Search

Even in light of all the multiport vector network analyzers (VNAs) that appear to be on the market at present, the measurement of noncoaxial multiport devices remains a critical challenge due to the lack of a multiport counterpart of the two-port thru-reflect-line (TRL) calibration. Accordingly, based on the concept of virtual auxiliary termination and the two-port TRL calibration technique, a scattering matrix reconstruction procedure is developed for the use of a two-port VNA to characterize noncoaxial multiport devices up to the intrinsic ports located at the ends of the planar transmission lines such as microstrips, coplanar waveguides, and substrate integrated waveguides (SIWs). The four-port scattering matrix of a 60-GHz SIW 0-dB coupler for experimental validation is reconstructed. The final results are in agreement with not only the simulation results, but also the fully corrected TRL results. Index Terms--Auxiliary termination, multiport network, scattering matrix measurement.

We present the analysis and experimental results of a beam-steering and-switching antenna array using coupled oscillators and phase-locked loops. With the use of a type-II coupled phase-locked loop array and an external reference signal, the antenna array can steer its beam by a single control voltage, reduce the beam-pointing error arising from the phase errors of the oscillator array, and hold its output frequency stably at the reference signal frequency in operation. Using a double-pole double-throw switch and a difference amplifier at the center element of the antenna array, one can switch the array radiation pattern between the sum pattern and the difference pattern. Moreover, the beam-scanning range is extended to [+ or -] 90[degrees] by properly using frequency prescalers in the phase-locked loops. The radiation characteristics of a three-element antenna array are measured to verify the array performance. Index Terms--Antenna arrays, beam steering, beam switching, coupled oscillators, phase-locked loops.

The analysis and experimental results of an antenna array using injection locked coupled oscillators with self-tuning of oscillator free-running frequencies are presented. With the use of coupled type-II phase locked loops for tuning oscillator free-running frequencies and an external injection signal for stabilizing the array operating frequency, this antenna array can steer its beam through a single control voltage and hold its output frequency at the injection signal frequency in operation. In addition, its beam-pointing error arising from phase errors in coupled oscillators can be reduced and the array works well over a certain frequency band. Phase dynamics and stability are studied and experimentally verified. Experimental results of a three-element injection locked coupled oscillator array show that its uniform phase progression ranges between -16[degrees] and 52[degrees], and the phase errors are less than 5[degrees] at 2.7 GHz. The operation bandwidth is shown from 2.68-2.72 GHz. By loading the injection locked coupled oscillator array with rectangular patch antennas, the beam-steering radiation characteristics are measured at various control voltages. Index Terms--Coupled oscillators, injection locked oscillators, phase locked loops, stability.

A novel approach is developed to design linear passive retro-directive antenna arrays by properly making use of 90[degrees] hybrids. For a 90[degrees] hybrid, when its direct port and coupled port are terminated with the same reflection coefficient, the phase difference of the reflected signals at the other two ports are reversed to that of the incident signals. This then becomes to be a phase difference reversal circuit (or a retro-directive circuit) and leads to the design of linear retro-directive antenna arrays using 90[degrees] hybrids. The formulas and a folding method on designing linear retro-directive array circuits with arbitrary element numbers are presented with the measurement results of three-element, four-element, and six-element retro-directive antenna arrays. Index Terms--90[degrees] hybrid, phase difference reversal, retro-directive antennas.

Almost all methods for measuring the scattering matrix of an n-port device with the use of a two-port vector network analyzer (VNA) require one to terminate the other n - 2 ports in the fully characterized auxiliary terminations and prefer auxiliary terminations with small or moderate reflection coefficients. In this paper, a technique is presented to measure the auxiliary terminations indirectly. It not only eases the measurement of auxiliary terminations, but also makes the concept of virtual auxiliary termination realizable. In practice, examples of virtual auxiliary terminations can be the inherent connectors or bonding pads of the test device. Also studied is the remedy for the numerical pitfall possibly coexisting with the use of strongly reflecting auxiliary terminations such as virtual auxiliary terminations. The scattering matrix of a four-port circulator is then acquired accordingly from measurements using a two-port VNA and virtual auxiliary terminations. Index Terms--Auxiliary termination, multiport network, scattering matrix measurement.

A near-field phasor beam measurement system is developed for the characterization of heterodyne receiver optics at submillimeter-wave frequencies. The system synthesizes a pair of submillimeter-wave signals as the RF and local oscillator (LO) sources from common reference sources. The synthesized harmonic numbers of the RF and LO sources are arranged with difference by one, which makes this a new configuration with a unitary harmonic diffrence. The coherent RF and LO signal are down-converted by the receiver under test, then mixed with the microwave-frequency common reference signal to generate the second-order IF signal around 100 MHz for amplitude and phase comparison. The amplitude and phase fluctuation of the measurement system at 683 GHz is within [+ or -] 0.2 dB and [+ or -] 4[degrees] in a 1-h period, respectively. The system dynamic range at 683 and 250 GHz can be as high as 43 and 47 dB, respectively. The system is then used to measure the receiver beam patterns at 683 and 250 GHz with different RF transmitting probe antennas. Index Terms--Frequency conversion, Gaussian beams, phase measurement, submillimeter-wave receivers.

