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Radioengineering

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Proceedings of Czech and Slovak Technical Universities

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December 2026, Volume 35, Number 4 [DOI: 10.13164/re.2026-4]

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A. G. Pakfiliz [references] [full-text] [DOI: 10.13164/re.2026.0425] [Download Citations]
An S-Band Radar Method for Early Detection of Stealth Fighters in Air-to-Air Combat

Stealth aircrafts reduce the detection ranges of traditional radar systems primarily by lowering Radar-Cross-Section (RCS) optimized for high-frequency X-band threat radars. To evaluate multi-band detection potential under airframe constraints, this study analyzes the integration of a wing-leading-edge S-band radar array to complement conventional nose-mounted X-band systems. Using link-budget modeling and simulation, detection performance across L-, S-, and X-bands is compared for low-observable target profiles under clear and jamming conditions. The analytical findings indicate that while nose-mounted X-band radars maintain high angular resolution, S- and L-band arrays provide extended burn-through and crossover detection ranges against stealth targets. Specifically, a fixed-aperture S-band wing-mounted configuration offers a balanced trade-off between element density and aperture constraints, providing actionable theoretical insights for future multi-band radar architectures in air-to-air scenarios.

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Keywords: Stealth fighters, air-to-air radar, radar absorbent material, S-band radar

Q. Guo, H. D. Zhao, W. J. Wang, W. K. Zhang [references] [full-text] [DOI: 10.13164/re.2026.0442] [Download Citations]
SAGA-YOLO: A High-Accuracy Detector for SAR Aircraft in Complex Environments

Synthetic Aperture Radar (SAR), utilizing its ability to actively transmit microwave signals, is widely used for military aircraft target reconnaissance and aviation safety monitoring. However, the complex background of ground-based aircraft targets, combined with the coherent imaging characteristics of SAR, results in speckle noise, which increases the difficulty of detection. To address this, we propose a high-accuracy SAR aircraft detection model, SAR-Adaptive Gated-Attention You Only Look Once (SAGA-YOLO). It includes the following three improvements: Firstly, the model constructs a robust gated convolutional backbone network that adaptively filters out noise and irrelevant clutter by dynamically adjusting feature channels, thereby extracting more robust semantic features of aircraft. Secondly, an attention module, C2-CAS, is introduced at the end of the backbone network to refocus features on the most distinctive scattering regions of the aircraft, enhancing its key structural features. Finally, a feature fusion network, Slim-neck, is introduced at the neck to improve the model’s ability to extract and fuse features of targets at different scales. Experimental results on the SAR-AIRcraft-1.0 and SADD aircraft datasets demonstrate that SAGA-YOLO achieves superior performance; compared to the baseline YOLO11, SAGA-YOLO improves mAP50 and mAP50-95 by 3.0% and 5.8% respectively on SAR-AIRcraft-1.0, and by 0.6% and 18.3% on SADD.

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Keywords: SAR image, YOLO11, aircraft detection, gated backbone

L. Ju, J.–J. Ren, Z.-Q. Zhang, C.-L. Dai, Y. Mao, K. Huo, K. Wei, W. Hu, W. Jiang [references] [full-text] [DOI: 10.13164/re.2026.0455] [Download Citations]
A Low-Scattering Tightly Coupled Dipole Array in a Triangular Lattice

A low-scattering tightly coupled phased array antenna based on a triangular lattice with resistive metallic strips is presented in this paper. Resistive metallic strips are loaded orthogonal to the dipole at the wide-angle impedance matching (WAIM) layer to achieve an in-band low-scattering array design. Compared to a rectangular grid, a triangular grid phased array reduces the total number of elements by approximately 13.4%. The proposed array achieves ±45° beam scanning in the E- and H-planes over the frequency band of 5.6-16 GHz, while exhibiting outstanding low-scattering characteristics for cross-polarized waves throughout the operational bandwidth. The monostatic RCS reduction exceeds 10 dB within the range of 6.6-16 GHz, and it also demonstrates excellent bistatic RCS reduction under oblique incidences. A 10×10 array prototype is fabricated and measured. Both simulated and measured results are in good agreement, validating the feasibility of the design.

