Aiming at the application requirements of brightness temperature calibration of the hot calibration target of spaceborne microwave radiometer, and based on the temperature gradient characteristics of the absorbing coating of the calibration target and the mechanism of brightness temperature deviation, combined with practical temperature measurement and experimental methodology, a brightness temperature metrological calibration technology solution applicable for in-orbit use is studied. Given the current background of high emissivity design and determination technology of the calibration target being basically perfected, this work focuses on summarizing the methods for determining the temperature gradient characteristics of the calibration target coating. The goal is to construct an in-orbit available brightness temperature calibration method that uses multiple parameters, such as the measurable temperature values of the metal inner core of the calibration target and that near the radiation aperture of the calibration target. Based on feasible electromagnetic simulation technology, thermal simulation technology, platinum resistance and infrared temperature measurement techniques, the paper preliminarily summarizes the implementation path of the brightness temperature calibration technology system for space-borne calibration targets. This involves first constructing a basic brightness temperature calibration model considering uniform background brightness temperature and improving the mapping relationship from the inner core temperature of the calibration target and the equivalent background brightness temperature to the longitudinal temperature gradient of the coating. Subsequently, an application model for brightness temperature calibration considering the installation environment is constructed, improving the mapping relationship from the temperature measurements of the inner core and that of the radiation aperture area of calibration target to the overall brightness temperature deviation. Finally, the validation and application of the brightness temperature calibration model are discussed. The research on brightness temperature calibration of space-borne calibration source is an important technical basis and reference for further improving the accuracy of brightness temperature of calibration target and even developing space microwave radiation measurement standards.

microwave calibration targets (MCT) are widely applied in on-orbit and pre-launch radiometric calibrations due to its compact size. However, it is well known that array-shaped MCT suffers from the temperature gradient at tips that leads to radiating brightness temperature (BT) bias. Therefore, it is vital to estimate the equivalent physical temperature of the MCT given the tip temperature gradient for the BT bias correction. In this work, the authors numerically investigate the TMCT estimation based on possible temperature measurement techniques, specifically considering a combination configuration of platinum resistor temperature (PRT) detector at the metal base and the infrared camera detector for the tipbottom temperature difference. By considering the possible variation of coating material parameters and thermal measurement errors, it is possible to evaluate TMCT estimation accuracy. Numerical results indicate that this temperature measurement configuration can lead to accurate TMCT estimation at the level of 0.1 K (1-sigma). Factors that notably impact on the estimation accuracy are discussed. This investigation can be a direct reference for MCT BT correction applications in the pre-launch calibration process.

Metasurfaces with high absorption characteristics can serve as blackbody calibration targets for microwave radiometers. Resonant metasurfaces with thin structures are especially attractive candidates due to their high cost-effectiveness and excellent temperature uniformity. However, such metasurfaces usually exhibit spatial dispersion in reflection, since their resonance conditions are strongly wave-vector dependent. This phenomenon affects the radiation behavior of metasurfaces when used as calibration targets and presents challenges to the calibration process, which are difficult to detect through monostatic measurements. This work first characterizes the highly directional, non-Lambertian radiation behavior of metasurface microwave calibration targets (MMCTs), then clarifies its impact on near-field brightness temperature (BT) transfer. Finally, the appropriate antenna configuration for effective deployment of MMCTs is identified. A specific metal-insulator-metal prototype is taken as a representative example to illustrate this common issue. Near-field simulations and experiments are conducted. Results are obtained for the actual BT transfer from MMCT to near-field antennas of different apertures, as well as the engineering estimated BT with monostatic measurements. It is revealed that, collimated antennas are required for efficient BT transfer and accurate performance estimation of the MMCT.

