Showing papers on "Radiation pattern published in 1978"

Reactively controlled directive arrays

Abstract: The radiation characteristics of an N -port antenna system can be controlled by impedance loading the ports and feeding only one or several of the ports. Reactive loads can be used to resonate a real port current to give a radiation pattern of high directivity. The theory of resonance is extended to include complex port currents and impedance loads. The initial design of an array is obtained by resonating a desired port current vector, which is then improved by an optimum seeking univariate search method. The direction of maximum gain can be controlled by varying the load reactances. Several numerical examples are given for a circular array of seven dipole elements.

Basic theory of probe-compensated near-field measurements

Abstract: The general problem concerning the interaction of a probe antenna with the near field of an arbitrary antenna is considered. The application of the Lorentz reciprocity theorem to the problem of determining antenna characteristics, including the far-field pattern, is presented. The data required to correct for the directional effects of the probe, the effect of probe correction on the measured data, and the attendant mathematical computations in rectangular systems are described. Extensions to cylindrical and spherical systems are discussed.

Estimates of brightness temperatures from scanning radiometer data

Abstract: In the analysis of antenna temperature maps of the earth obtained by satellite-borne microwave radiometers, estimates of brightness temperatures or averages of brightness temperatures over areas considerably smaller than the region sensed at a given position of the radiometer antenna are often needed. An application of the Backus-Gilbert methodology is made to obtain an objective criterion of the best resolution (in a least squares sense) obtainable from a given system and to investigate the trade-off between resolution and noise in the derived average brightness temperatures. The mathematically related problem of simultaneously analyzing antenna temperature measurements made at different frequencies by antennas with noncoincident antenna patterns is also considered.

Multimodal Far-Field Acoustic Radiation Pattern Using Mode Cutoff Ratio

Abstract: The far-field sound radiation theory for a circular duct was studied for both single-mode and multimodal inputs. The investigation was intended to develop a method to determine the acoustic power produced by turbofans as a function of mode cutoff ratio. This information is essential for the design of acoustic suppressors in engine ducts. With reasonable simplifying assumptions, the single-mode radiation pattern was shown to be reducible to a function of mode cutoff ratio only (modal indices removed). With modal cutoff ratio as the dominant variable, multimodal radiation patterns can be reduced to a simple explicit expression. This approximate expression provides excellent agreement with an exact calculation of the sound radiation pattern using equal acoustic power per mode. Radiation patterns for cases other than equal modal power are presented using the approximate radiation equation. An approximate expression for the duct termination losses as a function of cutoff ratio also is included.

Superdirective receiving arrays

Abstract: Some of the characteristics of arrays designed for maximum directive gain subject to a tolerance constraint are described. Numerical data are presented which illustrate trade-offs between array size, directive gain, SNR, efficiency, bandwidth, tolerances, and farfield patterns. The results of an experimental study are found to be in good agreement with theoretical predictions.

Windshield mounted half-wave communications antenna assembly

Abstract: A half-wave length communications antenna assembly especially adapted to be mounted on non-conductive surfaces, such as on a window of a vehicle. The antenna assembly desirably includes an electrically shortened half-wave inductively loaded radiating whip loaded at its base end by a loading capacitor plate to be fixed to a non-conductive surface. The whip is coupled through the non-conductive surface to a transmission line internally of the vehicle by a coupling capacitor plate which, with the loading capacitor plate, forms a coupling capacitor. A tuned circuit which is tuned to the nominal resonant frequency of the whip is connected to the coupling capacitor plate and serves as an impedance matching circuit between the half-wave whip and the transmission line. The tuned circuit also affects the radiation pattern of the whip to produce a pattern more typical of a five-eighths wave length antenna to provide somewhat greater gain.

