Showing papers in "IEEE Antennas and Propagation Magazine in 2003"

An overview of fractal antenna engineering research

Abstract: Recent efforts by several researchers around the world to combine fractal geometry with electromagnetic theory have led to a plethora of new and innovative antenna designs. In this report, we provide a comprehensive overview of recent developments in the rapidly growing field of fractal antenna engineering. Fractal antenna engineering research has been primarily focused in two areas: the first deals with the analysis and design of fractal antenna elements, and the second concerns the application of fractal concepts to the design of antenna arrays. Fractals have no characteristic size, and are generally composed of many copies of themselves at different scales. These unique properties of fractals have been exploited in order to develop a new class of antenna-element designs that are multi-band and/or compact in size. On the other hand, fractal arrays are a subset of thinned arrays, and have been shown to possess several highly desirable properties, including multi-band performance, low sidelobe levels, and the ability to develop rapid beamforming algorithms based on the recursive nature of fractals. Fractal elements and arrays are also ideal candidates for use in reconfigurable systems. Finally, we provide a brief summary of recent work in the related area of fractal frequency-selective surfaces.

A survey of various propagation models for mobile communication

Abstract: In order to estimate the signal parameters accurately for mobile systems, it is necessary to estimate a system's propagation characteristics through a medium. Propagation analysis provides a good initial estimate of the signal characteristics. The ability to accurately predict radio-propagation behavior for wireless personal communication systems, such as cellular mobile radio, is becoming crucial to system design. Since site measurements are costly, propagation models have been developed as a suitable, low-cost, and convenient alternative. Channel modeling is required to predict path, loss and to characterize the impulse response of the propagating channel. The path loss is associated with the design of base stations, as this tells us how much a transmitter needs to radiate to service a given region. Channel characterization, on the other hand, deals with the fidelity of the received signals, and has to do with the nature of the waveform received at a receiver. The objective here is to design a suitable receiver that will receive the transmitted signal, distorted due to the multipath and dispersion effects of the channel, and that will decode the transmitted signal. An understanding of the various propagation models can actually address both problems. This paper begins with a review of the information available on the various propagation models for both indoor and outdoor environments. The existing models can be classified into two major classes: statistical models and site-specific models. The main characteristics of the radio channel - such as path loss, fading, and time-delay spread - are discussed. Currently, a third alternative, which includes many new numerical methods, is being introduced to propagation prediction. The advantages and disadvantages of some of these methods are summarized. In addition, an impulse-response characterization for the propagation path is also presented, including models for small-scale fading, Finally, it is shown that when two-way communication ports can be defined for a mobile system, it is possible to use reciprocity to focus the energy along the direction of an intended user without any explicit knowledge of the electromagnetic environment in which the system is operating, or knowledge of the spatial locations of the transmitter and the receiver.

Wideband monopole antennas for multi-band wireless systems

Abstract: The planar monopole antenna is shown to provide extremely wideband impedance characteristics. Recently, many techniques to tailor and optimize the impedance bandwidth of these antennas have been investigated. These include the use of bevels, slots and shorting posts. These antennas are becoming popular, and have been proposed for modern and future wideband wireless applications. The radiation performance is also shown to be acceptable over a wide frequency range.

Design of small-size wide-bandwidth microstrip-patch antennas

Abstract: Several designs for small-size wide-bandwidth microstrip antennas are examined through simulation and experiment. Designs are presented based on two wideband patch antennas: the U-slot patch antenna, and the L-probe-fed patch antenna. Several techniques are utilized to reduce the resonant length of these wideband microstrip-patch antennas: increasing the dielectric constant of the microwave substrate material, the addition of a shorting wall between the conducting patch and the ground plane, and the addition of a shorting pin between the conducting patch and the ground plane. Simulation and experimental results confirm that the size of the antennas can be reduced by as much as 94%, while maintaining impedance bandwidths in excess of 20%.

