Scattering from periodic arrays of crossed dipoles

An analysis is presented for calculating the scattering from periodic arrays of symmetrical crossed dipoles. It is shown that in general the reflection coefficient versus frequency of symmetrical crossed-dipole arrays exhibits two distinct resonances, which occur when the dipole elements are on the order of a half-wavelength long. The analysis reveals also the presence of an antiresonance, which occurs at a frequency between the two resonances. At the antiresonant frequency the array reflection coefficient is zero. Thus, within a relatively narrow frequency band, the array reflection characteristics traverse the extremes of complete reflection followed by no reflection, and finally by complete reflection again. The anomalous reflection versus frequency behavior in such arrays is shown to be directly attributable to two distinct resonances excited in the crossed elements. For an isolated crossed dipole, it is shown that the total induced current can be approximately represented by two uncoupled current components. The current components induced on the same elements in an array, however, are coupled to one another through interaction with neighboring elements. The coupling results in complete cancellation of the total bistatic array scattering, at a frequency which lies between the two component current resonant frequencies.     

Analysis of finite phased arrays of printed dipoles

The problem of a finite array of printed dipoles is treated, and results are presented in the form of reflection coefficient magnitudes, element patterns, and efficiency (based on power lost to surface waves). Various sized arrays are considered, and are compared with infinite array solutions. The excitation of surface waves is discussed in relation to the scan blindness phenomenon and the transition to an infinite array. Techniques for computational efficiency are also presented.     

Mutual coupling between microstrip dipoles in multielement arrays

An analytical method for evaluating mutual coupling in multielement microstrip arrays is discussed. The elements of the array are strip dipoles covered by an overlayer and excited by microstrip lines embedded in the substrate. In addition, two different ways of excitation are considered to evaluate the effect of the feeding lines on the coupling between the dipoles. Throughout the analysis, the transmission line and dipole widths are taken to be a fraction of the free-space wavelength, so that the longitudinal current component is the dominant contributor to the characteristics of the array. Under this assumption, the transverse component of the current is neglected without introducing appreciable error. The current distribution along the longitudinal direction is evaluated by solving an appropriate integral equation using Galerkin's method with piecewise continuous sinusoidal functions. All possible interactions between the currents on the feeding transmission lines and dipoles are included. Using the computed current distribution, transmission-line theory is applied on the feeding microstrip lines to evaluate self- and mutual impedances as well as scattering coefficients at chosen reference planes     

Near fields of resonant circular arrays of cylindrical dipoles

The far fields of resonant circular arrays of cylindrical dipoles with only one or two dipoles driven have been examined in a number of recent works. The discrete currents on such arrays can be thought of as a slow standing or traveling wave. In the present paper, the near fields of such arrays are studied. In order to better understand the somewhat unusual behavior of the near fields, we first discuss some simpler two-dimensional problems involving slow traveling current waves. We find that the associated fields possess certain surface-wave characteristics. We also find important differences from the case of linear arrays, which has been extensively studied in the past. Our discussions include methods to measure the efficiencies of our radiators, the extent of the near-field region, and the onset of the far-field region.     

Scan blindness in infinite phased arrays of printed dipoles

A comprehensive study of infinite phased arrays of printed dipole antennas is presented, with emphasis on the scan blindness phenomenon. A rigorus and efficient moment method procedure is used to calculate the array impedance versus scan angle. Data are presented for the input reflection coefficient for various element spacings and substrate parameters. A simple theory, based on coupling from Floquet modes to surface wave modes on the substrate, is shown to predict the occurrence of scan blindness. Measurements from a waveguide simulator of a blindness condition confirm the theory.     

Low sidelobe arrays of parallel dipoles with finite reflector

The effects of a finitely sized reflecting plane on the radiation patterns and on the input impedances of linear low sidelobe arrays of parallel dipoles are analysed. The differences between arrays with a finite reflector and those with an infinite one are studied and the design criteria are discussed.<>     

The optimum directivity of uniformly spaced broadside arrays of dipoles

The optimum directivity of various types of uniformly spaced broadside arrays of dipoles is investigated theoretically in this paper. The formulation is processed with the aid of an array matrix. The expression for the optimum directivity and the corresponding excitation are expressed directly in terms of the elements of the array matrix. The computed values are assembled in several sets of curves, and the results are compared with the directivity of uniformly excited arrays.     

