Principles of Planar NearField Antenna Measurements
This single volume provides a comprehensive introduction and explanation of both the theory and practice of 'Planar NearField Antenna Measurement' from its basic postulates and assumptions, to the intricacies of its deployment in complex and demanding measurement scenarios.
Inspec keywords: Maxwell equations; planar antennas; electromagnetic wave propagation
Other keywords: electromagnetic wave propagation; planar nearfield antenna measurement; Maxwell equation
Subjects: Antennas; Electromagnetic wave propagation; Antenna theory
 Book DOI: 10.1049/PBEW053E
 Chapter DOI: 10.1049/PBEW053E
 ISBN: 9780863417368
 eISBN: 9780863413339
 Page count: 424
 Format: PDF

Front Matter
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1 Introduction
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In this article, this volume has been penned for a very specific purpose; to explain clearly, concisely and in an understandable form the theory and practice of planar nearfield antenna measurements. Again, as stated in the preface, to do this the volume will confine itself to considering the radiative coupling between electronic systems in free space for a number of very sound reasons.

2 Maxwell's equations and electromagnetic wave propagation
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This chapter dealt with a theory that describes propagation through free space as a process of the propagation of EM waves, these waves being directly related to the acceleration of charged particles. A description of these waves is then provided based on Maxwell's equations and the derivation of the scalar Helmholtz equations. Then a description of the sources of these waves as being retarded potentials that then produce fields was provided. Although the basis of this explanation is charge and current densities on the geometrical structure that constitutes an antenna, direct measurement methods to assess these sources are not viable, therefore an alternative equivalent fields model was developed which in turn suggested other measurement methodologies.

3 Introduction to nearfield antenna measurements
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In this chapter, the practical aspects of measuring the nearfield of an antenna in terms of scanning and RF subsystems is discussed.the farfield radiation pattern is characterised by spatial amplitude variation, spatial phase variation, spatial polarisation variation.

4 Plane wave spectrum representation of electromagnetic waves
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The theoretical development of planar nearfield antenna measurements is usually based on this plane wave spectrum (PWS) representation of electromagnetic (EM) fields. This generalized interpretation can be shown to stem from the freespace solution of the scalar wave equation, which itself follows directly from classical EM theory and Maxwell's equations where the four Maxwell equations are postulated, mathematical generalizations of a great many macroscopic experimental observations of electricity and magnetism. The validity of these phenomenological physical laws is attested to by the extraordinarily good agreement attained between measurement and prediction.

5 Measurements  practicalities of planar nearfield antenna measurements
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In Chapter 4 we presented a detailed derivation of the coordinate free form of the near field to farfield transform employing the plane wave spectrum representation. We now take this result and consider the various practical issues needed to transform this into a viable planar nearfield measurement process. In Chapter 6 we will consider the auxiliary issues of probe pattern characterisation and in this chapter we will address the remaining issues.

6 Probe pattern characterisation
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As shown in Chapter 5. for plane rectilinear nearfield scanning, probe pattern characterisation errors contribute to the overall facility error budget as a singular mapping. Essentially then, this means that an error in the probe pattern at a particular direction in space will correspond to a similar error at that angle being introduced into any antenna pattern that has been corrected with this data. Furthermore, this can potentially constitute one of the largest and most repeatable, measurement uncertainties. Thus it is clear that in order to obtain reliable measurements, the electromagnetic (EM) properties of the nearfield probe must be known very accurately indeed. Throughout this work, it has been assumed that relative measurements are taken, that is, without reference to absolute gain. If an absolute gain is required, then it is assumed that a substitution method is used employing a calibrated gain standard in order that the measurements can be correctly normalized. The importance of this for the characterisation of the nearfield probe is that if the same probe and probe pattern used to correct the measurement of the antennas under test (AUTs) and the standard gain horn (SGH) then the gain of the probe will be unimportant. Thus, within this chapter, characterisation of the gain of the probe is not discussed and the gain of the probe will be normalized to unity for convenience. This chapter briefly describes the properties that are desirable in a nearfield probe before proceeding to describe methods for obtaining and then using reliable probe pattern data.

