Servo-Performance Parameters of the NASA Deep Space Network Antennas

The performance of an antenna control system is evaluated using performance parameters such as settling time, bandwidth, steady-state error in rate offsets, and antenna root-mean-square servo error while tracking in wind gusts. The performance parameters are measured in the field, or obtained through analysis and simulations. Limited access time to antennas, incomplete test equipment, limited test/analysis time, and partial models do not allow determination of all the parameters. However, field practice and analytical results indicate correlations among them; hence, even incomplete knowledge of the performance parameters would allow for estimation of the missing parameters. This paper investigates the relationships of the antenna performance parameters as a function of controller gains. It also establishes the interrelationships among the parameters. It does this for an idealized (or rigid) antenna, and extends the relationships to the NASA Deep Space Network antennas (flexible structures with dish sizes of 34 or 70 meters). The results obtained should simplify antenna testing, and allow for better performance evaluation from limited data.     

The SNFGTD method and its accuracy

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.     

The calculation of radiation characteristics of antennas near a rectangular cylinder reflector by GTD

The horizontal radiation patterns of dipoles and double-loop antennas near a rectangular cylinder reflector are calculated and analyzed by the geometrical theory of diffraction (GTD). The magnitudes of the direct field, the reflected field and the diffracted field at a field point are studied. The effects on the resultant field pattern of the size of the reflecting cylinder, of the relative position between the antennas and the reflecting cylinder, and of the methods of feeding of antennas are analyzed. The results obtained show that in order to obtain the optimum omni-directional characteristics of rediation, the optimum design for the parameters of antenna system is needed. The method and results in this paper provide means and bases for the optimum design of this kind of antenna system.     

Diffraction coefficients for a slot-excited conical antenna

First-order diffraction coefficients for a slot-excited conical antenna are calculated, beginning with integral expressions for the radiation field. Simple formulas based on these coefficients are presented for the radiation patterns that correctly reproduce the interference structure as well as predict the magnitude of the field down to quite low frequencies. Plots of the diffraction coefficients are given for both symmetric and asymmetric excitation of a circumferential slot. Radiation fields are compared with those obtained by summing the exact normal mode series.     

Pattern distortion of aperture antennas radiating in the presence of conducting platforms

An analytically simple and numerically efficient technique for calculating the pattern distortion of aperture antennas radiating in the presence of conducting platforms located in the near or far field of the antenna is presented. The technique presented, based on uniform geometrical theory of diffraction (UGTD), is also applicable for large aperture antennas (aperture area> 15 lambda^{2}). An excellent agreement between the calculated and measured results obtained for a typical aperture antenna mounted on a conducting platform confirms the validity of the analytical technique developed.     

Asymptotic transition region theory for edge diffraction. II.Calculation of diffraction losses in multireflector antennas

For pt.I see ibid., vol.38, no.9, p.1350-8 (1990). Asymptotic formulas that can be used to calculate diffraction losses in a multireflector antenna without having to integrate rapidly varying fields over the reflector surfaces and the aperture are presented. Two kinds of losses caused by edge diffraction are considered: the reduction in antenna efficiency and the increase in spillover. The asymptotic formulas are obtained from the standard transition region field introduced in part I and are expressed in terms at the Δρ defined there     

Investigation of the parasitic loop counterpoise antenna

The radiation field produced by a parasitic loop counterpoise antenna is investigated. This is a new antenna configuration which consists of a small circular loop in the presence of a large parasitic loop and is placed above a circular ground plane or counterpoise. Geometrical theory of diffraction is applied to obtain the theoretical expressions for the far field produced by such an antenna. Within the range of approximation the agreement between theory and experiment has been found to be very good. The introduction of a parasitic loop, in general, reduces the ground plane edge diffraction effects on the far-field patterns and makes the overall pattern in the half-plane above the counterpoise less directional. Possible application of the antenna is also discussed.     

圆口径正馈抛物面天线的近场分析

分析了圆口径正馈抛物面天线的近场.采用口径场绕射积分求取了天线的近场及远场方向图,绘制了波束宽度、方向性和最大副瓣电平三个关键参数随场点距离的关系曲线.通过与物理光学法的计算结果对比,验证了该方法的正确性.仿真结果是:随场点距离的减小,波束宽度展宽,方向性减小,最大副瓣电平在一定范围内波动.场点在D2/λ外,远场近似的波束宽度误差小于1%;在0.5D2/λ外,远场近似的波束宽度误差小于10%;在0.5D2/λ内,不能采用远场近似.     

Characteristics of near field of the horizontal vibrator located above a square screen

The 3D vector problem concerned with the diffraction of the horizontal half-wave vibrator field by a perfectly conducting and infinitely thin rectangular screen is solved during observations at a finite distance from the vibrator. The technique for calculating the diffracted field, which is based on the laws of geometric diffraction theory and uniform diffraction coefficients obtained from the rigorous solution to the problem of the diffraction of the field of a Hertzian electric dipole at the edge of a perfectly conducting semiplane, is proposed. Fast-acting programs for calculating and analyzing the amplitudes and phases of the components of diffracted and complete vibrator fields in the entire space of observation angles at the set distance of the observation point from a rectangular screen and along the normal to the screen under changes in the distance between the screen and the observation point are developed. The character of the transformation of the spatial amplitude distribution of three field components depending on removal of observation point from an antenna is studied. It is shown that the distance to the far field zone of the investigated radiating system exceeds two wave lengths.     

