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In this paper, we have introduced a low-profile electronically steerable parasitic array radiator (ESPAR) antenna that can successfully be used to estimate the direction-of-arrival (DoA) of incoming signals in wireless sensor network (WSN) applications, in which the height of the complete antenna has to be low. The proposed antenna is over three times lower than high-profile ESPAR antenna designs currently available in the literature for the DoA estimation; it can provide eight unique main beam directions and relies on simplified beam steering, which makes it applicable to simple and inexpensive WSN nodes. Measurements using our fabricated ESPAR antenna prototype indicate that relying solely on the received signal strength values recorded at the antenna output port, it is possible to achieve accurate DoA estimation results with error levels similar to those available for high-profile ESPAR antennas relying on the similar energy-efficient simplified beam steering concept and having 12 unique main beam directions. As a consequence, the overall time required for the DoA estimation using the proposed antenna can be reduced by 33%. INDEX TERMS Switched-beam antenna, electronically steerable parasitic array radiator (ESPAR) antenna, direction-of-arrival (DoA), received signal strength (RSS), Internet of Things (IoT), smart city.

In this paper, we present a low-cost energy-efficient electronically steerable parasitic array radiator (ESPAR) antenna-based wireless sensor network (WSN) node designed for IEEE 802.15.4 standard that is capable of performing direction of arrival (DoA) estimation in real-life outdoor environments. To this end, we propose the WSN node architecture, design and realization that utilizes NXP JN5168 radio frequency (RF) wireless transceiver and a microcontroller integrated with ESPAR antenna beam-switching circuits. To incorporate DoA estimation capability into the developed single-board WSN node, power-pattern cross-correlation (PPCC) algorithm, that relies solely on received signal strength (RSS) values measured by the transceiver at the antenna output for every considered directional antenna radiation pattern, has been adapted and implemented in a simple microcontroller embedded within NXP JN5168 integrated circuit. Measurements conducted in an outdoor environment show that the proposed low-cost WSN node can successfully provide DoA estimation results, which may be used to enhance WSN capabilities in practical applications. The obtained root mean square (RMS) DoA estimation errors are 7.91 • , 6.58 • and 9.47 • for distances between WSN nodes equal to 3 m, 5 m and 10 m respectively. INDEX TERMS Internet of Things (IoT), wireless sensor network (WSN), switched-beam antenna, electronically steerable parasitic array radiator (ESPAR) antenna, direction-of-arrival (DoA), received signal strength (RSS).

In this paper, we have proposed a new directionof-arrival (DoA) estimation algorithm, which uses power pattern cross-correlation (PPCC) estimator and received signal strength (RSS) values recorded at electronically steerable parasitic array radiator (ESPAR) antenna's output port, providing more accurate DoA results for unknown signals impinging the antenna. PPCC estimator provide accurate DoA estimation results for directions lying within the horizontal plane but the estimation accuracy drops for other elevation angles as the estimator relies on ESPAR antenna's radiation patterns measurements in horizontal plane only. The proposed modification allows one to measure ESPAR antenna's radiation patterns at two different elevation planes during the calibration phase, which improves PPCC-based DoA estimation algorithm's overall accuracy when signals impinging the antenna arrive from arbitrary elevation angles. The proposed algorithm have been verified during anechoic chamber measurements using our ESPAR antenna prototype.

In this letter, a concept of high-gain circularly polarized X-band antenna employing a partially reflecting surface (PRS) has been presented. In the initial antenna analysis, the influence of parasitic elements size in the PRS structure on antenna radiation pattern parameters has been investigated and the optimal arrangement of the elements has been identified. The proposed antenna provides wide bandwidth of return loss above 10 dB of 20% (8-9.8 GHz) and circular polarization in a frequency range 8.35-8.95 GHz. The final design is compact (62 × 62 × 22.2 mm) and lightweight (29.7 g), which makes it suitable for use in CubeSat X-band communication systems but also drone and high-altitude pseudo-satellite (HAPS) applications.