Phased Arrays in Radar and Communication Systems. Pattern Characteristics and Synthesis of Linear and Planar Arrays. Patterns of Nonplanar Arrays. Elements, Transmission Lines, and Feed Architectures for Phased Arrays. Summary of Element Pattern and Mutual Impedance Effects. Array Error Effects. Special Array Feeds for Limited Field of View and Wideband Arrays.

Phased Array Antenna Handbook

One property of electromagnetic waves that has been recently explored is the ability to multiplex multiple beams, such that each beam has a unique helical phase front. The amount of phase front ‘twisting’ indicates the orbital angular momentum state number, and beams with different orbital angular momentum are orthogonal. Such orbital angular momentum based multiplexing can potentially increase the system capacity and spectral efficiency of millimetre-wave wireless communication links with a single aperture pair by transmitting multiple coaxial data streams. Here we demonstrate a 32-Gbit s−1 millimetre-wave link over 2.5 metres with a spectral efficiency of ~16 bit s−1 Hz−1 using four independent orbital–angular momentum beams on each of two polarizations. All eight orbital angular momentum channels are recovered with bit-error rates below 3.8 × 10−3. In addition, we demonstrate a millimetre-wave orbital angular momentum mode demultiplexer to demultiplex four orbital angular momentum channels with crosstalk less than −12.5 dB and show an 8-Gbit s−1 link containing two orbital angular momentum beams on each of two polarizations. High speed data transmission using orbital angular momentum beams has been recently demonstrated. Here, Yan et al. demonstrate a 32 Gbit/s millimetre-wave communication link using eight coaxially propagating independent orbital angular momentum beams with four orbital angular momentum states on two orthogonal polarizations.

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High-capacity millimetre-wave communications with orbital angular momentum multiplexing

Explains set theoretic estimation, which is governed by the notion of feasibility and produces solutions whose sole property is to be consistent with all information arising from the observed data and a priori knowledge. Each piece of information is associated with a set in the solution space, and the intersection of these sets, the feasibility set, represents the acceptable solutions. The practical use of the set theoretic framework stems from the existence of efficient techniques for finding these solutions. Many scattered problems in systems science and signal processing have been approached in set theoretic terms over the past three decades. The author synthesizes a single, general framework from these various approaches, examines its fundamental philosophy, goals, and analytical techniques, and relates it to conventional methods. >

The foundations of set theoretic estimation

Coaxial line reflection methods for measuring dielectric properties of biological materials at radio (RF) and microwave (MF) frequencies are reviewed and compared from the point of view of their relative uncertainties of measurement of lossy substances with high dielectric constant. Advantages and limitations of different methods and some practical recommendations are presented.

Coaxial Line Reflection Methods for Measuring Dielectric Properties of Biological Substances at Radio and Microwave Frequencies-A Review

Microwaves and millimeter waves have been used extensively to image dielectric bodies. The application of microwaves in biomedical imaging and diagnostics, however, remains a field with many uncharted territories. This article is an overview on medical imaging using microwave imaging for breast cancer and its challenges, hopes, and outlook.

Microwave Imaging for Breast Cancer

It is shown that the electromagnetic (EM) field, radiated or scattered by bounded sources, can be accurately represented over a substantially arbitrary surface by a finite number of samples even when the observation domain is unbounded. The number of required samples is nonredundant and essentially coincident with the number of degrees of freedom of the field. This result relies on the extraction of a proper phase factor from the field expression and on the use of appropriate coordinates to parameterize the domain. It is demonstrated that the number of degrees of freedom is independent of the observation domain and depends only on the source geometry. The case of spheroidal sources and observation domains with rotational symmetry is analyzed in detail and the particular cases of spherical and planar sources are explicitly considered. For these geometries, precise and fast sampling algorithms of central type are presented, which allow an efficient recovery of EM fields from a nonredundant finite number of samples. Such algorithms are stable with respect to random errors affecting the data.

Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples

Starting from the observation that fields differing less than a prescribed error cannot be resolved as distinct entities, the degrees of freedom of the scattered field are introduced and then computed. The degrees of freedom are shown to be practically equal to the Nyquist number appropriate to the effective (spatial) bandwidth of the scattered field and to the extension of the observation domain. Accordingly, a finite number of elements of information can be used to determine the scattered field to a prescribed approximation error. It is also shown that the field representation can be made in terms of field values and simple sampling functions, provided that a marginal increase in the approximation error is tolerated. The results not only completely justify the use of sampling interpolation for representing scattering fields, but also demonstrate that such representation is practically an optimal one. An algorithm for the reconstruction of scattered fields, given the maximum allowed error, is then produced. >

On the degrees of freedom of scattered fields

It is shown that the scattered fields are almost space bandlimited functions. The effective bandwidth W is introduced and evaluated for a very general scattering system, as well as the error made using functions bandlimited to w > W for representing the scattered field. The effective bandwidth is very simply related to the maximum dimension of the scattering system; the error drops to negligible values for modest increases of w compared to W , in the case of large scatterers. Important consequences of the above general results are finally stressed.

On the spatial bandwidth of scattered fields

With reference to inverse scattering from an unknown object of limited extension embedded in a homogeneous background at a fixed frequency, we show that only a finite-dimensional representation of the unknown contrast can be hopefully retrieved. Exploiting the quasi-band-limitedness property of scattered fields, an accurate upper bound to the dimension of such a space is evaluated in both the single incidence and multiview cases. Moreover, effective schemes are given to collect all the information available from the scattering experiments in a nonredundant manner. As a by-product, an optimal (minimally redundant) sampling strategy for the monostatic radar cross section is also provided. Finally, we briefly discuss how the requirement for a globally effective and reliable solution scheme can lead to a reduction of the actually retrievable information.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97RS01826

Electromagnetic inverse scattering: Retrievable information and measurement strategies

The array synthesis problem is of utmost importance and therefore a large number of papers have been devoted to it. A new synthesis method based on the intersection approach is presented: the synthesis is viewed as the problem of finding the intersection of sets. This is a more natural, flexible and effective formulation of the problem but despite this it has not previously received much attention. The search for a point belonging to the intersection of two sets allows a very effective solution procedure, based on the projector concept and on a suitable choice of the starting point of the iteration. A number of one and two dimensional examples are presented to highlight the various aspects of the method. Emphasis is put on constrained array synthesis which is important when the practical realisation of the synthesised array is of concern. The method permits the synthesis of constrained patterns without appreciable degradation of the pattern desired.

Ovidio Mario Bucci

Papers

Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples

On the degrees of freedom of scattered fields

On the spatial bandwidth of scattered fields

Electromagnetic inverse scattering: Retrievable information and measurement strategies

Intersection approach to array pattern synthesis