Dipole Probe Research Articles

Microwave diffraction by a conducting object for various shapes*

We have studied microwave diffraction by plane, conducting screens having rotational symmetries about their axes. The dimensions of the objects were of the order of a wavelength. Sample shapes were, for twofold symmetry, a rectangle; for threefold symmetry, a triangle; for fourfold symmetry, a square, cross, and cuspate figure; and for fivefold symmetry, a pentagon. The measurements of intensities were taken along the axes of the objects on the shadow side with the electric dipole probe parallel to the incident electric field. For objects of three- or more-fold symmetry, the observed intensities were independent of the orientation of the object about its axis. Only for the object of twofold symmetry was the intensity dependent on orientation. For scattering objects of the order of a wavelength in dimensions, and for which exact electromagnetic theory solutions have never been obtained, this is a useful generalization. We have presented an elemental model to explain the independence of intensity upon the rotation of the screens of three- or more-fold symmetry, as well as the unique behavior for twofold symmetry.

The m = 1 motion of a screw pinch measured by external magnetic probes

Measurements of the m = 1 growing mode of a linear screw pinch, by external dipole probes, are compared with the recorded plasma motion on streak photographs. Good agreement is found up to a maximum amplitude which depends on plasma density and growth rate.

The VLF complex permittivity of deep Antarctic ice measured in situ

The results of the first in‐situ measurement at VLF of the complex permittivities of deep Antarctic ice are presented as functions of depth, projected along the vertical. Measurements made with an electrically short dipole probe at five frequencies to a depth of 1500 m below the ice surface are reported. Temperature is identified as a dominant factor affecting the ice permittivity. This fact, coupled with the measurements of complex permittivity to 1500 m depth, is used to estimate ice permittivities from 1500 m to the bottom of the ice sheet at 2164 m. Comparison of the in‐situ measurements with those of surface measurements is made; it is concluded that for the purpose of VLF antenna and propagation studies the Antarctic ice sheet cannot be considered to be an isotropic homogeneous material, but must be treated at least as a horizontally stratified medium whose dielectric and loss properties vary in the vertical direction with temperature.

The Dielectric Properties of Antarctic Ice

AbstractTwo 0.5 m cores from "Byrd" station, Antarctica have been studied in the laboratory, one from a shallow depth (155 m) and the other from the zone where recrystallization has given a vertical c-axis fabric, and the air in situ is thought to be in clathrate form (1 424 m). The dielectric response has been studied in the frequency range 60 Hz to 10 kHz, and in the temperature range — 6° C to —6o° C. The behaviour observed is markedly different from that of "pure" polycrystalline ice such as may be made by slowly freezing distilled de-ionized water and is thus at variance with the conclusions of Rogers (unpublished) who deduced, from measurements of the admittance of a dipole probe lowered through the fluid-filled drill hole at "Byrd", that the ice surrounding the hole had a dielectric response similar to that of "pure" ice. The Antarctic ice is shown to have properties similar to those of the ice from "Camp Century" and "Site 2" in Greenland studied by Paren (1973). In an attempt to discover what factors determine the difference in electrical behaviour between polar ice and pure ice, some samples were melted and subsequently refrozen slowly. Their dielectric response was similar to that of pure polycrystalline ice. These results are discussed in connection with the impurity content and growth conditions of the ices.

The Dielectric Properties of Antarctic Ice

AbstractTwo 0.5 m cores from "Byrd" station, Antarctica have been studied in the laboratory, one from a shallow depth (155 m) and the other from the zone where recrystallization has given a verticalc-axis fabric, and the airin situis thought to be in clathrate form (1 424 m). The dielectric response has been studied in the frequency range 60 Hz to 10 kHz, and in the temperature range — 6° C to —6o° C. The behaviour observed is markedly different from that of "pure" polycrystalline ice such as may be made by slowly freezing distilled de-ionized water and is thus at variance with the conclusions of Rogers (unpublished) who deduced, from measurements of the admittance of a dipole probe lowered through the fluid-filled drill hole at "Byrd", that the ice surrounding the hole had a dielectric response similar to that of "pure" ice. The Antarctic ice is shown to have properties similar to those of the ice from "Camp Century" and "Site 2" in Greenland studied by Paren (1973). In an attempt to discover what factors determine the difference in electrical behaviour between polar ice and pure ice, some samples were melted and subsequently refrozen slowly. Their dielectric response was similar to that of pure polycrystalline ice. These results are discussed in connection with the impurity content and growth conditions of the ices.

