EuCAP 2006 - European Conference on Antennas & Propagation

 
Session: Session 5A07A - Conformal Antennas (15g)
Type: Oral Antenna
Date: Friday, November 10, 2006
Time: 08:30 - 12:20
Room: Gallieni 2
Chair: Sipus & Persson
Co-chair:
Remarks:


Seq   Time   Title   Abs No
 
1   08:30   Development of a Hemi-Spherical Wideband Antenna Array for Breast Cancer Imaging
Craddock, I.1; Preece, A.1; Leendertz, J.1; Klemm, M.1; Benjamin, R.1; Nilavalan, R.2
1University Of Bristol, UNITED KINGDOM;
2Brunel University, UNITED KINGDOM

Breast cancer is the most common cancer in women. X-ray mammography is currently the most effective detection technique, however it suffers from a relatively high missed- and false-detection rates, involves uncomfortable compression of the breast and also entails exposure to ionizing radiation.

Microwave detection of breast tumours is a potential non-ionising alternative being investigated by a number of groups. In these microwave-based systems, in a similar fashion to Ground Penetrating Radars, microwaves are transmitted from an antenna or antenna array, and the received signals, which contain reflections from tumours, are recorded and analysed.

A pre-requisite for all of these systems is a suitable antenna array. Recent work by the authors has included the development of a planar stacked-patch array for phantom measurements. In the quest for greater experimental realism, a curved breast phantom has recently been developed along with an approximately hemi-spherical conformal array.

This contribution presents, for the first time, details of the conformal array design and initial results from this unique experimental imaging system.

 
 
2   08:50   Closed-Form Green's Functions in Cylindrically Stratified Media for Method of Moments Applications
Karan, S.; Erturk, V.B.; Altintas, A.
Bilkent University, TURKEY

There has been a wide range of military and commercial applications for microstrip antennas/arrays, and/or various type printed structures that can conform to curved host platforms. Among them, cylindrically curved geometries are one of the most important types since they may sufficiently model the geometry of many complicated applications. Besides, they may be treated as the canonical geometries, whose solutions are regarded as the building blocks toward the development of analysis tools for arbitrarily convex curved bodies. As a result, rigorous analysis of such structures becomes crucial. However, as the number of layers deposited on the host platform increases, rigorous investigation of these structures becomes quite challenging.

The method of moments (MoM) based solutions are widely used for the rigorous analysis of planar layered printed structures. However, available tools for their curved counterparts are limited. Furthermore,majority of them suffer from the efficiency and accuracy problems due to the difficulties in the evaluation of the Green's function representation (both in spatial and spectral domains), which is the kernel of the integral equation.

The use of closed-form Green's function representations in conjunction with the MoM has been a good candidate in the design and rigorous analysis of many printed antennas/arrays and other geometries embedded in an arbitrary layer of a cylindrically stratified media. Therefore, in this paper closed-form Green's function representations in cylindrically stratified media are developed to be used in conjunction with MoM. The Green's function for cylindrically stratified media involves a summation over the cylindrical eigenmodes, and a Fourier integral. As the first step, the summation over the cylindrical eigenmodes is evaluated for all possible field and source points. Note that this summation is very slowly convergent when both the field and source points lie on the same interface, which is the case for all MoM applications. Therefore, several techniques are performed to sum the cylindrical eigenmodes efficiently and accurately. Once this summation is performed, the discrete complex image method (DCIM) is applied to the Fourier integral to obtain closed-form solutions with the help of generalized pencil of function (GPOF) method. Unlike the previous works on this subject, when the field and source points lie on the same interface along the axial line, an accurate expression for the closed-form Green's function representation is obtained in this paper. Furthermore, the present method can work even for fairly large coated cylinders.

Numerical examples will be presented in the form of mutual coupling between arbitrarily oriented current modes for all previously problematic cases reported in the literature. Moreover, the current distribution on several printed geometries as well as the input impedance and mutual impedance of microstrip antennas will be presented.

