Showing papers on "Dipole antenna published in 1988"

Subpicosecond photoconducting dipole antennas

Abstract: Photoconducting antennas have been demonstrated which are capable of generating and coherently detecting subpicosecond electrical pulses. These antennas, when illuminated with femtosecond optical pulses, radiate electrical pulses which have frequency spectra that extend from 2 THz. Microscope dipoles measuring 50, 100, and 200 mu m have been fabricated and tested. Integrated photoconductors of radiation-damaged silicon-on-sapphire were used both for impulsive current excitation of the transmitting antennas as well as for gating the receiving antennas. >

Near-field probe used as a diagnostic tool to locate defective elements in an array antenna

Abstract: Results of an experimental study are presented in which the near-field probe was used as a diagnostic tool to locate the defective elements in a planar array. The near-field data were processed not only to obtain the far-field patterns of the array under the test, but also to reconstruct the aperture field for diagnostic purposes. The backward transform enables the near-field probe to identify accurately aperture faults at a distance, free of interactions and couplings with the array elements. In practice, to recover the aperture field properly from the near-field distribution, the evanescent components in the computed far-field spectrum must be excluded from the inverse process with fast-Fourier-transform (FFT) techniques. For low-gain array antennas, a correction on the far-field spectrum is required to remove the contribution of the probe and the element factor before the inverse transform, strongly enhancing the resolution. >

A microwave powered high altitude platform

Abstract: A system is described for the reception of microwave power from a remote transmitter and its conversion to DC power when movement of the receiver system relative to the transmitter may occur. The operation of the receiver/converter is demonstrated through the flight of a microwave-powered aircraft. A high-power transmitter at 2.45 GHz was used to beam energy to the aircraft circling overhead. A custom printed-circuit array of dipole antennas with associated rectifying diodes coating the underside of the plane converted the microwave energy to direct current to power the electric motor. >

Extendable antenna for portable cellular telephones with ground radiator

Abstract: An antenna for a portable cellular telephone includes a quarter-wavelength ground radiator and a helical coil capacitively coupled to an extendable half-wavelength radiator. The extendable half-wavelength radiator includes a metallic coil molded in plastic. The ground radiator includes a serpentined transmission line on a flexible circuit board. The helical coil and ground radiator are coupled by a transmission line to a duplexer. The duplexer couples transmitter signals from a radio transmitter to the antenna and receiver signals from the antenna to a radio receiver. The antenna may be advantageously utilized in any portable radio applications where small size and immunity to hand induced radiation losses are desired.

Design of log-periodic dipole antennas

Abstract: The goal of the paper is to improve the log-periodic dipole antenna (LPDA) design procedure introduced by Carrel (1961). A computer-aided design technique is introduced, providing an easy-to-use design method of LPDAs. Corrections to Carrel's design contours, and new constant input resistance and truncation coefficients diagrams are presented. The influence of the feeder characteristic impedance, and dipole length-to-diameter ratio on LPDA parameters is studied. Design diagrams for LPDA's typical of HF communications are included.< >

Beam powered antenna

Abstract: The invention relates to an antenna for receiving and modulating transmitted signals having a very low power density on the order of below 1 watt per meter squared, at a selected center frequency above 2,000 MHz The antenna uses a folded planar dipole section having two separated oppositely disposed U-shape elements, each of said elements having a pair of ends A capacitor is series-coupled between two of the oppositely disposed ends of the elements, and a diode having a high impedance at the selected center frequency is series-coupled between the other two oppositely disposed ends of the U-shape elements A metallic backplane is mounted parallel to the planar folded dipole section and spaced from the dipole section a predetermined distance selected to reduce the antenna impedance The impedance of the diode, the shape of the folded planar diode section and the predetermined amount of spacing of the backplane from the planar folded dipole section are all selected to increase the dc power output of the antenna

High-Tc superconducting short dipole antenna

Abstract: A recently discovered rare-earth copper-oxide material, which is superconducting above the temperature of liquid nitrogen, is used to fabricate a high-efficiency electrically short dipole antenna. Thus the necessity to cool a lead antenna structure to liquid helium temperatures is avoided. Results are presented which show the gain enhancement achieved using this new material when compared with a conventional copper structure at room temperature.