The electrically short sheathed dipole: Experimental relationship between its measured admittance and the permittivity of the external medium

Motivated by a study of the electrical properties of deep polar ice, a technique was developed to interpret the measured admittance of an electrically short sheathed dipole probe in terms of the complex relative permittivity of the surrounding material. With the related analytical problem unsolved, the experimental study described herein provides new information for evaluating theoretical models as well as guidelines for the full-scale design of systems to measure the properties of ice or other materials in similar circumstances, and bounds on the measured sample volume. Reduction of the admittance data is accomplished with the aid of a three-element equivalent circuit resulting from a laboratory model study that is also described. Potential applications include measure-merit of the in situ properties of rock and naturally occurring ice masses surrounding deep drill holes. Preliminary results from the Antarctic ice sheet at Byrd Station are included by way of illustration.

An Experiment for Determining the VLF Permittivity of Deep Antarctic Ice

An experiment that has been designed to yield the dielectric and loss properties of the ice cap near Byrd Station, Antarctica, in the very low-frequency range, subsequent to proper interpretation of the input admittance of an electrically short dipole probe, is described. The probe is lowered into a deep drill hole, together with appropriate measuring and telemetering equipment, in order that the parameters of interest may be recorded as functions of vertical depth and frequency. The motivation for such an experiment is discussed and the environmental conditions described, insofar as they restrict the kinds of experimental procedures that can be carried out with reasonable safety and economy. The measurement technique itself is outlined, together with some background material concerning the use of a dipole antenna as a probe for exploring the properties of the medium in which it is immersed. Also discussed is a related experiment performed to provide a check on the method. It involved the use of the same probe and instrumentation to measure the complex permittivity of sawdust, a lossy dielectric material. The results are shown to be in good agreement with those obtained by a more conventional laboratcry method.

Linear and Nonlinear Response of a Plasma Sheath to Radio Frequency Potentials

The linear and nonlinear response of a plasma sheath is studied by subjecting it to rf potentials at frequencies corresponding to certain natural modes of oscillation of the sheath-plasma system. The sheath investigated is one that exists around a spherical dipole probe, immersed in an essentially collisionless uniform plasma and excited with sinusoidal signals at frequencies between 50 and 250 MHz. It is shown that the linear rf current-voltage characteristics agree with both the simple sheath model of Mayer and Harp and with the more refined model of DeAngelis and Baldwin. The nonlinear response (which is observed as the generation of higher harmonics) is quantitatively explained as being due to an oscillation of the sheath-plasma boundary induced by the applied rf field. This oscillation, in effect, acts as a time varying sheath capacitance.

Modulated Dipole Plasma Probes

Plasma density profiles are determined by microwave reflection from a modulated dipole probe. The applications of this method to quiescent plasmas and in other plasma explorations are discussed.

DIFFRACTION OF MICROWAVES BY LONG METAL CYLINDERS

A parallel-plate transmission line has been applied to the study of the amplitude and phase of the diffracted electric field near metal cylinders of circular, square, and rectangular cross-section. The transverse dimensions of the cylinders are comparable with the wavelength (3.280 cm.). A travelling dipole probe inserted in the line through a slot in one of the plates permits investigation of the field. Absorbing wedges at the boundary of the line make the plates effectively infinite in extent. Essentially plane wave conditions exist in the region utilized in the measurements. Good agreement is obtained between calculation and experiment for circular cylinders. Measured results are given for square and rectangular cylinders. For one orientation of the rectangles the measured results are compared with approximate calculations.

Microwave Diffraction Measurements in a Parallel-Plate Region

Many infinite two-dimensional electromagnetic-scattering problems are strictly equivalent to the configuration obtained from the image principle by placing a finite-scattering obstacle between two parallel, infinite, and perfectly conducting planes and illuminating it with a plane or cylindrical incident wave confined to the parallel-plate region. An experimental approximation to this arrangement is described for use in the 3-centimeter wavelength region. Finite rectangular parallel metal plates support a transverse incident electromagnetic wave with the electric field oriented perpendicular to the plates. Absorbing wedges at the boundary of the region reduce reflections from this discontinuity. The electromagnetic field scattered from any particular obstacle may be investigated by means of a dipole probe introduced into the region through a moveable section of one of the plates. The success of this technique is demonstrated by comparing measurements of the total field scattered by infinite conducting wedges of included angle 0, 45, and 90 degrees with the known theoretical expressions, for perpendicular incidence. Agreement with theory is good. The effect of finite thickness of the diffracting screen (case of 0° wedge) in increasing the amplitude of the scattered wave is noted and compared to theoretical results derived for scattering from a thin conducting half-plane with a cylinder superimposed on the edge.