A design method of$N$-way reconfigurable power divider (RPD) with$N$transmission modes and$2^{N}$-1 states based on reconfigurable impedance matching network (RIMN) is proposed. The RIMN is composed of$M$parallel$\lambda $/4 impedance transformers with different characteristic impedances and PIN diodes and can realize$2^{M}$-1 different characteristic impedance. The$\lambda $/4 impedance transformers are selected for impedance matching by the PIN diodes. Any output port can be the transmission port through the PIN diodes. For multi-way power divider/combiner, the size of reconfigurable matching network can be reduced greatly. For verification, an eight-way RPD is designed and fabricated. The proposed eight-way RPD realizes 8 transmission modes and 255 states. The measured results show that the insertion losses are less than 2.8 dB. The isolations between transmission ports are higher than 16 dB. The size of the eight-way RPD is$0.23\ast 0.4~\lambda _{g}^{2}$.

In this article, an efficient radar fusion-before-imaging (RFBI) method for heterogeneous borehole radar systems is proposed. Different from conventional fusion-after-imaging processes, the sample sets collected from heterogeneous borehole radar systems (monostatic, bistatic, or multiple-input multiple-output) are first merged into one data set before the imaging process in RFBI, and a single imaging operation is demanded to obtain the target space image with high precision. Specifically, the diversity in heterogeneous borehole radar sample sets is taken into consideration, and the radar sample sets are inserted and fused into a high-dimensional sample set before imaging. The target space spectrum is generated according to the echo space–frequency constraint relationship, and the target space is extracted from one imaging process. The influence of clutters in RFBI results is reduced, and the imaging accuracy is satisfactory. Meanwhile, due to the fusion process ahead, the computational time of RFBI hardly increases with the number of radar sample sets. The synthetic and field experiment results show that RFBI demonstrates comparable accuracy as Kirchhoff migration and higher efficiency in processing large amounts of data sets at the cost of large memory requirement, which is suitable for the joint imaging of heterogeneous radar systems.

This paper presents a mechanical reliability study of 3-dB waveguide hybrid couplers in submillimeter and terahertz bands. To show the necessity of improving the mechanical properties of the coupler’s branch in submillimeter and terahertz bands, a comprehensive study regarding the displacement of hybrid branch variation with varying width-length ratio and height-length ratio has been completed. In addition, a modified 3-dB waveguide hybrid coupler is designed and presented. Compared with the traditional branch structure, the proposed hybrid consists of a modified middle branch with circular cutouts at the top and bottom on both sides instead of the traditional rectangle branch, which increases the branch size and improves its mechanical reliability while achieving the same performance. Simulation results show that the deformation of the modified hybrid branch is 22% less than those of other traditional structure designs under the same stress. In practice, a vibration experiment is set up to verify the mechanical reliability of hybrid couplers. Measurement results show that the experiment deteriorates the coupling performance. Experimental results verify that the performance of the novel structure coupler is better than that of a traditional structure branch hybrid coupler under the same electrical properties.

This paper presents an efficient multiple-input multiple-output (MIMO) borehole radar imaging method based on a high degree of freedom for subsurface sensing. The variable separations between the different transmitter and receiver pairs in MIMO borehole radar are considered and introduced as imaging coefficients into the expanded unified MIMO sample set, which gives the MIMO sample set desired characteristics of a high degree of freedom. By exploiting the high degree of freedom in the unified MIMO expanded sample set, the sample interpolation and energy migration of target reflections can be done in once efficient imaging processing for all transmitters of the MIMO radar system, and finally, accurate target image with low sidelobe level (SL) can be delivered. The computational cost of the proposed method will barely increase with the number of transmitters in the MIMO radar survey, which enables the proposed method to handle MIMO borehole radar imaging in an accurate and efficient manner. The imaging properties of the proposed method are proven to be superior to the conventional imaging methods in SL and computational cost, which is suitable for large MIMO borehole imaging surveys in subsurface sensing applications.

The integral equation model (IEM) is considered as a promising algorithm for soil moisture retrieval from active microwave data over bare soil and sparsely vegetated conditions. However, the soil dielectric constant is implicitly embedded in the complicated IEM; inversion of soil moisture is often accomplished through iteration and is thus computationally expensive, particularly when it is applied to retrieve soil moisture from active microwave data on a large scale. To simplify the inversion process of soil moisture directly from the active microwave data, basic math functions were adopted to fit the simulation results of the original IEM so that the radar backscattering coefficient becomes an explicit function of soil dielectric constant or the soil dielectric constant is an explicit function of radar backscattering coefficient. Soil moisture is then calculated directly from radar backscattering coefficient without iteration. We called this model empirically adopted IEM (EA-IEM). The accuracy of the EA-IEM to the original IEM and its applicability are analyzed through three processes: model intercomparison, sensitivity analysis, and model comparison withinsitumeasurements. The average differences of backscattering coefficients between the EA-IEM and the original IEM are 0.14 dB for HH-polarization and 0.12 dB (Gaussian correlation function) and 0.2 dB (exponential correlation function) for VV-polarization. The sensitivity of soil moisture variation is examined under the consideration of absolute and relative calibration errors. A comparison between the soil moisture estimated and the measurements is performed, and the root-mean-square (rms) error is found to be 3.4%, suggesting that the EA-IEM performs well in these real cases. All these analyses indicate that the EA-IEM is a good representative of the original IEM and can be used to retrieve soil moisture under the tested range of model parameters: incidence angles between 10degand 60deg, soil dielectric constants between 4 and 42, surface rms height from 4 to 31 mm, and correlation length from 50 to 250 mm.