Electrically short monopole antennas with dielectric or ferrite coatings

Abstract: A comprehensive analysis of the electrically-small material-clad monopole is performed by representing the various fields by discrete and continuous eigenvalue spectrums. The cladding creates a dominant quarter-wavelength resonant-cavity effect whereby the leakage of radiation at the terminal plane is transformed into a low impedance at the annular feed. The mode coupling taking place at both feed and terminal plane is solved by variational methods to yield the antenna radiation pattern, bandwidth, efficiency and input impedance as a function of electrical size and frequency. The agreement between the computed and measured results establishes the usefulness of the analysis, and design curves are presented; other factors that influence the system performance of the antennas in practice are discussed. It is concluded that the optimisation of this type of antenna centres on the choice of suitable low-loss material with comparable relative permeability and permittivity values at the frequency of interest. In general, this calls for research into improved materials, but useful antennas can be made with commercially available materials.

Coaxial phased array antenna

Abstract: Disclosed is a coaxial antenna array for communicating circularly polarized electromagnetic radiation. A pair of open ended antenna cavities is coaxially constructed and operates by excitation of linear radiation elements arranged within each of the cavities. A pair of crossed-dipole radiation devices are centered within the inner cavity and operated by means of a phase-shifting network circuit to transmit as well as receive circularly polarized radiation. Four monopole radiation devices are symmetrically arranged to operate in the outer cavity in phase quadrature by means of the phase-shifting network circuit to also both transmit and receive circularly polarized electromagnetic radiation. Combined operation of the two antenna cavities with a 180° phase differential between the fields related to the two antenna cavities provides a broad beam, relatively wide frequency bandwidth communication capability. Particular embodiments disclosed feature a generally square cavity array as well as a circular cavity array.

Antenna pattern synthesis and shaping

Abstract: An antenna utilizes components of spatial harmonics to synthesis and shape radiation patterns. A counterpoise mounts a monopole radiator as well as a plurality of arrays of parasitic antenna elements excitable by the monopole radiator in a manner to produce the selected harmonics and consequently the desired radiation pattern. The parasitic antenna elements are compensated and are provided with switching to cause their activation. A cone radiator cooperates in exciting the parasitic antenna elements.

Method for determining antenna near-fields from measurements on a spherical surface

Abstract: A method for determining antenna near-fields from measurements on a spherical surface. Phase and intensity values are measured by scanning a probe antenna over a spherical surface. The method applies both to planar or conformal antennas. The spherical surface may be in the Fresnel or Fraunhofer regions. The field at the antenna is computed utilizing the complex conjugate of the measured field. As a result, the source antenna at the point at which the antenna radiating field is evaluated is inside the measurement surface. Far-field patterns can be computed from the near-fields reconstructed from the spherical hologram surface.

Theory of the circular-disc printed-circuit antenna

Abstract: A planar conducting structure, an electrically small distance above a ground plane, can be constructed to radiate in the direction normal to its plane while still retaining its low-profile characteristics. A circular disc structure is analysed theoretically to provide aid in the design of such antennas. The currents, fields, total radiated power, directive gain, losses, Q-factor and efficiency are all calculated at several frequencies for various values of the thickness, permittivity, and loss tangent of the dielectric material that separates the antenna from the ground plane.

Fresnel focussed imaging system

Abstract: An imaging system, particularly useful for acoustic medical diagnosis of a human subject, utilizes an array of radiating elements or sonic transducers located side-by-side and positioned along the subject. Signals received by the transducer are applied to a pair of pattern generation circuits which weight the individual signals by factors of +1, -1 or 0. Graphs of the weighting factors as a function of transducer location have the likeness of cosinusoidal and sinusoidal Fresnel patterns, these patterns being produced by the two circuits. Upon reception of signals, the weighted signals of each pattern are summed together, multiplied by cosinusoidal and sinusoidal reference signals and then summed together to provide a radiation pattern which converges from the array to a focal point in front of the array while eliminating a diverging pattern from a virtual focus behind the array. For transmission, sinusoidal and cosinusoidal signals are weighted by the Fresnel factors, summed together and applied to the transducers.

