Active Filters for UHF and Microwave Frequencies

A technique is described for using transistors directly as high-Q inductors at microwave frequencies. Several experimental band-pass filters have been built and tested to verify usefulness of the inductive transistor circuit. Stable filters with unity insertion loss have been realized at UHF. Observations made during temperature cycling show that environmental stabilization can also be achieved. Analysis has been made of noise figure and nonlinear distortion, and supporting experimental data are provided. The inductive transistor circuit is expected to be practical for a variety of small-signal filtering and multiplexing applications.     

Television Broadcasting at Microwave Frequencies

Over 125 Multipoint Distribution Service (MDS) transmitting stations have been constructed over the past seven years. Each of these stations is equivalent to a low power television broadcast station operating on a microwave frequency at approximately 2,150 MHz. The problems encountered in the design and construction of an MDS station differ markedly from a VHF or UHF television station. The propagation characteristics, likewise, are significantly different than for either UHF or VHF frequencies. The solution to the problem of effective coverage and reception throughout a service area requires innovative designs and techniques, and these are described.     

Duplexing Systems at Microwave Frequencies

The paper reviews the various methods of duplexing at microwave frequencies. General principles, including the use of passive and solid-state devices, are first discussed. The characteristics of gaseous-discharge duplexing tubes of both self- and externally-excited types are examined and data for typical examples given. The various arrangements of discharge tube duplexers and methods of measuring their performance are described. The survey concludes with a bibliography.     

Measurement and Control of Microwave Frequencies by Lower Radio Frequencies

From the fields of nuclear and paramagnetic resonance comes a relation between precession frequency and magnetic field strength for nuclei and unpaired electrons. The relation is such that f/sub n/=K/sub n/H for nuclei and f/sub e/=K/sub e/H for electrons. Thus if the frequency of one oscillator is set for f/sub n/ and the frequency of another oscillator is adjusted so that simultaneous nuclear and electronic resonance occurs in the same magnetic field, the frequency ratio of the oscillators is given by the ratio of K/sub e/ to K/sub n/. Values of K/sub e/ and K/sub n/ have been tabulated for many substances and therefore allow frequency comparisons to be made. For example, protons in mineral oil and electrons in hydrogen have a precession frequency ratio of 658.228; hence for an f/sub e/ in x band, f/sub n/ is about 14 mc when the magnetic field is 3300 Gauss. Changing the value of H causes the frequencies to move up or down the frequency scale but their ratio is always constant. By this method microwave frequencies may be measured with equipment of a much lower frequency range. The precision of measurement is limited by the widths of the nuclear and electronic resonance curves and runs between one part in 10/sup 4/ to 10/sup 5/. This frequency measurement method may be made the basis of automatic control of microwave frequencies by quartz crystals or very stable lower frequency oscillators. An experimental model of such a system has been constructed and operated.     

Biological Tissues Characterization at Microwave Frequencies

The present work is concerned with the measurement of dielectric permittivity and conductivity of various high loss tissues from freshly sacrificed animals. The measurement makes use of the 'infinite sample' technique which involves mounting of the sample in a rectangular waveguide system excited in the TE/sub10/ mode at 9.4 GHz. A more complex system consisting of skin-fat-muscle combination is also studied. An evacuation of relaxation times is made in all the cases. It is hoped that these data will be relevant in further quantifying the available results in this frequency range.     

基于准周期阵列可调微波吸收体

在传统准周期阵列吸收体单元图形间集成一系列PIN二极管,通过控制PIN二极管偏置电流大小,可以实现吸收体反射特性的动态调节.自主设计了3种不同单元图形的可调微波吸收体,采用正弦信号和直流偏置相结合的激励方法,实现了吸收体PIN二极管阵列的有效驱动.微波反射率测量结果表明:在2～8GHz和8～17GHz两个频段,具有反射率动态可调特性.通过比较不同图形动态反射率曲线,发现由于不同单元图形对应的电感电容不同,反射曲线各异,通过对单元图形设计,可以实现特定的吸波要求.     