Applicator array and positioning system for hyperthermia

Abstract: The use of an externally located dipole array mounted upon a high dielectric material for heating biological tissues is disclosed. This dipole array provides improved flexibility and adaptability to varying tissue surface contours. The adjustable dipole antenna array or other type of electromagnetic antenna applicators provides an ability to adapt the heating pattern to meet the needs of the different tissue zones, applicator positions, and applicator mechanical scanning by adjusting individual power amplitude and phase to the various elements of the applicator array for controlling the heat distribution of heat in target tissue having areas of varying power absorption and cooling effect (blood flow). The array design may also include capacitively coupled metal elements in the form of strips, patches or tubes. The electromagnetic heating elements are attached to a transparent flexible dielectric fluid filled envelope, tube, or bag for viewing both the tissue surface being heated and positioning the applicators and temperature sensors. The flexibility of this array provides uniform heating of a diversity of surface contours. Also, an electromechanical positioning system for the applications allows the applicators to be selectively positioned in the tubes, and thereafter repositioned during treatment.

Implantable helical coil microwave antenna for interstitial hyperthermia

Abstract: An implantable helical coil microwave antenna has been developed for improved localization and control of interstitial hyperthermia for deep-seated tumours. A helical coil structure was employed as an extension of the inner conductor at the terminal portion of a miniature semi-rigid coaxial cable. The antennas were constructed with three different connection configurations of the helical coil to the feedline, and with several coil turn densities during the optimization of heating characteristics. In order to compare relative antenna heating performance, a set of quantitative parameters was introduced. Power deposition profiles of 2450 MHz helical coil antennas were studied in both phantom models and muscle tissue in vivo, and compared to those of commonly used dipole antennas. Optimal antenna performance was obtained with a 10-turn per 1 cm helical coil connected to the inner conductor at the tip and separated from the outer conductor by a 0.1 cm gap (HCS-10). These antennas produced a well-localized heating pattern with a sharp falloff of temperature in both directions axially from the coil element. For half-wavelength insertion depths, the effective heating length (50 per cent of maximum SAR) of HCS-10 antennas matched that of standard dipole antennas, but was shifted down towards the tip. For shorter and deeper antenna insertion depths the HCS-10 heating pattern remained similarly localized to the region surrounding the helical coil with minimal cold zone at the tip. In contrast, the dipole antenna heating pattern changed significantly depending on insertion depth, with an unavoidable 0.2-0.7 cm cold region at the antenna tip and elevated surface temperatures for short insertion depths.

Planar TV receiving antenna with broad band

Abstract: A TV receiving antenna with broad band and capable of connecting with a coaxial cable comprising a sheet of electrically non-conductive synthetic plastic material; a pair of antenna elements of metallic foil in the shape of substantially a triangle or a combination of a triangle and a rectangle being superimposed on one side of said sheet, a pair of frequency compensating members of snake-shaped strip line of metallic foil connected to said antenna elements respectively being superimposed on the one side of said sheet; a pair of antenna elements of metallic foil in the shape of a trapezoid or a combination of a trapezoid and a rectangle being superimposed on the opposite side of said sheet; a pair of frequency compensating members of snake-shaped strip line of metallic foil connected to said antenna elements respectively being superimposed on the opposite side of said sheet; and impedance transformers of microstrip lines of metallic foil on each side of said sheet also superimposed for matching the antenna elements and the coaxial cable. Each antenna element can have a plurality of parallel slots for the selection of polarization. The antenna can receive TV signals both in UHF and in VHF bands and eliminate ghosts. It is suitable for mass production by etching the patterns on a laminated sheet for printed circuits.

Broadband directional antenna

Abstract: A broadband antenna includes a center fed active radiating/receiving element formed of two contiguous conductive spirals. The two planar antenna spirals are secured to a cavity loaded with a wave absorbing lossy material such that the antenna is operative only in its front lobe direction opposite the loaded cavity. In accordance with the present invention, the cavity wall is formed of varying pattern of conductive and insulative material. The resultant composite antenna is characterized by a relatively narrow beam pattern for both horizontally and vertically polarized electric waves across a broad spectrum of frequencies.

Analysis of mutual coupling between a finite phased array of dipoles and its feed network

Abstract: The mutual coupling effects between a finite phased array of dipoles and its feed network are analyzed. The feed network is typically a corporate feed consisting of split-tee power dividers cascading to form a certain power distribution over the aperture. A simple iterative approach is used to solve the interaction between elements and feed. The radiation of a finite dipole array are first found for a given voltage excitation. These radiation impedances are then used as loads for the feed network, and the n+1 port network problem is analyzed. Due to the interaction between the feed network and dipoles, the antenna parameters such as mismatch, antenna pattern, and gain are all affected. These effects can be determined from the analysis of the network representation. Numerical results for a typical phased array with a corporate feed show that the resultant VSWR of the feed pattern degradation is due to the mutual coupling effects. >

