Thick Dipoles Research Articles

Benchmarking Computational Electromagnetics With the Large Resonant Circular Array of Electrically Short and Thick Dipoles

The phenomenon of resonances in large circular arrays of electrically short and thick dipoles was studied intensively in the nineties by Harvard’s Antenna Group, led by R. W. P. King and T. T. Wu. Here, we propose these arrays as a benchmark for modern 3-D computational electromagnetics (CEM) solvers. Our proposed benchmark is challenging because it exhibits resonances which, for judicious choices of the parameters, can be extremely narrow. We present exploratory simulations of our proposed benchmark using COMSOL multiphysics and ANSYS HFSS and give detailed comparisons with previously published results.

Using Ultra-High Expansion Orders of Max-Ortho Basis Functions for Analysis of Axially Symmetric Metallic Antennas

Implementation of max-ortho basis functions is proposed in a method for analysis of axially symmetric metallic antennas based on exact kernel of electric field integral equation in combination with Galerkin testing. High-precision evaluation of matrix elements is enabled by: a) representing them as a linear combination of impedance integrals due to the Legendre polynomials and their first derivatives; b) using the singularity cancelation techniques; and c) evaluating the Legendre polynomials and their first derivatives by well-known recurrent formulas. Applicability of max-ortho bases up to expansion order of $n =128$ is illustrated on a full-wave thick dipole antenna.

Analysis of axially symmetric wire antennas by the use of exact kernel of electric field integral equation

The paper presents a new method for the analysis of wire antennas with axial symmetry. Truncated cones have been applied to precisely model antenna geometry, while the exact kernel of the electric field integral equation has been used for computation. Accuracy and efficiency of the method has been further increased by the use of higher order basis functions for current expansion, and by selecting integration methods based on singularity cancelation techniques for the calculation of potential and impedance integrals. The method has been applied to the analysis of a typical dipole antenna, thick dipole antenna and a coaxial line. The obtained results verify the high accuracy of the method.

Wideband feeds for the upgraded GMRT

This paper describes the existing feeds in use at the GMRT Observatory and details the ongoing development of next generation wideband feeds for the upgraded GMRT. The existing feeds include: feed with folded thick dipoles (for 150 MHz), dipole-disc feed (for 325 MHz), dual-band coaxial feed (for 233 MHZ and 610 MHz), and corrugated horn feed (for 1400–1450 MHz). The new broadband feeds covered in this paper are: cone-dipole feeds for 250–500 and 500–1000 MHz, wideband horn feed for 550–900 MHz, and dual ring feed for 130–260 MHz. Design techniques and performance results for these are described.

Antennas for Digital Television Receivers in Mobile Terminals

The incorporation of new services in handheld devices, such as the Digital Video Broadcast-Handheld (DVB-H) operating at the lower ultrahigh-frequency (UHF) band poses a challenge for antenna designers. Wideband small antennas or electrically tunable narrowband small antennas are needed to fulfil the performance requirement. It is well known that below 900 MHz the operation of embedded mobile terminal antennas is based on utilizing the whole structure of the terminal as a radiator. However, even this way reaching the whole required impedance bandwidth of about 46% at about 0.61-GHz center frequency is possible only either with clamshell-type terminals used as thick dipoles by feeding them from the hinge or with “large tablet”-sized terminals. With the popular smartphone-sized terminals with a monoblock structure the available bandwidth with good total efficiency is clearly smaller. We study the options to implement antennas for smartphone-type mobiles for receiving digital TV broadcasts at about 0.47-0.75-GHz frequencies. The mainly studied technology is the nonresonant capacitive coupling element (CCE)-type antennas having one of the smallest achieved volume-to-bandwidth ratios. We show that for a fixed-frequency antenna with a volume of less than a few cubic centimeters the total efficiency will become rather low due to moderate matching level, but the requirement of the DVB-H standard for the realized gain can easily be met. Additionally, we show that by having switching between two bands, one can implement a dual-antenna configuration with small total volume and significant multiple-input-multiple-output (MIMO) gain. Furthermore, we study the very important effect of the user's hands on the antenna performance and find that the effect can range from some increase of the total efficiency due to improved matching to significant losses caused by the hands. Finally, we propose some possible ways ahead in solving this very challenging antenna design problem.

Approximate Kernel Orthogonalization for Antenna Array Processing

We present a method for kernel antenna array processing using Gaussian kernels as basis functions. The method first identifies the data clusters by using a modified sparse greedy matrix approximation. Then, the algorithm performs model reduction in order to try to reduce the final size of the beamformer. The method is tested with simulations that include two arrays made of two and seven printed half wavelength thick dipoles, in scenarios with 4 and 5 users coming from different angles of arrival. The antenna parameters are simulated for all DOAs, and include the dipole radiation pattern and the mutual coupling effects of the array. The method is compared with other state-of-the-art nonlinear processing methods, to show that the presented algorithm has near optimal capabilities together with a low computational burden.

