Search Results (344)

This paper presents two broadband four-way power dividers based on a novel T-coil topology, operating in the 22–32 GHz band (covering the K/Ka bands). Type I adopts a cascaded power division structure, while Type II employs a direct-feed integrated architecture. The innovation lies in the introduction of isolating capacitors at the input and output ports, which significantly shortens the critical transmission line lengths in both topologies. This effectively reduces the equivalent inductance and raises the self-resonant frequency, achieving wideband response while maintaining structural simplicity, compact size, and ease of integration. Both circuits were fabricated using a standard 45 nm CMOS process. The measured core chip areas (excluding pads) are only 0.125 mm² for Type I and 0.066 mm² for Type II, demonstrating excellent integration density. Through even-mode and odd-mode theoretical analysis and full-wave electromagnetic simulation verification, both power dividers exhibit good impedance matching and port isolation across the target frequency band. Measurement results further confirm their performance: across the entire 22–32 GHz band, both power dividers achieve a return loss better than 11 dB and isolation exceeding 15 dB; the insertion loss is 1.1–1.4 dB for Type I and 0.8–1.3 dB for Type II; the amplitude imbalance is below ±0.3 dB and ±0.1 dB, respectively; and the phase imbalance is less than ±5° and ±3°, respectively. All measured data show good agreement with simulation results. In summary, Type I offers advantages in layout flexibility and isolation performance, while Type II excels in insertion loss and chip size. Both provide practical circuit solutions for broadband, high-performance, and compact power division systems. Full article

This paper proposes a millimeter-wave miniature on-chip bandpass filter (BPF) implemented using a 0.18 μm CMOS process. To address the issues of insufficient coupling capability, limited control of transmission zeros, and excessive chip area in traditional on-chip filters, a folded cross-interdigital coupling structure is proposed to enhance coupling efficiency and reduce size. The design incorporates metal–insulator–metal (MIM) capacitors to increase the coupling capacitance between resonators without increasing the area, and utilizes a defected ground structure (DGS) to modify the current distribution at the ground plane, generating additional transmission zeros to improve selectivity. An LC equivalent circuit model was established and verified through full-wave electromagnetic simulation, and the design was validated through chip fabrication and on-wafer measurements. The measurement results show an insertion loss of 3.36 dB and a fractional bandwidth of 49.1% at 32 GHz, with two transmission zeros. The core dimensions are 0.25 mm × 0.18 mm. This design achieves a good balance between miniaturization, selectivity, and insertion loss, making it suitable for millimeter-wave SoC applications. Full article

Lightweight multi-rotor unmanned aerial vehicles (UAVs) have shown great potential in flexible Earth observation, but they impose strict restrictions on payload, volume, and power consumption. Traditional pulse-Doppler synthetic aperture radar (SAR) systems offer high imaging performance but suffer from high peak power and large volume, making them unsuitable for lightweight UAV platforms. To meet the low-power demand, most existing lightweight UAV SAR systems adopt frequency-modulated continuous-wave (FMCW) schemes, which are compact and low cost yet limited by a low range resolution, poor anti-interference ability, and single imaging modes. Therefore, it is urgent to develop an SAR system that combines the high performance of pulse radar with the lightweight advantage of FMCW radar. To this end, this paper proposes a compact, low-power Ka-band pulse-Doppler SAR system for multi-rotor UAVs. With 1.2 GHz bandwidth and highly integrated RF and antenna design, the system achieves miniaturization and low power consumption while maintaining high-resolution imaging capability. Furthermore, two-step waveform error correction and a signal predistortion method are presented to compensate amplitude and phase errors and improve the purity of the transmitted signal. Experimental results show that the proposed system can obtain clear SAR images with a resolution better than 0.3 m, providing a practical high-performance pulse-SAR solution for lightweight UAV platforms. Full article

Ice clouds play a significant role in the Earth’s radiation balance due to their unique microphysical and radiative properties, which vary with formation mechanisms and regions and influence the local energy budget. In this study, six years of Ka-band Zenith Radar (KAZR) observations from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) and the Southern Great Plains (SGP) sites, combined with the Fu–Liou radiative transfer model, were used to examine the macrophysical and microphysical properties of ice clouds, their radiative effects, and contributions to the surface energy budget. The results show that the frequency of ice cloud occurrence at SACOL is 40%, significantly higher than the 27% observed at SGP. At both sites, ice cloud altitudes exhibit an increasing trend in the context of recent warming, with a more pronounced increase at SGP. Seasonal variations are evident, with spring characterized by relatively thick and widespread ice clouds, while summer is dominated by high-altitude, optically thin clouds. Ice cloud occurrence peaks at night and decreases during the day at both sites; however, cloud diurnal variations in summer are much greater at SGP than at SACOL. Radiative analysis indicates that longwave radiation-induced warming dominates ice cloud radiative forcing. Net radiative forcing at the top of the atmosphere is 6.08 W/m² at SACOL and 3.06 W/m² at SGP, contributing to atmospheric heating within and beneath cloud layers. At the surface, sensible heat dominates the energy budget at SACOL (over 63%) due to its arid climate, whereas latent heat dominates at SGP (about 67%) because of abundant moisture; and ice clouds have the greatest impact in winter, reducing surface net radiation by 29% at SACOL and 26% at SGP, producing a cooling effect. Full article