 
 
3   09:10   Efficient UTD Representation for the Collective Radiation and Surface Fields Produced by Large Conformal arrays on Convex Surfaces
Janpugdee, P.; Burkholder, R.J.; Pathak, P.H.
The ElectroScience Laboratory, The Ohio State University, UNITED STATES

A collective description of the near and far fields, as well as surface fields, that are radiated by a large array conformal to a very large smooth, arbitrary convex metallic surface is obtained in terms of just a few uniform geometrical theory of diffraction (UTD) rays. The conformal array may consist simply of many slots in the convex surface, or may consist of a large number of complex array elements embedded in a multi-layered medium with a radome, etc., thus creating a large array window that is flush with the perfectly conducting convex surface. The presence of the aperture/window formed by the array configuration is replaced by equivalent magnetic surface current sources that reside over the flush array aperture surface, but with the array aperture now closed by a perfect conductor via a well-known electromagnetic surface equivalence theorem. In the conventional UTD analysis of conformal arrays, the equivalent surface source distribution is quantized into elemental sources, and each elemental source then launches radiation and surface fields according to the UTD ray solution obtained previously by Pathak et al. (P. H. Pathak et al., IEEE Trans. AP., AP-29, pp. 609-622, Jul. 1981; P. H. Pathak et al., IEEE Trans. AP., AP-29, pp. 911-922, Nov. 1981). For a large aperture window, the number of such elemental sources can be extremely large (in the thousands, or even tens of thousands) and so this conventional UTD procedure becomes intractable because a correspondingly large number of rays launched radially out from each elemental source have to be traced over the convex surface. In contrast, the present approach directly provides a collective UTD radiation and surface fields in terms of only a few rays emanating from some interior and boundary points on the "entire" array aperture/window. Hence, the present collective UTD approach is highly efficient as compared to the conventional UTD. In most practical applications, the large array is situated conformally on an even larger platform, e.g. an aircraft, a spacecraft, or a naval ship, etc. In these realistic cases, the very large platform may be modeled efficiently by the UTD, while the array may be modeled locally by numerical (FEM, FE-BI, etc.) methods. The equivalent magnetic surface currents can be obtained from the numerical solution to the local array problem. Subsequently, the coupling of the array fields to the external platform is then achieved by the present collective UTD analysis which launches the collective ray fields from the array aperture/window to then interact with the platform. Such a hybrid numerical and collective UTD method provides a useful, tractable and relatively efficient analysis of large arrays on a very large platform; whereas conventional numerical methods can become rapidly intractable to solve the whole multi-scale large array plus platform problem at once. Numerical examples illustrating this hybrid approach will be presented for conformal arrays on fairly general convex shapes.

 
 
4   09:30   TTC Patch Antennas Made in a Conformal Form with Small Radius
Kabacik, P.; Opalka, P.; Hornik, P.
Wroclaw University of Technology, POLAND

In this paper we will demonstrate some results of research carried out on cylindrical and spherical antenna arrays. Investigated arrays have a radius of about half a wavelength, consist of up to four antenna elements and are to operate onboard spacecraft at the TTC communication and some intersatellite links at the S-band. Mass scale applications of the developed technology cover wireless access nodes, transportation systems, remote monitoring and identification. In our studies we analyzed dozen main concepts of configurations made with two to four patches arranged on small cylinders or on a hemispherical ground. Method of Moment numerical tools were applied CONCEPT and CYMPHA (developed at Univ. of Hamburg and Univ. of Zagreb respectively). Patches are up to 12 mm thick and their surface conform to the ground. The objective of the study was to analyze properties of a combined radiation pattern. The optimization aim was to minimize depths of nulls or to eliminate nulls at all. Accomplishment of this goal is further complicated by complexity of wave polarization in arrays comprising differently pointed antenna elements. One example of our results is plotted in Fig. 1. In a case of the spherical ground, patches have circular or donut shape. In presented experimental work, circular patches placed on the hemispherical ground are shaped as the conical Chinese straw hat (well suited for manufacture). Measured impedance properties are consisted with computed electrical properties by the Method of Moment (see plots in Fig. 2). Tolerance sensitivity analysis have showed that maintaining of geometrical dimensions is more important in spherical than in cylindrical antenna shapes.

Fig. 1. (a) spacecraft model and (b) calculated radiation pattern for one configuration of three element conformal array located in a top corner of a 'washing machine' size spacecraft and fourth element located on the cut off corner.

Fig. 2. Calculated (a) and measured (b) return loss of a patch shaped as the conical Chinese straw hat and placed atop the hemispherical ground.

