Analysis of a second order endfire line array using SYSNOISE

Abstract

It can be shown canonically that two equal-amplitude dipoles configured linearly can be summed to yield a directivity index of 9 dB if the directional axes are pointed in 180 deg opposition, and the rear-facing dipole is delayed by the appropriate amount of time. For a 9 mm end-to-end aperture, the rear facing dipole should be delayed by 7.25 microsec before it is summed with the front-facing dipole in order to achieve the 9 dB. The purpose of this study is to compare the canonical results to those of SYSNOISE. The two dipoles are modeled within SYSNOISE as: 1) four monopoles, 2) four meshed spheres with ka1, and 3) a 9 mm by 1 mm meshed sheet of zero thickness. For each of these three cases, the amplitude and phase of the two dipoles is summed as described above, and the directivity pattern and directivity index is computed by integrating the responses from 614 field points located at equal 10 deg azimuth and elevation angles at 1 m radius from the line array origin. Although the four monopoles and four meshed spheres yielded excellent results at 1 kHz and 6.3 kHz, the results from the meshed sheet was degraded. Further analysis indicated that the phase computation from SYSNOISE was the culprit.