Based on monolithic microwave integrated circuit (MMIC) technology, this paper presents the design and implementation of a low-cost, highly integrated Ka-band sixteen-channel transmit/receive (T/R) module, specifically tailored to meet the application requirements of phased array antennas in airborne and spaceborne radar systems, satellite communications, and 5G/6G millimeter-wave networks. The proposed module employs an MMIC-based single-channel dual-chip discrete architecture, optimally integrating amplitude-phase multifunction chips and transmit-receive multifunction chips in terms of both fabrication process and performance characteristics, achieving a favorable balance between high performance and high-integration density. Using low-cost, low-temperature co-fired ceramic (LTCC) substrates, full-silver conductive paste, and a nickel–palladium–gold plating process, a novel “back-to-back” thin-slice packaging technique is presented to improve integration, lower manufacturing costs, and boost long-term reliability. Furthermore, the design incorporates glass insulators and a direct array interconnection scheme, which significantly minimizes transmission losses and reduces interface dimensions. The final module measures 70.3 mm × 26.2 mm × 10.9 mm and weighs only 34 g. Experimental results demonstrate a transmit output power of at least 23 dBm, a receive gain exceeding 26 dB, and a noise figure below 3.5 dB, achieving a 22.5–58% reduction in volume per channel while maintaining competitive RF performance. To improve testing effectiveness and guarantee data consistency, an automated radio frequency (RF) test system based on Python 3.11.5 was also developed. This work provides a practical technical approach for the engineering realization of Ka-band phased array systems. Full article

To address the stringent cost and performance requirements of commercial Satellite-on-the-Move (SOTM) terminals, we propose a Genetic Algorithm (GA)-based design for a millimeter-wave Phased-Array-Fed Lens (PAFL). This antenna is specifically intended to be the electronic scanning module within a hybrid mechanical–electronic steering architecture. In this hybrid configuration, wide-angle coverage is handled by mechanical positioning, while the PAFL is responsible for high-precision fine tracking and jitter compensation within a critical ±15° field of view. By utilizing a small-scale active array to illuminate a large passive planar lens, this design significantly reduces hardware costs compared to full phased arrays. To mitigate phase aberrations and gain loss inherent in such compact focal-to-diameter (F/D) systems, a two-stage co-optimization strategy is introduced. It globally optimizes the lens phase distribution and subsequently synthesizes feed excitation codebooks to dynamically correct residual errors. A Ka-band prototype comprising an 8 × 8 active feed and a 28 × 28 transmitarray lens was fabricated. Measurements demonstrated stable scanning within the required ±15° range with a gain variation of less than 1.5 dB, achieving a peak directivity of 28.9 dBi and sidelobe levels below −12 dB. Full article

In response to the significant challenges posed by the rapid progress of multi-spectral detection technologies to traditional stealth techniques, this paper presents a flexible transparent metasurface structure that is compatible with radar and infrared stealth. It consists of multi-layer functional patterned indium tin oxide (ITO) films and a flexible polydimethylsiloxane (PDMS) substrate. The metasurface uses a high-duty-cycle multi-scale circular ring to achieve a microwave absorption bandwidth of 30 GHz and low infrared emissivity of 0.33 in an optimized ultra-thin 2.65 mm thickness system. The simulation results show that the metasurface achieves absorption exceeding 90% in the frequency range of 10.8–40.8 GHz, which covers common radar bands like X, Ku, K, and Ka. Furthermore, the structure exhibits polarization insensitivity and sustains stable absorption in a wide range of 60 degrees of transverse magnetic (TM) fields. Meanwhile, it decreases the radar cross-section (RCS) by more than 10 dB over a wide angular range even when bent. This study presents a feasible metasurface with ultra-thin, flexible, transparent, and multi-spectral compatibility for the next generation of stealth systems. Full article

