Near-field Far-field Transformation Technique Research Articles

A fully probe corrected near-field far-field transformation technique employing plane-wave synthesis

Abstract. The far-field behavior of an antenna under test (AUT) can be obtained by exciting the AUT with a plane wave. In a measurement, it is sufficient if the plane wave is artificially generated in the vicinity of the AUT. This can be achieved by using a virtual antenna array formed by a probe antenna which is sequentially sampling the radiating near-field of the AUT at different positions. For this purpose, an optimal filter for the virtual antenna array is computed in a preprocessing step. Applying this filter to the near-field measurements, the far-field of the AUT is obtained according to the propagation direction and polarization of the synthesized plane wave. This means that the near-field far-field transformation (NFFFT) is achieved simply by filtering the near-field measurement data. Taking the radiation characteristic of the probe antenna into account during the synthesis process, its influence on the NFFFT is compensated. The principle of the plane-wave synthesis and its application to the NFFFT is presented in detail in this paper. Furthermore, the method is verified by performing transformations of simulated near-field measurement data and of near-field data measured in an anechoic chamber.

Directivity Computation by Spherical Spiral Scanning in Near Field Region

A procedure for evaluating the directivity of an antenna under test from near-field measurements on a spherical scanning surface is proposed. It uses a formula in closed form for computing the total radiated power from the knowledge of the radiation intensity samples on the far-field sphere. These last are determined via an efficient probe compensated near-field–far-field transformation technique based on a nonredundant sampling representation of the probe voltage. The samples are collected on a spiral wrapping the near-field spherical surface from pole to pole. With respect to the classical spherical scanning, this innovative way of collecting the data allows the reduction of the time required for the samples acquisition by means of a continuous movement of the positioning systems. The effectiveness of the proposed procedure is assessed by numerical simulations.

Nonredundant Nf-Ff Transformation With Helicoidal Scanning

A probe compensated near-field-far-field (NF-FF) transformation technique with helicoidal scanning, which makes use of a minimum number of probe output voltage samples, is developed in this paper. This target is achieved by using a nonuniform helix as scanning curve, i.e., a circular helix with a constant angular elevation step, and by developing an efficient sampling representation over it. Then, by choosing the step coincident with the spacing needed to interpolate the probe voltage along a generatrix, it is possible to reconstruct such a voltage at any point over the cylinder. This allows the recovering of the data required by the standard probe compensated NF-FF transformation technique with cylindrical scanning. As compared with the previous technique employing a uniform helix with a constant linear elevation step, it allows one to reduce remarkably the number of needed samples without affecting the numerical efficiency. Numerical examples assess the accuracy and the stability of the FF reconstruction process.

A two probes scanning phaseless near-field far-field transformation technique

An innovative and effective technique to determine the far-field of a radiating system from near-field intensity data is introduced, analyzed, and tested. The approach is based on the simultaneous measurement of the amplitude of the voltages received by two different probe antennas moving over a single scanning surface in the near zone and performs the phase retrieval of the near-field by assuming as unknown the plane wave spectrum of the field. The radiated field is then straightforwardly evaluated. As compared to the existing phaseless measurement techniques, the use of two different probes makes it possible to avoid the need for a second scanning surface and thus allows the use of smaller (and cheaper) anechoic chambers. Furthermore, the measurement time is essentially equal to that required by conventional techniques based on the measurement of the complex near-field. The reliability and the effectiveness' of the approach are investigated and discussed and the key factors affecting its behavior are highlighted. In particular, the relevance of the difference between the plane wave spectra (PWS) of the two probe antennas in ensuring an acceptable reliability of the solution, with respect to the starting point of the procedure, is outlined. Finally, the effectiveness of the approach is confirmed by an extensive numerical analysis, which also shows the stability of the solution against data noise.

An effective near-field far-field transformation technique from truncated and inaccurate amplitude-only data

A general approach to the near-field far-field transformation from amplitude only near-field data is presented. The estimation of the far field is stated as an intersection finding problem and is solved by the minimization of a suitable functional. The difficulties related to the possible trapping of the algorithm by a false solution (common to any nonlinear inverse problem) are mitigated by setting the problem in the space of the squared field amplitudes (as already done in a number of existing papers) and by incorporating all the a priori knowledge concerning the system under test in the formulation of the problem. Accordingly, the a priori information concerning the far field, the near field outside the measurement region and the accuracy of the measurement setup and its dynamic range are properly taken into account in the objective functional. The intrinsic ill conditioning of the problem is managed by adopting a general, flexible, and nonredundant sampling representation of the field, which takes into account the geometrical characteristics of the source. As a consequence, the number of unknowns is minimized and a technique is obtained, which easily matches the available knowledge concerning the behavior of the field. The effectiveness of the approach is shown by reporting the main results of an extensive numerical analysis, as well as an experimental validation performed by using a very low cost near field facility available at the Electronic Engineering Department, University of Napoli, Italy.

Near-field far-field transformation in time domain from optimal plane-polar samples

This paper presents a new approach to the time-domain near-field far-field transformation technique introduced by Hansen and Yaghijan (1994) and is based either on a time-domain or frequency-domain scheme. The approach presented here attempts to overcome the main drawbacks of this technique related to the computer time and memory requirements, which could make unrealistic the application of the technique to cases of practical interest. To this end, the advanced representation of the (time and frequency domain) near field previously introduced by the authors, which requires a minimum number of nonequispaced field samples, are exploited. This leads to new relationship between the near-field measured samples and the far field, which requires a minimal set of time-space measurements. Various computational schemes are considered and compared showing that the presented algorithm requires a reduced measurement effort, computer time, and memory occupancy, while allowing a lower far-field reconstruction error for a fixed number of measurements.

Far-field computation from amplitude near-field data on two surfaces: cylindrical case

A near-field far-field transformation technique involving the amplitude of the measured data has been recently proposed. It requires two sets of near-zone amplitude measurements and relies upon a mean-square fitting of the measured amplitude by an iterative projection algorithm. In the paper the method is applied to the case of cylindrical scanning, for which all the details are explicitly worked out. Furthermore the probe effect is accounted for and the relative correction procedure is considered. The relevance of a rough phase estimate and/or a general knowledge of the source is also discussed to overcome the difficulties related to the presence of local minima which can trap the procedure. Numerical examples validating the effectiveness of the technique are provided.