Planar Electrically Small Antenna With Omnidirectional Endfire Pattern for IoT Sensors

An electrically small antenna based on a planar loop conductor and operating in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band is presented. The antenna design is guided by characteristic modes analysis through which a dual excitation strategy is proposed to reinforce the antenna's magnetic mode while suppressing its electric modes. By proper design of its feeding structure, the inductance of the loop is resonated to give rise to an antiresonance in its impedance response. Frequency tuning and impedance matching are then facilitated by reactive loading using lumped capacitors. Measurements show a ka of 0.57 at 2.47 GHz with an endfire omnidirectional pattern and a total efficiency of 33%. The antenna is suitable for compact, low-profile, and low-cost wireless terminals, such as Internet of Things sensors, in scenarios where 360$^{\circ }$ spatial connectivity is required.