Antenna system for electronic security installations

Abstract: An antenna system for an electronic security installation comprising a pair of substantially identical planar multiple loop antennas respectively connected to the transmitter and receiver of the security system and providing an electromagnetic field of high intensity only in the interrogation region of the system while also discriminating against interferring signals originating outside of the interrogation region. Each planar antenna includes two or more loops lying in a common plane with each loop being twisted 180° with respect to each adjacent loop to be in phase opposition. The transmitting antenna and receiving antenna are substantially identical in size and configuration, and each antenna has a total loop area of one phase equal to the total loop area of opposite phase.

Design of superdirective arrays with high radiation efficiency

Abstract: Superdirective arrays have been considered unrealizable due to the high elemental currents required and their sensitivity to small variations in amplitude and phase. A new approach to superdirective array design based on polynomials which optimize radiation efficiency has been devised. An example of the optimization procedure is given for a 2-wavelength 11-element array, and the sensitivities of radiation patterns optimized for directivity and for radiation efficiency are compared. The relationship between radiation efficiency and the number of elements within a given array length is investigated, and an optimum number is proposed. Finally, the variation of radiation efficiency with beamwidth reduction of optimally filled arrays of different lengths is given.

Conformal spiral antenna

Abstract: An electrically small, microstrip radiator designed for small-diameter mile applications. The preferred embodiment comprises a cylindrical tube of epoxy fiberglass dielectric having a spiral conducting strip formed thereon. The tubular construction permits the antenna to be conformally mounted to the surface of the missile. RF input coupling may be achieved by an inductive post, and high radiation efficiency is obtained by strongly coupling RF currents to the body of the missile and exciting the dipolar mode of radiation. The design includes means for mechanically tuning the antenna over a narrow frequency range. The resultant spiral-slot antenna produces an axially polarized radiation field and a dipole radiation pattern with isotropic gain in a low cost and rugged construction.

Loaded horizontal antenna over an imperfect ground

Abstract: The Fresnel reflection coefficient technique is employed to establish an E -field integral equation for the antenna current. A resistive loading of the form \Lambda(x) = \Lambda_{0}/(1 - |x|/L) is used to load the antenna. An optimization technique is discussed for determining the value of critical loading \Lambda_{0}^{c} , which enforces a traveling wave current on the antenna. Results are given for the critical loading parameters, antenna currents, input impedances and radiation patterns versus different antenna dimensions and ground permittivities and conductivities. Some representative time-domain results for such loaded antennas are also included.

A broadband experimental null-steering antenna system for mobile communications

Abstract: The paper describes a novel and particularly simple form of null-steering antenna suitable for the base station of a mobile communication system which is subject to high levels of directional interference. The antenna provides omnidirectional coverage in the horizontal plane and has the facility of introducing a directional null which may be electronically steered through 360° of azimuth. This can therefore be used to minimize an interfering signal without affecting communication in other directions. The antenna employs a 4 or 5 element circular array and a simple null steering network requiring only one phase changer to steer the null. Various configurations of this antenna are examined theoretically and supported by experimental measurements on a system operated at 153 MHz. Experimental values of null depth lie in the range of 25?50 dB when operating over a bandwidth of ±30% of the centre frequency. Null widths (measured at the ? 10 dB points) may be electronically varied within the range 28°?68°. Applications for such a null-steering antenna and methods to increase the operating bandwidth are also discussed.

Planar antenna with tightly wound folded sections

Abstract: A broad-band antenna system capable of receiving VHF, FM, and UHF bands, having highly desirable directional properties, providing sharp nulls for the rejection of unwanted reflections, and with broad directional properties, usually obtainable only with large tunable dipoles or loops, many times the dimensions of this very compact antenna unit. This is strictly a receiving antenna, since it has no radiation capabilities, and consequently minimal loss of received signal due to reradiation.