The wind and wave-induced Doppler shift (WDS) significantly affects the accuracy of ocean surface current fields retrieved from synthetic aperture radar (SAR). Understanding how different factors affect WDS is therefore essential for improving current inversion accuracy. Existing studies have predominantly focused on single-band WDS, mainly in the C-band, while investigations across other radar bands remain limited. In this study, we simulate the dynamic ocean surface height field and velocity field, and the radar backscatter from the ocean surface that includes the effect of breaking waves. Based on the Doppler shift theory of ocean surface motion proposed by Chapron, we develop a WDS simulation model with potential applicability to multiple radar bands. The performance of the model is verified by comparing its results with those from the CDOP, KaDOP and KuMOD models. The correlation coefficient between the proposed model and the CDOP model reaches 0.97, with mean deviation (MD), mean absolute error (MAE), and root-mean-square error (RMSE) not exceeding −2.07 Hz, 3.35 Hz, and 4.49 Hz, respectively. For comparisons with the KaDOP model, the correlation coefficient is 0.93, and the MD, MAE, and RMSE are within −21.23 Hz, 42.37 Hz, and 52.20 Hz. For comparisons with the KuMOD model, the correlation coefficient is 0.98, and the MD, MAE, and RMSE are within −2.60 Hz, 7.13 Hz, and 9.08 Hz. These results demonstrate that the proposed model can effectively predict the WDS for both C-, Ka-, and Ku-band radar returns. Furthermore, we investigate the impacts of radar parameters, including frequency band, polarization, and incidence angle, as well as wind field forcing on WDS, showing the model’s applicability across multiple radar bands. Finally, the proposed model is applied to current retrieval using Sentinel-1 ocean (OCN) data, and the inversion accuracy is assessed against collocated high-frequency (HF) radar observations. The MD, MAE, and RMSE of the current retrieval using the proposed model are −0.04 m/s, 0.26 m/s, and 0.32 m/s, which are close to those from the CDOP-based retrieval (MD, MAE, and RMSE of −0.02 m/s, 0.25 m/s, and 0.30 m/s). These results demonstrate that the proposed model performs well in ocean surface current inversion and shows potential for further application to ocean current retrieval based on radar data across different frequency bands. Full article

Sub-band errors are inevitable in synthetic bandwidth synthetic aperture radar (SAR) systems due to differences in signal paths and frequency responses of the components used for different sub-bands, which degrade imaging performance if not properly compensated. In this paper, a prominent-reflector-based sub-band error estimation method is proposed for synthetic bandwidth SAR. Based on the analysis of the sources and impacts of sub-band errors, the proposed method estimates and compensates the errors in three steps, corresponding to time-delay error, amplitude error, and phase error. By leveraging the stable reflective properties of prominent reflectors in the scene, the proposed method directly derives sub-band error estimates from focused sub-band images in the time domain. Compared to existing methods, the proposed method achieved robust, high-accuracy performance while requiring less execution time. The effectiveness and efficiency of the proposed method are validated using real data collected by a Ka-band synthetic bandwidth SAR system. Full article

This paper presents a thermal management solution for a Ka-band gyrotron traveling wave tube (gyro-TWT) with non-superconducting magnets. At present, the miniaturization and non-superconductivity of gyro-TWT have become a trend, but miniaturization leads to a significant increase in power density and a severe limitation in heat sink volume, which critically limits power capacity. To address this challenge, a joint microwave–thermal management evaluation model is used to investigate the heat transfer process and identify the crucial factors constraining the power capacity. A cylindrical heat sink with narrow rectangular grooves is introduced. Based on this, the cooling efficiency has been enhanced through structural optimization. The beam–wave interaction, electrothermal conversion, and heat conduction processes of the interaction circuit are analyzed. The compact heat sink achieves a 1.2-fold increase in coolant utilization and reduces the overall volume by 27.4%. Meanwhile, this heat sink improves the cooling performance and power capability of the gyro-TWT effectively. At 29 GHz, the gyro-TWT achieves a pulse power of 150 kW. Simulation results show that the maximum temperature is 348 °C at a 45% duty cycle, reduced by 159 °C. The power capacity of the Ka-band gyro-TWT increases by 40.6%. Full article

To address the diverse requirements of satellite communication applications involving medium-/low-rate reliable links and high-rate high-capacity services, an integrated S/Ka dual-band dual-reflector antenna is proposed as an effective solution. Owing to the stringent spatial constraints of satellite platforms, the longer operating wavelengths in the S-band lead to oversized feed horns in the integrated antenna design, which induces severe secondary aperture blockage, thus degrading aperture efficiency and impeding practical mechanical layout implementation. To alleviate this critical drawback, the proposed antenna achieves multi-band aperture reuse by deploying an array with four miniaturized S-band radiating elements around a broadband Ka-band feed horn. A frequency-selective surface (FSS)-based sub-reflector is further designed to effectively enhance the effective aperture size for the S-band operation, while ensuring unobstructed electromagnetic propagation in the Ka-band, thus enabling simultaneous dual-band high-gain radiation. After comprehensive electromagnetic simulation and parametric optimization for the antenna feed and the FSS sub-reflector, experimental measurements verify that the S-band left-hand and right-hand circularly polarized (LHCP/RHCP) channels achieve more than 20.2 dBic gains with more than 6° half-power beamwidths (HPBWs), and the Ka-band channel yields gains exceeding 41.2 dBic, with HPBWs greater than 0.8°. Full article

