A scattering matrix-based beamformer for wide-angle scanning in hybrid antennas

This paper presents a new approach to beamforming in hybrid antennas. Using a scattering matrix model for the hybrid antenna system, a bidirectional transformation is developed that relates the signals at the hybrid system feed to the signals that would be present in a planar array at the location of the reflector aperture. For example, the received fields at the feed of a hybrid antenna system may be transformed into the fields at the reflector aperture, and these reflector aperture fields may then be processed as if they were received by a planar or linear array. Similarly, the desired field or current distribution across the reflector aperture when transmitting may be transformed into the required field or current distribution at the hybrid system feed. This method allows standard linear or planar array analysis and synthesis techniques to be used with the hybrid system. Examples are provided for transmit and receive weight synthesis.     

Radiation pattern synthesis for circular aperture horn antennas

A set of radiation pattern functions, suitable for synthesis of radiation patterns from circular aperture horn antennas, is obtained by assuming an aperture distribution consisting of the fields of cylindrical waveguide modes. A technique is presented for using a linear combination of the radiation pattern functions to approximate a desired radiation pattern. Linear combinations of the radiation pattern functions resulting in maximum secondary gain, when used to illuminate a paraboloidal antenna, are obtained empirically. Using spherical wave theory, maximum performance theoretically obtainable from an antenna is derived as a function of the aperture size of the feed system; the feed efficiency resulting from these theoretical limits on performance is compared to the feed efficiency of patterns obtainable from circular aperture horn antennas, and to experimental results of attempts to realize optimum circular aperture horn patterns.     

The efficiency of near-field Cassegrainian antennas

The aperture efficiency of a near-field Cassegrainian antenna is approximated with a Fresnel field analysis. Curves are presented which give the efficiency of the system in terms of the amplitude distribution on the feed aperture and the Raleigh distance to the subreflector. It is found that separation between feed aperture and subreflector can be as great asfrac{1}{2}d^{2}/lambdawithout excessive loss.     

Maximization of the directive gain of a parabolic reflector antenna

The directive gain of a parabolic reflector antenna is maximized by optimizing the feed aperture distribution. The feed aperture distribution is specified by a set ofNbasis functions weighted by coefficients to be determined. This approach is different from the conventional method where, given a particular feed, the directive gain is maximized by subjecting the reflector aperture parameters to optimization.     

Scalar horn with shaped lens improves Cassegrain efficiency

It is shown that the aperture efficiency of a classical Cassegrain antenna can be considerably improved if the radiation pattern of the feed is optimally shaped. The corresponding optimum field distribution Over the aperture of the feed consists of a circular main lobe which is surrounded by concentric sidelobe rings. This optimum field distribution with one sidelobe ring included is realized by shaping of a dielectric lens which is positioned in the aperture of a corrugated horn antenna. The design can provide a theoretical aperture efficiency of 90.5 percent when subreflector diffraction and aperture shadowing are neglected, i.e., an improvement of 0.4 dB compared to an optimum conventional feed. Measurements of a model at 22.8 GHz gave a practical result of 85.6 percent. If the model is used to feed a 30 m radiotelescope the overall antenna efficiency becomes about 71 percent.     

Synthesis of a laterally displaced cluster feed for a reflector antenna with application to multiple beams and contoured patterns

When a feed is displaced from the focus of a reflector, phase distortion results in the effective aperture distribution, which in turn gives rise to secondary beam distortion. In multiple beam or contour beam antennas, the feed normally consists of an array of identical elements located on a triangular lattice. Taking advantage of this arrangement, a "cluster" of feed elements instead of a single element may be used to control each beam. By adjusting the relative excitations of the elements in a cluster, the aperture phase distortion due to the feed displacement may be partially compensated. Two general methods for synthesizing the excitations for a laterally displaced feed cluster are presented. In the first method the excitations are chosen to minimize the weighted phase error in the effective aperture by analytical means. The second method determines the excitations by a gradient optimization algorithm which minimizes the weighted error between an objective and the actual power patterns in the secondary pattern space. The first method is roughly two orders of magnitude more efficient computationally than the gradient optimization algorithm, but not as flexible in application or as precise. Numerical results are presented for cluster feed designs and their application to the synthesis of contour patterns.     

