Transmission Of Microwave Signals Research Articles

Microwave sensing of increased intracranial water content.

A new technique to monitor brain edema is described that uses microwave energy to detect increased intracranial water content. By recording the phase of a transmitted microwave signal, increases in intracranial water content on the order of 1% are detected in vivo. These findings suggest that the progression of brain edema on a long-term basis can be assessed by progression of brain edema on a long-term basis can be assessed by our technique. Furthermore, phase changes related to small intracranial pressure variations are observed, indicating that the detection of pathologic pressure variations such as Traube-Hering-Mayer waves and plateau waves is possible by a noninvasive microwave technique.

Effects of sizes of nonspherical raindrops on crosspolarization of transmitted microwave signals

The effect of raindrop shape on the crosspolarization of microwaves has been investigated using a modification of Brussard's meteorological rain model. It is shown that in a rainfall situation the drops that mainly affect the crasspolarization signal level are the small- to medium-size ones.

Experimental system for random propagation measurements

An experimental system that is used to study the transmission of electromagnetic waves through a randomly ionised plasma is described. The primary advantage of the system is that the statistics of the propagation medium may be controlled and therefore repeated. Measurements of phase fluctuations suffered by transmitted microwave signals are used to study random propagation effects in plasmas having background pressures in the range 50-1300 Pa (0.4-10 Torr).

Transmission of microwave signals through an e.h.v. insulator for application to e.h.v. electronic current-sensing systems

It is shown how a conventional solid-core e.h.v. post insulator can be adapted for use as a fairly low-loss, physically robust, microwave transmission link for use in unconventional e.h.v. electronic current-sensing systems. Coupling into and out of the insulator is by way of an interesting adaptation of the short backfire antenna.

Detection and generation of magnetic bubble domains using ferrimagnetic resonance

Ferrimagnetic resonance within the bubble layer is used to couple two microwave transmission lines. If a bubble is switched into the coupling region which is comparable in size with a bubble domain, the transmitted microwave signal changes. For a 5-μm bubble a Zero/One ratio of 6:1 has been obtained. It is also possible to influence the resonance transmission in a reference layer adjacent to the bubble layer by the magnetic stray field of a bubble. At high power levels a nonlinear interaction between the microwaves and the magnetic domains is observed which can be used for bubble generation.

Ferroelectrics properties of Tri-glycine sulphate thin films: J T Jacobs and K L Keester, (Proc 19th Nat Symp Am Vac Soc) J Vac Sci Technol, 10 (1), Jan/Feb 1973, 231–234

This study investigates the potential of microwave technique for online monitoring and evaluation of biofilms in the pipelines. A microwave vector network analyser and an in-house built transmitting and receiving coaxial-line transducer were employed to transmit microwave signals in the pipe. The brass pipe specimen was tested by adhering different volumes of polymeric tape layers onto its internal surface simulating the biofilm build-up. By taking the pipe as a circular waveguide of microwave, the frequency domain measurements were conducted in the 45–47 GHz range with TM01 dominant wave mode. The permittivity of the biofilm-contained area has been expressed as a function of the resonance frequency after establishing the resonance condition in the waveguide. It was realized that the resonance frequencies experience systematic shifts with the growth of biofilm layer length and thickness. The effects of dielectric material properties and the volume of the added biofilm layer on the resonance frequency records were then explained using the cavity perturbation theory which confirmed the experimental findings. Measurement results indicated a high degree of sensitivity to the small amounts of introduced biofilm which proves the potential of the microwave technique for online biofilm monitoring in both closed-end and open-end terminal conditions.

Airborne collision avoidance and other applications of time/frequency

Time/frequency technology provides a reliable aircraft collision avoidance system (CAS) that can operate in either synchronous or asynchronous modes. Precision time-ordered techniques of CAS provide both range and range-rate measurements in a one-way sense to all aircraft as well as ground stations within range of transmitted microwave signals. The cooperative system utilizes exact frequency references coupled with precise synchronization: control of frequency to 1 part in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> and time to less than 1 µs. In addition to performing specific functions of protecting aircraft, the time/frequency CAS provides a means for wide dissemination of submicrosecond timing. Flying clocks, which are an integral part of the airborne CAS, have been providing transcontinental and intercontinental transfer of time since 1964. CAS ground stations can serve as depositories of time and frequency derived from flying clocks, satellites, Loran-C and Omega navigation systems, or television transfer referenced to national and international time/frequency standards. Airborne relay of CAS time provides a ready means for local transfer of time to users that are not within line of sight to a ground station. Thus time and frequency can be disseminated for applications other than collision avoidance. In the process of developing the technology and originating CAS, new systems for communication, position determination, navigation, and vehicle surveillance evolved. Such systems can be structured to operate at high data rates, provide improved accuracy and use less RF spectrum than existing techniques.

