Showing papers by "John F. Nye published in 1995"

Diffraction by a black half-plane: theory and observation

Abstract: It is clear what properties should be possessed by an ideal black screen that is plane, infinite and thin, namely, no reflection and no transmission of an incident electromagnetic wave. If an aperture is cut in this infinite screen, however, it is by no means clear what diffracted field should be expected, there being no compelling local definition of blackness, but only rival theoretical models. Such a screen is realizable approximately by using a highly absorbing material, and blackened screens are present, for example, in most optical instruments. To explore the region very close to a black screen it is more practical to use microwaves. Here, for a half-plane screen geometry, we present measurements of the field closer to the diffracting edge than a few wavelengths, made by using 30 mm microwaves and the modulated scatterer technique, and we compare them with two theoretical models, namely, Kirchhoff diffraction and Sommerfeld black half-plane diffraction. The fields from both models are rederived in unconventional ways, and both are found to be in good general agreement with the measured field on the side of the screen away from the source, but the Sommerfeld field matches the experiment much better on the source side. It is plausible that such differences as remain are due to the imperfect absorption of the screen and its finite thickness.

Mapping a diffraction field close to an obstacle

Abstract: Demonstrates the effectiveness of the optically modulated scatterer technique as a probe for EMC measurements. As a practical test, the authors have measured the spatial distribution of the known microwave field caused by diffraction by a half-plane. The field was produced by directing linearly polarized 10 GHz radiation from a horn antenna on to the edge of a plane conducting sheet with an angle of incidence of approximately 45/spl deg/ and with the electric field parallel to the edge. The amplitude and phase of the electric field were measured on both sides of the sheet at distances from the edge down to a small fraction of a wavelength, and the results were compared with a modified form of Sommerfeld's classical theory in which the source is a line current parallel to the edge. The spatial phase pattern matched well everywhere; in regions of small phase gradient the phase agrees to within 10/spl deg/. The amplitude pattern was reproduced with an accuracy of about 5%. >