A comparison among 1-, 3-, and 7-horn feeds for a 37-beam MBA

A very common multiple beam antenna (MBA) configuration consists of a collimating device illuminated by an array of feeds. The collimating device is usually a reflector or a lens. The feeds are usually horn antennas with a circular aperture. The reflector is usually offset-fed to eliminate aperture blockage; the lens is center fed. The antenna's feeds are excited to produce a finite number of beams, so as to provide contiguous coverage of the field of view. The designer is forced to minimize the angular spacing between adjacent beams in order to maximize the minimum gain over the antenna's field of view. On the other hand, the feed horn's aperture gain is maximized when the feed horn spacing and its aperture diameter are equal. This results in antenna efficiency of the order of 30% when a single feed horn is excited to produce a beam. When a cluster of 3 or 7 adjacent feed horns are excited to produce a single beam, antenna efficiency can be increased to 50%. When it is tolerable, several identical antenna apertures can be used to replace a single aperture configuration. In this case, each of M apertures produces approximately N/M beams of an MBA that produces N beams. Horns producing adjacent beams do not illuminate the same aperture. This permits the use of a much larger horn aperture for a given beam spacing. This results in reduced spillover, higher gain of each beam, and increased antenna efficiency of each aperture. This paper investigates the maximization of gain for several lens antennas. It shows that antenna gain is increased as its focal length is decreased. That is, a focal length-to-diameter ratio (F/D) less than 1 is preferred     

Synthesis of the fields of a transverse feed for a spherical reflector

The problem of designing a transverse feed for a spherical reflector is considered and a method is presented for synthesizing the fields on a surface of a sphere enclosing a feed that will produce a specified reflected field at the surface of a spherical reflector. The method identifies the reflector and a spherical surface enclosing the feed as a boundary value problem and uses a finite set of spherical waves to approximate the boundary conditions. A feed designed to excite this field will in turn produce the desired reflected field at the surface of the reflector, under the condition that that portion of the reflected field which is scattered by the feed may be neglected. It is shown that the feed need produce only a small part of the synthesized field to obtain an antenna efficiency of more than 70 percent. Some typical field distributions will be shown so as to indicate a method for designing a feed and to point out the correlation between the polarization of the synthesized field and the polarization of the reflected field at the surface of the reflector.     

Aperture feed for a spherical reflector

The problem of designing a feed system for illuminating a spherical reflector is examined. A method is proposed for specifying the required field distribution over the aperture of the feed system, and the primary illumination and gain resulting from this distribution are derived. The results indicate that a significantly smaller feed aperture can be employed than would be indicated by conventional ray tracing methods. Specific numerical results are obtained by taking the Arecibo antenna as an example, for which a calculated aperture efficiency of 67.5 percent is possible with approximately a 38-foot-diameter aperture feed.     

A theoretical study of a mulit-element scanning feed system for a parabolic cylinder

This paper is a theoretical investigation of a method of scanning a beam of an antenna consisting of a fixed reflector and an array of elements illuminating the reflector by appropriately controlling the phase and amplitude of the signal radiating from each element. For the purpose of determining the feasibility of such a systems the two-dimensional problem is explored where a specific antenna geometry and an element radiation pattern are assumed. It was found that although the system exhibits similar performance to a system with a single radiating element when radiating a beam on axis, it could be scanned some eight beamwidths off axis without significant deterioration in characteristics. Interestingly, it was discovered that there is a separation (about0.7lambdain this case) between the feed elements where the sidelobes become dramatically high, analogous to grating lobes in a phased array.     

Physical limitations on interference reduction by antenna pattern shaping

The problem of minimizing the signals received from interfering or undesirable signal sources by appropriately modifying the antenna radiation pattern is addressed. The solution is presented in terms of the modal expansions of the monochromatic electromagnetic fields outside the radiating structure, and appropriate optimization of these radiation patterns. Physical realizability of the results is assured by requiring that the allowable fields are derived from nonsuper gain antenna excitations; hence the results represent an upper bound on the performance. We consider two complementary, one-dimensional antenna structures, i.e., a circumferentially symmetric line source of lengtha, and an axially independent cylindrical antenna of radiusrho_{0}. We consider first the case ofNdiscrete interfering signals and compute the loss in antenna directivity when the radiation pattern is modified so as to place a null at each angular position of the interfering signal sources. The results indicate that if all interfering signals are located outside the main beam, the loss in directivity is negligible. When one or more undesired signal sources are in the main beam, the antenna directivity will be reduced appreciably. It is shown that when uniformly distributed noise is superimposed over the discretely located interfering signals, the same antenna pattern maximizes the signal/noise ratio for those practical cases when the interfering signal power is significantly greater than the uniformly distributed noise source power. It is further shown that introducing noise uniformly distributed over all space is equivalent to adding thermal noise at the antenna terminals, as would occur with the receiver. Finally, the effects of bandwidth on the system performance are considered.     

A microwave radome performance verification gauge

A microwave radome gauge (MWRG) design for verification of the microwave properties of a radome is presented. A mathematical model of the gauge is developed, and expected results are calculated and compared to measured results. These results indicate that the MWRG is relatively insensitive to its flange (choke) dimensions and the radiation resistance of the slot antenna formed by the flange and conducting foil that enclose the radome. The resonant frequency of the device shifts ≈0.1% for a 1% change in the radome's core thickness, or dielectric constant, and ≈0.01% for a 1% change in sheet thickness. The tool is virtually insensitive to a change in loss tangent of radome sheet material. Use of the MWRG shows that this test procedure is a satisfactory method of verifying an RF window's performance prior to shipment and installation with the antenna assembly. Experience with the measurement of standard panels indicates consistency in manufacturing and of the test method and equipment. The MWRG has also demonstrated its ability to detect defects in the RF window; that is, it accurately located bleeder strips and overlap areas between adjacent sheets