Recent developments in some of the more active areas of antenna and diffraction theory are summarize& Although concerned primarily with U.S.A. activity, some reference is made to the literature of other countries where closely allied work is in progress. Fields included in this survey are broad-band antennas, wide-angle microwave optics, antennas for ionospheric scatter propagation, traveling-wave antennas, slot radiators, diffraction, and scattering theory. Recent work in broad-band antennas has been concerned mainly with trying to hold both radiation pattern and impedance independent of frequency. The problem has been approached successfully through development of that class of antenna shapes which depends on angles only. The equiangular spiral is a simple example. In microwave optics the requirements for wide-angle scanning of antenna beams have been met largely by using new light-weight, low-loss dielectrics to construct suitable lenses such as a spherical Luneberg lens. Using the geodesic analog of the Luneberg lens in one plane a scan of 40 beamwidths without aberrations has been obtained. The very high gain required for antennas suitable for VHF ionospheric scatter propagation has been obtained through use of long horizontal rhombics and, more recently, by corner reflector antennas driven by collinear arrays of dipoles. The latter antennas have the important advantage for this application of much lower sidelobe level Traveling-wave antennas have received much attention in recent years because of their inherent adaptability to flush-mounted applications. Among the forms considered are corrugated surfaces and single or double dielectric-clad surfaces. The launching problem has been studied rather intensively and recent interest has been shown in the synthesis and scanning aspects for slow-wave structures. Progress has continued in the exploitation of fast-wave systems, with major advances centering on the launching problem and the polarization problem. Strip lines are taking an important place as feeding systems for traveling-wave antennas. Recent years have been marked by considerable research activity on flush-mounted microwave antennas. Such antennas often take the form of slots or apertures in the metallic surfaces of aircraft. The radiation pattern of the slot radiator depends upon the shape of the metallic surface in which it is cut, and for complicated shapes resort is made to experiment. However, certain simple geometric shapes have been treated mathematically. These shapes include the circular cylinder, half plane, wedge, sphere, elliptic cylinder, oblate and prolate spheroid, and cones. Various combinations of these shapes on which work also has been done are the semicircular boss on a flat ground plane, the cylindrically tipped wedge, and the spherically tipped cone. The problem of radiation from apertures in a metallic surface is closely related to the reciprocal problem of calculating the currents excited on the surface by an incoming wave. As a direct consequence of the reciprocity theorem there is an intimate connection between slot radiators, diffraction, and scattering. The result is that the extensive body of knowledge classed as diffraction and scattering must be considered as an integral part of antenna theory. During the past three years there has been a very considerable effort in obtaining new exact solutions, several new approximate solutions, and a better physical understanding of the mechanisms involved in scattering. All of the geometric shapes mentioned above have received attention, and much experimental work has also been done. For the future it can be expected that this intensive effort will continue with emphasis on the asymptotic approaches of Kline and Keller, the method proposed by Logan, and Fock's method of obtaining asymptotic results based on local analysis.
Read full abstract
Mar 1, 1961
J Mawdsley + 1
Open Access