In this paper, a two-dimensional mutually coupled oscillator array is studied for the application of a beam-scanning and polarization-agile antenna array. In the design of antenna array, a two-dimensional oscillator array is implemented in x-y plane, the polarization agility is provided by one dimension (or y-direction) and the other dimension (or x-direction) is for beam scanning. By properly tuning the free-running frequencies of these oscillators, the array radiation direction can be scanned at the selected polarization states including linearly polarized, left-hand and right-hand circularly polarized states. The maximal phase difference of [+ or -] 180[degrees] between coupled oscillating signals is acquired by utilizing their second-harmonic signals. This then gives well-defined phase differences among oscillators for beam scanning in addition to the required quadrature phase difference for circular polarization. The performances of polarization agility and beam scanning for a four-element antenna array are verified experimentally and shown to have the potential for adaptive antenna array applications. Index Terms--Coupled oscillator systems, injection locked oscillators, phased arrays, polarization control.

In this paper, mathematical analyses and physical interpretations for the mutual coupling mechanisms within a finite nonlinearly loaded antenna array are presented. These analyses are based on the method of nonlinear currents with the array mutual coupling effects treated by a power series expansion technique. Initially, the antenna input terminals in an antenna array are related by an equivalent multiport microwave circuit. Each port of this equivalent circuit network is then treated by the method of nonlinear currents. The antenna array mutual coupling effect is considered in this linear network and treated by a power series expansion technique. This study gives not only lull analyses, but also physical insight into the mutual coupling mechanisms of nonlinearly loaded antenna arrays. Index Terms--Loaded antenna, microwave circuit, mutual coupling,

In this paper, an effective usage of a vector network analyzer (VNA) for microwave imaging of conducting objects is presented. In the measurement arrangement, a novel four-port transmission test set is developed to combine with a laboratory VNA as a four-port transmission VNA. It is shown to effectively acquire four sets of the frequency-swept object scattered field in a multistatic arrangement for each object rotation angle. The image reconstruction formulations of this arrangement are derived under physical optics approximation. The measurement system, calibration method, and experimental results are presented. Reconstructed images of continuous and discrete conducting objects and a B-52 model aircraft measured in the frequency range 7.5-12.5 GHz are shown in good agreement with the scattering object geometries. These measured results demonstrate the effectiveness of the developed four-port microwave imaging system. Index Terms--Microwave imaging, microwave measurements, multistatic scattering.

In this paper, a quasi-monostatic frequency-swept microwave imaging system using illumination diversity technique is proposed to have an efficient scattered field acquisition arrangement. The measurement system, calibration method, and experimental results are presented. The image reconstruction principle is developed under physical optics approximation. Reconstructed images of the continuous, discrete conducting objects and B-52 model aircraft measured in the frequency range 7.5-12.5 GHz are shown to be in good agreement with the scattering object geometries. The images reconstructed with and without using illumination diversity technique are presented to illustrate the effectiveness of the developed microwave imaging system. Index Terms--Electromagnetic scattering, microwave imaging, microwave measurements.

In this paper, the injection locking performance analysis of dipole antenna array with each element loaded with a two-terminal oscillator is presented. The analysis is based on the nonlinear model of oscillator and the linear model of antenna array considering mutual coupling effects. The locking range of injection signal and the array radiated power are obtained by solving an equivalent multiport network. In general, the solutions include stable and unstable solutions. The Routh-Hurwitz stability criterion is then applied to remove the unstable solutions. Numerical results show that the array performance such as frequency locking range and radiated power by taking into account the array mutual coupling effects is quite different from that of an isolated antenna element. In addition, the influence of antenna element spacing upon array locking parameters in this paper is found to be consistent with other existing theories. Index Terms--Injection locked oscillators (ILO), loaded antennas, mutual coupling.

In this paper, a novel phase conjugation circuit and its application to retro-directive antennas are presented. The phase conjugation circuit uses a balanced circuit structure with subharmonically injection locked self-oscillating mixers oscillating at [omega]. In the operation, an input signal at [omega] /2 is converted to its conjugated signal with no external source required for LO signal pumping, and the output signal frequency is locked at the same frequency of the input signal. The developed phase conjugation circuit is implemented with active antennas to become a retro-directive antenna array. Both the theoretical and measured results of phase conjugation and retro-directive performance are shown in good agreement. Index Terms--Injection locked oscillators, phase conjugation, retro-directive antennas.

