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 near-field 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.
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.
near field antenna measurements pdf free
In this chapter, the practical aspects of measuring the near-field of an antenna in terms of scanning and RF subsystems is discussed.the far-field radiation pattern is characterised by spatial amplitude variation, spatial phase variation, spatial polarisation variation.
The theoretical development of planar near-field 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 free-space 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.
In Chapter 4 we presented a detailed derivation of the coordinate free form of the near field to far-field 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 near-field 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.
As shown in Chapter 5. for plane rectilinear near-field 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 near-field 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 near-field 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 near-field probe before proceeding to describe methods for obtaining and then using reliable probe pattern data.
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 three-dimensional 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 near-field 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.
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 near-field 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.
This chapter presents a brief introduction to a number of the more advanced, and more recently developed topics associated with planar near-field 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 poly-planar technique that is used to mitigate measurement truncation.
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), far-field plotting systems, direction cosine, azimuth over elevation, elevation over azimuth, elevation over azimuth, polar spherical, azimuth and elevation (true-view), 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 true-view coordinate system.
Networks of sensors placed on the skin can provide continuous measurement of human physiological signals for applications in clinical diagnostics, athletics and human-machine interfaces. Wireless and battery-free sensors are particularly desirable for reliable long-term monitoring, but current approaches for achieving this mode of operation rely on near-field technologies that require close proximity (at most a few centimetres) between each sensor and a wireless readout device. Here, we report near-field-enabled clothing capable of establishing wireless power and data connectivity between multiple distant points around the body to create a network of battery-free sensors interconnected by proximity to functional textile patterns. Using computer-controlled embroidery of conductive threads, we integrate clothing with near-field-responsive patterns that are completely fabric-based and free of fragile silicon components. We demonstrate the utility of the networked system for real-time, multi-node measurement of spinal posture as well as continuous sensing of temperature and gait during exercise.
Wireless technologies can be used to connect wearable sensors without physical constraints. In particular, radio-based wireless communication methods, such as Bluetooth and Wi-Fi, are widely used to enable sensors to wirelessly communicate around the body for monitoring health and providing real-time clinical notifications11,17,18,19,20,21,22,23,24. Unlike wired interconnects, however, these radio-based technologies require each sensor to be separately powered, typically using rigid batteries or bulky energy harvesters. These components limit the degree of skin conformability and user comfort that can be achieved, and require periodic replacement or availability of specialised energy sources for long-term function25,26,27,28. In addition, the radiative nature of data transmission results in vulnerabilities to eavesdropping and necessitates the use of cryptography techniques to address privacy concerns15,16,24,29,30. Near-field communication (NFC) is an alternative wireless technology in which sensors are inductively powered by a wireless reader25,31. Because NFC-based sensors can be battery-free, low-cost, and secure against eavesdropping, this technology has emerged as a versatile platform for developing skin-mounted or implanted electronics capable of measuring heart rate, tissue oxygenation, sweat electrolyte composition, ultraviolet light exposure, and other physiological parameters32,33,34,35,36,37,38,39. A major limitation of NFC technology, however, is that the sensors can function only within the near-field of the reader, which is at most a few centimetres for a mobile reader such as a smartphone25,31. This constraint has so far limited the use of wireless, battery-free sensors for continuous monitoring during exercise and other forms of unconstrained physical activity. Although prior studies have demonstrated multiple battery-free sensors for temperature/pressure mapping40 and neonatal vital signs monitoring41, the systems relied on large NFC readers integrated into bedding and is incompatible with use during upright activity. Recent work also demonstrated vital sign and body motion monitoring with multiple stretchable passive tags, but required clothing to incorporate an individually-powered readout circuit above each sensor42. 2ff7e9595c
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