 
 
5   09:50   Design and Development of a Spherical Array Antenna
De Witte, E.; Marantis, L.; Brennan, P.; Griffiths, H.
University College London, UNITED KINGDOM

Phased Array Antennas made it possible to develop smart, adaptive and reconfigurable antenna systems. High-tech Radar systems are seeing a transition from mechanically scanned antennas to antenna arrays with electronic beam-steering. For applications where a full azimuthal antenna coverage is required, circular antenna arrays have been deployed. Modal signal processing has emerged to exploit the symmetry of the circular array. In applications where full omni-directional coverage in both azimuth and elevation is required, such as LEO satellite communications, the natural extension of the circular array is the spherical array antenna. Only a limited number of spherical array antenna systems have been developed, and none of them have exploited modal processing in the way it is done for circular arrays.

Over the past 4 years, the capabilities and limitations of spherical array antennas have been studied at University College London. The project was started after a case-study on an antenna upgrade for the PROBA satellite ground segment showed that existing spherical array antenna designs could not offer an affordable solution for this kind of systems.

The studies have culminated in the design and development of LISA, a large spherical array antenna. In this paper, the design and development aspects of LISA will be described and the underlying theoretical principles will be briefly reviewed. UCL has acquired a spherical near-field antenna test facility to support the development of LISA and other antenna systems. The results and conclusions from these measurements will also be presented on the conference.

 
 
6   10:40   ITD Solution for a Canonical Geometry of Interest to Conformal Antennas
Albani, M.; Maci, S.
University of Siena, ITALY

The description of edge diffraction mechanisms in truncated structures can be approached both by ray techniques (i.e., Uniform Theory of Diffraction, UTD) or by incremental techniques. The incremental techniques like the Physical Theory of Diffraction (PTD) or the Incremental Theory of Diffraction (ITD) can overcome the impairment of the ray methods for observer close to the ray caustics. These techniques can be applied by using two different schemes. In the first scheme, the field is described in terms of Geometrical Optics (GO) plus incremental diffraction (ID) integration (let us denote this scheme as GO-ID). In the second scheme, the field is described in terms of Physical Optics (PO) plus incremental fringe (IF) contribution integration (PO-IF). In the GO-ID approach, the ID contributions are distributed and integrated along the edge discontinuities in order to reconstruct a diffracted field which augment the dominant GO contribution. When the ray regime is applicable, the ID integration recovers the diffraction contribution as predicted by UTD. The ID contributions exhibits singularities that, when integrated along the edge, produces discontinuities at the shadow-boundaries associated with the locally tangent canonical wedge; this discontinuities compensate for the opposite GO discontinuity leading a continuous field. Within this GO-ID framework, the ITD ID contributions have been demonstrated to provide excellent results for edges in planar surfaces. PTD cannot be used in this framework, since the singularities of the PTD total incremental contributions do not reproduces, when integrated, discontinuities at the appropriate shadow boundaries. This impairment is essentially due to the fact that the available closed-form PTD formulas are constructed by a plane wave (in place of a spherical wave) incidence. On the other hand, the PO-IF method can be applied successfully within both the ITD and the PTD framework. Indeed, the fringe contribution is constructed by subtracting from the total incremental diffraction contribution the PO end-point incremental contribution, both of them formulated on the basis of the same canonical wedge problem. Indeed, the wrong singularities associated to the total PTD edge waves are cancelled by the singularities associated to the incremental end-point PO contributions. Both PTD and ITD provides very similar results in the PO-IF scheme.

When dealing with curved surfaces, the PO-IF scheme is still applicable in both PTD and ITD framework. Although questionable in principle, the fringe correction to PO can be indeed applied still using the formulas relevant to the local wedge problem with flat faces. However, within the ID-GO scheme, curved surfaces cannot be treated neither with ITD nor with PTD. The purpose of this paper is to improve the ITD for application in the ID-GO scheme. Maintaining the ID-GO option available is important since many high-frequency software codes (especially in the reflector antenna framework) allows the UTD plus GO ray description, and then possess a GO ray-tracer. The formulation presented here, which leaves a part the creeping wave description, is based on a higher order asymptotic correction of the total ID first-order contribution provided by ITD for a wedge with planar surfaces. The solution is heuristically based on the analogous higher order contribution obtained by the PO-end point. Numerical results will be presented to show the accuracy of the formulation.