Circular arrays of radial and tangential dipoles for turnstile antennas

A method of obtaining a turnstile antenna from a circular array of radial or tangential dipoles is presented. Approximate current distributions, terminal admittances, and radiation fields are given for arrays of 3, 4, and 5 half-wavelength elements for various array diameters.     

Difficulties in MoM analyses of resonant circular arrays of cylindrical dipoles

Previous studies have shown that a properly dimensioned, large circular array of cylindrical dipoles possesses very narrow resonances. Only one dipole is driven and the rest are parasitic. Those studies made use of the 'two-term theory'. In the present Letter, the Numerical Electromagnetics Code is applied to the aforementioned circular array. At some frequencies, meaningless results - large, negative driving-point conductances - are found. An attempt is made to explain why such results occur and to propose remedies when possible.     

Scan blindness phenomenon in conformal finite phased arrays of printed dipoles

Scan blindness phenomenon for finite phased arrays of printed dipoles on material coated, electrically large circular cylinders is investigated. Effects on the scan blindness mechanism of several array and supporting structure parameters, including curvature effects, are observed and discussed. A full-wave solution, based on a hybrid method of moments/Green's function technique in the spatial domain, is used to achieve the aforementioned goals. Numerical results show that the curvature affects the surface waves and hence the mutual coupling between array elements. As a result, the array current distribution of arrays mounted on coated cylinders are considerably different compared to similar arrays on planar platforms. Therefore, finite phased arrays of printed dipoles on coated cylinders show different behavior in terms of scan blindness phenomenon compared to their planar counterparts. Furthermore, this phenomenon is completely different for axially and circumferentially oriented printed dipoles on coated cylinders suggesting that particular element types might be important for cylindrical arrays.     

Direction and polarization estimation using arrays with small loopsand short dipoles

It is shown that when polarization-sensitive arrays consisting of crossed small loops and short dipoles are used, one can eliminate the requirement in the ESPRIT algorithm that sensors must occur in matched pairs. The dipoles and loops are sensitive to the polarizations of incident electromagnetic plane waves. The dipoles are sensitive to the incident electric field components, and the loops to magnetic field components of the incident waves. The invariance properties among the dipole and loop outputs of an arbitrary array of orthogonal loops and orthogonal dipoles are exploited to compute both the two-dimensional arrival angles and polarizations of incoming narrowband signals. It is shown that with dipoles and loops, vertical arrays are not necessary to obtain good direction estimates for signals from low angles     

On the optimum directivity of general nonuniformly spaced broadside arrays of dipoles

In this letter the maximum directivity of general nonuniformly spaced broadside arrays of dipoles is examined. This is done with the help of formulas found by the method of orthonormalization of the base, which includes the array factor.     

Numerical simulation of annular phased arrays for anatomicallybased models using the FDTD method

Annular phase arrays (APAs) of aperture and dipole antennas used for hyperthermia are simulated in three dimensions by using the finite-difference time-domain (FDTD) method. A 17363 cell, 1.31 cm resolution, anatomically based model of the human torso surrounded by a bolus of deionized water is used for calculations of specific absorption rates (SARs). Test runs on the calculation of fields in the water-filled interaction space and with homogeneous circular- and elliptical-cylinder phantoms correlate well with the experimental data in the literature, lending support to the accuracy of the FDTD method for near-field exposure conditions. Results are given for APAs using different sizes of aperture and dipole antennas and for a subannular array to obtain higher SARs in the liver. It is concluded that, because of its flexibility, the proposed procedure may be useful for a variety of realistic radiofrequency applicators for hyperthermia and other biomedical applications     

Comments, with reply, on `Numerical simulation'' of annular-phasedarrays of dipoles for hyperthermia of deep-seated tumors by J.-Y. Chenand O.P. Gandhi

The commenter asserts that it would be dangerous to take computed local values of specific absorption rate (SAR) seriously and use them as a basis for patient treatment, as suggested by J.-Y. Chen and O.P. Gandhi (ibid., vol.39, p.209-16, Mar. 1992) in the above-titled paper. He lists 11 reasons that support his assertion. The authors refute the commenter's claims, based on his use of simple models and whole-body SAR or average SAR in large regions of the body     

Numerical simulation of magnetic induction heating of tumors with ferromagnetic seed implants

