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The PCB antenna used on the CC2511 USB dongle reference design is described in this application note. Even if the antenna presented is for a USB dongle design it can be used in all 2.4 GHz designs, especially where small space is required for the antenna. This application note describes the antenna dimensions, the RF performance and considerations for complying with regulatory limits when using this design.
The suggested antenna design requires no more than 15.2 x 5.7 mm of space and ensures a VSWR ratio of less than 2 cross the 2.4 GHz ISM band when connected to a 50 ohm source.
http://www-s.ti.com/sc/techlit/swra117