 
 
7   11:00   Multi-Polarized High-Gain Omni Directional Arrays
Herscovici, N.1; Sipus, Z.2
1Chelton Microwave Corp., UNITED STATES;
2University of Zagreb, CROATIA

Meeting the diverse demands and challenges of today wireless industry requires new types of antennas where improvements in radiation characteristics over increasingly, larger bandwidths can be obtained. In recent years, the need for high gain, multipolarized, omnidirectional (in the azimuth plane) antennas increased considerably. These types of requirements are often met in military as well as in commercial applications.

However, to date, not to many configurations that attempt to meet these of challenges, are shown in the open literature.

This paper proposes a novel antenna which in its final configuration, is omnidirectional, easily arrayable (to generate narrow beams in elevation) and capable of multiple polarizations.

The basic configuration consists of a patch that is wrapped around a cylinder which is a dielectric coated conductor. If the cylinder has a radius smaller than half of the wavelength, the radiation pattern in azimuth is omnidirectional. Feeding the patch appropriately, like in the planar case, one can generate any polarization. The same techniques for broadband that are used in the planar case are applicable for the cylindrical case. The only challenge is exciting the appropriate modes in the cylindrical plane which have different radiation patterns then the ones in axial plane.

A full-wave spectral domain analysis for multilayered structure based on the G1DMULT algorithm [2] is presented as the electromagnetic engine used to optimize the design. The paper describes the development of this type of antenna from its simple form (vertical polarization) [1] to its most complex variation, the circularly polarized, single- and dual fed element.

References:

[1] N. Herscovici, Z. Sipus, P.-S. Kildal, "The cylindrical omnidirectional patch antenna," IEEE Transactions on Antennas and Propagation, Vol. 49, pp. 1746-1753, Dec. 2001.
[2] S. Raffelli, Z. Sipus, P.-S. Kildal, "Analysis and measurements of conformal patch array antennas on multilayer circular cylinder," IEEE Transactions on Antennas and Propagation, Vol. 53, pp. 1105-1113, March 2005.

 
 
8   11:20   Synthesis of a Large Conical Array
Guy, R.
BAE SYSTEMS ATC, UNITED KINGDOM

Conformal Arrays offer many potential advantages over planar arrays, such as maintaining beam gain over wide fields of view and the possibility, of better integration into a platform with a predefined shape. For example, Airborne Intercept (AI) Radars have traditionally used planar arrays. However their field of view is restricted to a semi-cone scan angle of about 60°, and the beam gain falls by about 3dB at edge of scan. Where as, conformal arrays such as the sphere, cylinder or cone can be designed to scan and maintain beam gain over angles greater than the forward hemisphere.

Beam scanning in a planar array is relatively straight forward compared to a conformal array. In a planar array, long established aperture distributions can be used to provide low sidelobe sum and difference patterns. By providing the appropriate planar phase front, the beam can be scanned using these predetermined, fixed amplitude distributions. The polarization state of the elements can also remain fixed for all scan angles.

In the conformal array, both the amplitude and phase must change in order to maintain the pattern shape with scan. To scan over a large field of view the amplitude distribution must migrate across the array to illuminate the elements towards the direction of propagation. In order to maintain a given polarization state in the scan direction the individual element polarizations must be controlled, (except for the special case of the cylindrical array).

A large conical array capable of providing full hemispherical coverage is used as an example to illustrate the beam forming properties of a conformal array and make comparisons with a multifaceted planar array. Many alternative radiation pattern synthesis techniques can be used for conformal array synthesis. The techniques have been reviewed and the method of successive projections chosen as the most suitable for synthesis of a large conformal array scanning over a large field of view.

The array consists of cross dipole elements mounted above a conical ground plane. A method has been derived for positioning elements onto the conical array, preserving a regular planar array equilateral triangular lattice, [1]. The whole array, including mutual coupling effects, was modeled using the method of moments. The ability to form low sidelobe sum and difference patterns over the entire forward hemisphere has been demonstrated.

The attributes of the conical array and a multifaceted array designed to have similar performance are compared.

[1] R.F.E.Guy, 'Method of Obtaining a Regular Element or Beam Conformal Array Lattice applied to a Luneburg Lens with a Concave Spherical Array Feed', 4th European Workshop on Conformal Antennas, Stockholm, Sweden, May 23-24, 2005.

