in improved performance. In various implementations, the use of the supplemental antenna in conjunction with the internal antenna may enhance antenna performance and/or increase antenna efficiency. Other embodiments are described and claimed.