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In this letter, new slot antennas with an extended ground are proposed. They are easily built as internal antennas in a compact wireless device. Originated from lambda/4 slot antennas, an additional vertical L-shaped metallic sheet as an extended ground and a main radiator is added to reduce the antenna size. The sheet is designed close to a ground plane, and its height can be chosen according to the available space of a given device. These features offer more design flexibility for antenna and mechanical engineers. In this letter, antennas for WLAN are designed and implemented as examples, and a four-element antenna design using the resultant antennas is demonstrated, showing that the multiple-antenna design occupies only a small region in the system board. With these design examples the proposed structures can be a promising solution for multiple-antenna systems in compact wireless devices.