To meet the calibration requirements of the 50-70 GHz solar flare millimeter-wave spectrometer designed by Shandong University, this study develops a low-cost truncated-cone array blackbody radiation source. The source achieves an average emissivity of 0.9996 across the operational band and ensures surface temperature uniformity better than 0.3 degrees C. Electromagnetic simulations based on the finite integration technique (FIT) were conducted to optimize cone height, array periodicity, and absorber coating thickness, while thermal simulations confirmed that the truncated geometry effectively reduces axial temperature gradients and improves brightness temperature uniformity with only a slight reduction in emissivity. An FPGA-based brightness temperature control system further enabled stable regulation within +/- 0.27 degrees C. Experimental validation included both a two-point calibration and a solar-blackbody comparison. These results provide qualitative validation that the proposed truncated-cone array blackbody can be applied as a calibration reference for solar observations. Future work will focus on 3D-printed modular arrays to enhance uniformity, reproducibility, and multichannel adaptability.

The array-shaped microwave calibration target (MCT) is widely applied in the radiometer payloads, with the advantages of compact size and Lambertian thermal radiation. However, it has been noticed that the vertical temperature gradient at unit tips introduce notable brightness temperature (BT) bias, obstructing the achievement to high-accuracy calibration. Meanwhile, the direct testing on the temperature of the coating layer remains as a difficult task, especially when the MCT is mounted on the space-borne payloads, so the BT of the MCT is hard to be accurately determined. In this work, the authors explore the possibility to estimate the brightness temperature based on the widely applied PRT (platinum resistance thermometer) in the unit kernel. It is shown in the thermal measurement of a uniform background that, the PRT tested temperature at different height of metal kernel can be distinguishable, and the temperature difference variation agrees with the trend of target-ambient temperature difference. While the PRT has been widely applied to test the aperture temperature homogeneity of the array-shaped MCT, the results in this work indicate that the PRT has the potential to practically detect the vertical temperature gradient in units which is directly related to the BT bias. Further, the possible manners and main obstruction factors in projecting the PRT tested temperature difference to the BT bias estimation, are discussed in this work.

In this work, the authors discuss the mono-static measurement for the array-shaped microwave calibration target (MCT), specifically on the C-g variation in the middle frequency range. Based on former studies, it is known that, in the monostatic measurement, the measurable backscattering reflectivity r0 shall be compensated by the C-g factor towards the totalreflectivity, and in the middle frequency range the C-g is hard to be concluded for common MCT parameters. In this work, the authors numerically simulate the backscattering measurement configuration, and investigate the Cg variation properties. It is found that, the C-g can vary with incident beam condition, and it is advised to use specifically calculated C-g with specific parameters for practical applications.

This work proposes a method for modeling brightness temperature (BT) transfer in the near-range calibration procedure of microwave radiometers. The approach allows 3-D analysis of the BT at the antenna that received from the calibration target. Unlike classic far-field methods, the proposed model allows direct evaluation of near-field characteristics in the calibration links. The theoretical foundation of the model relies on the derived near-field reciprocity relationships between the incoherent radiation of lossy materials and the coherent radiation of the antenna. To deal with the realistic absorbing materials, both electric and magnetic losses are incorporated into the model. The method is first validated and analyzed at the simulation level by verifying reciprocity and comparing results with analytical solutions. Subsequently, a fundamental issue in the calibration process is investigated: the optimal form matching between the typical calibration targets and the antenna. A comparative study is conducted within a 3-D full-wave simulation scenario, on the near-field radiation and far-field Lambertian radiation characteristics of pyramid array and coated cavity, along with their BT transfer performance under different receiving antenna configurations. A validation experiment is designed to observe the concerned near-field effect in practice. Results demonstrate that the cavity exhibits reduced transferred BT when received by a diffuse antenna at close range. The proposed model provides a new approach for analyzing near-field characteristics in calibration links. It is expected to open new avenues for optimizing calibration procedures in practical radiometric applications.

Near-field measurement is an effective approach for evaluating antenna performance. Multiple reflection interference between the probe and the antenna under test (AUT) is a common error source in near-field measurement. The recently introduced scanning optic-induced plasma scattering (SOPS) technology exhibits significant susceptibility to interference, which adversely impacts its measurement precision. The SOPS technique enables the efficient acquisition of near-field distribution data across a sequence of multiple planes. In this communication, we take advantage of this high efficiency and propose a 3-D plane wave spectrum filtering-assisted near-field measurement method. The measured near-field data at different distances were first decomposed through plane wave expansion theory. Then, based on different behaviors between the original wave and the reflected waves at various distances, multiple reflected wave components were filtered out in the spatial spectrum along the distance dimension to suppress the multiple reflection interference between the AUT and the silicon wafer. A typical Ka-band antenna was simulated and measured using the proposed method. The filtered near-field distribution and far-field pattern agree well with the simulation results. The 3-D plane wave spectrum filtering-assisted SOPS technology enables near-field measurements to be completed within a few minutes. The proposed method is particularly valuable for measuring high-frequency antennas with a large aperture, and it presents an effective solution to deal with multiple reflections associated with near-field measurement.