Synthesis of offset dual reflector antennas transforming a given feed illumination pattern into a specified aperture distribution

The problem of transforming a given primary feed pattern into a desired aperture field distribution through two reflections by an offset dual reflector system is investigated using the concepts of geometrical optics. A numerically rigorous solution for the reflector surfaces is developed which realizes an exact aperture phase distribution and an aperture amplitude distribution that is accurate to within an arbitrarily small numerical tolerance. However, this procedure does not always yield a smooth solution, i.e., the reflector surfaces thus realized may not be continuous or their slopes may vary too rapidly. In the event of nonexistence of a numerically rigorous smooth solution, an approximate solution that enforces the smoothness of the reflector surfaces can be obtained. In the approximate solution, only the requirement for the aperture amplitude distribution is relaxed, and the condition on the aperture phase distribution is continued to be satisfied exactly.     

Multiple beam feed networks using an even number of beam ports

An aperture illumination compatible with the use of an even number of adjacent beam ports in a multiple beam feed network is discussed. The antenna pattern characteristics of near-in sidelobe levels, half-power beamwidth, aperture efficiency, and feed network loss are evaluated. Maximization of the available antenna gain at adjacent beam crossover points is shown to be possible for either sequential or simultaneons operation of a receiving system. The results presented indicate that lossy feed networks are quite suitable for certain array antenna applications.     

Optimum low sidelobe high crossover multiple beam antennas

Simultaneous multiple beam antennas (MBA's) with low sidelobe high crossover beams are considered. A simplified proof is presented which shows dissipative loss of typically -3 dB must be accepted. The optimum efficiency for the constrained fed MBA is shown to be achieved by placing attenuation in the aperture of a Butler matrix (BM) in accordance with the desired tapered aperture distribution. A combination of aperture attenuation and feed design, including the use of overlapping orthogonal feed distributions, is considered for lens type MBA's. The optimum efficiency is shown to be achieved with relatively simple feeds for a broad class of desired beam shapes.     

Reflection-lobe dispersion in a phased-array antenna

Reflection lobes in phase-scanned arrays with traveling-wave feeds can significantly degrade sidelobe levels. These lobes can be dispersed if the regularity of the feed system can be destroyed. In this communication, an aperture phase distribution is described that provides the maximum reflection-lobe dispersion for an arbitrary reasonably smooth amplitude distribution. This phase distribution is an explicit function of the amplitude distribution. The theoretical limitations on reflection-lobe dispersion in one- and two-dimensional arrays are determined and compared with the calculated properties of a sample linear array. The edge effects are also indicated.     

Analysis of a cluster feed for the Arecibo trireflector systemusing forward ray tracing and aperture integration

A cluster feed for the Gregorian dual-reflector feed of the Arecibo radio telescope is studied. The system is analyzed by forward ray tracing from each individual feed horn via the three reflectors to the aperture. The radiation pattern is determined by aperture integration. The analysis method is very time efficient compared with physical optics (PO) integration, the latter being unusable at high frequencies because the computer time increases by a factor proportional to the frequency to the power 4. Results obtained by the ray tracing method and PO integration are in very good agreement at frequencies low enough to be used for PO calculations. A special algorithm is developed to determine the squint and directivity of an individual main beam without computing the radiation pattern. This makes it possible to use the program very efficiently when searching for optimum cluster feed geometries. The computer programs are used to design a seven-element cluster feed for the Arecibo trireflector system     

Experimental verification of the effect of aperture shape on prompt IRA response

This article presents experimental measurements verifying the relationship between aperture shape and prompt response in an impulse radiating antenna (IRA). Various aperture configurations were realized by constructing a lens IRA with focusing aperture significantly larger than the transverse electromagnetic (TEM) feed structure and then eliminating portions of the aperture with metal blocks. Virtually any prompt response can be realized by a suitable construction. Experimental results agree well with predictions made by evaluating the aperture integral of the TEM mode distribution.     

