H.-H. Viskum

Abstract: The spherical near-field geometrical theory of diffraction (SNFGTD) method is an extended aperture method by which the near field from an antenna is computed on a spherical surface enclosing the antenna using the geometrical theory of diffraction. The far field is subsequently found by means of a spherical near-field to far-field transformation based on a spherical wave expansion of the near field. Due to the properties of the SNF-transformation, the total far field may be obtained as a sum of transformed contributions which facilitates analysis of collimated beams. It is demonstrated that the method possesses some advantages Over traditional methods of pattern prediction, but also that the accuracy of the method is determined by the quasioptical methods used to calculate the near field.

Abstract: Established theoretical techniques for inferring the constitution of more-than-one-dimensional bodies probed by linear scalar (or singly polarized vector) wave motion are fitted into a systematic theoretical development. It is concluded that what are now most needed are computationally manageable (and efficient) approaches of accuracy intermediate between the simplistic algorithms employed in virtually all important practical systems (e.g., radar, sonar, computed tomography, seismology) and the computationally intensive exact methods. A possible basis for such an approach is suggested.

Abstract: Incremental length diffraction coefficients (ILDCs) for the half-plane are integrated around the rim of a paraboloid reflector antenna to obtain well-behaved far fields of the nonuniform current for all angles of observation. These far fields, when added to the physical optics far field, produce a more accurate total far field of the reflector. Excellent agreement with the far fields obtained from a method-of-moments solution to the electric field integral equation applied to a 20-wavelength-diameter reflector shows that the cross polarization, farther-out sidelobes, and fields near nulls of reflector antennas can be appreciably modified by the fields of the nonuniform currents. ILDCs are also used to investigate the effect of cracks on the surface of reflectors that can result from the imperfect fitting together of panels to form large reflectors. Three models of cracks are studied. Significant pattern effects are found, depending on the model and orientation of the cracks. >

Abstract: The authors demonstrate the importance of incremental length diffraction coefficients (ILDCs) for accurately calculating the pattern of reflector antennas and for investigating the effect of cracks on the surfaces that can result from the imperfect fitting together of panels to form a large reflector. The three models of cracks considered here are 1) an infinitely long slit in a PEC (perfectly electrically conducting) screen, 2) an infinitely long channel with a semicircular cross section in a PEC screen; 3) an infinitely long boss (ridge) with a semicircular cross section in a PEC screen. Significant pattern effects of the cracks are found depending on the crack model and on the orientation of the cracks. >

Abstract: To enable surface coils to be adapted to a wide variety of examinations and anatomical features, the authors present a flexible detector coupled with an auto tuning device. The conductive loop, which is a flexible mercury filled tube, can be used to obtain various shapes, such as a single flat 22-cm diameter loop or an 11-cm diameter two-turn coil. Tuning is carried out at 21 MHz (0.5 T field) by a microprocessor controlled varactor. Studies conducted on a phantom are used to evaluate the signal/noise ratio and the spatial sensitivity of the coil according to different geometrical arrangements (saddle-shaped and two-turn). The results are compared with those obtained with the constructor's spine coil. The studies of patients described show good image resolution, even in the case of short sequences with thin slices and restricted fields of view. The clinical results and those obtained on the phantom demonstrate the feasibility and value of the device, particularly in examining anatomical regions of different forms.

Abstract: Geometrical defects of a high gain reflector antenna can cause the radiation pattern of the antenna to fail to meet its specifications. These defects give rise to loss of gain, widening of the main beam and raising of sidelobes. The geometrical defects can be identified, and subsequently corrected, by utilizing information contained in the phase of the copolar aperture field distribution. For technical reasons, this phase can be difficult or inconvenient to measure directly. Therefore, indirect methods of deducing the phase are often preferred. This thesis introduces an iterative algorithm, called the modified Gerchberg-Saxton algori thm, which has been developed for retrieving the copolar aperture field phase distribution from the far field copolar amplitude pattern. In order to aid convergence of this algorithm, it incorporates information concerning the design and any known aspect of the antenna. The modified Gerchberg-Saxton algorithm is based on the conventional Gerchberg-Saxton algorithm, originally developed for electron microscopy, but incorporates features of Fienup's phase retrieval algorithms. This thesis reviews radio engineering theory with an emphasis on high gain reflector antennas. In particular, the Fourier transform relationship between the copolar aperture field distribution and the copolar radiation pattern is critically examined. The problem of retrieving the copolar aperture field distribution from the amplitude of its Fourier transform is called a Fourier phase problem. The Fourier phase problem, the uniqueness of its solutions and iterative algorithms for solving it are discussed. Other established methods for determining geometrical defects of an antenna are descri bed and their relative advantages and disadvantages are assessed. The main advantage of the modified Gerchberg-Saxton algorithm is that it requires measurement of only a single copolar amplitude pattern. The modified Gerchberg-Saxton algorithm is evaluated by applying it to computer simulated data and to measured amplitude patterns of an acoustic antenna. This evaluation illustrates the relationship between the accuracy of the data to which the algorithm is applied and the accuracy of the retrieved copolar aperture field phase distribution. The performance of the algorithm appears to be insensitive to the location and dimensions of the geometrical defects of the antenna. The optimum form of the algorithm seems to be versatile and robust enough to offer real hope of being able to retrieve, to a useful level of accuracy, the phase of the aperture field from a single measured radiation pattern amplitude (i.e. there is no need to measure the phase of the radiation pattern).