Surface diagnosis of large reflector antennas using microwave holographic metrology: An iterative approach

Abstract

Recent investigations have shown that it is quite possible to accurately characterize the surface profile of large reflectors using microwave holographic techniques. In these techniques the complex (amplitude and phase) far‐field pattern of the antenna is measured first. The surface profile is then constructed using the Fourier transform relationship which exists between the far field and a function related to the induced surface current. In this paper the concept of the Fourier transform relationship is first investigated to demonstrate that it is, in general, a summation of many Fourier transforms. However, for large reflectors with small beam widths, only the first term of the series has the major contribution. Furthermore, an iterative scheme is described to analytically/numerically continue the far‐field pattern outside the measurement window. This, then, results in an improved resolution of the surface map data and, in particular, reduces the amplitude artifacts outside the boundary of the reflector. A novel and efficient simulation model has been developed to properly evaluate the accuracy of the technique in recovering the simulated surface errors. Finally, results of a recent measurement are summarized.

An Alternative Solution for the Reflector Surface Retrieval Problem

“…Note that for large and focusing reflectors, as radioastronomical ones, we can assume θ 1 (radians) (cos θ ∼ = 1 and sin θ ∼ = 0), and find an approximate expression for (11) …”

“…On the other side, the inverse methods, based on microwave holographic measurement, examine the entire optics chain. They obtain the current induced on the reflector surface, and then the information about the surface misalignments, by using the antenna far-field pattern which is measured by pointing the reflector towards a satellite or a radio source [11]. The amplitude and phase of the far-field can be measured by using a second antenna to obtain a stable phase reference [11].…”

“…Up to now, this problem has been solved by using an extrapolation procedure, based on a simple Fourier transform [11] inversion. This is computationally efficient, but is limited to the paraxial case.…”

“…They obtain the current induced on the reflector surface, and then the information about the surface misalignments, by using the antenna far-field pattern which is measured by pointing the reflector towards a satellite or a radio source [11]. The amplitude and phase of the far-field can be measured by using a second antenna to obtain a stable phase reference [11]. As an alternative, only the amplitude of the far-field can be used as input data [12][13][14].…”

Reduction of truncation errors in planar near-field aperture antenna measurements using the method of alternating orthogonal projections

“…It is worth noting that the method described in this paper also provides an accurate representation of the aperture field distribution. In fact, a similar iterative procedure has already been applied to determine, from far-field data, surface distortions in large reflector antennas [17] and the individual element excitations of planar arrays for pattern synthesis [18] or array antenna diagnosis and calibration [19]. This contribution is organized as follows: first, the mathematical formulation of the method of alternating orthogonal projections is briefly summarized in Section 2; then, its application to the problem of pattern reconstruction is described in Section 3.…”