Capacitively excited and fully planar small size printed antenna

A very small, efficient, omnidirectional planar antenna is presented. It is composed of a printed loop surrounded by a folded ring. This new structure has a radiation pattern similar to a λ/2 dipole, but its size is only around one tenth of the wavelength. It has a gain of 1.68 dBi, and an efficiency of 99.8%, and it is scalable. Two antenna design examples are discussed herein: one if for WLAN (2.45 GHz) with a gain of 1.68 dBi and 39 MHz impedance bandwidth, and another is for WiMAX (3.5 GHz) with a gain of 1.52 dBi and 73 MHz impedance bandwidth. Both designs are printed on FR-4 substrate 0.8 mm thick and exhibiting an efficiency greater than 98%. The CST Microwave Studio was used for the design simulations.