Hosung Choo

This paper proposes a differential frequency domain reflectometry (FDR) microwave soil moisture sensor employing an exponential tapered line to obtain improved impedance matching characteristics in a non‐uniform microstrip line structure. In the proposed sensor, the width of the central microstrip line is set to 2 mm to obtain a distinct peak value in the sensing‐band, while the feed microstrip line width is designed as 5 mm to achieve high skirt characteristics in the reference‐band. To improve the impedance matching characteristics of the non‐uniform microstrip lines, an exponential tapered line is applied to the feeding line structure. As a result, the proposed sensor simultaneously achieves the distinct peak value and high skirt characteristics. In addition, triangular spiral resonator and rectangular spiral resonator structures are employed to implement the differential FDR method, making the sensing‐ and reference‐bands, respectively. The fabricated sensor is experimentally evaluated under various volumetric water content (VWC) conditions. When the VWC (%) varies from 0% to 20%, the frequency difference between the sensing‐ and reference‐bands shifts from 3150 MHz to 1925 MHz with a coefficient of determination R ² of 0.926. These results demonstrate that the proposed sensor is suitable for smart farming applications requiring accurate soil moisture monitoring.

In this paper, we propose a receiver deployment optimization method that ensures robust localization performance under Direction of Arrival (DoA) estimation errors arising in practical environments. The proposed method accounts for DoA estimation errors and optimizes receiver deployments by incorporating Position Dilution of Precision (PDoP) to mitigate their amplification into localization errors. To accurately characterize DoA estimation errors under realistic propagation and installation constraints, a ray-tracing-based Wireless InSite simulator is employed to model environments including actual terrain and buildings. Based on this model, a cost function incorporating both the DoA estimation errors at each receiver and the PDoP is defined, and a Genetic Algorithm (GA) is applied to minimize this cost function and determine the optimal receiver deployment. The optimal deployment achieves low localization RMSE and maintains high robustness against environmental and system uncertainties, particularly when the standard deviation of the DoA estimation error exceeds approximately 8°. These results demonstrate that the proposed deployment method provides reliable localization performance in realistic propagation environments and remains robust against increasing DoA estimation errors.

This paper proposes a method for analyzing propagation environments using terrain information and for predicting the communication coverage between a control tower and airborne platforms. This method involves collecting terrain data and implementing it in a Wireless InSite simulator. Digital elevation model data are used to model a communication environment, and mounting analysis allows the antenna pattern of an airborne platform to be generated. The analysis assumes flight scenarios in which the airborne platform follows a given trajectory at various altitudes. At altitudes of 600 m, 1,200 m, and 1,800 m, communication coverage increases with altitude, and communication is possible for 44.91%, 81.99%, and 95.45% of the total communication time in the predefined flight scenario, respectively. A dust particle attenuation coefficient model is used to observe changes in communication coverage due to weather conditions at the Ku-band. At altitudes of 600 m, 1,200 m, and 1,800 m, when dust particles are present, communication is possible for 37.48%, 61.91%, and 62.52%, respectively. A rainfall case is also evaluated using an ITU-R model-based rain attenuation calculation at 30 mm/hr. With rain, communication is possible for 29.72%, 40.16%, and 34.47% at 600 m, 1,200 m, and 1,800 m, respectively. These results demonstrate that terrain and weather conditions should be considered when analyzing the communication coverage between a control tower and airborne platforms.

In this paper, we propose an analytical geometry−based jammer trajectory optimization method that can maintain a stable Jamming-to-Signal (J/S) ratio. Maintaining a stable J/S ratio requires the interference signal to preserve a constant angular direction relative to the fixed sidelobe of the ground station. For this purpose, trajectory derivation is based on two geometric elements: a cone with its vertex at the ground station and a horizontal plane corresponding to the jammer’s operating altitude. The intersection of the two elements forms an ellipse, and the optimal jammer position is selected as the farthest point from the ground station to reduce detection risk. By connecting these points over time as the satellite moves, a jammer trajectory is obtained. Applying this procedure to different target sidelobe angles generates a set of candidate trajectories. Among these candidate trajectories, the optimal trajectory is selected based on a jamming performance criterion, defined as a mean J/S ratio of 0 dB with a standard deviation of 3 dB. The optimal jammer trajectory derivation is carried out under a free−space assumption, and the results are validated using STK simulations incorporating environmental effects. The J/S ratio results show that the optimal trajectories derived for each date satisfy the jamming performance criterion, with differences between the free-space-based model and STK results within 0.6 dB in the mean J/S ratio and 1.12 dB in the standard deviation. These results demonstrate that the proposed method can effectively derive optimal trajectories that maintain a stable J/S ratio with low variation under realistic operational conditions.