10 million unknowns: is it that big? [computational electromagnetics]

Abstract: At the Center for Computational Electromagnetics at the University of Illinois, we recently solved a very-large-scale electromagnetic scattering problem. We computed the bistatic radar cross-section of a full-size aircraft at 8 GHz, involving the solution of a dense matrix equation with nearly 10.2 million unknowns. We regarded this as the "ultimate test" of a massively parallel implementation of the multilevel fast multipole algorithm (MLFMA), called ScaleME. In this paper, we narrate the technical difficulties faced and the experience gained from a very informal point of view. We describe the various methods developed for surmounting each of the obstacles.

Reconfigurable antennas for wireless devices

Abstract: New technologies in communications electronics, such as software-defined radio (SDR) and RF switches implemented using micro-electromechanical systems (MEMS), present new challenges and opportunities for antenna design. In sharp contrast to digital technology where Moore's law reigns, a fundamental law of physics constrains the ability to realize electrically small antennas that are both efficient and broadband. As a result, covering several frequency bands concurrently with a single antenna having enough efficiency and bandwidth is a major challenge. One possible solution to this problem is to use reconfigurable antennas that tune to different frequency bands. Such an antenna would not cover all bands simultaneously, but provides narrower instantaneous bandwidths that are dynamically selectable at higher efficiency than conventional antennas. Such tunable-antenna technology is an enabler for software-definable radios, the RF front ends of which must be reprogrammable on the fly. This paper discusses the practical implementation issues, limitations, and measured results of small, narrowband, tunable antennas within portable handsets. Many of the concepts discussed in this paper will likely become practical and cost effective in the near future because of recent advances in RF MEMS switches.

Coming soon to a Wal-Mart near you

Abstract: According to the Web site of the Association for Automatic Identification and Data Capture Technologies [http://www.aimglobal.org/technologies/rfid], "radio frequency identification (RFID) technology is an automatic way to collect product, place, and time or transaction data quickly and easily without human intervention or error." With the ability to track everything from crates of disposable razors to individual peanut-butter jars on the store shelves, RFID technology offers the potential of "real-time supply chain visibility." Promoters of RFID technology feel [C. Humer, 2003] that "RF tags are to this decade what the Internet was to the 1990's-a promise of radical change in the way business is done." However, before the full potential of RFID technology can be realized, several hurdles need to be overcome: reliability, cost, lack of standards, and security. As these hurdles gradually diminish, Wal-Mart publicly embraces the technology.

Parametric design and analysis of multiple-beam reflector antennas for satellite communications

Abstract: This paper presents the parametric design and analysis of multiple-beam reflector-antenna systems employed for satellite communications. It is based on extending the earlier work of Rao (see IEEE Antennas and Propagation Magazine, vol.41, no.4, p.53-59,1999) by taking into account the efficiency of the horn and pointing error of the satellite in the design of the multiple-beam antennas (MBAs), and by analyzing the edge-of-coverage directivity and co-polar isolation (C/I) performance. Design and analysis equations are developed for the multiple-beam antennas using offset parabolic-reflector antennas by including various design parameters such as the number of reflectors, the number of frequency cells, the focal-length-to-diameter (F/D) ratio, the horn efficiency, and the pointing error. The analysis employs a quasi-Gaussian beam representation for the primary and secondary patterns in order to take into account the effect of the sidelobes. Results of the analysis given in this paper agree well with rigorous computations based on physical optics analysis of the antenna radiation. Design curves showing the impact of horn efficiency on the C/I performance of multiple-beam antennas are presented for various frequency-reuse schemes.

Engineering Entrepreneurship: does entrepreneurship have a role in engineering education?

Abstract: Many engineering programs have recently added courses and material on "engineering entrepreneurship". These programs represent a diverse understanding of what engineering students should receive in the way of instruction in the area of entrepreneurship. This paper examines various definitions of engineering entrepreneurship, and also examines the pedagogical justification for including entrepreneurship in engineering education. The authors use as a context the engineering programs at The University of Texas at Austin, particularly the Department of Mechanical Engineering. The paper examines educational objectives and criteria discussed in documents produced by the National Science Foundation, the American Society of Mechanical Engineering International, the Accreditation Board for Engineering and Technology, and other sources, to develop one approach for entrepreneurship education.

