An electrically small 3-d folded grounded loop antenna for omnidirectional connectivity

Abstract

—Electrically small antennas are of intense and increasing academic and industrial interest due to the advent of ubiquitous RFID devices and more generally within the Internet of Things (IoT) applications. For most of these applications, antennas will have to be as small as possible, when being integrated within a transceiver, while maintaining significant efficiency values. Of particular interest are antennas that can radiate omnidirectionally along a planar surface, thus establishing optimal connectivity capabilities for devices surrounding the corresponding transmitter. Such antennas are important not only for energy harvesting but also for near-field wireless charging applications. In this paper, we report an electrically small antenna of size ka ≈ 0.25, where a is its effective radius and k the wave vector at operating frequency. The antenna geometry is a 3-dimensional folded meandering loop and contains its own ground, so that it becomes insensitive to the integration environment. The radiation efficiency of the antenna is 70%, and it radiates as a vertically polarized dipole. The operating frequency chosen in this paper targets RFID/IoT applications at 915 MHz, and the impedance matching bandwidth, as realized, is narrow but appropriate for such applications and may be further increased if appropriate matching networks are used.