James K. Hsiao

Properties of a nonisosceles triangular grid planar phased array

An array with a general triangular grid structure (not necessarily isosceles) is considered, and methods of generating such an array are described. An analysis of its properties is presented. It is shown that this array has a grating lobe of -6 dB relative to the main lobe. The necessary conditions to avoid grating lobes are presented, with the conventional rectangular and isosceles triangular grids included as special cases. Furthermore, it is shown that one of the grid dimensions can be extended as compared to that of an isosceles triangular grid. However, when this is done, the optimal, grid area is reduced.

Patterns and polarizations of simultaneously excited planar arrays on a conformal surface

A conformal array on a surface of small curvature can be approximated by a number of planar arrays, several of which may be excited simultaneously so as to achieve a performance similar to that of a conformal array. Since the main beam of a planar array can be steered to any direction in the visible Space, several arrays, each oriented in a different direction, can be steered cooperatively to form a single beam in a desired direction. A general formulation of the radiation field of such an array of arrays is developed with the aid of vector recomposition formulas between two orthogonal co-ordinate systems. This formulation does not involve the integration of the current source but is solely dependent upon the knowledge of the far-field expressions of elementary radiators. Using this formulation, it can be shown that within each array, the conventional row and column phase setting can be used, although each array requires an additional phase shift to compensate for the phase difference caused by its position on the curved surface. As examples, the radiation patterns and polarizations of multiple arrays of short dipoles are studied using the present formulation. A comparison of the multiple planar arrays with the conventional conformal array is also presented.

Computer aided impedance matching of an interleaved waveguide phased array

The active admittance of a reduced-size dielectric-loaded waveguide in an interleaved phased array has a low conductance component and a high VSWR. Matching the impedance of such an element poses a difficult problem, in particular if it is desired to achieve matching over a wide frequency band. In this communication, a method to design an impedance transformer for matching such a load based on an optimization approach is presented. The computer results show that a load with a VSWR of 7 to 8 across a 10 percent bandwidth can be matched to achieve a VSWR of about 1.25 across the same band.

On the accuracy of a numerical integration procedure for computing diffraction fields using table lookup

In this communication we have analyzed the errors introduced by table lookup when an electromagnetic diffraction integral is performed numerically. We have shown that in order to set the 3\sigma error of both the real and imaginary components equal to \varepsilon , the required table should have at least K entries uniformly distributed within a 2\pi range, where K \geq 3.85/\varepsilon\sqrt{N} and N is the number of terms used for numerical integration. The maximum deviation of the amplitude ( \bar{R} ) of this integral in this case is also less than \varepsilon and the maximum phase deviation is \tan^{-1} (\varepsilon/\bar{R}) . A numerical example is presented which confirms this prediction.

Statistical properties of a staggered PRF MTI system

In this paper, the statistical properties of improvement factors of a staggered PRF MTI system for the cases either using optimal filter weights or Binomial filter weights, are presented. It is shown that the degree of deviation of samples is a function of both the standard deviation of the clutter spectrum density function and the amount of variation of interpulse durations.

Performance of a range-ambiguous MTI and doppler filter system

In this report the clutter rejection performance of a range-ambiguous MTI and doppler filter system is analyzed. The reduction of correlation time in a range-ambiguous system in general improves the clutter rejection performance. However, due to the fold-over phenomenon, a far-away target has to compete with the near-in clutter; hence, a higher improvement factor is required. Examples presented here which take into account these effects show that the improvement factor of a range-ambiguous MTI system is slightly better than that of a range-unambiguous system. However, the performance of a doppler filter processor in a range-ambiguous system is worse.