In this work, the brightness temperature (TB) transfer from the microwave calibration target (MCT) to the feeder antenna in the near-field region is investigated, which is the fundamental physical process in microwave radiometer calibration. As in this scenario, the MCT and antenna cannot be separately considered as points like in far fields, it is interesting and important to study the TB characteristics of the target in cases of feeder antennas at different positions and with different aperture sizes. Recently, as reciprocity in the near field has been established, this fundamental issue can be now investigated. The study starts at a high frequency of 89 GHz when the free-space lambda is much smaller than the unit period of MCT; then, the distributions of the local TB contribution rate are calculated to understand the possible TB transfer variation. It is found that the key factor for the TB variation is the illumination area upon the array-type MCT, and as the footprint is sufficiently large to cover several pyramid units, the TB can be stable versus relative position.

For microwave radiometer calibration, higher main beam efficiency reduces interference from environmental brightness temperature (BT). A key challenge for high-frequency microwave sounders is achieving high main beam efficiency with limited mirror size. Although efficiency has improved to around 95-98%, there is still room for improvement. This study investigates the main beam efficiency of ground-based radiometer antenna systems, considering feeders with different radiating qualities and mirror conditions. A quasi-ellipsoidal mirror with feeders operating at multiple discrete frequencies is designed to optimize the optical path for microwave calibration. The key focus is transmission efficiency, which affects BT leakage. Numerical results for various feeders are compared - straight wall corrugated horns with 98.2% Gaussian content, dual-mode horns with 98.5%, curved wall corrugated horns with 99.8%, and ideal Gaussian beams - highlighting achievable transmission levels in different polarizations with different feeders. Design curves for the high main beam efficiency quasi-optical reflector antenna system are provided. Such information is beneficial and of direct referencing value for practical radiometer reflector antenna designs.

In this work, an interesting phenomenon is investigated in the scenario of calibration link in microwave radiometers, where a feeder antenna is directly harvesting the radiated brightness temperature (BT) from the microwave calibration target (MCT). As reciprocity has been established for the near-field BT transfer, it is important to investigate the performance variation of the MCT in case of different antenna illumination. In this work, it is found and analyzed that at the frequencies where lambda is near to the unit period of periodic array-shaped calibration target, the reflectivity may be notably increased in case of small aperture feeder illumination. This is a disturbing phenomenon that should be avoided for practical applications.

For the applications of radiometer calibration, the microwave calibration target is a significant device which provides with an accurate reference brightness temperature. The total reflectivity, as an important indicator, has to be determined by measurements. In this work, the remaining issue in the mono-static measurements is to be numerically investigated, which is the compensation factor Cg in the frequency range of lambda

In this work, we report the investigation on the methodology for accurately determining the reflectivity of microwave calibration target (MCT), which is a significant parameter for precise radiometer calibration. Specifically, in the common scenario of monostatic measurement, the antenna illumination effects from the feeder T/R antenna were studied, on the reflectivity determination. Since it is almost impossible to get truth value in actual measurement, a numerical evaluation process is established to address this problem, covering the antenna illumination, target scattering, and measurable scattering acquisition, with a reference truth value according to the physical definition of reflectivity. Then, in the numerical results, it can be concluded that the edge diffraction and periodical Floquet interference effects may lead to notable measurement error. For the most convenient measurement setup of monostatic with a nonfocusing feeder antenna, the proper range of application can be concluded, and the corresponding disturbing mechanisms are revealed for practical reference.

