A Compact Tri-Band Antenna Based on Inverted-L Stubs for Smart Devices

We designed and constructed a novel, compact tri-band monopole antenna for intelligent devices. Multiband behavior was achieved by placing inverted-L shaped stubs of various lengths in a triangular monopole antenna fed by a coplanar waveguide. The resonance frequency of each band can be controlled by varying the length of the corresponding stub. Three bands, at 2.4 (2.37-2.51), 3.5 (3.34-3.71), and 5.5 (4.6-6.4) GHz, were easily obtained using three stubs of different lengths. For miniaturization, a portion of the longest stub (at 2.4 GHz) was printed on the opposite side of the substrate, and connected to the main stub via a shorting pin. To validate the concept, the antenna was fabricated on a low-cost 1.6-mm-thick FR-4 substrate with dimensions of 20 x 15 x 1.6 mm(3). The antenna exhibited a moderate average gain of 2.9 dBi with an omnidirectional radiations over the bandwidths required for RFID, Bluetooth, ISM, WiMAX, and WLAN-band applications. These features make the antenna suitable for compact smart devices.