A novel method for acquiring the scattering matrix of an n-port network from measurements using a reduced-port network analyzer is developed. This method can obtain the scattering matrix of a nonreciprocal or reciprocal n-port network with the use of a three, or two-port network analyzer. The formulation of this method considers the imperfection of terminators used in the measurement, and only two of the terminators are required to be known. Experimental results from a four-port microstrip circuit show good accuracy using the developed method. Index Terms--Multiport network, scattering matrix measurement.

Subharmonically injection locking to an oscillator is an approach to obtain a stable source operated in microwave or millimeter-wave regions. In this paper, we present the formulation and results on the locking performance of a metal-semiconductor field-effect transistor (MESFET) subharmonically injection-locked oscillator (SILO). Nonlinear-state equations of SILO are derived based on the use of power series expression for MESFET nonlinear elements. The locking performance includes SILO output power, conversion gain, locking bandwidth, and the critical injection-locking signal level. The derived formulation is useful in the design of a MESFET SILO. Simulation results are shown in good agreement with those of experimental test measurement. Index Terms - Locking performance, nonlinear-state equations, subharmonically injection-locked oscillator.

A microwave diversity imaging system conventionally uses a vector network analyzer (VNA) to directly measure the object scattered field (amplitude and phase) over a selected frequency range and viewing angles, then reconstructs the scattering object characteristic function through two-dimensional Fourier inversion. In this paper, we present a cost-effective microwave diversity imaging system using a six-port reflectometer, which measures four amplitude (or power) values to acquire the object scattered field indirectly. One can then eliminate the coherent detectors in a VNA. The calibration procedure for this microwave diversity imaging measurement is also described. Experimental results of three types of scattering objects, a metallic cylinder, four distributed line scatterers, and a 72:1 scaled B-52 aircraft model, are presented using the described six-port microwave imaging system. Index Terms - Microwave diversity imaging, six-port reflectometer.

The principle of operation, calibration procedure, and experimental results of a wideband six-port polarimetric measurement system are described in this paper. This measurement system can measure the polarization states of incoming waves and the polarimetric scattering matrices of scattering targets from the wideband power data. The imbalances and coupling between antenna polarization channels and the characteristics of microwave components used in the measurement system are calibrated using three partly unknown scattering calibrators that are illuminated by four unknown-polarized calibrating waves. Index Terms - Measurement, polarimetry.

In this paper, nonlinear analyses of MESFET oscillator in free-running and injection-locked states are studied based on the Volterra series method and stability analysis. The approach shown is capable of quantitatively analyzing the injection locking performance of oscillator using three-terminal devices in the fundamental mode of operation. Both the transmission-type and reflection-type injection locked oscillators (ILO) are simulated and experimentally verified.

The basic principle, methodology and experimental results of frequency-swept microwave imaging of continuous shape conducting and discrete line objects in a bistatic scattering arrangement are presented. Theoretical analysis is developed under the assumptions of plane wave illumination and physical optics approximation. The measurement system and calibration procedures are implemented based on the plane wave spectrum analysis. Images of three different types of scattering objects reconstructed from the experimental data measured in the frequency range 7.5-12.5 GHz are shown in good agreement with the scattering object geometries. The results demonstrate that the developed bistatic frequency-swept microwave imaging system has potential as a cost-effective tool for the application of remote sensing, imaging radar, and nondestructive evaluation.

The time-harmonic scattering properties of wire scatterers with nonlinear or time-harmonic loads are analyzed in the frequency domain. First, the scattering problem is treated as a network problem that is analyzed by the method of moments. Then, the harmonic balance technique is applied in the nonlinear load case, and the conversion matrix technique is applied in the time-harmonic load case. The analysis is shown efficient for the calculation of steady state response compared with the conventional time-domain approach. Numerical simulation shows the effect of nonlinear loads on changing the scatterer radar cross section (RCS), the intermodulation phenomenon as the incident wave containing two different frequencies, and the effect of time-harmonic loads on modulating the scattered field of wire scatterer.

The port reduction method (PRM) is a method to acquire the scattering matrix of an n-port network from the scattering matrix measured at a reduced port order by terminating certain ports. This then relaxes the instrumentation requirement and calibration procedure. As the port order is reduced to two, the scattering matrix of an n-port network can be obtained from the measurement using a conventional two-port vector network analyzer. In this paper, we describe two novel PRM's, which can reduce the order of measured ports to two. The experiment results show good accuracy. These two PRM's can provide a simpler calibration procedure and instrumentation than those directly using an n-port network analyzer. In addition, they give more accurate results than those measured by a two-port network analyzer with the assumption of using ideal terminators. Index Terms--Multiport network, scattering matrix measurement.