This paper presents a compact RF energy harvesting (RFEH) system with an improved impedance matching network. The antenna includes a matching network consisting of periodic stubs, making it possible not only to adjust $S_{11}$ when it changed due to fabrication errors but also to design the antenna to be more compact. We initially analyzed the effects of the fabrication errors and variations of $S_{11}$ with respect to an adjustment of the matching network using an EM simulation. The antenna dimensions overall are 14.5×18.5 $mm^{2}$ , and it is integrated with an RFEH circuit consisting of a rectifier and power management unit (PMU), which compose the RFEH system. The proposed system was implemented, charging a load capacitor at the received power of -15 dBm and -5 dBm; therefore, the proposed system with the impedance matching network contributes to compactness and can be utilized as the power source for compact Internet of Things (IoT) sensors.

This letter presents an antennadiagnosis method using infinitesimal dipole modeling. For antenna diagnosis, it is important to estimate the current distribution of the antenna aperture. By using the measured near-field data of the antenna, the infinitesimal dipole current distribution that can generate the same near-field measurement result is extracted. This current distribution can be considered as a back-projection image on the antenna aperture. Using the infinitesimal dipole modeling, accurate antenna diagnosis is possible even by using only the measured near-field which is limited over the antenna aperture. We compared the results of the conventional antenna diagnosis method based on Fourier transform and the proposed method for the near-field measurement results of the standard gain horn antenna. Although the limited near-field measurement over the antenna aperture, the back-projection image for the antenna diagnosis of the proposed method has a very high resolution.

This paper presents a curved-retrodirective beamforming system for improving microwave power transmission efficiency in the Fresnel region. Since microwave power transmission in the far-field region has very low efficiency, studies on the Fresnel region are being actively conducted. In these studies, a retrodirective beamforming (RDB) technique is popular. The RDB system with a sub-array structure was a realistic structure that reduced system complexity. However, the transmission efficiency is lowered because the beamwidth of the transmitter antenna element is narrow. To solve this problem, this paper proposes a curved-retrodirective beamforming system that can focus the microwave power on the receiver. The proposed system uses the peak gain of the transmitter antenna element by using tilted beams to improve transmission efficiency. The system design method that can maximize the transmission efficiency is also presented depending on the given conditions such as transmission distance, characteristics of the transmitter and receiver antenna. The simulation showed a reduction in power leakage compared to the conventional system. The fabrication and measurement validated the efficiency improvement of the proposed system for IoT devices in the Fresnel region.

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 x 107 mm (0.19 λ ₀ x 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.

This paper presents a pattern reconfigurable dual-polarized dipole antenna with staggered parasitic elements operating in 3.5 GHz band. The proposed antenna is designed by assembling two linearly polarized antennas to be orthogonally polarized, and each antenna includes two parasitic elements and a single dipole element. A circuit including a varactor is inserted into each parasitic element, and the current phase of the parasitic element changes according to the capacitance of the varactor. Each antenna has a beam steering range of about ±30° in the E-plane, and the beamwidth and gain are maintained uniformly regardless of the steering angle due to the angled parasitic element. Also, because of the continuously changing characteristic of the varactor, the beam of the proposed antenna is steered continuously, and only four varactors are used. The size of the fabricated dual-polarized antenna is $1.40\lambda _{0}\times 1.40\lambda _{0}\times 0.47\lambda _{0}$ , and the measured results confirm that the antenna has an average gain of around 6 dB with 0.3 dB gain variation.