Thermal distribution studies of helical coil microwave antennas for interstitial hyperthermia

Abstract: An implantable 915 MHz helical coil antenna was developed for improved localization and control of interstitial microwave hyperthermia. The radiating element consisted of a fine wire coil wound back over the inner conductor of a miniature semi-rigid coaxial cable in place of the terminal portion of outer conductor. The power deposition profiles from single helical coil antennas were studied both in homogeneous phantom and in muscle tissue in vivo and compared to those of single half-wavelength linear dipole antennas. The effects of variable coil length, turn density, and antenna insertion depth in tissue were characterized. The helical coil antennas produced a well-localized heating pattern with a sharp falloff of temperature in both directions axially from the coil element. One of the best heating patterns was obtained with a 35 turn, 35 mm long helical coil element which was separated from the antenna feedline outer conductor by a 1 mm gap (HCS-35(1)/36). This antenna showed a marked shift of the effectively heated volume toward the antenna tip and essentially no dependence of the heating pattern on insertion depth. In contrast, the axial power deposition profiles of dipole antennas were strongly affected by insertion depth and exhibited an inadequately heated area at the antenna tip even with 14−34 wavelength insertion. Thermal distribution studies showed that the single helical coil microwave antenna provided more predictable, well-localized heating of deep-seated tissues, with minimal requirement for over-implanting of the treatment volume.

Antenna patterns of nonsinusoidal waves with the time variation of a Gaussian pulse. III

Abstract: The development of antenna theory for nonsinusoidal electromagnetic waves has been based on the idealized rectangular pulse. In practice, an antenna that is designed to operate in the mode of an electric hertzian dipole would radiate a pulse that best approximates a Gaussian one when the driving current consists of a linear transient. The principle of radiation of nonsinusoidal electromagnetic waves with the time variation of Gaussian pulses is discussed. The properties of the Gaussian pulse are presented, i.e., the autocorrelation function, energy spectral density, and spectrum. Antenna patterns, such as peak-amplitude pattern, peak-power pattern, energy pattern, and slope pattern are derived for a Gaussian pulse received (or radiated) by a linear array antenna. Computer plots of the derived antenna patterns are presented that show a considerable improvement in the angular resolution capability over that of the antenna patterns that have been derived for a rectangular pulse. >

The antenna cable as a source of error in EMI measurements

Abstract: Radio-frequency current on the exterior of an antenna cable, which is derived from the signal being measured or transmitted, can significantly alter the results of antenna calibration and EMI (electromagnetic interference) measurements. Two distinct phenomena are observed. The first is related to the behavior of the cable as a parasitic element of the antenna system; the second, which is more serious, is related to leakage between the antenna and cable due to imperfect balance, leaky connectors, and other problems. These phenomena are examined, and a method to measure the leakage between an antenna and its cable is developed. Several types of commonly used antennas are characterized with regard to cable interaction. The use of a ferrite choke on the exterior of the cable is proposed as an effective solution to the problem. >

Use of broadside twin element antennas to increase efficiency on electrically thick dielectric substrates

Abstract: This paper describes a technique for using twin element antennas (dipoles and slots) to increase the efficiency of antennas fabricated on electrically thick dielectric substrates. We present calculations which show that the efficiency of both the slot and dipole antennas can be increased by the proper spacing of elements placed broadside to each other. We consider the use of substrates that are odd integral multiples of a quarter of a dielectric wavelength thick and give results for an e=4 substrate with thicknesses of one, three, and five quarter wavelengths. These thicknesses can be used when working at millimeter wave frequencies and yield substrate dimensions which can be handled and processed easily, while still yielding radiation-to-air efficiencies of about 70%. We also show calculated beam patterns for the elements which appear to be suitable for imaging array applications.

The determination of the electromagnetic field and SAR pattern of an interstitial applicator in a dissipative dielectric medium

Abstract: The spatial distribution of microwave energy absorbed per unit mass (the specific absorption rate or SAR) in biological tissue is calculated for a class of interstitial antennas. The insulated interstitial applicator is simulated as an asymmetrically drive antenna. An expression for the electric field intensity near the antenna is derived and calculated by direct numerical evaluation of a surface integral over the insulation. The predicted SAR patterns obtained using the calculated electric field intensity and the tissue conductivity agree very well with the measured SAR distributions around three different applicators in muscle-equivalent phantoms. >

ICRF heating with mode control provided by a rotating field antenna

Abstract: A rotating field antenna set — a pair of two closely spaced dual half-turn antennas — is used to tailor the azimuthal spectrum of the antenna field. It is demonstrated that the azimuthal mode of ICRF waves can be controlled by proper phasing of the antenna set. Ion heating is shown to be strongly dependent on the azimuthal mode number. When the antenna set is operated in the m = −1 (left rotating) mode, the m = −1 modified ion cyclotron wave is selectively excited, resulting in efficient ion heating.

