A Compact Broadband Stepped Bow-Tie Antenna for Ambient RF Energy Harvesting

This paper presents a compact broadband stepped bow-tie antenna for ambient RF energy harvesting (RFEH). The proposed antenna consists of stepped shapes for wide bandwidth, making it possible to investigate ambient RF energy in entire cellular bands. In addition, the antenna is composed of a bow-tie shape to achieve compactness for internet of things (IoT) sensors. We explain the design steps taken to achieve this wide bandwidth, analyzing the surface current distributions with respect to the number of stepped shapes through an EM simulation. In addition, we cut the broadband antenna in half and integrated it with the RFEH circuit for compactness, causing impedance matching to deteriorate. The novel modified bow-tie shaped half antenna improved impedance matching, which is analyzed with the surface current distribution. We found that the proposed antenna has a fractional bandwidth of 125% of
$S_{11} < -10$
dB, ranging from 0.85 GHz to 3.66 GHz with a compact dimension of 68 mm
$\times107$
mm (
$0.19\,\,\lambda _{0} \times 0.30\,\,\lambda _{0}$
). With the proposed antenna, we investigated ambient RF energy at several outdoor sites and chose the feasible frequency bands (0.9 GHz and 1.8 GHz). For the bands, the RFEH circuit was designed consisting of a rectifier and power management unit (PMU), and integrated into the proposed antenna. In one site, we experimented and verified the RFEH with the antenna. The broadband and compact antenna in this paper can contribute to implementing the RFEH prototype for the capability to be used as a power source for IoT sensors.