Limitations of the Thevenin and Norton equivalent circuits for a receiving antenna

Abstract: The investigation carried out in this paper was stimulated by a recent paper published by Love (2002), in which the appropriateness of the use of the Thevenin and Norton equivalent circuits for a receiving antenna was questioned. A review of the available literature led to the conclusion that the limitations inherent in the Thevenin and Norton equivalent circuits had not been adequately examined, and this led to the investigation that is reported on in this paper. The Thevenin and Norton equivalent circuits are useful in the reduction of the equivalent circuit for a transmitting-receiving antenna system to simpler networks that facilitate the evaluation of the received power. One finds in the literature that the calculated power dissipation within these equivalent circuits is often equated to the reradiated and scattered power from the receiving antenna. Such calculations are not correct, because power dissipation in the network from which the Thevenin and Norton equivalent circuits were obtained cannot be made using the Thevenin and Norton equivalent circuits. However, as we will show, the Thevenin and Norton equivalent circuits can be used to find a reradiated electromagnetic field that is a part of the total field scattered by a receiving antenna. As part of the derivation of this new result, we develop a derivation of the Thevenin and Norton equivalent circuits from the basic principles of uniqueness and superposition applied to electromagnetic fields.

A review of electromagnetic scattering analysis for inlets, cavities, and open ducts

Abstract: This paper is a review of all methods that have been used in the literature for the solution of a very important and challenging set of problems, namely electromagnetic scattering from inlets, cavities, and open ducts. The techniques examined have been grouped together according to their fundamental principles, so that easy and direct comparisons among them are facilitated. The complexity and difficulty of the problems are clearly inferred from the very long list of references, which includes several very recent articles. The purpose of the paper is to highlight the advantages and limitations of the techniques invoked, draw useful conclusions, and trigger initiatives for future improved approaches to the topic.

Overview of an image-based technique for predicting far-field radar cross section from near-field measurements

Abstract: For the last 18 years, our group has been developing a variety of near-field-to-far-field transformations (NFFFTs) for predicting the far-field (FF) RCS of targets from monostatic near-field (NF) measurements. The most practical and mature of these is based on the reflectivity approximation, commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is also the most computationally efficient because - despite its theoretical underpinnings - it does not explicitly require image formation as part of its implementation. This paper presents a formulation and implementation of the image-based NFFFT that is applicable to two-dimensional (2D) spherical and one-dimensional (1D) circular near-field measurement geometries, along with numerical and experimental examples of its performance. We show that the algorithm's far-field RCS pattern-prediction performance is quite good for a variety of frequencies, near-field measurement distances, and target geometries. In addition, we show that the predicted RCS statistics remain quite accurate under conditions where the predicted far-field patterns have significantly degraded due to multiple interactions and other effect.

Suitability of FDTD-based TCAD tools RF design of mobile phones

Abstract: This paper discusses the general suitability and possible limitations of an enhanced finite-difference time-domain (FDTD) simulation environment for straightforward and efficient RF design of complex transmitters. The study was conducted using a current commercially available multi-band mobile phone. Simulations were conducted in free space and with various dielectric loads, whereby different parameters were evaluated such as impedance, efficiency, far-field as well as near-field distributions of e-fields and h-fields, and the specific absorption rate (SAR). The results were compared to measurements. In addition, mechanical-design issues that showed a significant influence on the electromagnetic (EM) field behavior could be predicted by simulations and were experimentally reproduced. The accurate prediction of all essential performance parameters obtained by straightforward simulations suggests that appropriately enhanced software packages are suitable for device design in industrial research and development environments with few limitations, provided flexible graphical user interfaces (GUIs) and graded meshes combined with local grid-refinement schemes are available.