Microwave Transmission Through Permalloy Films

A new interference effect has been observed in the transmission of microwave signals through Permalloy films. This effect, which involves interference between the eddy-current microwave transmission and the magnetic field associated with the excited spin waves, permits identification of the symmetry and surface pinning behavior of the standing spin waves. Previous studies of spin waves in Permalloy films have involved a determination of the surface impedance in the vicinity of spin-wave excitation. The present measurements show a decrease in transmitted microwave power for the lowest-order spin-wave mode and for those higher-order modes whose magnetic excitation has even symmetry with respect to the plane of the film. However, corresponding to intermediate modes, which have odd symmetry, we observe a transmission maximum. Thus we observe alternate minima and maxima on transmission. The measured fields correspond exactly with the field positions of the alternate strong and weak maxima in surface impedance, as observed on reflection. This correspondence between transmission and reflection supports the generally accepted interpretation of spin-wave mode symmetry. In addition, the direction and amplitude of this interference as a function of the angle between the magnetic-field direction and the plane of the film agree with the partial-pinning model of Nisenoff and Terhune.

Oscillation Modes of Oscillistors I Uniform Magnetic Field.

In order to investigate the dominant mode in the oscillistor oscillation, the density oscillations of m=0 and helical modes have been studied by observing simultaneously both the transmitted microwave signal through a sample whose diameter was smaller than the skin depth and the potential difference between two probes alloyed axial-symmetrically on the sample. Experiments showed the following results: (1) The m=0 mode oscillation of plasma density is predominant in oscillistor oscillation, and (2) the helical mode oscillation is excited at larger magnetic field or current than that of the m=0 mode oscillation. (3) The terminal voltage oscillation is equivalent to the m=0 mode oscillation. (4) The oscillation is excited only in a limited region 2–4 mm from the injecting junction, and this density oscillation travels along the drift direction of the minority carriers. (5) The drift velocity of the density wave increases with the distance from the exciting position.

Recombination Coefficient of Electrons with NO+ Ions in Shock-Heated Air

A measurement of the electron-positive-ion recombination coefficient has been made in air of density near 0.1 atm. and of temperature about 2900°K, yielding the value, α = (4 ± 2) × 10−8 cm3 sec−1. The results indicate that dissociative recombination, NO+ + e− → N + O, is the dominant electron depletion process. Air behind a reflected shock wave at a temperature of 3600°K is expanded rapidly into a thin-wall quartz tube. The tube serves as a post in three X-band waveguides. Electron densities are computed from the measured phase shifts of transmitted microwave signals. The relation between electron density and phase shift is determined with discharge plasmas of known electron densities. The recombination coefficient is computed from the measured electron depletion rate and the time variation of the temperature and velocity of the gas in its unsteady expansion down the quartz tube. The effects of diffusion, attachment, and impurities are also investigated. The measured value of α is in reasonable agreement with that of Lin et al., at 5000°K when a T−3/2 temperature dependence is assumed. This result is also compared with other laboratory measurements near 300°K.

Multidiode Switches (Correspondence)

The impedance of crystal diodes is known to depend on the applied bias voltage. This has suggested the use of diodes as switching elements in the control of microwave signal transmission. In the simplest form, the diode switch consists of a transmission line which is shunted by a diode. Coaxial cables as well as waveguides have been used for the transmission lines that are shunted by point-contact and p-i-n diodes. Slab line or coaxial switches where a diode is inserted in series with the center conductor of the line have also been developed.

THE GROUP VELOCITY OF PLANE SURFACE WAVES

The velocity of transmission of a modulated microwave signal over a dielectric-covered plane conductor has been measured over a range of dielectric thicknesses for which the first order TM surface wave mode is known to propagate. An exact theoretical treatment of plane surface wave group velocity for both TM and TE surface wave modes of any order has been developed and calculations carried out for the first two orders. The experimentally determined signal velocities are in good agreement with the group velocities calculated for the first order TM mode. Effects of possible surface wave propagation on the accuracy of microwave distance-measuring methods are discussed.

A Practical Method for Estimating Rainfall Rate Frequencies Directly from Climatic Data

In evaluating the environmental limitations on microwave signal transmission, it was necessary to determine the occurrence of rainfall rates for a number of regions in different parts of the world. Clock-hourly precipitation data were used where available. Where these data were not available, a known empirical technique was employed which correlates clock-hourly rainfall at individual rates to a climatological index. This index uses parameters which are almost universally obtainable—namely, mean annual rainfall and number of days with measurable rain. In the course of this work, the authors found a more general relation between clock-hourly rainfall rate frequencies and the climatological index. Using this relation, rainfall frequencies at any rate are obtained directly from a linear equation and two nomograms. It is also possible to synthesize the complete frequency distribution of rainfall rates and obtain, as a check, the mean annual rainfall. Examples, supplemented by illustrations, are given in the development of this technique and its utilization.

Microwave Studies of the Electron Loss Processes in Gaseous Discharges

A method for studying electron loss processes in gaseous discharges by means of the free transmission of microwave signals through the discharge is discussed. This method has the advantage that it is easily adaptable to any size or shape of discharge tube. The electron loss processes in pure hydrogen have been studied using this method. For electron densities of 5\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, the electron-ion recombination coefficient has been found to be approximately 5.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. An electron-neutral collision frequency of 3.97\ifmmode\times\else\texttimes\fi{}${10}^{9}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ at a pressure of 0.29 mm Hg has been measured. The dominant loss process proved to be attachment, with a probability of attachment of approximately 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$.