Asymmetric long and thick dipoles for in‐flight telemetry of projectiles

AbstractLong and thick dipoles are presented, with a feeding point located off the centre. Parameter studies are performed, showing that asymmetric radiation patterns with a main lobe in a preferred direction can be achieved. Experimental results confirm the feasibility of these antennas and illustrate possible applications. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 508–511, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23105

Calculable Dipole Antenna for EMC Measurements with Low-Loss Wide-Band Balun from 30 MHz to 2 GHz

A half-wavelength calculable dipole antenna has been developed. The Antenna Factor (AF) of this dipole is evaluated between 30 MHz and 2 GHz. Generally the proposed formula by ANSI-63.5 (ANSI, 1988) method for EMI antenna calibration is for a resonant dipole and assumes a perfect sinusoidal current density along the dipole. Indeed, the mutual effects on AF in this case could be taken into account with some approximate correction factors. The perfect sinusoidal form of current density is not correct for the real thick dipoles. For these dipoles, i.e., nonresonant and thick, assuming a perfect sinusoidal current density introduces errors around 1 dB/m on AF. An alternative method is presented: an analytical method based on a simple new current distribution along the dipoles to evaluate their radiation characteristics, and this is compared with a numerical approach based on MoM (the Method of Moments) by NEC software, to model two dipoles in a standard site. The entire frequency range, 30 MHz to 2 GHz, is covered by only two baluns. For the frequency range, 350 MHz to 2 GHz, we have used for the first time a low-loss wide-band tapered balun. The experimental results are presented where a good agreement is observed with the calculated ones.

A disk-loaded thick cylindrical dipole antenna for validation of an EMC test site from 30 to 300 MHz

We propose a new broad-band, short (80-cm-long) disk-loaded thick cylindrical dipole (DTCD) antenna for validation of an electromagnetic compatibility (EMC) test site for the frequency range between 30-300 MHz. In order to achieve broad-band characteristics, we have chosen a thick dipole as DTCD element. Disk loading has been applied to make this dipole shorter. To eliminate any ambiguity in balun characteristics, a commercially available hybrid junction is used as an antenna balun. Thus, the DTCD proposed in this paper is a very short, simple, and well-defined broad-band antenna that covers the frequency range between 30-300 MHz. The DTCD has an azimuthally symmetrical radiation pattern and, thus, is very well suited for the validation of an EMC test site. This paper presents the DTCD antenna characteristics such as input impedance, antenna gain, antenna factor, and classical site attenuation, which are calculated by use of the method of moments (MoM). These theoretical antenna factors and classical site attenuations of the DTCD are compared with experiments over the frequency range of 30-300 MHz. They agree with each other to within /spl plusmn/1 dB in antenna factor and to within /spl plusmn/2 dB in site attenuation. Our preliminary investigation indicates that an additional DTCD (25 cm long) can cover the frequency range of 300-1000 MHz. Therefore, only two DTCDs will be needed for very accurate calculable classical site attenuation values in order to validate an EMC test site for the entire frequency range between 30-1000 MHz.

The influence of the electrode size of cochlear implants on the spatial selectivity

Introduction. The authors aimed to study the influence of the electrode size on the spatial selectivity with cochlear implants. Material and methods. The Leiden cochlear implant project studies the key factors that determine the clinical outcome with cochlear implants. For this purpose the electrical field in the cochlea is computed, and from this the neural response is predicted. The present cochlear model consists of a 30 turn tapered spiral (guinea-pig). In this study we present calculations for both point sources and electrodes with realistic dimensions, comparable with the often-used Nucleus electrode. The size of the electrode relative to the scala tympani is varied. Furthermore, the consequences of the application of three poles (positive, negative, positive) instead of two poles (positive, negative) were investigated. Results. A higher degree of scala tympani filling results in the reduction of current thresholds without much influence on the spatial selectivity. Only when thin electrodes are used does the application of three poles result in an improved spatial selectivity. Such tripoles, however, exhibit relatively high thresholds. Conclusion. Favourable use of tripole stimulation (thin electrodes) easily results in high current densities. Thick dipoles result in reduced thresholds and current densities with comparable spatial selectivity relative to thin dipoles.

A hybrid procedure to solve Hallen's problem

The knowledge of the electromagnetic fields in the neighborhood of an antenna needs the accurate evaluation of the current distribution on it. This is a subject which deserves particular attention mainly for sensors. It is called Hallen's problem and is relevant to the current distribution on a cylindrical antenna, two hollow cylinders separated by an infinitesimal gap. In this paper, we show that it is possible to extract the singular part of the current, and to obtain a reasonable precision with a finite number of terms regardless of the distance from the gap. The method seems to be suitable for thick dipole antennas. The procedure has been defined hybrid because we first resort to a finite number of steps of the iterative solution, and then the nth integral equation is solved by the Bubnov-Galerkin projection method.

An entire-domain Galerkin analysis of the moderately thick dipole

A simple approximate method for the numerical analysis of the moderately thick tubular dipole is presented. It is based on an entire-domain Galerkin procedure using a polynomial representation of the current distribution. The singularity occurring in the kernel of the integral equation is extracted in a form allowing approximate analytical evaluation of the triple integrals. A comparison between computed and previously reported experimental results is given.

