Near-field (NF) measurement techniques and the related NF to far-field (FF) transformations are gaining interest for their ability to allow an accurate evaluation of the radiation characteristics of those antennas whose electric sizes do not make it possible to perform direct FF measurements in a controlled and reflection-free environment, such as the anechoic chamber.
This book provides a comprehensive treatment of classical NF-FF transformation techniques and describes the significant improvements achieved in their performance by correctly applying the non-redundant sampling representations of antenna radiated electromagnetic (EM) fields.
Non-Redundant Near-Field to Far-Field Transformation Techniques is designed to meet the needs of students of antenna NF measurements, as well as of engineers and physicists working in the area. It has been written keeping in mind the fulfilment of two main objectives. The former is to provide all the analytical details on the derivation of the classical NF-FF transformation techniques without and with probe compensation, which are not easily available elsewhere in the literature. The latter is to give a comprehensive description of effective representations of the EM fields radiated over arbitrary rotational surfaces, which make use of a non-redundant (i.e., minimum) number of samples collected on these surfaces or along proper spirals wrapping them.
Inspec keywords: near-field communication; antenna radiation patterns; antenna testing; electromagnetic fields; scanning antennas
Other keywords: microwave holography; reflector antennas; antenna testing; antenna radiation patterns; anechoic chambers (electromagnetic); scanning antennas; near-field communication; satellite antennas; electromagnetic fields; radar antennas
Subjects: Other radio links and systems; Electromagnetic compatibility and interference; Education and training; General electrical engineering topics; Single antennas; Antenna theory; Holography
The large spreading of high-performing antennas, adopted in satellite communication systems and radar equipment, has given rise to an increasingly demand for accurate measurements of the antenna radiation characteristics, which, as well known, are defined in the antenna far-field (FF) region. To ensure that the reflections from the surroundings and incoming electromagnetic (EM) interferences do not affect the accuracy, as in the case of the outdoor FF ranges, the measurements are performed in controlled indoor environments able to emulate the free-space propagation conditions, i.e., shielded anechoic chambers. However, due to the limited sizes of an anechoic chamber, the direct FF measurements are possible only for antennas under test (AUTs) whose electric dimensions allow the fulfilment of the FF distance requirements. When these requirements are not satisfied, only near-field (NF) measurements are made possible there and, accordingly, the evaluation the antenna radiation characteristic can be obtained by resorting to a NF-FF transformation technique [1-13], which allows the accurate computation of the amplitude, phase, and polarization of the antenna far field from the knowledge of the acquired NF data. Besides the evaluation of the complete antenna FF pattern, NF measurements can be employed to obtain, through a back transformation, the field at the antenna surface (microwave holography [14, 15]) and this information can be exploited for diagnostic purposes to detect, e.g., faulty elements in an array or surface deformations in a reflector antenna.
Aim of this chapter is to give a comprehensive description of effective sampling representations of the electromagnetic (EM) fields, radiated by sources contained in an arbitrary convex domain bounded by a surface with rotational symmetry on a surface having the same rotational symmetry. These representations are very appealing since they require a non-redundant, i.e., minimum, number of field samples. It will be shown that such a number is always finite also for an unbounded observation surface, depends only on the geometry of the source, and essentially coincides with the degrees of freedom of the field.
It is useful to note that the sampling representations of the EM fields are typically more convenient than those using asymptotic or modal expansions, because the related coefficients are the field samples, namely, the directly available computed or measured values at the sampling points, and the basis functions are universal and simple. Therefore, they can be exploited to obtain very efficient representations of the radiated EM fields on quite arbitrary surfaces. To achieve these results, a suitable phase factor is extracted from the field expressions and proper parameterizations are adopted for describing the observation surface. Then, optimal sampling expansions of central type are used to accurately reconstruct the radiated EM field on such a surface from the knowledge of its samples.
In the previous chapter, the non-redundant (NR) sampling representations of the radiated electromagnetic (EM) fields have been described in detail and it has been stressed that they can be conveniently exploited to get an effective sampling representation of the voltage, acquired by the probe on the scanning surface, using a NR number of its samples. This allows the development of NR near-field to far-field (NF-FF) transformation techniques (see the next chapters), requiring a number of NF data remarkably lower than that needed by the corresponding classical NF-FF transformations, so that a considerable acquisition time saving is gained.
In this chapter, efficient sampling representations of the probe voltage over a rotational scanning surface, from a NR number of its samples collected along a proper spiral wrapping such a surface, are presented. These representations allow, as will be shown in the following chapters, the development of effective NR NF-FF transformations which adopt innovative spiral scannings. As a matter of fact, the massive NF data needed by the corresponding traditional NF-FF transformation are accurately determined, using an optimal sampling interpolation (OSI) expansion, from the NR samples acquired along the spiral. A further considerable measurement time saving can be so achieved, since the acquisition of these samples is sped up by gathering them on fly and adopting continuous and synchronized motions of the antenna under test (AUT) and probe. The NR representations and related OSI expansions are obtained by adopting a suitable model of the AUT, by considering a spiral such that its step coincides with the sample spacing needed for the interpolation along the related meridian curve, and by determining the NR sampling representation along the spiral.
The near-field to far-field (NF-FF) transformation techniques with planar scannings have received over the years large attention as testified by a lot of papers concerning them appearing in the open literature [15, 24-51, 108, 109, 112, 122-130, 143]. They are usually adopted to characterize highly directive quasi-planar antennas, which radiate a pencil beam pattern, well within the angular region specified by the edges of the antenna under test (AUT) and of the scanning area, so that the truncation (see Sect. 4.4) does not severely affect the pattern reconstruction in the region of greatest interest.
The near-field to far-field (NF-FF) transformation techniques in cylindrical scanning geometry [52-65, 113-126, 142], due to their own features, are commonly employed to characterize elongated antennas, which concentrate their radiated field mainly in the plane orthogonal to their predominant dimension and usually exhibit a main beam, which is narrow in elevation and broad in azimuth. As a matter of fact, these transformations make possible to accurately reconstruct the antenna far field except for the regions specified by the spherical polar caps corresponding to the height of the measurement cylinder (see Sect. 5.4).
The near-field to far-field (NF-FF) transformation techniques in spherical scanning geometry [64-101, 122-126, 131-141] are conveniently employed to obtain the full radiation pattern reconstruction of the antenna under test (AUT) from a single set of NF measurements, even though the involved analytical complexity and computational effort grow considerably as compared to those required by the NF-FF transformations in planar and cylindrical scanning geometries.
No Abstract available.