Comparison of monostatic and bistatic radar images

Abstract: Relationships between monostatic and bistatic radar images are reviewed and discussed, both from the point of view of identical receiver locations, and from the conventional point of view where the monostatic radar is located at the angular bisector between the bistatic transmitter and receiver. Example images are computed using scattered fields from two three-dimensional (3D) body-of-revolution (BOR) geometries. A Method-of-Moments solution is used to calculate scattered fields so that no scattering interactions are neglected. In the conventional comparison, sample bistatic images show direct-scattering mechanisms similar to those of monostatic images as expected, although significant differences are observed in higher-order scattering effects. With identical. receiver locations, the sample bistatic images generated are very different from the monostatic images, and illustrate the practically important fact that geometries designed to minimize monostatic scattering can produce large bistatic returns.

A discussion on the significance of geometry in determining the resonant behavior of fractal and other non-Euclidean wire antennas

Abstract: Fractal-shaped wire antennas have been shown to exhibit resonance compression and multi-band behavior that has primarily been attributed to the space-filling properties of the fractal geometry. Exhibiting a lower resonant frequency than a same-sized Euclidean antenna, fractal-shaped antennas can be made smaller than a Euclidean antenna that is resonant at the same frequency. While numerous fractal-shaped antennas - such as Minkowski Island fractal loops and Koch fractal monopoles -have been described in the literature, most discussions regarding these antennas have primarily focused on comparing their resonant behavior to those of simple Euclidean antennas. To understand the significance of any fractal geometry in determining the resonant behavior of the fractal-shaped antenna, it is also necessary to consider the other physical properties of the antenna. For the wire-loop antenna, these physical properties include the loop area, the total wire length, and the wire. diameter. For the wire monopole antenna, these physical properties include the monopole height, the total wire length, and the wire diameter. In the previous discussions of fractal-shaped antennas, there has been little or no comparison of the fractal antenna's performance properties with those of another wire antenna having a different non-Euclidean geometry, but otherwise having the same physical properties. While the antennas would have different geometries, they would have the same physical area or height, the same total wire length, and the same wire diameter. In this paper, the performance properties of the Minkowski Island fractal loop and the Koch fractal monopole are compared with those of other non-Euclidean-geometry antennas. It is demonstrated through numerical modeling and measurement that antennas having other non-Euclidean wire geometries offer similar and, in some cases, improved, performance over their fractal-antenna counterparts. From the analysis presented here, it is also demonstrated that antenna geometry alone - fractal or otherwise - is not the significant factor determining the resonant behavior of wire antennas.

Transmitting, receiving, and scattering properties of antennas

Abstract: In most modern textbooks on antennas, it is stated that receiving antennas scatter as much as they absorb under matched-load conditions. This is incorrect, as was shown 50 years ago. The reason for this discrepancy is an apparently incorrect use of the equivalent circuits. In this tutorial paper, a simple example of a two-element Yagi antenna and a test dipole in the far field is analyzed for transmission, reception, and scattering, through the use of the symmetric impedance matrix. It is shown that the correct equivalent circuit for the scattering case is more complicated, owing to the existence of more sources than in the transmitting case. Through a numerical example, it is also shown that the scattered power may be larger, equal to, or smaller than the absorbed power.

Maxwell, Hertz, the Maxwellians, and the early history of electromagnetic waves

Abstract: In 1864, Maxwell conjectured from his famous equations that light is a transverse electromagnetic wave. Maxwell's conjecture does not imply that he believed that light could be generated electromagnetically. In fact, he was silent about electromagnetic waves, and their generation and detection. It took almost a quarter of a century before Hertz discovered electromagnetic waves and his brilliant experiments confirmed Maxwell's theory. Maxwell's ideas and equations were expanded, modified, and made understandable by the efforts of Hertz, FitzGerald, Lodge, and Heaviside, the last three being referred to as the "Maxwellians." The early history of electromagnetic waves, up to the death of Hertz in 1894, is briefly discussed. The work of Hertz and the Maxwellians is briefly reviewed in the context of electromagnetic waves. It is found that historical facts do not support the views proposed by some, in the past, that Hertz's epoch-making findings and contributions were "significantly influenced by the Maxwellians.".

