Amr A. Ibrahim

Experimental Characterization of Polarimetric Radar Backscatter Response of Distributed Targets at High Millimeter-Wave Frequencies

Subterahertz frequencies between 100 and 300 GHz remain an untapped portion of the frequency spectrum for many radar- and radiometer-based remote sensing applications. This can be attributed in part to the lack of knowledge of the phenomenology of signal interaction with terrain at these frequencies. This paper examines recently acquired polarimetric radar backscatter data of different types of surfaces using a newly constructed polarimetric instrumentation radar operating at 222 GHz. At this frequency bare surfaces such as asphalt and dirt are electrically rough with subsurface aggregate sizes comparable with the wavelength resulting in substantial volume and surface scattering. The data show strong backscatter responses from bare surfaces with angular dependence proportional to the cosine square of the incidence angle along with significant depolarization (between -8 and -4 dB). The data for vegetation-covered surfaces show weak dependence on incidence angle and appreciable depolarization (between -12 and -6 dB). Empirical models for bare and vegetation-covered surfaces are proposed.

Temperature Dependence of GaN HEMT Small Signal Parameters

This study presents the temperature dependence of small signal parameters of GaN/SiC HEMTs across the 0–150°C range. The changes with temperature for transconductance (𝑔m), output impedance (𝐶ds and 𝑅ds), feedback capacitance (𝐶dg), input capacitance (𝐶gs), and gate resistance (𝑅g) are measured. The variations with temperature are established for 𝑔m, 𝐶ds, 𝑅ds, 𝐶dg, 𝐶gs, and 𝑅g in the GaN technology. This information is useful for MMIC designs.

AlGaN/GaN HEMT With Distributed Gate for Channel Temperature Reduction

Self heating in electronic devices reduces their performance and lifetime. A novel high electron-mobility transistor (HEMT) layout that reduces the channel temperature is presented. To decrease self heating, the new distributed gate (DG) HEMT is configured with multiple, active, and nonactive sections along each gate-stripe. Simulations and experimental results indicating the improved performance of the new layout are presented. Compared to a conventional HEMT, the fabricated novel DG GaN HEMT demonstrated a decrease in channel temperature from 178 °C to 150 °C, accompanied by a 3-dB increase in output power, and 13-fold increase in lifetime.

A Broadband 1-to-

A novel 1-to- N broadband “ iπ-wave” power divider/combiner with reflection cancellation is presented and demonstrated. The new iπ-wave structure provides reflection cancellation and output port isolation with 1-to- N (arbitrary N) signal splitting or N-to-1 summing. It has low loss due to its use of low-impedance transmission lines and provides a relative bandwidth of 50%-200%. The concept is studied theoretically and demonstrated experimentally with several 1-to-4 dividers. The divider/combiner pair provides power amplifiers with broadband operation and well-matched input/output impedances. The concept can be implemented in hybrid circuits or monolithic microwave integrated circuits (MMICs).

N

The problem of long distance wave propagation in a sparse random medium is considered in this paper. A mathematical technique for modeling the behavior of electromagnetic wave propagation as a function of distance in a 2-D sparse random media at millimeter wave (MMW) regime is presented. The proposed model is a field method based on Maxwells equation and, thus, the phase and magnitude of the field in the random medium can be tracked accurately. The random media is characterized by low volume fraction but electrically large scatterers having relatively high dielectric constant. The technique relies on discretizing the random media into thin slabs and relating the forward and backward scattered plane wave spectra from the individual slabs by an equivalent spectral bistatic scattering matrix. By cascading the scattering matrices of the individual slabs, the statistics of the overall scattered wave in both forward and backward directions are obtained. This technique will be referred to as statistical S-matrix approach in spectral domain, or SSWaP-SD. Using this method, it is shown that after propagation to a critical range inside the random media, the incoherent component of the forward propagating wave overcomes the mean-field component resulting in a dual-slope attenuation curve as a function of distance. The accuracy of the model is examined against a full wave Monte Carlo simulation and a very good agreement is observed. Finally, based on the proposed model, analytical expressions for predicting the forward path-loss as well as the back scattered power from a sparse, translational invariant discrete random media are derived. The analytical expressions are tested against Monte Carlo simulation for a very long random medium and very good agreements are demonstrated.

Power Divider/Combiner With Isolation and Reflection Cancellation

An advanced power divider/combiner concept is presented and demonstrated. The new concept provides significant isolation and enhanced reflection cancelation for 1:N dividers/combiners. It is demonstrated with a 1:4, 2-8 GHz, with 0.4 dB insertion loss, 20 dB isolation, and 10 dB return loss improvement over conventional (e.g. Wilkinson) dividers. Additionally, a 1:3, 4-12 GHz (100% bandwidth) divider with low insertion loss, high isolation and high return loss improvement is demonstrated.

Simulation of Long Distance Wave Propagation in 2-D Sparse Random Media: A Statistical S-Matrix Approach in Spectral Domain

This paper reports on complex permittivity values of different natural and synthetic materials at J-band (220-325 GHz). The materials considered in this study include different types of fabrics (wool, polyester, acrylic, jeans, vinyl, and cow leather), in addition to human skin. For the wool and jeans materials, we consider both dry and wet conditions. The knowledge of dielectric constant of these materials is needed for evaluation of radar based concealed object detection methods. The measurement setup used consists mainly of a vector network analyzer (VNA) combined with two -band frequency extenders. A free space transmission only measurement technique is used to obtain both the complex dielectric constant and the effective thickness of all the fabric materials. Such approach is adequate for fabric materials as they do not have a well-defined thickness. The major factors affecting the uncertainty in the measurements are shown to be related to the sample thickness inhomogeneity, and to the unrepeatability of the network analyzer measurement itself. For the human skin case, a reflection measurement method from a terminated waveguide is adapted.

Divider/combiner with enhanced isolation and reflection cancellation

A novel power divider/combiner concept is presented and demonstrated at Ka-band. The new offset-divider/combiner concept provides reflection cancelation, and 1-to-N or N:/1 (arbitrary N, including odd numbers) splitting or summing with equal or unequal power division/summation. Additionally, it has inherently low loss due to its use of low impedance transmission lines. It can be implemented in most technologies (microstrip, CPW, stripline, waveguide, etc.). In this paper, the new concept is demonstrated with a 27-33 GHz divider/combiner using microstrip technology. The bandwidth can be easily expanded to cover an octave.

Sub-Terahertz Dielectric Measurement and Its Application to Concealed Object Detection

Characterization of different thin materials at millimeter-wave frequency, 240 GHz, is presented. This includes vegetation leaves as well as different types of fabrics. The complex permittivity retrieval algorithm is based on fitting the measured transmission coefficient at different incident angles to the corresponding analytical transmission coefficient of a simple dielectric slab. The extracted dielectric permittivity values for the fabrics are compared with the corresponding values measured at low microwave frequency (~1 GHz), using a standard material analyzer, where they are found to be nearly the same. For the leaf measurements, it is found that a simple dielectric mixing formula can be used to predict the value of dielectric permittivity.

Novel Ka-band ‘offset-divider/combiner’ with reflection cancellation

This paper reports on a set of radar experiments conducted at 222 GHz in support of envisioned application of radars to autonomous vehicles. The measured radar backscatter data of road surfaces at incident angles between 80 and 88o are presented. The polarimetric radar response level from different surfaces encountered in roads and highway environments and the angular dependency are discussed. The results are helpful in developing models for different surfaces and in the design process and assessment of autonomous vehicles sensors.