Prague, Czech Republic – July 01, 2015EMSCAN will present a paper at the PIERS 2015 conference on July 6 at 9 am in room I. The title of the paper is "Full-sphere Radiation Pattern Measurement of 3D Antennas Using a Compact Planar Very-near-field Scanner".

Far-field test facilities for antenna pattern measurements are often very large, costly, and slow. An alternative to direct far-field measurement exists based on near-field scanning and near-field to far-field transformations. The compact size of these near-field measurement systems allows for integration of measurement probes in planar, circular and other array configurations to reduce the required movements of the antenna-under-test (AUT) and hence the measurement time. Planar very-near-field scanning has already been successfully commercialized for small antennas with an array of 1600 H-field probes printed on a 45 cm by 45 cm printed circuit board [1]. Radiation pattern in a hemisphere is predicted in a matter of a few seconds using plane wave spectrum expansions. The accuracy of these far-field patterns however, degrades as the “apparent antenna dimension”-to-“scanner aperture” ratio increases. Hence, a single planar scan fails to accurately predict the pattern of 3D antennas that have high profiles relative to the dimensions of the scanner. Alternatively, multiple planar scans that close the measurement surface around the antenna can improve pattern accuracy [3]. As suggested in [3], a cubic measurement surface composed of 6 planar scans is considered herein. Phase coherence between the six measurements is crucial and is ensured by feeding a sample of the input signal to the scanner via a power divider. For fast far-field predictions, measured fields on each face are independently transformed to far-field in a matter of a few seconds. Subsequently, the far-field contributions of different faces are superimposed after proper coordinate translation/rotation.
This works investigates the optimum setup required for accurate radiation pattern measurement of high profile antennas. It is concluded that different box dimensions are required at different frequency bands. A 40 cm × 40 cm × 40 cm box provides good far-field results below 1 GHz whereas a smaller 20 cm × 20 cm × 20 cm box is required at higher frequencies. Two test fixtures are prototyped to facilitate movements of the AUT for all 6 measurements. The rather large height-to-scan-area ratio in these setups leads to two spurious artifacts; first, the consequent large truncation error results in ripples in the radiation pattern. A raised cosine window with a quadratic phase window is optimized for elimination of these pattern ripples. Secondly, the large separation of the AUT above the scanner results in inaccurate far-field phase of a single scan beyond the solid angle formed between AUT and the corresponding face [4]. At each point in the far-field, before superimposing the contributing fields, the phase of the fields are corrected according to the phase of the field that is most accurate at that point. Figure 1 (a) depicts the near-fields of a 900MHz antenna measured on faces of a 40cm×40cm×40cm box. Only the magnitude of Hx is shown in Figure 1 (a). The predicted far-field pattern of Figure 1 (b) is in good agreement with chamber measurements.

                             Fig. 1 (a)                                                                              

                                                   Fig. 1 (b)

EMSCAN is the world leading developer of FAST magnetic very-near-field measurement technologies and applications since 1989, providing real-time test solutions to antenna and PCB designers and verification engineers, without the need for a chamber.
The EMxpert, a family of compact EMC and EMI diagnostic tools, and the RFxpert, a family of antenna measurement tools, enable engineers to quickly optimize their designs. EMSCAN solutions dramatically increase designer productivity and substantially reduce time-to-market and project development costs.