Pattern synthesis for TechSat21 - a distributed space-based radar system

Abstract: The TechSat21 space-based radar employs a cluster of free-floating satellites, each of which transmits its own orthogonal signal and receives all reflected signals. The satellites operate coherently at the X band. The cluster forms essentially a multielement interferometer, with a concomitantly large number of grating lobes and significant ground clutter. A novel technique for pattern synthesis in angle-frequency space is proposed, which exploits the double periodicities of the grating lobes in the angular domain and of the radar pulses in the frequency domain, and allows substantial gains in clutter suppression. Gains from 7 to 17 dB relative to the normal random, sparse array appear feasible.

Design and construction of a multipurpose wideband anechoic chamber

Abstract: An electromagnetic anechoic chamber has been constructed at the Multimedia University. It is designed to operate over a very wide frequency range from 30 MHz through 18 GHz. It can be used for electromagnetic compatibility (EMC) tests, antenna measurements, radar cross section (RCS) measurements, testing RF transceivers, calibration of scatterometers, and other electromagnetic research experiments. The geometry of the chamber is asymmetrical, consisting of a combination of rectangular and tapered volumes. The size of the chamber is about 64 ft /spl times/ 32 ft /spl times/ 24 ft high. Ease of construction and ease of lining of the absorbers were preserved so that the actual quietness is comparable to the theoretical simulation result. The transmitting source is placed at one end of the chamber, and the receiving antenna or object under test is placed in a designated quiet zone at the other end. The walls and ceilings are configured such that no first-order and or second-order reflections (except those reflected from the floor) propagate into the quiet zone. Hence, less-expensive absorbers can be used to achieve the required wideband performance. Reflection from the floor is required by the international EMC standards for radiated electromagnetic interference (EMI) emission tests; therefore, the chamber is designed for this capability. However, absorbing material can be placed on the floor to convert a semi-anechoic chamber into a fully anechoic chamber for radar and antenna measurements.

Cataracts and cell-phone radiation

Abstract: A concern that is often expressed about microwave radiation is the induction of cataracts. The formation of lens opacities in the eyes of laboratory animals following acute microwave exposure is well established. It is generally accepted that acute exposure to higher levels of CW radiation causes various degrees of lens opacification in laboratory animals at many microwave frequencies. However, the exact conditions under which these changes may occur in human beings are a subject of debate. Nevertheless, linear extrapolations of computed results indicate that the incident density required for the human eye to reach the cataractogenic threshold may only be slightly lower than that needed for rabbits.

Broadband wireless local-area networks at millimeter waves around 60 GHz

Abstract: During the past few years, research covering propagation, channel characterization, and wireless system performance has yielded a substantial knowledge of the 60 GHz channel. The unlicensed 60 GHz frequency band presents many attractive properties for wireless communications. This paper addresses some wideband propagation characteristics for broadband wireless LANs (BWLANs). Important system-design characteristics, from measured results obtained from two wideband 60 GHz LOS radio links, are presented. Measurements were undertaken using the swept-frequency channel-sounding method. Analysis from the complex frequency responses in a worst-case scenario has yielded a lower-coherence-bandwidth value of 5 MHz. Minimum and maximum B/sub 0.9/ coherence bandwidths, obtained with a directional-horn transmitting antenna and an omnidirectional receiving antenna, were 1.10 MHz and 105.33 MHz, respectively. It was observed that the coherence bandwidth fluctuated significantly with the location of the receiver with respect to the base station. These results can be used for the modeling and design of future BWLANs.

A universal matrix solver for integral-equation-based problems

Abstract: A new method has been developed for compressing the matrices that occur in most integral-equation-based computer programs. This method is easy to interface with existing computer programs, and allows them to run significantly faster and with significantly less memory. This method applies not only to electromagnetic and acoustic computation, but also to most programs involving a Green's function or any integral equation with a kernel having some smoothness properties. Our numerical computations, running on a high-end personal computer, have achieved compression ratios of fifty times, and compressed inversion of the matrices fifty times faster than by previous methods. For larger problems, solved on high-performance computers, these ratios would improve to about one thousand to one for larger moment method problems.