한국전자파학회논문지 (The Journal of Korean Institute of Electromagnetic Engineering and Science)

This paper analyzes performance degradation caused by mechanical errors in rib-type deployable mesh reflectors to propose error tolerance levels that can be used as a reference when designing antennas composed of these reflectors. In rib-type deployable mesh reflectors, five types of errors may occur: joint stop angle error, feeder alignment error, feeder pointing error, defective surface error, and wrinkled surface error. In this study, antenna performance degradation stemming from these mechanical errors are analyzed, and error tolerances corresponding to a reduction in the boresight gain by 1 dB (20.6%), 2 dB (36.9%), and 3 dB (50%) are suggested. These boresight gain reduction levels are widely adopted as intuitive indicators of antenna performance degradation. In addition, the corresponding variations in half-power beamwidth, sidelobe level, and main lobe direction are calculated for each error case. The error tolerances at which a performance degradation of 1 dB occurs for each error type are as follows: 0.32° at 10 GHz for joint stop angle error, 9.5% at 3 GHz for defective surface error, 0.0071 m at 10 GHz for wrinkled surface error, 0.016 m at 10 GHz for feeder alignment error, and 23.6° at 3 GHz for feeder pointing error. These results reveal that the most sensitive and critical error is the joint stop angle error, which must be significantly minimized when designing and fabricating rib-type deployable mesh reflector antennas.

We propose a wire-based parabolic reflector antenna capable of electrically controlled beam steering through the integration of voltage-programmable RF switches. By toggling the switch modes, the surface current distribution on the reflector is reconfigured in real time, enabling angular adjustments of the main beam within the azimuthal plane. The antenna was designed and optimized using the FEKO electromagnetic simulator, with performance verified through experimental measurements. Key parameters including reflector diameter, focal length, and feed dimensions were carefully selected to maximize gain and demonstrate controllable beam steering. The fabricated prototype operates at 1.7 GHz with a 50 cm aperture and a 75 cm focal length, achieving a measured gain of 7.5 dBi. Beam steering up to 1° was demonstrated without mechanical reconfiguration. For low Earth orbit (LEO) altitudes of around 1500 km, this angular variation corresponds to approximately 26 km of ground spatial coverage, whereas for medium Earth orbit (MEO) altitudes of around 5200 km, it corresponds to approximately 90 km. The proposed antenna offers a compact, lightweight, and electronically reconfigurable solution suitable for integration into future spaceborne and mobile communication platforms.

In this paper, we propose a signal intelligence (SIGINT) drone swarm system with a three-dimensional (3-D) volumetric self-complementary array configuration. In the proposed system, multiple drones form two array layers separated along the boresight direction of the system, providing sufficient spacing between drones mounting an antenna element. The antenna elements in one array layer are arranged in a complementary manner to fill empty spaces in the other layer, allowing the system to maximize the number of drones deployed within the aperture area. As a result, the effective electrical spacing at 300 MHz is reduced from 1.7λ and 0.9λ to 0.85λ and 0.45λ along the x- and y-axes, respectively. The array gains of the proposed system are 3.96 dBi, 6.40 dBi, and 15.3 dBi at 100 MHz, 200 MHz, and 300 MHz, and the side-lobe levels (SLLs) are −13.0 dB, −12.7 dB, and −13.0 dB. In addition, the proposed drone swarm SIGINT system is evaluated in a practical SIGINT environment that considers terrain features, and then the detection performance is compared with those of conventional ground-based and airborne SIGINT systems. In this SIGINT scenario, the proposed system can detect signals over an extended detection range of 150 km than those of ground-based and airborne systems.