Parametric oscillation in nonlinear dipole arrays

An experimental investigation of large nonlinear dipole arrays is presented, using a diode-loaded dipole placed between two parallel conducting plates. Using this topology the authors were able to demonstrate enhanced conversion efficiency into the second harmonic and obtain spontaneous parametric oscillation for certain array geometries. Parametric oscillation was achieved by strongly driving the antenna at a single pump frequency and adjusting the cavity length for resonance at a subharmonic. A pump threshold for parametric down-conversion was observed, below which no sub-harmonics were produced. These results suggest using quasi-optical techniques for frequency multiplication and parametric up-conversion or amplification     

Plane-wave scattering from half-wave dipole arrays

A matrix equation for determination of plane-wave scattering from arrays of thin short-circuited dipoles of lengths about half a wavelength is derived. Numerical and experimental results are presented for linear, circular, and concentric circular arrays.     

Efficient analysis for infinite microstrip dipole arrays

A moment method analysis of infinite microstrip dipole arrays which uses an efficient technique to evaluate the generalised impedance matrix is described. A particularly simple formulation is obtained through the use of the periodic Green function. Results for the reflection coefficient magnitude against scan angle are given for a typical array.     

Feed region modes in dipole phased arrays

Feed region modes are derived for a class of dipole phased arrays. The dipole and its balun are a linearly polarized version of the perimeter array radar (PAR) antenna element, and are modeled in strip-line geometry. Knowledge of the feed region modes is essential in determining the influence of supports on the element scan performance, and should shed light on the formation of blind spots in dipole arrays. It is shown that for practical spacings, the balanced strip-line feed structure supports a propagating transverse magnetic (TM) mode in addition to two transverse electromagnetic (TEM) modes. The propagation constant of this mode is scan dependent, and under inappropriate conditions its cutoff occurs before the onset of the grating lobe. Pending further analysis, it is conjectured that this mode cutoff may cause blind spots which limit the array scan coverage.     

对数周期偶极天线扇形阵的分析

采用矩量法对对数周期偶极天线扇形阵进行理论分析，在计算广义阻抗矩阵元素时，利用矩阵的互易性、天线阵的结构对称性和对数周期天线的工作原理进行了算法简化，从而在保证计算精度的前提下，大大提高了计算速度。     

Design of dipole arrays with equal element impedances

A simple method for synthesising a given radiation pattern from a new type of dipole array, an unequal element array, is described. The input impedances of the elements are the same and the lengths and the radii of the dipoles are different.<>     

Planar dipole arrays with equal element input impedances

Planar antenna arrays of unequal dipoles in which all the input impedances of the elements are the same are proposed to simplify the matching networks or to improve the matching condition for broadside arrays. A method to design this type of array is described. The array produces a given radiation pattern. The elements are of different lengths and different radii     

K-pulse techniques for short-pulse radiation from dipole arrays

An investigation of K-pulse techniques to transmit short-time duration pulses from dipole arrays is presented. The array is analyzed from a radiation standpoint using the singularity expansion method. The analysis also makes use of the method of moments to express the array's radiation in terms of the terminal voltages and pole singularities. Following a numerical search in the complex frequency plane (from which the dominant singularities of the array are located), the time-domain waveform of the approximated K-pulse voltage is constructed. Calculated plots of the far-zone radiated electric field resulting from applying these voltages at the array terminals are presented, and their energy concentrations are compared to those of the optimum solutions     

Transient electromagnetic fields of a vertical magnetic dipole on a two-layer earth

The transient fields of a vertical magnetic dipole on a two-layer earth model are expressed in analytical form using two different approaches. In the first, the fields in the time domain are obtained as the inverse Laplace transforms of derived full wave time-harmonic solutions, while in the second, the concept of natural frequencies of the stratified earth is utilized. Comparison with a previously obtained approximate solution reveals that the latter is the late time part of the present solution. Important features in the waveforms of the surface fields due to step and pulsed current excitations are demonstrated by a variety of numerical examples. These features provide diagnostic means of sensing the earth's stratification, overburden thickness, and the ratio of conductivities of the layers.     

Design of transversely fed EMC microstrip dipole arrays includingmutual coupling

Design techniques and procedures for microstrip dipole arrays transversely fed by proximity coupled microstrip lines are presented. Two design equations which include the effects of mutual coupling are developed, and the corresponding design curves are obtained by a rigorous integral equation solution. A seven-element standing wave linear array is designed to illustrate the developed design procedures. The design data are checked by a complete integral equation solution of the array, with good agreement. The measurements of radiation pattern and input impedance are found to be in good agreement with the design goal     

Estimating brain conductivities and dipole source signals with EEG arrays

Techniques based on electroencephalography (EEG) measure the electric potentials on the scalp and process them to infer the location, distribution, and intensity of underlying neural activity. Accuracy in estimating these parameters is highly sensitive to uncertainty in the conductivities of the head tissues. Furthermore, dissimilarities among individuals are ignored when standarized values are used. In this paper, we apply the maximum-likelihood and maximum a posteriori (MAP) techniques to simultaneously estimate the layer conductivity ratios and source signal using EEG data. We use the classical 4-sphere model to approximate the head geometry, and assume a known dipole source position. The accuracy of our estimates is evaluated by comparing their standard deviations with the Cramér-Rao bound (CRB). The applicability of these techniques is illustrated with numerical examples on simulated EEG data. Our results show that the estimates have low bias and attain the CRB for sufficiently large number of experiments. We also present numerical examples evaluating the sensitivity to imprecise assumptions on the source position and skull thickness. Finally, we propose extensions to the case of unknown source position and present examples for real data.     