 
 
9   11:40   Design of a Triple Patch Antenna Element for Double Curved Conformal Antenna Arrays
Knott, P.
FGAN-FHR, GERMANY

Conformal antenna arrays fitted to the surface of a non-planar part of modern aircrafts, vehi-cles or ships are considered an attractive alternative for certain applications where planar ar-rays or reflector antennas have definite drawbacks. Some of the potential advantages are im-proved aerodynamics, increased payload, large field of view (LFOV) and low observability (LO). However, the usage of conformal array technology in commercial applications is still comparably rare. Recent examples of antenna arrays on curved apertures are typically of cir-cular cylindrical shape and mostly exhibit a weak degree of curvature only [1].

The number of antenna elements on a planar or curved antenna aperture required for high di-rectivity and two-dimensional beam steering can easily become very large, especially if an element spacing around half the free space wave length is required. The proposed paper de-scribes the design of a sub-array structure carrying 3 circularly polarized patch antennas and a phase corrected feed network on a common substrate that can be seen as a trade-off between antenna gain vs. beam granularity.

The aim of the triple patch antenna sub-array is to facilitate integration into a double curved aperture geometry and to reduce the number of required array channels at the same time. The paper also includes detail on the development of a spherical antenna array designed at a centre frequency of f = 9.4 GHz with 95 sub-arrays, a diameter of 300 mm and an opening angle of 63°. The array which is currently being built as antenna front end for a conformal antenna array demonstrator [2] shall demonstrate two-dimensional beam scanning with full hemispherical coverage. Numerical and Experimental results on antenna matching and radiated far field patterns will be presented as well as simulated beam forming results.

References

[1] P. Knott, "Conformal Antenna Arrays - Design and Technology for Military Applica-tions", Military Sensing Symposium (MSS), Dresden, Germany, October 2004.

[2] H. Gniss, "Digital RX-only Conformal Array Demonstrator", International Radar Sym-posium (IRS), DGON, Munich, Germany, pp. 1003-1012, September 1998

 
 
10   12:00   Hybrid Spectral Domain-UTD Method Applied to Conformal Antennas Analysis
Bosiljevac, M.1; Persson, P.2; Sipus, Z.1
1Faculty of Electrical Engineering and Computing, Zagreb, CROATIA;
2Royal Institute of Technology, Stockholm, SWEDEN

This paper presents a hybrid spectral domain - UTD method applied to the analysis of various conformal antennas (UTD - Uniform Theory of Diffraction). The research of this method has been made possible and is a part of the ACE program for structuring the research on conformal antennas in Europe. The developed method is a result of joining research activities of University of Zagreb, Croatia and KTH, Sweden. The basic idea behind the hybrid method is to combine different analysis methods for conformal antennas and, at the same time, preserve the advantages of the considered methods.

The usual approach to the analysis of circular-cylindrical and spherical structures is to use a modal expansion approach together with the moment method, i.e. to transform the three-dimensional problem into a spectrum of one-dimensional problems. This is done by performing a two-dimensional (2D) Fourier transformation in the coordinates for which the structure is homogeneous. In the cylindrical case, we perform the Fourier transformation in the axial direction and the Fourier series in the phi direction, and in the spherical case we perform the vector-Legendre transformation. The goal with the developed hybrid method is to reduce the number of terms in the Fourier series or vector-Legendre series and to reduce the length of integration in the Fourier transformation. The basic idea is to subtract the asymptotic part of the Greens function and to calculate the asymptotic part using UTD. Hence, we will use the advantages of the spectral domain method (possibility of analyzing multilayer structures) and of the UTD (possibility of analyzing electrically large structures). This will generate a code possible to analyze large multilayered conformal structures at high accuracy and with great speed.

The abilities of the hybrid spectral domain - UTD method will be demonstrated on different types of conformal arrays of waveguide elements. First, multilayer circular-cylindrical structures will be analyzed, followed by the extension of this approach to the analysis of cylinders with elliptical cross-section and spherical structures. The accuracy of this approach will be verified through the comparison of the results with rigorous spectral method results and measurements. As an example of a waveguide array on a circular-cylinder we have considered an 18x3 array built at Ericsson Microwave Systems AB, Mlndal, Sweden. Figure 1. shows mutual coupling results for the first row of waveguide elements when all array elements are present and terminated with their characteristic impedance. Results show very good agreement with spectral method results and measurements.

Figure 1. The amplitude of the mutual coupling in the E-plane for the 18x3 array of waveguide elements.