Modeling of Vegetation Permittivity at Microwave Frequencies

When materials are to be treated with microwaves for different purposes such as drying, online moisture measurements, disinfestation, and remote sensing, thorough knowledge of the material dielectric properties becomes extremely important. Unlike for other materials, measurement of vegetation dielectric properties is very complicated due to the nature of the materials themselves. Hence, vegetation dielectric models that require fewer accurately and easily measurable physical quantities are of great importance. Therefore, dielectric models that only require the measurement of moisture content (MC) have been investigated for alfalfa, a plant material that has high commercial value in national and international markets. The models were developed by measuring the dielectric properties by using an open-ended coaxial probe at frequencies ranging from over 300 MHz to 18 GHz at 22 degC and at an MC ranging from 12% to 73% in wet basis     

Nonlinearities in Metal Contacts at Microwave Frequencies

Generation of intermodulation products arising from nonlinearities at metal-to-metal contacts for microwave frequencies has been investigated. Detailed studies concentrated on the intermodulation products generated at contacting faces between both similar and dissimilar metals, and included contact materials of copper, beryllium-copper, brass, oxygen-free nickel, mild steel, stainless steel, aluminium, and twelve home-made steels with various compositions. The surface properties of the materials were found to be of prime importance. Measurements were carried out at frequencies around 1.5 GHz. The variations of the 3rd- and 5th-order intermodulation product levels with power levels of the fundamental signals and with the axial force applied to the contact were studied.     

A Graphical Method for Measuring Dielectric Constants at Microwave Frequencies

This paper describes a graphical method for measuring the real and imaginary parts of the dielectric constant epsilon/epsilon/sub 0/ = epsilon' - j epsilon" of materials at microwave frequencies. The method is based on the network approach to dielectric measurements proposed by Oliner and Altschuler in which the dielectric sample fills a section of transmission line or waveguide. In contrast to their method, the network representing the dielectric sample is analyzed in terms of the bilinear transformation Gamma'=(aGamma+b)/(cGamma+d); ad-bc=4. The analysis proceeds from the geometric properties of the image circle in the r plane obtained by terminating the output line in a calibrated sliding short. The technique described retains the desirable features of the network approach but avoids the necessity of measuring both scattering coefficients. As a result the procedure is more direct and, in the case of the TEM configuration, leads to an entirely graphical solution in which the complex dielectric constant can be read from a Smith chart overlay.     

Ferrites with Planar Anisotropy at Microwave Frequencies

Materials with an easy plane of magnetization (planar anisotropy) have recently been discovered. The large anisotropy field that tends to keep the magnetization in the easy plane reduces the field required to cause ferromagnetic resonance, which makes the material promising for microwave applications. Equations are derived for the susceptibility, taking into account losses and a finite medium. Propagation in a longitudinal and transverse static field is considered. The location of a slab in a rectangular waveguide for minimum loss in the forward direction, and the use of the material as a phase shifter, are discussed. Experimental microwave data on some materials are given, and also data on an isolator and phase shifter incorporating these materials.     

Carrier Pulses at Microwave and Millimeter-Wave Frequencies

This paper discusses some recent experimental results obtained using special gallium arsenide point-contact diodes for the generation of phase-locked carrier pulses in the microwave and millimeter-wave bands. Several methods of generating such pulses are described. 11.2-Gc microwave phase-locked carrier pulses of about 1.0-nanosecond base duration have been generated at a 160-megabit/second rate. These microwave pulses, which are generated directly from a baseband signal, normally have peak power levels in excess of 0.5 mw. Millimeter-wave phase-locked carrier pulses have also been generated at 56 Gc. These very high frequency pulses have a base duration as short as 0.25 nanosecond and occur at a 160-megabit/second rate. Furthermore, phase-locked carrier pulses have been generated at frequencies as high as 89.6 Gc. A simple method of generating nonphase-locked 0.3-nanosecond millimeter-wave carrier pulses directly from 1.92 gigabit/second rate baseband pulses has also been investigated. The experimental arrangement used to demonstrate the "turn on" and "turn off" principle of transient carrier pulse generation is described.     

Design of Ferrite-Impregnated Plastics (PVC) as Microwave Absorbers

This paper is concerned with the modeling of absorption of microwaves in a composite containing a random distribution of Fe/sub 3/O/sub 4/ particles embedded in PVC. The theoretical model based on a self-consistent multiple scattering formalism, including the effect of statistical correlations in the positions of the particles. A T-matrix is used to characterize the response of individual ferrite particles to any incident excitation. An analytical expression is obtained for the complex propagation constant in the composite in the long wavelength limit. In addition to presenting results for a variety of materials including Ni ferrite compounds, it is shown that a particular set of assumed values of the complex magnetic permeabllity and dielectric function leads to very good agreement with the experimental data of Ueno et al [2].     