Calculations based on the bioheat transfer equation have been carried out to determine the temperature distributions to be expected from the use of inductively heated ferromagnetic implants to heat deep-seated tumors. Two types of ferromagnetic implants are considered: constant power seeds, for example, those constructed from Type 430 stainless steel; and constant temperature seeds which pass through a Curie transition to the nonmagnetic state at a specified temperature. The temperature distributions are studied as a function of the size of the implant array, its geometrical relationship to the tumor, the density of implants within the array, and the blood perfusion characteristics of the tumor and its surrounding normal tissue. Two tumor models are considered: a uniformly perfused model which is indistinguishable from the surrounding normal tissue, and an annular perfusion model with a necrotic core surrounded by intermediately and highly perfused shells. Temperature distributions are considered acceptable if the minimum temperature in the tumor is greater than 42°C and the maximum temperature does not exceed a maximum allowable value (either 48 or 60°C). The results of over 200 combinations of the above parameters are presented in a compact format. General conclusions drawn are that the tumor should lie entirely within the implanted array if the tumor periphery is to be heated adequately, and that the constant temperature seeds, which are self-regulating in temperature, give better tumor temperature distributions.     

Analysis of wide-band infinite phased arrays of printed folded dipoles embedded in metallic boxes

A theoretical method is presented to analyze the scan performance of infinite phased arrays of printed folded dipoles embedded in metallic boxes. By using the equivalence principle, each unit-cell is divided into an inhomogeneous interior region and a homogeneous exterior region. The exact dyadic Green's function for both regions is derived. The current distribution on the antenna elements and the corresponding active impedance are found by solving the pertaining integral equations with the method of moments. Numerical results show that the E-plane scan performance is improved significantly by placing the folded dipoles inside metallic boxes. Without the use of additional matching circuits a bandwidth of more than 16% was obtained over a wide scan range. A disadvantage of the use of metallic boxes is the strong appearance of a blind scan angle at the higher frequencies. Measurements of the element pattern of an 1152-element prototype show a very close agreement with the presented theoretical computations.     

Scan performance of infinite arrays of microstrip-fed dipoles withbent arms printed on protruding substrates

E-plane scan results are presented for infinite arrays of microstrip-fed dipoles printed on protruding dielectric substrates. Active impedance values are calculated for dipoles with bent arms and for the array with a near-field dielectric radome cover. It is found that bending the dipole's arms produces a small improvement in the scan range, as compared to the element with the arms straight, and that a dielectric radome with a low permittivity causes a small shift in the impedance curves. The scan limitation of the array is dictated, however, by the presence of the coplanar feedlines with improvement achievable by varying the substrate's thickness and permittivity as well as the dipole/feed geometry. A dipole element with a modified coplanar feed design is introduced for wider scan coverage     

Analysis of finite arrays of axially directed printed dipoles on electrically large circular cylinders

Various arrays consisting of finite number of printed dipoles on electrically large dielectric coated circular cylinders are investigated using a hybrid method of moments/Green's function technique in the spatial domain. This is basically an "element by element" approach in which the mutual coupling between dipoles through space as well as surface waves is incorporated. The efficiency of the method comes from the computation of the Green's function, where three types of spatial domain Green's function representations are used interchangeably, based on their computational efficiency and regions where they remain accurate. Numerical results are presented in the form of array current distributions, active reflection coefficient and far-field pattern to indicate the efficiency and accuracy of the method. Furthermore, these results are compared with similar results obtained from finite arrays of printed dipoles on grounded planar dielectric slabs. It is shown that planar approximations, except for small separations, can not be used due to the mutual coupling between the array elements. Consequently, basic performance metrics of printed dipole arrays on coated cylinders show significant discrepancies when compared to their planar counterparts.     

Equivalent network of transverse dipoles in inset dielectric guide:application to linear arrays

The inset dielectric guide (IDG) is an alternative to the image line, retaining most if its advantages without its main disadvantages. In particular, IDG makes it possible to realize low-cost leaky wave antennas with very pure polarization properties by laying thin metal strips (dipoles) on the air-dielectric interface. The scattering properties of thin metal strips are analyzed by means of a Ritz-Galerkin variational approach, taking into proper account the edge singularities. A variational expression for the equivalent circuit of the radiating dipole is derived, and this information is applied to the design of a tapered linear array by network methods. The array was built and tested, and it showed excellent performance     

The radiation patterns of antenna arrays consisting of dipoles coupled to a two-wire transmission line

In this paper the radiation patterns of antenna arrays consisting of dipoles coupled electromagnetically to a two-wire transmission line are reported. The numerical calculation was made on an IBM 650 computer.