Differential Near Field Holography for Small Antenna Arrays

Janice, Brian A

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Differential Near Field Holography for Small Antenna Arrays

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Near-field diagnosis of antenna arrays is often done using microwave holography; however, the technique of near-field to near-field back-propagation quickly loses its accuracy with measurements taken farther than one wavelength from the aperture. The loss of accuracy is partially due to windowing, but may also be attributed to the decay of evanescent modes responsible for the fine distribution of the fields close to the array. In an effort to achieve better resolution, the difference between these two phase-synchronized near-field measurements is used and propagated back. The performance of such a method is established for different conditions; the extension of this technique to the calibration of small antenna arrays is also discussed. The method is based on the idea of differential backpropagation using the measured/simulated/analytical data in the near field. After completing the corresponding literature search authors have found that the same idea was first proposed by P. L. Ransom and R. Mittra in 1971, at that point with the Univ. of Illinois. This method is basically the same, but it includes a few distinct features: 1. The near field of a (faulty) array under test is measured at via a near field antenna range. 2. The template (non-faulty) near field of an array is simulated numerically (full-wave FDTD solver or FEM Ansoft/ANSYS HFSS solver) at the same distance - an alternative is to use measurements for a non-faulty array. 3. Both fields are assumed (or made) to be coherent (synchronized in phase). 4. A difference between two fields is formed and is then propagated back to array surface using the angular spectrum method (inverse Fourier propagator). The corresponding result is the surface (aperture) error field. This approach is more precise than the inverse Rayleigh formula used in Ransom and Mittra's paper since the evanescent spectrum may be included into consideration. 5. The error field magnitude peaks at faulty elements (both amplitude and phase excitation fault). 6. The method inherently includes all mutual coupling effects since both the template field and the measured field are full-wave results.

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