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Keywords: Antenna array, triangular lattice, radar cross section (RCS)

C. Liu, L. Guo, Y. Tian, S. Tian, S. Zhang [references] [full-text] [DOI: 10.13164/re.2026.0465] [Download Citations]
A Novel Iterative Method for I/Q Imbalance Estimation and Calibration in Direct-Conversion Transmitters

Direct-conversion transmitters (DCTs) have been widely employed in modern communication systems due to their advantages of low cost and high spectral efficiency. However, the inherent problem of in-phase/quadrature (I/Q) imbalance in this architecture significantly limits overall system performance. This paper addresses the I/Q imbalance problem in DCTs by proposing an efficient estimation and calibration method based on an improved swarm intelligence optimization algorithm, which is implemented in software. The algorithm dynamically optimizes the iterative process of pre-distortion coefficients, thereby improving the efficiency of imbalance parameter estimation. To further mitigate the limitations of conventional particle swarm optimization (PSO) algorithms in terms of convergence speed and accuracy, we design a nonlinear exponential parameter update strategy with adaptive focusing characteristics. This strategy adaptively adjusts learning factors during iteration by dynamically evaluating changes in the image rejection ratio (IRR), effectively balancing local exploitation and global exploration capabilities to enhance convergence efficiency. Experimental results demonstrate that the proposed method achieves efficient and precise I/Q imbalance calibration for transmitters, with the IRR improved to over 65 dB for single-tone signals and over 53 dB for broadband signals.

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Keywords: I/Q imbalance, parameter estimation, direct-conversion transmitter, iterative, frequency-dependent, improved PSO

S. Q. Nguyen, M. X. Pham, T. H. Dang, H. N. Nguyen, C. C. Pham, T. M. Hoang, L. T. Tu [references] [full-text] [DOI: 10.13164/re.2026.0477] [Download Citations]
UAV-Assisted Hybrid Power-Frequency Multiple Access with Transmit Antenna Selection over Double Nakagami-m Channels

This paper investigates the performance of an unmanned aerial vehicle (UAV)-assisted hybrid power--frequency multiple access (HPMA) system with transmit antenna selection (TAS) over double Nakagami-m fading channels. While existing literature has largely focused on conventional non-orthogonal multiple access (NOMA) or single-antenna setups, the novelty of this work lies in the joint integration of HPMA and TAS within a multi-antenna UAV network to combat the severe cascaded scattering effects modeled by the double Nakagami-m distribution. By avoiding the complexity of successive interference cancellation, this joint framework offers a highly reliable and low-complexity transmission strategy. In the considered model, a multi-antenna UAV employs TAS to serve ground users over these cascaded fading channels. Closed-form expressions for the outage probability (OP) are derived by characterizing the statistical distribution of the equivalent channel gain. To provide further insights, asymptotic OP expressions are also obtained in the high-SNR regime, from which the diversity order of the considered system is determined. In addition, the throughput under delay-limited transmission is evaluated. Monte Carlo simulations are presented to validate the accuracy of the theoretical analysis, and numerical results demonstrate that the proposed HPMA scheme achieves better outage performance than the conventional NOMA scheme under the same system settings.

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Keywords: UAV communication, hybrid power-frequency multiple access, transmit antenna selection, double Nakagami-m fading, outage probability, diversity analysis

S. Q. Nguyen, M.-H. H. Do, L.-D. Huynh, T. N. Nguyen [references] [full-text] [DOI: 10.13164/re.2026.0487] [Download Citations]
Computational Analysis of Secrecy Outage in Energy-Harvesting Multi-Backscatter IoT Networks

This paper studies the secrecy outage performance (SOP) of an energy-harvesting multi-backscatter-device (BD) network in the presence of a passive eavesdropper. A dedicated radio-frequency source illuminates multiple BDs employing power splitting (PS), and the BD with the strongest source-to-BD channel is opportunistically selected for confidential backscatter transmission. Unlike conventional secure backscatter communication models that assume an always-active BD, we explicitly impose a practical circuit-activation constraint requiring the selected BD to harvest sufficient energy before transmission. Consequently, the selected source-to-BD channel jointly determines circuit availability and the strengths of the legitimate and wiretap cascaded links. A tractable analytical framework for the SOP is developed by accounting for activation failure, secrecy-rate outage, opportunistic selection, PS, and Rayleigh fading. Since the activation constraint yields a non-zero lower integration bound, we derive a finite-sum approximation using the binomial expansion and Gaussian -Chebyshev quadrature. We further obtain a high-signal-to-noise-ratio expression that reveals a non-zero secrecy outage floor caused by the proportional scaling of the legitimate and eavesdropping links. Monte Carlo simulations validate the analysis and show that increasing the number of candidate BDs improves secrecy performance, particularly in energy-limited regimes, but with diminishing returns. The results also reveal the trade-off between circuit activation and backscattered-signal strength and quantify the effects of the activation threshold, PS coefficient, and channel conditions on secrecy performance.

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Keywords: Energy harvesting, backscatter communication, physical layer security, secrecy outage probability, power splitting