Radiation Patterns of TV Transmitting Dipole Arrays

When designing a dipole 'panel', a unit building block for a TV transmitting dipole array, there are several constraints which need to be met viz. beamwidth, sidelobe levels, front to back ratio and input impedance. Theories on reflector backed dipole arrays have so far dealt with idealised or thin dipoles and with infinite reflector. They are not very useful for predicting sidelobe levels and front to back ratios nor would they be of any use to calculate the input impedance of an array of relatively thick dipoles which are often used in practice. Experimental procedures are usually employed by the designer and these have proved to be very time consuming.

Further comments on the design of log-periodic dipole antennas

The design procedure for the log-periodic dipole (LPD) antenna is discussed, by extending the results presented in a previous paper by the authors. The results of gain calculations, already published, are summarized and extended to cases of very thick dipoles. The gain analysis is completed with a discussion of the optimum antenna input feed line <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z_{0}'</tex> the achievable VSWR, the voltage gradient, and the effects of the presence of the ground. All the results of calculations given here are presented as a function of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z_{0}</tex> (the characteristic impedance of the transmission line feeder), <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h/a</tex> (the ratio between the half-length and the radius of the dipoles), <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau</tex> (the logarithmic decrement of the geometrical configuration), and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sigma</tex> (the spacing ratio of dipoles), i.e., the parameters, which usually describe the geometry of an LPD array. The results of calculations are presented in many diagrams, for different values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau,\sigma, Z_{0}</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h/a</tex> , which can be useful for accurate design of LPD antennas including those for high-power applications. Measurements of VSWR performed on antenna models have confirmed the computed results.

On numerical convergence of moment solutions of moderately thick wire antennas using sinusoidal basis functions

Wire antennas are solved using a moments solution where the method of subsectional basis is applied with both the expansion and testing functions being sinusoidal distributions. This allows not only a simplification of near-field terms but also the far-field expression of the radiated field from each segment, regardless of the length <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</tex> . Using sinusoidal basis functions, the terms of the impedance matrix obtained become equivalent to the mutual impedances between the subsectional dipoles. These impedances are the familiar impedances found using the induced EMF method. In the induced EMF method an equivalent radius is usually used in the evaluation of the self-impedance term to reduce computation time. However, it is shown that only for very thin segments that the correct equivalent radius is independent of length. When the radius to length ratio ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a/L</tex> ) is not small, an expansion for the equivalent radius in terms of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a/L</tex> is given for the self-impedance term. The use of incorrect self-term, obtained by using a constant equivalent radius term, is shown to be responsible for divergence of numerical solutions as the number of sections is increased. This occurrence is related to the ratio of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a/L</tex> of the subsections and hence becomes a problem for moderately thick wire antennas even for a reasonably small number of segments per wavelength. Examples are given showing the convergence with the correct self-terms and the divergence when only a length independent equivalent radius is used. The converged solutions are also compared to King's second- and third-order solutions for moderately thick dipoles.

The possibility of an interaction anomaly in acoustic receiving arrays and radio arrays

The conditions whereby interaction anomalies which occur in transmitting arrays could arise in acoustic or radio receiving arrays are established. It is concluded that the effect will only occur in special cases: for close packed resonant acoustic arrays and for radio arrays with thick dipoles arrayed asymmetrically.

The control of electromagnetic scattering by impedance loading

Electromagnetic scattering from a conducting object can be controlled by loading portions of its surface with distributed or lumped impedances. The scattered field from a loaded object is expressed as a superposition of fields scattered by the unloaded object and fields reradiated by the loading. The interference of these fields can be used to produce the required scatter control. Techniques for evaluating the reradiated field are discussed. A general form for the scattered field as a function of load impedance is shown, and techniques for studying this form are discussed. Theoretical and experimental results for thin, impedance loaded dipoles are compared. For dipole lengths not much greater than λ/2, broadside backscatter can be reduced 30 to 40 dB with a passive center load. Greater lengths and other aspects require multiple passive loads. Backscatter reduction of up to 20 dB has been reported for thick dipoles at broadside incidence, planar surfaces at normal incidence, and long thin objects near end-on incidence.

The Mutual and Input Impedance of Strips Between Parallel Planes

The input impedance of a thick dipole antenna between parallel planes, or the mutual impedance between two such elements, each carrying an assumed sinusoidal current distribution, is obtained by first considering filamentary dipoles. The longitudinal or tangential field intensity about a single dipole of length L between parallel planes, as indicated in Fig. 1, is therefore obtained by considering the infinite series of images in the two planes, from which the longitudinal field is simply written as an infinite summation of the fields due to the individual dipole elements. This series converges too slowly to be of any practical use, however, and a transformation of series must be applied, which results in the infinite series of exponential terms with imaginary arguments becoming a series of modified Bessel functions of zero order of the second kind. The final expression is given as Eq. (1) below and converges rapidly in most cases.

Antenna Impedance Measurement by Reflection Method

Two methods of measuring the impedance of radiators are described. They are based on measurement of the power reflected by the radiators. These methods are particularly useful for finding the radiation resistance of thick half-wave dipoles.