Generalized, field-corrected antenna measurements (GFAM)

• Applicant: Professor Dr.-Ing. Dirk Heberling
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 406228537

• Final Report Year: 2021

Final Report Abstract

The aim of this project was to compensate scattered signals of an antenna measurement carried out in an imperfect TZF. The technique is composed of two steps: first, the SMCs of the TZF have to be obtained by measurements on a spherical surface around the test zone with a known and well-calibrated probe. This is achieved via the matrix-vector representation of the test-zone measurement and the subsequent solution of the linear system of equations by a direct (matrix inversion) or iterative method. Secondly, the measurements of the AUT are compensated by again solving the linear system of equations but now with the known SMCs of the test zone. Using spherical waves as basis functions, the method is generally applicable to any spherical surface, whether in the SNF or in the far field. It has been shown that care must be taken in the choice of the TZF probe. With a dipole probe, for example, it is technically not feasible to reconstruct certain spherical modes of the spectrum due to the zeros of the spherical Bessel function and the nulls in the radiation pattern of the dipole. With a dipole probe, the measured input error is amplified by a factor of about 33 compared to a directive probe. A directive probe is, thus, more suitable since it leads to a wellconditioned problem. Moreover, the scattering of the rotating positioning system which cannot be modelled by the SWE is minimised by the spatial filtering property of the directive pattern. To perform the spherical TZF measurements, a scanning arm has been mounted onto the rollover-azimuth positioner. Since the characterisation of the field is crucial for the success of the compensation method, the scanning arm is designed to be stable to enable a precise sampling on the spherical surface. Furthermore, glass-fibre reinforced plastic has been used to minimise the scattering of the additional hardware brought into the test zone. However, the rotational motion of the azimuth positioner limits the accuracy of the TZF measurements and can be avoided by designing a measurement setup in which the metal tower of the azimuth positioner does not rotate during the acquisition of the TZF. To validate the compensation technique, an additional flat metal plate with a size of 800 mm× 800 mm (W × H) is installed in the measurement chamber. The scattering of the metal plate significantly distorts the TZF. As exemplary AUT, a BTS antenna has been characterised at a frequency of 2.4 GHz. To create the reference pattern, the AUT has been measured without the metal plate. It has been demonstrated that with the current measurement setup a reduction of the maximum error of approximately 20 dB is achieved and that the directivity of the BTS antenna can be retrieved with an error of only 0.001 dB compared to the reference. The results prove that the current measurement setup is well suited for performing spherical measurements of the test zone in order to correct unwanted field components from directive antenna patterns. The work of this project has been presented at several conferences and the theoretical background as well as the emulation of an antenna measurement has been published in a highly renowned journal. In the latter, it has been proven that the IHF’s in-house algorithm to emulate an antenna measurement can compete with commercially available software and even outperform it in terms of computing speed.

Publications

• “Effect of the test zone field on the radiation characteristics of antennas”. In: Proceedings of the 13th European Conference on Antennas and Propagation (EuCAP). Mar. 2019, pp. 1–5
  T. M. Gemmer and D. Heberling
  
• “Generalized test-zone field compensation”. In: Proceedings of the 41st Annual Meeting and Symposium of the Antenna Measurement Techniques Association (AMTA). Oct. 2019, pp. 1–6
  T. M. Gemmer and D. Heberling
  
• “Accurate and efficient computation of antenna measurements via spherical wave expansion”. In: IEEE Transactions on Antennas and Propagation 68.12 (2020), pp. 8266–8269
  T. M. Gemmer and D. Heberling
  
• “Spherical test-zone field measurements of a compact antenna test range”. In: Proceedings of the 42nd Annual Meeting and Symposium of the Antenna Measurement Techniques Association (AMTA). Nov. 2020, pp. 1–4
  T. M. Gemmer and D. Heberling