To extend the wideband performance of high-order band-pass filtering applications, optimized designs with knitted structures based on traditional miniaturized frequency-selective surfaces (FSSs) are proposed in this paper. The presented miniaturized FSSs consist of multiple metallic capacitive layers, knitted inductive layers, and substrates. In contrast to the conventional high-order miniaturized FSSs composed of metallic frames, patches, and substrates, the optimized miniaturized FSSs replace the original metallic wire frames with knitted structures. Both proposed modified miniaturized FSSs achieve a flat pass-band from 5.5 GHz to 10.3 GHz with a 3 dB bandwidth of 71.6% under vertical incidence. The unit cells have dimensions of 0.16 λ0 × 0.16 λ0 × 0.284 λ0 and 0.16 λ0 × 0.16 λ0 × 0.279 λ0, respectively, where λ0 is the free space wavelength at 7.9 GHz, which is the center frequency of the operating band. Numerical simulations and measurements demonstrate that the proposed modified miniaturized FSSs exhibit excellent wideband performance with clean transition bands around the pass-band during oblique incidence and are suitable for applications such as radomes, where wideband filtering is essential for covering multi-band functions of radar or communication instruments.

In this letter, the deviation of calibration target EM performance due to coating material property is discussed. At the typical operating frequencies for the microwave radiometers, namely 10.65 GHz and 89 GHz, simulations were conducted for the uniformly coated pyramid, and the optimized pyramid black body target under different thermal conditions. The dielectric constant and permeability were subjected to random perturbations of +/- 5%. The total reflectivity and local absorption were calculated. Furthermore, the directionally radiated brightness temperature was computed with the longitudinal temperature distribution inside the coating, while analyzing the uncertainties of brightness temperature arising from the material property variation.

Brightness temperature (BT) transfer in near field region is the typical physical process in the microwave radiometer calibration procedure. In this work, the authors report the numerical framework for calculating BT transfer from the calibration target to antenna inversely based on near field concepts. Specifically, the near field reciprocity is derived between incoherent thermal radiation from the calibration target and the coherent radiation from the receiver antenna, based on the fundamental mode property at the antenna port, which is common in real-world applications. Then, the reciprocity-based calculation routine can be established in the near field, including both the antenna and calibration target in the same computation region. The reciprocity-based calculation is then verified and discussed versus direct calculation results, in the cases of homothermic and heterothermic, close/open scenarios. The reciprocity-based calculation routine has wide potential applications in analyzing realistic calibration problems.

The quasi-optical reflector antenna is widely applied in millimeter-wave radiometers, the leakage and thermal loss of which are important for the calibration accuracy. In this work, the authors investigates the power transmission efficiency of a typical quasi-optical reflector antenna. The analysis includes the power leakage due to the spill over at 89 GHz, and the ohm loss of reflector of specific polarization state at 54,89,118GHz. The overall results gives an intuitive sight of the quasi-optical antenna performance for radiometer applications.

In the current development context of microwave radiometer calibration, the focus for the calibration target has shifted from the EM and thermal characteristics of the target itself to the optimization of total brightness temperature characteristics; As for the calibration link, with the focus on brightness temperature transfer in the near-field, it has gradually evolved from 2D qualitative analysis to 3D quantitative analysis. In this letter, a numerical simulation platform development for analyzing near-field microwave brightness temperature transfer is reported. Introduced the application background and development status of the numerical simulation platform, including the FDTD solver platform for scattering & reflectivity problems and near-field brightness temperature transfer. In addition, prospects and outlooks were made for future applications.

The quasi-optical mode converters (QOMC) is the key device to improve the efficiency of high-power fusion gyrotron transmission. In this work, the authors report an efficient QOMC design for a compact three-mirror layout in 140 GHz, TE22,8. Based on geometrical optics and iterative phase correction technique, the design of the first-stage focusing mirror and the second and third-stage phase mirrors is completed. The full vector physical optics integration is adopted as the main computational means, and the three-level phase correction surface is iterative optimized around the main polarization field component to realize the mode field conversion with high outgoing Gaussian purity. The outgoing Gaussian purity eta(v) of the phase-corrected converter system is improved from 94.1% to 99.7%, which, combined with a power transfer efficiency eta(p) of more than 98.7%, realizes a high-performance prototype design of a higher-order cyclotron QOMC.