V-conical antenna

Abstract: A new angular antenna called the V-conical antenna was investigated. The characteristics of the antenna are frequency-independent, and the electromagnetic fields are pure spherical waves (transverse electromagnetic mode) even in the near region. The theoretical analysis makes use of conformal mapping. Closed-form solutions for the fields, currents, and characteristic parameters are obtained. The formulas are simple and precise. An experimental study of the antenna was conducted under pulse excitation. The shape of the measured electric-field pulses is the same as that of the source pulse from a picosecond pulse generator. The measured field patterns are very close to those predicted by the theory. Such a structure with a tapered resistive termination is an ideal directional pulse antenna or electromagnetic pulse simulator. >

Antenna with enhanced gain

Abstract: The present invention will yield enhanced gain over a conventional antenna system by taking advantage of combining directional antennae. In one embodiment, the invention utilizes four 10 dB dipole, vertically-polarized, omni-directional antennae having a reflector added to each one to limit the horizontal beamwidth to 90° and increase the gain to 16 dB. The vertical beamwidth remains at 7°. By utilizing four of these antennae and utilizing power combining hybrids to connect each of the two opposed antennae together, excess gain over a 10 dB omni-directional system will be obtained. The effective antenna gain is equal to the directional antenna gain minus the omni-directional antenna gain minus the hybrid loss. Thus, the vertical beamwidth and physical height of the antenna are preserved with the increase in gain at the cost of a little antenna complexity.

Deterministic thinned aperture phased antenna array

Abstract: A phased array antenna (10) includes a plurality of radiating elements (14) arranged in concentric rings (11, 12) to form a deterministically thinned antenna aperture which facilitates heat removal from the array, while minimizing side lobe signals and thereby increasing directively of the antenna for a preselected antenna gain. The radiating elements (14) in any one of the rings (11, 12) are the same radiating size, and the spacing (L, L') between elements in the same ring and between elements in adjacent rings (S, S') is determined by the number of elements in each ring. The rings may be any of several shapes, including circular or polygonal.

Conformal array antenna

Abstract: A conformal array antenna system is disclosed comprising a structural base body (11) having a shape suitable for a surface of an airplane or a ship, and a plurality of antenna units (12) disposed on the structural base body. Signals received by these antenna units are converted into digital signals and fed to a digital beam forming circuit (14) which synthesizes such digital signals to form a multiplicity of beams. The antenna units and the digital beam forming circuits may be connected by electrical transmission lines (13) or optical fibers.

Non-invasive quarter wavelength microwave applicator for hyperthermia treatment

Abstract: A printed circuit board structure fabricated to define a plurality of quarter wavelength microstrip antennas, each radiating microwave energy in a dipole pattern. The ends of the microstrip antennas are formed overlapping the ends of adjacent microstrip antennas so that a resultant additive effect of the radiated microwave energy is exhibited near the ends of the antennas. The overall effect is to produce a more uniform radiation pattern which does not have the typical radiation minima located at the ends of the dipole antennas.

An optically linked electric and magnetic field sensor for Poynting vector measurements in the near fields of radiating sources

Abstract: A unique, single-element antenna measurement scheme that can simultaneously measure the electric, magnetic, and time-dependent Poynting vectors of electromagnetic (EM) fields is described. The electric and magnetic responses of the antenna sensor are separated by a O degrees /180 degrees hybrid junction. The resulting two RF voltages, along with relative phase and frequency information, are transmitted to a remotely located vector analyzer by a pair of well-matched fiber optic downlinks. The remote receiver measures and displays the electric dipole response, the magnetic loop response, and the time phase difference between the two. This information is sufficient to determine the time-dependent Poynting vector. Both a theoretical analysis and a discussion of experimental measurements performed, which describe the capabilities and performance of a working prototype of the antenna measurement scheme, are presented. The results demonstrate that a three-axis (isotropic) version of this system could be used to measure the near fields of EM sources, as well as to completely describe the resultant flow of energy. >

Mobile telescoping whip antenna with impedance matched feed sections

Abstract: An antenna element including a loading coil is telescopically extendable and retractable from within a housing tube. A characteristic impedance of a transmission line from a lower end part of the antenna element to a cable is equal to one of a cable. A part of the loading coil is reinforced. A branching filter, which is operatively connected between the antenna and a communication device using a different frequency band, suppresses a mutual interference between signals for the communication device. An antenna circuit, which is operatively connected between the antenna or a branching filter and the communication device, converts an impedance of a lower part in a frequency band, and reduces a loss due to a capacitive antenna impedance.