Multiple beam antenna array

Abstract: The multiple beam, short wave planar antenna array system, which simultaneously exhibits low loss, low side lobe levels, high efficiency, and low volume in a design affording independent control of the E and H plane radiation pattern shapes is particularly applicable in microwave radiometric systems such as airborne mapping systems. The antenna system generates symmetrical, matching narrow pencil beam sensitivity patterns by using symmetrically spaced sectorial receiver horns fed energy through a hyperbolic dielectric lens, the horn array and lens residing in a TE01 mode parallel plate guide, the lens being illuminated by signals collected by a broad wall slotted wave guide array supplied with a thin, planar sheet radome mounted on the energy receiving face of the array.

Beam scanning using radiation pattern distortion

Abstract: A laser beam scanner in which a single-lobe propogates radiation pattern through an electrically variable asymmetric electrical charge distribution. Because the electrical charge distribution determines the real and imaginary parts of the refractive index of the material through which the radiation pattern propogates the radiation pattern may be deflected by changing the charge distribution profile.

Base band speed sensor

Abstract: A base band radio speed sensor, for use with the path of travel of the craft whose speed is to be sensed advantageously disposed at right angles to the antenna patterns, utilizes a transmitter flanked by equally spaced receivers for determining the times of passage of the craft with respect to symmetrically disposed receiver antennas.

Electromagnetic Properties and Effects of Advanced Composite Materials: Measurement and Modeling

Abstract: : This report covers several areas: (1) Electromagnetic properties of composite materials are given for the range DC to 30 MHZ; (2) Techniques of measurement are discussed in the range DC to 1 GHz; (3) Electromagnetic shielding theory is discussed; Surface transfer impedance and the two loop method are related to the conductivity. A matrix method for the calculation of shielding effectiveness for laminated anisotropic materials (perpendicular incidence) is presented; (4) The modification required by the method of moments to allow calculation involving non-perfect conductors are discussed; and (5) A comparison of antenna patterns over a graphite/epoxy ground plane and an aluminum ground plane for dipole and monopole antennas at 370 MHz and 837 MHz is given.

Antenna system to reduce fading caused by multipath transmission

Abstract: An antenna system including a single directional antenna having a single directional gain pattern for receiving electromagnetic waves in a certain direction. The orientation of the directional gain pattern is varied to vary the gain of the antenna in directions other than the certain direction in which electromagnetic waves are to be received. The gain is varied at a frequency higher than that of amplitude variations superimposed on the received electromagnetic waves while maintaining the gain substantially constant in the certain direction in which electromagnetic waves are to be received. The antenna gain variations in the directions other than the certain direction are effective to reduce the amplitude of received undesired electromagnetic waves and thereby diminish amplitude variations caused by these undesired electromagnetic waves.

Antenna employing curved parasitic end-fire directors

Abstract: An antenna assembly comprising elementary driven antenna elements and curvilinear end-fire directors on a conventional supporting structure to obtain an omnidirectional or other desired antenna pattern, even for supporting structures of relatively large transverse dimensions. The end-fire directors employ one or more curved conductive members.

Antenna having radiation pattern with main lobe of generally elliptical cross-section

Abstract: An antenna designed to emit a beam of generally elliptical cross-section has a reflector with a parabolic-elliptical concave surface conforming to the formula (x/p)+(y/q)=2z where p and q are the parameters of its parabolic cross-section in the xz and yz planes, respectively. A microwave feed located on a line interconnecting the foci of these two parabolas, preferably at the outer focus separated from the surface vertex by a distance q/2, generates a beam parallel to the axis whose ellipticity depends on the ratio q/p.

Microstrip network having phase adjustment

Abstract: In an array antenna system having a coupling network interconnecting a plurality of element groups, the coupling network is provided with phase adjustments to shift the angular location of the effective element radiation pattern. An effective technique for this phase adjustment in a microstrip coupling network makes use of a field altering structure positioned adjacent the microstrip.