A Dual-ISM-Band Antenna of Small Size Using a Spiral Structure With Parasitic Element

This letter presents a compact, single-feed, dual-band antenna covering both the 433-MHz and 2.45-GHz Industrial Scientific and Medical (ISM) bands. The antenna has small dimensions of 51 ×28 mm 2 . A square-spiral resonant element is printed on the top layer for the 433-MHz band. The remaining space within the spiral is used to introduce an additional parasitic monopole element on the bottom layer that is resonant at 2.45 GHz. Measured results show that the antenna has a 10-dB return-loss bandwidth of 2 MHz at 433 MHz and 132 MHz at 2.45 GHz, respectively. The antenna has omnidirectional radiation characteristics with a peak realized gain (measured) of -11.5 dBi at 433 MHz and +0.5 dBi at 2.45 GHz, respectively.