This article proposes a novel microstrip directional coupler with high directivity. The proposed coupler was configured using three coupled transmission lines and two reflectors to cancel the leakage components in the isolation ports. To demonstrate the validity of the proposed coupler, we designed and fabricated the microstrip directional coupler with 15 dB coupling at 5.8 GHz. The measurement showed the directivity of 50.0 dB at 5.83 GHz, which was improved by 35.9 dB compared with that of conventional one.

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 × 107 mm (0.19 λ 0 × 0.30 λ 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.INDEX TERMS RF energy harvesting, broadband antenna, compact antenna, rectifier, Internet of Things.

This paper proposes new microstrip directional couplers with high directivity using current path detour to compensate for unequal even-and odd-mode phase velocities in the U-NII upper band (5.725～5.850 GHz). The proposed method involves matching the phase velocities of even and odd modes by designing a longer current path for the odd mode, whose phase velocity is faster than that of the even mode. The proposed idea is demonstrated by the design of two directional couplers: one is a directional coupler with triangular edges on each side of the coupling area, and the other is a coupler with circular patterns in the coupling gap. The measurement results of the proposed two directional couplers exhibited an isolation of more than 38.6 dB and 46.0 dB, respectively, and a directivity of more than 25.7 dB and 29.7 dB, respectively. Compared to the conventional structures, the directivity of the proposed structures was enhanced by more than 10.8 dB and 14.8 dB, respectively.