Antenna diagnostics for spherical near-field antenna measurements

Personnel

PhD student Cecilia Cappellin
Professor Olav Breinbjerg (supervisor)
Senior engineer Aksel Frandsen, TICRA (external supervisor)

Period

October 2004 - September 2007 

Funding

This project is part of the Danish Industrial PhD Programme, and is sponsored by TICRA and the Ministry of Science, Technology, and Innovation.

Background

Electrical and mechanical errors in an antenna can be identified by use of an efficient antenna diagnostics technique. Examples of such errors, which can originate from the manufacturing as well as the operation, are excitation of higher-order modes in a waveguide antenna, malfunction of microwave components in the feed network of an array and surface deviation on a reflector antenna. The presence of such errors is usually observed in the measured far-field pattern, however their causes can only be explained by analyzing the radiated extreme near-field amplitude and phase.

So far several techniques, commonly based on far-field, planar or spherical near-field measurements have been developed, all presenting limits in their practical and theoretical realization: they are limited either in terms of the type of antennas for which they can be used, or in terms of the accuracy that they can provide. There is thus a need for an antenna diagnostics technique that applies to general types of antennas, that is intrinsically accurate and that takes advantage of the high accuracy provided by the spherical near-field antenna measurements conducted in the DTU-ESA Spherical Near-Field Antenna Test Facility. The measurement technique employed at the DTU-ESA Facility is based on the spherical wave expansion (SWE) of the field radiated by the antenna. Such expansion is mathematically valid in any source-free region of space outside the smallest sphere centered at the origin of the measurement coordinate system and completely enclosing the antenna, and thus does not readily allow the calculation of the aperture field in the extreme near-field region.

Description

One way to overcome this is to transform the SWE of the radiated field into a plane wave expansion (PWE). It can be shown that the plane wave spectrum can be computed from the knowledge of the coefficients of the SWE, on any aperture plane in the antenna source-free region. The computation of the PWE from the SWE gives two main advantages. The first is that the plane wave spectrum can in principle be evaluated also in part of the spectrally invisible region, and the second is that the aperture field can be computed as an Inverse Fourier Transform (IFT) of this spectrum on any plane in front of the antenna, thus also inside the minimum sphere. In this way the spatial resolution achieved in the aperture field can theoretically exceed the traditional limit of half a wavelength, providing a much more accurate diagnostics.

The theoretical study of the SWE-to-PWE transformation has shown important properties of general importance. However, in implementing the technique in a real measurement facility, non-ideal aspects, such as measurement errors and finite dynamic range of the receivers, play an important and limiting role. While the first application to real measured data has shown satisfactory and good results, see Fig. 1-2, further and better investigations are necessary and under development.

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Figure 1. Arm of the SMOS antenna mounted on the tower of the DTU-ESA Spherical Near-Field Facility: only two elements are radiating.

antenna diagnostics 02a       antenna diagnostics 02b

Figure 2. X-component of the electric field detected on the plane at 0.01 wavelength from the antenna aperture: the radiating elements and the diffraction from the edges, as well as the junctions of the arm, are detected. 

[1] Cappellin, C., Nielsen, J. M., Breinbjerg, O., A high-resolution antenna diagnostics technique for spherical near-field antenna measurements, JINA 2004, International Symposium on Antennas, Nice, France, pp 310-311, 2004 [Published]

[2] Cappellin, C., Breinbjerg, O., Frandsen, A., A high-resolution antenna diagnostics technique for Spherical Near-Field Measurements, 28th ESA Antenna Workshop on Space Antenna Systems and Technologies, ESTEC, Noordwijk, The Netherlands, pp 899-906, 2005 [Published]

[3] Cappellin, C., Frandsen, A., Breinbjerg, O. , On the relationship between the spherical wave expansion and the plane wave expansion for antenna diagnostics, AMTA Europe Symposium, Munich, Germany, 2006 [Published]

[4] Cappellin, C., Breinbjerg, O., Frandsen, A., The influence of finite measurement accuracy on the SWE-to-PWE antenna diagnostics technique, EuCAP European Conference on Antennas and Propagation, Nice, France, 2006.

 

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