Cross polarizing effects of a water film on a parabolic reflector at microwave frequencies

The cross polarizing effects introduced by a uniform water film on the surface of a parabolic reflector are evaluated at 10 and 34 GHz. The reflector is considered to be illuminated by an isotropic linearly polarized source at the focus, but the analysis can be applied for any given primary feed aperture field distribution. The aperture cress polarized field is obtained, and a worst case diffraction field (when opposite quadrants of the reflector are wet) is then calculated. A relatively low level cross polarizing effect is predicted, dependent on diameter-to-focal-length ratio.     

单脉冲低副瓣天线馈源系统

介绍了一种天线副瓣为－２７ｄＢ效率又较高的圆极化馈源系统。     

A shaped offset-fed dual-reflector antenna

A shaping scheme based on geometric optics for offset-fed dual-reflector antennas is presented. A ray tube emerging from a symmetric feed horn is transformed, after reflections, into a circular beam with a uniform phase and a prescribed radial power distribution on the aperture. In this scheme, Snell's law was not imposed on the main reflector. Based on this approximate solution, computer runs were taken for a 5.5-m dish baseline system, and very satisfactory results were obtained. The system so designed not only gives very low sidelobes but also provides a very high aperture efficiency. At 12 GHz an estimated 84 percent of aperture efficiency was achieved in spite of the severe constraint that the ray intersecting the edge of the main reflector meet a -10-dBi criterion.     

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     

Analysis of mutual coupling between a finite phased array ofdipoles and its feed network

The mutual coupling effects between a finite phased array of dipoles and its feed network are analyzed. The feed network is typically a corporate feed consisting of split-tee power dividers cascading to form a certain power distribution over the aperture. A simple iterative approach is used to solve the interaction between elements and feed. The radiation of a finite dipole array are first found for a given voltage excitation. These radiation impedances are then used as loads for the feed network, and the n+1 port network problem is analyzed. Due to the interaction between the feed network and dipoles, the antenna parameters such as mismatch, antenna pattern, and gain are all affected. These effects can be determined from the analysis of the network representation. Numerical results for a typical phased array with a corporate feed show that the resultant VSWR of the feed pattern degradation is due to the mutual coupling effects     

非对称副镜卡塞格伦天线辐射场的计算

深空探测天线的多频段特性要求天线能够实现馈源间的快速切换。针对以旋转非对称副镜方式切换频段的大型卡塞格伦天线不再满足原有的口面场分布,利用口面场分布函数积分求解天线远场辐射方法失效的情况,提出了一种计算该型天线的有效方法。给出了一种适合于数值计算的镜面网格划分方法,并利用线性相位近似的物理光学法计算了副镜散射场及天线远区辐射场。计算结果与软件仿真结果的对比表明这种方法是准确实用的。     

Dual parabolic cylindrical reflectors employed as a compact range

Compact ranges using dual parabolic cylindrical reflectors are investigated, and the dependence of the aperture field on the feed pattern and system geometrical parameters is studied. A uniformity factor is defined to indicate the aperture field uniformity and the significance of the diffracted fields is explored. Offset configurations are also considered and studied. It is shown that their feed location and orientation can be optimized to minimize the geometrical-optics cross polarization. The effects of offset angle on the edge diffraction and aperture shape are also studied     

Scanning properties of large dual-shaped offset and symmetricreflector antennas

The scanning properties of shaped reflectors, both offset and circularly symmetric, are examined and compared to conic section scanning characteristics. Scanning of the pencil beam is obtained by lateral and axial translation of a single point source feed. The feed is kept pointed toward the center of the subreflector. The effects of power spillover and aperture phase error as a function beam scanning are examined for several different types of large reflector design including dual-offset, circularly symmetric large f/D, and smaller f/D dual reflector antenna system. It is shown that the Abbe-sine condition for improved scanning of an optical system cannot, inherently, be satisfied in a dual-shaped reflector system that is shaped for high gain and low feed spillover. The gain loss, with scanning, of a high-gain shaped reflector pair is demonstrated to be due to both aperture phase error loss and power spillover loss