Preface. 1: ELECTRICALLY SMALL ANTENNAS. 1.1 Introduction. 1.2 Fundamental Limitations. 1.3 Electrically Small Antennas: Canonical Types. 1.4 Clever Physics, but Bad Numbers. 1.5 Pathological Antennas. 1.6 ESA Summary. References. Index. 2: SUPERDIRECTIVE ANTENNAS. 2.1 History and Motivation. 2.2 Maximum Directivity. 2.3 Constrained Superdirectivity. 2.4 Bandwidth, Efficiency, and Tolerances. 2.5 Miscellaneous Superdirectivity. 2.6 Matching Circuit Loss Magnification. 2.7 Non-Foster Matching Circuits. 2.8 SD Antenna Summary. References. Index. 3: SUPERCONDUCTING ANTENNAS. 3.1 Introduction. 3.2 Superconductivity Concepts for Antenna Engineers. 3.3 Dipole, Loop, and Patch Antennas. 3.4 Phasers and Delay Lines. 3.5 SC Antenna Summary. References. Subject Index.

Electrically small, superdirective, and superconducting antennas

In this paper, the directivity and gain of several types of time-modulated linear antenna arrays are obtained. Three types of array elements are considered: arrays with isotropic elements, parallel short dipoles, and collinear short dipoles. Curves of directivity as functions of inter-element spacing and sidelobe levels (SLLs) are presented. A comparison between the computed and measured gains of a time-modulated printed dipole linear array shows reasonable agreement. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 167–171, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20241

Evaluation of directivity and gain for time‐modulated linear antenna arrays

It is noted that the advent of high-T/sub c/ superconducting materials has prompted a reexamination of the opportunities for improving antenna performance. Areas where superconductors have potential are considered, including superdirective arrays; large millimeter wavelength arrays; electrically small antennas; matching of electrically small antennas, including large transmitting antennas, and of superdirective arrays; switched line or single line phasers for electronic scanning of arrays; and traveling wave arrays where the transmission line phase velocity controls the beam angle. It is concluded that arrays above 5 GHz will benefit from low conduction, and probably also from low dielectric losses. Matching of electrically short antennas, both small (high-frequency) and large (low-frequency), and of arrays of moderate superdirectivity, will allow significant efficiency improvement. >

Superconducting antennas

Information channels supported by two orthogonal radiation modes in a pair of closely spaced dipole antennas are investigated. The radiation impedances of the two modes are closely related to the coupling behavior of the 2-element array as a function of the antenna spacing. Through a multiport impedance matching network, the common and difference modes of the coupled antenna system can be separated into different driving ports. Conjugate match can thus be independently performed for each port, resulting in a fully matched 2-element array. Such a 2-element array with closely spaced antennas can potentially offer a significant array gain or an ideal 2 times 2 multiple input multiple output channel capacity. Full-wave simulations of a pair of half-wave dipoles with 0.1 lambda spacing have demonstrated the feasibility of the approach for narrowband applications. By combining the common mode and difference mode, approximately 3 times of the gain of a single half-wave dipole can be achieved. Experimental validation with a test bed consisting of two lambda/4 monopoles and a rat-race coupler has been carried out. The concept may be extended onto closely coupled antenna systems with different numbers of elements.

Mode-Based Information Channels in Closely Coupled Dipole Pairs

Coherent multiple-input multiple-output (MIMO) radar is capable of forming a specific transmit beam pattern favorable for radar operation, but the transmitted waveforms from different antenna elements need to be orthogonal for coherent beamforming. In this work, an innovative approach is developed for MIMO radar waveform design in space-time domain, rendering MIMO radar able to satisfy transmit beamforming constraint in space-domain as well as the waveform orthogonality requirement in time domain. The approach is validated through simulations.

Mimo radar waveform design for transmit beamforming and orthogonality

For a linear uniform array of n elements, an expression is derived for the directivity as a function of the spacing and the phase constants. The cases of isotropic elements, collinear short dipoles, and parallel short dipoles are included. The formula obtained is discussed in some detail and contour diagrams of the directivity as a function of the spacing and the phase constants in the above-mentioned cases are exhibited.

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Directivity of basic linear arrays

The spherical near-field geometrical theory of diffraction (SNFGTD) method is an extended aperture method by which the near field from an antenna is computed on a spherical surface enclosing the antenna using the geometrical theory of diffraction. The far field is subsequently found by means of a spherical near-field to far-field transformation based on a spherical wave expansion of the near field. Due to the properties of the SNF-transformation, the total far field may be obtained as a sum of transformed contributions which facilitates analysis of collimated beams. It is demonstrated that the method possesses some advantages Over traditional methods of pattern prediction, but also that the accuracy of the method is determined by the quasioptical methods used to calculate the near field.

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The SNFGTD method and its accuracy

The commenter states that the technique described by M.S. Narasimhan and S. Christopher (ibid., vol.AP-32, no.1, p.13-19, 1984) of combining a spherical near-field (SNF) transformation and a near-field computation based on the geometrical theory of diffraction (GTD) is identical to the SNFGTD method originally used by F. Jensen and F.H. Larsen in 1977. Aside from this, various other criticisms are made on the presentation and on the results, specifically, the H -plane radiation pattern. In their reply, the authors note that the original paper seeks to determine accurately both the near and far fields of a paraboloid. They suggest that the editorial board review the computer programs used by themselves and by the commenter to pass a verdict on this controversy

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Comments on "A new method of analysis of the near and far fields of paraboloidal reflectors"

Definitions of scattered and diffracted fields, originally given by R. F. Millar, are reviewed and supplemented. The definitions are used to discuss relations between results obtained by commonly used pattern prediction methods for reflector antennas.

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H. Bach

Papers

Directivity of basic linear arrays

The SNFGTD method and its accuracy

Comments on "A new method of analysis of the near and far fields of paraboloidal reflectors"

A note on antennas: Definitions and methods