Electronically steered phased array radar antenna

Abstract: An electronically steered phased array antenna system. The system includes phased array radar antenna having a microwave phased shifter for inserting a predetermined amount of path delay into each of the radiated elements of the antenna. It also includes a beam steering controller connected to the microwave phase shifter which is adapted to control the phase shifter according to predetermined parameters relating to the antenna to maximize its performance. An electronically erasable programmable read only memory device is disposed on the antenna itself for storing the predetermined parameters relating to the antenna. The memory device is connected to the controller and by inputting its data into the memory of the controller converts the controller from a generic device to a dedicated controller for the antenna in question. The memory device can be reprogrammed without removing it from the antenna for changes in the antenna configuration data.

Limitations in microwave holographic synthetic aperture imaging over a lossy half-space

Abstract: Experimental results are presented which illustrate the capabilitity of a single-frequency microwave holographic scheme to image both metallic and plastic pipes at a depth of 0.33 m in wet soil with a covering layer of concrete paving stones. The data acquisition system utilises a crossed dipole antenna scanning above the interface recording two orthogonal polarisations at each point on a two-dimensional grid, from which improved surface rejection is obtained. Maximum imaging depth is limited primarily by the ratio of soil attenuation to surface and environmental effects on the antenna response, thereby preventing any significant increase in the conditions quoted. Theoretical investigation into the influence of the lossy dielectric half-space on the synthetic aperture in a one-dimensional model has illustrated that scan height is an important consideration for resolution and antenna system design. It is shown that scanning in close proximity (?0.1 ?) to the interface increase both lateral and depth resolution. To take advantage of this effect, it is concluded that a noncontacting antenna operating in close surface proximity must be insensitive to height fluctuations.

Electronically tunable antenna

Abstract: A resonant helical antenna capable of being tuned electronically over a broad range of frequencies. The helical turns of the radiating portion of the antenna are formed of tubular material which may be in the form of a single length of tubing or may comprise a number of parallel coaxial cables with their outer conductors in electrical contact. The antenna is tuned by a series of oppositely poled pairs of diodes that are connected at spaced points to the radiating coils of the antenna. When the diodes are biased to be conducive, a section of the radiating helix is short-circuited. Bias voltages to control the diodes are provided by leads inside the radiating turns of the helix. Each lead for a pair of diodes emerges at a point electrically balanced between the two spaced points that are connected to the associated diodes. No r-f potential exists between the outer and inner conductor of the radiating runs at the point where the bias lead emerges, so no r-f current flows in the bias lead wires. The antenna may include spaced cpacitance elements and may be in the form of a monopole helical antenna or it may be in the form of a dipole antenna with two oppositely-disposed arms.

Global position satellite antenna

Abstract: A low profile, low drag, broad-banded antenna system includes four substantially identical antenna elements in intimate contact with a hemisphere of high dielectric constant material. The antenna elements are equally spaced from each other around the hemisphere and are connected to a power divider circuit for reception of incoming signals from a global position satellite having circular polarization. A dielectric phase equalizing member of variable thickness surrounds the antennas in intimate contact therewith for improving the axial ratio particularly at low angles of radiation. The dielectric phase equalizing member is also formed from a high dielectric constant material. A radome of a radio transparent material covers the assembly to provide an aerodynamic configuration and to protect the antenna from the weather. The antenna has a gain from approximately -2.5 dBic at 10° above the horizon to approximately +3 dBic at 30° above the horizon.

Radiation by a straight thin-wire antenna embedded in an isotropic chiral medium

Abstract: Radiation characteristics of a straight thin-wire antenna embedded in an isotropic chiral medium are explored. The radiation field contains both left-circularly polarized (LCP) and right-circularly polarized (RCP) components. The radiation pattern is found to be symmetric about the plane theta = pi /2, azimuthally isotropic, and TEM (transverse electromagnetic) in nature; however, all three of these features are found as well for isotropic achiral media. Some specific features of the radiation field on certain constant spherical surfaces are also examined. If the antenna is reduced to a point dipole, it is shown that the radiation resistance is enhanced by the presence of chirality in the embedding medium, and the antenna LCP gain differs from the RCP gain. >