The Role of Chirality and Resonance in Synthetic Microwave Absorbers

Microwave absorption by a lossy dielectric material containing thin metal wires is considered. The wires are bent to create either chiral, non-chiral or racemic unit cells. No physical mechanism is found to support patents which were granted between 1990 and 1993, and related claims in the engineering literature, that chirality is the key to improved microwave absorbers. Instead, in synthetic composites which employ thin metal wires in a lossy dielectric host, half-wave resonance of the inclusions – not their geometric shape – is identified as the mechanism responsible for enhanced absorption. It is also found that – even in an essentially lossless host – resonant steel wires, whether chiral or not, can strongly absorb electromagnetic waves.     

Substitution Method for Swept-Frequency Measurements of Dielectric Properties at Microwave Frequencies

A solid-state computer-controlled system has been used to make swept-frequency measurements of the insertion loss and reflectance of biological specimens and other media. A substitution procedure was used for direct comparison of samples to allow determination of small differences (on the order of 0.1 dB) in insertion loss and reflectance.     

Dielectric Loss in Biogenic Steroids at Microwave Frequencies

Dielectric loss in steroids has been measured in solid format 9.4 GHz and in nonaqueous solutions at 3.3 and 9.4 GHz. The method for solutions consists of measurement on standing-wave pattern in front of a column of liquid of varying length and concentration, contained in a short-circuited dielectric cell. Keeping the concentration within the limits of dilute solutions, dipole moment and relaxation time of Cholesterol, Progesterone, and Testosterone have been evaluated. Mechanisms responsible for dielectric loss and its trend of variation in the three steroids are presented. The solid phase measurements were carried out by cavity perturbation technique on powders and crystal values, for epsilon' and epsilon" were evaluated. The difference in epsilon" values of the three steroids in the two phases is attributed to the difference in the mechanism of microwave absorption. However, identical values of epsilon' are obtained.     

On Noise in Distributed Amplifiers at Microwave Frequencies

Formulas for the noise figure, and the minimum noise figure of a multi-link distributed amplifier have been developed. In addition, a relatively simple approximation formula has been devised that predicts the minimum noise figure of a practical amplifier design with good accuracy up to frequencies of 9 GHz. Finally, after the dependence of the noise characteristics on the circuit parameters is discussed, the noise figures of a 2--18-GHz three-link module are computed and compared with those measured on an actual amplifier. The measured data across the 2--18-GHz band compare favorably with the computed results. Measurements and theory agree that only small improvements in noise figure may he achieved, when noise matching the module's input impedance.     

Measurement of Relative Phase Shift at Microwave Frequencies

A method is described for measuring the relative phase shift of microwave devices, such as traveling-wave tubes, which utilizes the serrodyne technique to transfer the measurements into the audio-frequency range. The method is used to measure the phase shift incidental to the variation of the dc potentials applied to the several electrodes of a 2- to 4-kmc traveling-wave tube. This method is particularly useful in coaxial systems, where accurately calibrated phase shifters (and attenuators without phase shift) are not available.     

Monitoring Variations of Biological Impedances at Microwave Frequencies

Two interferometer systems for monitoring time-varying biological impedances at 915 MHz and 913 GHz were developed. The systems compare the phase of the signal scattered from a region of biological tissue to that of a reference signal, using a phase sensitive detector. The phase changes of the scattered signal are an indication of the net changes within the test region due to various physiological processes, for example, the displacements of blood vessels during the cardiac cycle. The systems were tested with simulation models and their detection characteristics were found to be a linear function of the phase changes for return losses within the test region as high as 60 dB. Because of the interference between the signal scattered from the test region and the signal reflected at the antenna-tissue interface, the maximum range of operation of the systems was limited to a fraction of a wavelength in the test medium.     

Power and Area-Efficient Adaptive Equalization at Microwave Frequencies

We present a low power analog adaptive equalization technique suitable for combating inter-symbol-interference at very high data rates. The proposed technique, which we term the lumped parameter equalizer, addresses several of the problems associated with conventional microwave equalizers based on the tapped delay line structure. The theory is given, and simulation results comparing it with the performance of ideal tapped delay line filters are shown. Circuit implementations are discussed, along with the effect of nonidealities on equalizer performance.