Feeding in-phase dipole arrays: a tutorial and a MATLAB program

Here, we revisit a textbook example of how to design a dipole array and its feed network. The effects of mutual coupling are taken into account, so that a desired set of in-phase excitations is fed to the antenna and radiated efficiently. A MATLAB M-code program, implementing the procedure and plotting the resulting radiation power pattern, is also provided.     

SAR distributions in interstitial microwave antenna arrays with asingle dipole displacement

The use of interstitial microwave antenna array hyperthermia (IMAAH) as a treatment for cancer, in conjunction with radiation therapy and chemotherapy, has been investigated widely. The heating pattern produced by a coherently phased 915 MHz asymmetric antenna array displays the maximum power deposition in the array center. The authors report on the effect of variable insertion depth between antennas of an array on the heating patterns produced. The study of this heating behavior demonstrates a similar effect to that of the variably phased arrays, showing a shift of the heating peak towards the periphery of the tumor, offering a more simple method for the clinical treatment of such tumors     

Accurate pattern synthesis of parallel dipole arrays and itsapplication to unequal elements

A new simple method for accurate radiation pattern synthesis of parallel dipole arrays is described and its applications to low-sidelobe arrays with equal and unequal elements are presented     

High-gain series fed printed dipole arrays made of high-Tc superconductors

High-gain arrays are designed from closely spaced printed dipoles, alternately crossed by an integrated series feed. Both the array elements and the feeding and matching network are made of thick film YBa ₂Cu₃O_7-x material for increased efficiency. The optimization of directive gain in terms of spacing and dipole length, under constraints in array size, is performed by combining the space domain method of moments (MM) with transmission line theory. A practical design is experimentally demonstrated, and its performance is compared to simulated data. The superconducting array presents gain higher than that of similar cryogenically cooled Cu and Ag arrays tested. Supergain potential is discussed     

A scale-model study of down-hole VHF dipole arrays with applicationto subsurface exploration

The cross-borehole measurement technique at VHF frequencies typically employs winched single transmitting and receiving dipoles in various offset configurations to detect a buried target and estimate its vertical position from the pattern of forward scatter. Signal-to-noise ratio (SNR) and clutter from random scatterers in the host medium are practical limitations to target detection that can be modified by means of synthetic collinear arrays whose electric fields are determined as a function of vertical position from the measured amplitude and phase of a single element as it moves in the borehole. The present paper compares theoretical calculations based on a thin bare dipole in a moderately lossy host medium with a combination of laboratory scale model measurements and simple synthetic array techniques to indicate the potential advantages of the latter to geophysical exploration     

高辐射强度的带THz扼流圈的偶极天线阵列模拟分析

为了提高太赫兹辐射强度, 设计了带THz扼流圈的偶极天线阵列.模拟结果表明, 增加直线阵的阵元数对平均匹配效率影响很小, 却能线性增加相干辐射强度.加入THz扼流圈可减小进入到传输线的交流分量, 进而减小共振频率的偏移, 使平均匹配效率提升了两倍.相比于网格排列的平面阵, 交错排列的阵元在垂直方向上具有更小的耦合, THz发射谱更窄.通过使用聚酰亚胺透镜代替硅透镜, 可有效提高输入电阻, 并将总效率由25%提高到35%.     

Electromagnetic fields due to a horizontal electric dipole antenna laid on the surface of a two-layer medium

With applications to geophysical subsurface probings, electromagnetic fields due to a horizontal electric dipole laid on the surface of a two-layer medium are solved by a combination of analytical and numerical methods. Interference patterns are calculated for various layer thickness. The results are interpreted in terms of normal modes and the accuracies of the methods are discussed.     

Constructions and properties of Costas arrays

A Costas array is an n × n array of dots and blanks with exactly one dot in each row and column, and with distinct vector differences between all pairs of dots. As a frequency-hop pattern for radar or sonar, a Costas array has an optimum ambiguity function, since any translation of the array parallel to the coordinate axes produces at most one out-of-phase coincidence. We conjecture that n × n Costas arrays exist for every positive integer n. Using various constructions due to L. Welch, A. Lempel, and the authors, Costas arrays are shown to exist when n = p - 1, n = q - 2, n = q - 3, and sometimes when n = q - 4 and n = q - 5, where p is a prime number, and q is any power of a prime number. All known Costas array constructions are listed for 271 values of n up to 360. The first eight gaps in this table occur at n = 32, 33, 43, 48, 49, 53, 54, 63. (The examples for n = 19 and n = 31 were obtained by augmenting Welch's construction.) Let C(n) denote the total number of n × n Costas arrays. Costas calculated C(n) for n ≤ 12. Recently, John Robbins found C(13) = 12828. We exhibit all the arrays for n ≤ 8. From Welch's construction, C(n) ≥ 2n for infinitely many n. Some Costas arrays can be sheared into "honeycomb arrays." All known honeycomb arrays are exhibited, corresponding to n = 1, 3, 7, 9, 15, 21, 27, 45. Ten unsolved problems are listed.