7 Computational electromagnetic model of a planar nearfield measurement process
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Within this chapter a number of simulation techniques have been developed, each offering a different balance of sophistication and effort. Ultimately, it would perhaps be preferable to utilize a full wave threedimensional EM solver to tackle these problems and with the passage of time this is becoming an ever more feasible option.However, until these methods can be deployed on a sufficiently large scale and can provide results in a sufficiently compact timescale, other alternative solutions will remain attractive. For many applications, the comparatively simple vector Huygens' method is sufficient and indeed it was used in the preparation of many of the sim ulated data sets that are utilized within Chapter 9 to illustrate and verify some of the more advanced transformation and correction techniques. Unfortunately, it is not possible to use these techniques to model every phenomenon that can be observed in a nearfield measurement system, for example, multiple reflections between the AUT and the probe, but there is perhaps sufficient choice detailed that many situations can be accommodated.

8 Antenna measurement analysis and assessment
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The concept of measurement is interpreted differently in different sciences and there fore by definition in different areas of engineering and technology. As described in Chapter 1 the information extraction model is a particularly applicable concept for the cognitive evaluation of antenna measurements. In nearfield scanning we are primarily concerned with microwave frequencies and as millions of years of human evolution have left our species without sense organs that respond to such frequencies, sensory cognition is largely irrelevant, leaving only rational cognition as a tool for the evaluation of microwave phenomena.

9 Advanced planar nearfield antenna measurements
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This chapter presents a brief introduction to a number of the more advanced, and more recently developed topics associated with planar nearfield antenna measurements. These include (1) active alignment correction which seeks to improve the accuracy with which the boresight direction of an antenna is known, (2) position correction techniques for improving the flatness and straightness of the sampling grid, (3) phase recover which enables measurements to be made, where obtaining a direct phase reference would be impractical, (4) microwave holographic metrology (MHM) which is used for performing aperture diagnostics, (5) auxiliary translation and auxiliary rotation which are used to minimise truncation and (6) the polyplanar technique that is used to mitigate measurement truncation.

Appendix A: Other theories of interaction
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This list and very short description of postulated alternative mechanisms of interaction is not meant to be exhaustive or comprehensive. It is included to show that a wide range of interpretations of the interaction of charged particles across space and time are available. All of them can appropriately be used over a range of applicable circumstances and they all fail outside their range of applicability. Certain of them lend themselves to more than one formal structure, for example, it is possible to describe classical electromagnetism in terms of the Maxwell tensor in a fourvector relativistic space time and many of them fit more or less neatly into more over arching concepts for example, local gauge invariant field theories or Fisher information exchange concepts. However, the material in this overall text has been written so that while it is specific to classical electromagnetism, as this is anticipated to be the most widely used model recognized by the target readership, none of the above postulated mechanisms are inconsistent with the material in the book.

Appendix B: Measurement definitions as used in the text
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Presented are the list of measurement definitions as used in the text.

Appendix C: An overview of coordinate systems
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Within this text reference is made to a great many coordinate systems and the transformations between them. Implicit within this is the assumption that the tabulating grids are plaid, monotonic and equally spaced. Whilst not necessary from a theoretical stand point these conditions greatly simplify the recording process for a robotic positioner as well as simplifying the tasks of numerical integration, differentiation and interpolation. The following section presents a concise description of the most important coordinate systems and then goes on to discuss methods for representing the relationships between them like antenna mechanical system (AMS), antenna electrical system (AES), farfield plotting systems, direction cosine, azimuth over elevation, elevation over azimuth, elevation over azimuth, polar spherical, azimuth and elevation (trueview), range of spherical angles, transformation between coordinate systems, coordinate systems and elemental solid angles, relationship between coordinate systems, azimuth, elevation and roll angles, euler angles, quaternion, elemental solid angle for a trueview coordinate system.

Appendix D: Trapezoidal discrete Fourier transform
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An equation is evaluated numerically using the conventional discrete Fourier transform (DFT), the fast Fourier transform (FFT) and the trapezoidal transforms.

Appendix E: Calculating the semimajor axis, semiminor axis and tilt angle of a rotated ellipse
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This appendix discusses the calculation of semimajor axis, semiminor axis and tilt angle of a rotated ellipse.

Back Matter
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