Equalization for antenna-pattern measurements: established technique - new application

Abstract: This paper describes a novel system that overcomes the inaccuracies in antenna radiation-pattern measurements caused by multipath propagation. The system operates by specifically correcting for the effects of unwanted signals, rather than by attempting to remove, or minimize, them through the use of screens, or baffles, or an anechoic chamber. An equalization technique is used, the parameters of the equalizer(s) being determined from a special measurement of the antenna range under consideration. The method is generally applicable; it may be implemented ab initio in new indoor or outdoor ranges, or retrofitted to existing ranges to improve accuracy. Most importantly, however, the basic idea leads to the design of a completely new type of real-time three-dimensional range, in which sensors are placed on the surface of an imaginary sphere surrounding the antenna under test (AUT), and an anechoic chamber is not required.

Pillbox antenna design for millimeter-wave base-station applications

Abstract: A pillbox antenna is a linearly polarized, waveguide-fed, cylindrical reflector, sandwiched between parallel plates. It is well suited to be a base-station antenna for the millimeter-wave, local-to-multipoint distribution service (LMDS). We can form the reflector-surface profile to provide a shaped radiation pattern, such as a cosecant-squared pattern in elevation, and we can design the radiating aperture to provide a broad-beam azimuth pattern. In this article, we begin by discussing LMDS requirements for base-station antennas, and we then briefly compare pillbox antennas with other antennas. Next, we describe in detail a highly accurate and efficient procedure for designing a pillbox antenna, using a combination of ray-tracing theory and commercially available three-dimensional electromagnetic analysis software. In particular, we discuss methods for designing the reflector surface, the waveguide feed horn, the radiating aperture, the polarizer, and the radome. Measured results of a prototype antenna at 29.4 GHz compare very well with our predictions.

Study of deployed and modular active phased-array multibeam satellite antenna

Abstract: A deployed, self-cooled and modular (tile-based) active phased array for multibeam applications has recently been proposed in the literature. This novel antenna facilitates the implementation of large array antennas in space with many beams from the same aperture, offering a flexible payload with the potential for high traffic capacity. However, the configuration also raises some questions about degradation in antenna performance, due to constraints such as non-circular aperture, frame gap, uniform amplitude excitation, and a calibration boom deployed in front of the array. Methods dealing with these issues are discussed in this paper. It is shown that a phase-only optimization technique can effectively reduce interference between same-frequency spatially reused beams, and can compensate for EIRP and C/I degradation for tile-correlated errors and failures.

EMC and BEM engineering education: physics-based modeling, hands-on training, and challenges

Abstract: Educational challenges in EMC-BEM engineering and related issues are discussed in this paper. The importance of physics-based modeling and hands-on training are emphasized. Characteristic cases in EMC tests, measurements, modeling, and simulation are presented. Finally, some suggestions and a short-course outline are given for modern EMC-BEM education.

Comment on "Limitations of the Thevenin and Norton equivalent circuits for a receiving antenna"

Abstract: The author comments that Collin (see IEEE Antennas and Propagation Magazine, vol.45, p.119-124, 2003) points out some of the attributes of the constant-power equivalent circuit for a receiving antenna. However, he disagrees with the writer's statement that the internal power dissipated in the circuit can be equated to the reradiated, or scattered, power from the antenna. A reflector antenna is not at all like the slightly lossy dielectric bodies and the perfectly conducting targets treated in the classical literature. It is a highly "lossy" structure that absorbs nearly all the power incident upon it.

Remarks on "Comments on the limitations of the Thevenin and Norton equivalent circuits for a receiving antenna"

Abstract: Collin (see IEEE Antennas and Propagation Magazine, no.45, p.119-124, 2003) remarks that Love (see IEEE Antennas and Propagation Magazine, vol.45, no.4, p.98-99, 2003) has misinterpreted his. In the paper under discussion, he did not discuss the efficiency of a receiving antenna or give any comment on the maximum efficiency a parabolic receiving antenna could have. Love's statement that it appears that Collin believes most receiving antennas cannot have an efficiency greater than 50% is speculation on his part.