Supercooled liquid water (SLW) in mixed-phase clouds significantly influences precipitation efficiency and aviation safety. However, a comprehensive understanding of its vertical structure has been hampered by a lack of sustained, vertically resolved observations over the North China Plain. This study presents the first systematic analysis of SLW vertical distribution and microphysics in this region, utilizing a year-long dataset (2022) from synergistic ground-based instruments in Beijing. Our retrieval approach integrates Ka-band cloud radar, microwave radiometer, ceilometer, and radiosonde data, combining fuzzy-logic phase classification with a liquid water content inversion constrained by column liquid water path. Key findings reveal a distinct bimodal seasonality: SLW primarily occurs at mid-to-upper levels (4–7.5 km) during spring and summer, driven by convective lofting, while winter SLW is confined to lower altitudes (1–2 km) under stable atmospheric conditions. The temperature-dependent occurrence probability of SLW clouds has an annual maximum at −12 °C. The diurnal variation in SLW in summer shows peaks in the afternoon and at night, corresponding to convective cloud activity. Spring, autumn, and winter do not exhibit strong diurnal variations. Retrieved microphysical properties, including liquid water content and droplet effective radius, are consistent with in situ aircraft measurements, validating our methodology. This analysis provides a critical observational benchmark and offers actionable insights for improving cloud microphysics parameterizations in models and optimizing weather modification strategies, such as seeding altitude and timing, in this water-stressed region. Full article

Time-division duplex (TDD) transceivers have found broad utility in millimeter-wave 5G communication, radar and imaging applications. The co-design of the switch and transmit/receive (T/R) amplifiers becomes essential in optimizing the passive loss and chip size. This work presents a Ka-band T/R amplifier with an embedded switch topology. The amplification cores from the TX and RX channels reuse the matching network to the T/R common port, and the full combination of switching and matching structures is enabled within a compact two-winding transformer. Implemented in 40 nm CMOS technology, the proof-of-concept Ka-band T/R amplifier occupies a core chip area of 0.163 mm². Experimental results show that it achieves a peak gain of 17.2 dB with a −3 dB bandwidth of 22.6–30.2 GHz in TX mode and a peak of 17.1 dB with a −3 dB bandwidth of 23.4–31.0 GHz in RX mode. The compact size and wideband gain response make the proposed T/R amplifier suitable for Ka-band TDD applications. Full article

Orthomode transducers (OMTs) are critical for communications, enabling frequency reuse through orthogonal polarization separation, yet traditional manufacturing faces challenges in cost, weight, and complexity at high frequencies. This study explores additive manufacturing (AM) for fabricating high-performance OMTs for X-band and Ka-band applications. Two turnstile-junction-based OMTs were designed, optimizing a symmetrical architecture for low return loss, high isolation, and broadband operation. Both OMTs were fabricated using 3D printing technologies, with a comparative analysis of different AM techniques on performance. Measurements validated simulation results, achieving good return loss for both OMTs and isolation above 35 dB for the X-band and above 25 dB for the Ka-band. These designs meet the requirements of modern communication antenna feed systems across multiple frequency bands. Additionally, 3D printing demonstrates the promising potential of AM in RF component manufacturing, offering performance comparable to traditional metal-machined parts. Full article

A compact fork-shaped four-element multiple-input multiple-output (MIMO) antenna system with wide bandwidth for 5G millimeter-wave (mmWave) applications is presented. The antenna elements are arranged orthogonally to achieve a compact footprint of $20\times 26{\mathrm{mm}}^2$. To enhance the gain, a frequency selective surface (FSS) is placed above the MIMO system, providing an average gain improvement of 1.5 dB across the entire operating band and achieving a peak gain of 7.5 dB at 41 GHz. The proposed design operates in the Ka-band (22–46 GHz), making it well suited for 5G communications. The antenna exhibits an isolation greater than 20 dB and radiation efficiency exceeding 80% across the band. Moreover, key MIMO performance metrics, including diversity gain (DG ≈ 10) and envelope correlation coefficient (ECC < 0.05), meet the required standards. A prototype of the proposed system was fabricated and measured, with the experimental results showing good agreement with simulations. Full article