稳定型准光腔外衍射场计算

稳定型准光腔外衍射场的计算是实现准光功率合成与天线一体化设想的关键性问题。本文在A.L.Cullcn提出的准光腔复源点理论和L.B.Felsen提出的复射线理论基础上结合几何分析和计算,得到了E和H平面远区方向性图。本文的计算与实验结果基本吻合。     

Field correlation diffraction theory of the symmetrical cassegrainian antenna

The received field as focused by the parabolic main reflector of a Cassegrainian antenna at the surface of an arbitrary profile subreflector is calculated by a spherical wave expansion. This facilitates the application of the field correlation principle and leads to an expression for aperture efficiency taking into account diffraction effects. A comparison is made with numerical results previously published or obtained by other methods. The potential advantage of the technique is the speed of computation and the capability for synthesis as well as analysis of reflector shapes.     

Scattering of the electromagnetic wave by a phased antenna array with consideration for the radome

A model for calculating the field scattered by the antenna of an aircraft radar with consideration for the radome is described. Backscattering patterns of phased antenna arrays with and without a radome are presented. Substantial contribution of diffraction lobes into the radar visibility of the antenna is noted.     

On the use of antenna weight functions in field strength prediction and interference reduction

In this paper two methods for calculating the received electromagnetic field by a single-parabolic reflector antenna in the shadow region behind a finite-width screen are proposed and analysed. The first one is referred to as the far-field approach and treats the obstacle and reflector antenna diffraction separately. The antenna simply is replaced by a point source having the receiving properties of the reflector antenna considered. The second method is called the near-field approach and considers the combined effect of obstacle and antenna diffraction. It is shown that considerable differences between the results of both methods may exist, even for an obstacle-antenna separation large compared to the Rayleigh distance of the antenna, and both for a CW and broadband analysis of the communications channel. It is concluded that the near-field method gives the best results and can be applied to many practical problems such as interference reduction and searching the optimal position of VSATs in urban environments.     

An aperture-matched compact range feed horn design

The moment method and the uniform geometrical theory of diffraction are used to obtain two separate solutions for theE-plane far field pattern of an aperture-matched horn antenna. This particular horn antenna consists of a standard pyramidal horn with the following modifications: a rolled edge section attached to the aperture edges and a curved throat section. The resulting geometry provides significantly better performance in terms of the pattern, impedance, and frequency characteristics than normally obtainable. The moment method is used to calculate theE-plane pattern and voltage standing-wave ratio (VSWR) of the antenna. However, at higher frequencies, the moment method requires large amounts of computation time. On the other hand, the uniform geometrical theory of diffraction provides a quick and efficient high frequency solution for theE-plane field pattern. In fact, the uniform geometrical theory of diffraction may be used to initially design the antenna; then the moment method may be applied to "fine tune" it. In both methods, a two-dimensionalE-plane model of the antenna is used, but these two-dimensional solutions yield excellent agreement with measured data of the actual three-dimensional antenna. This procedure has been successfully applied to design a compact range feed horn.     

Metrological characteristics of antenna system for measuring electromagnetic field parameters

Errors in measuring the electromagnetic field parameters of a five-element antenna system have been analyzed. Analytical relationships for main errors were derived, and maximum values of measurement errors were calculated.     

Calculation of Near Field of Circular Aperture Antenna Using Geometrical Theory of Diffraction

A tutorial account of the geometrical theory of diffraction is presented, and as an application of the theory the near field of a circular aperture antenna is calculated. This method is useful in calculating quite accurately the field in regions far from the antenna axis. The method finds appeal in its simple mathematical formulation which lends itself to obtaining numerical results with relative ease.     

Near-field scattering by physical theory of diffraction andshooting and bouncing rays

This paper proposes a method to compute the near-field RCS and Doppler spectrum of a target when the distances to the antennas are comparable to the target size. By dealing with a small piece of the target surface at a time, the transmitting antenna, and the receiving antenna are in the far-field zone of the small piece of the induced currents. The electromagnetic field produced by this small piece of induced currents can be written as a spherical wave. Sum up all spherical waves produced by every small piece of induced currents and we can obtain the total scattered field at the receiving antenna. The physical theory of diffraction (PTD) and the method of shooting and bouncing rays (SBR) are modified to evaluate the received signals. Numerical results based on these techniques are obtained and discussed. The formulation applies the simple concepts of “equivalent” image and vector effective height, which are believed to be novel     

硅基光学相控阵扇形天线优化设计

在光学相控阵芯片中，天线单元的性能直接决定了波导中光波能量向外辐射的效率。在此基础上，主要针对扇形结构天线进行优化设计，通过对扇形天线增加浅刻蚀区域和硅薄带的设计，对天线向下衍射损失以及背反射损失进行了抑制，大幅提高了天线向上衍射效率。此外，分别对提高向上衍射效率和减小设计尺寸的设计要求制定了设计方案，对于高衍射效率方案，天线向上衍射效率达到81.6%，向下衍射减少到4%，背反射减少到1.4%；对于小型化方案，天线向上衍射效率达到67.2%，向下衍射减少到5.6%，背反射减少到1.2%，大幅提高了光学相控阵芯片集成度及发射效率。     

Monopulse Operation with Continuously Variable Beamwidth by Antenna Defocusing

Monopulse tracking operation with continuously variable beamwidth may be obtained by antenna defocusing. To demonstrate this property, antenna patterns (including phase) of a square aperture for large amounts of defocusing have been computed by diffraction theory for the case of highly tapered illumination in both the sum and difference antenna modes. From an examination of the computed patterns over a 10 to 1 beamwidth range, it is shown that with the increasing defocusing, the patterns in the sum and difference modes exhibit smoothly increasing width. Some change of shape is noted, but the pattern amplitudes and phase relationships are still suitable for on-axis tracking or off-axis location by monopulse techniques. To achieve monopulse operation with the defocused patterns two modifications to ordinary monopulse receivers are necessary. A phase shifter, inserted in the difference or sum channel and ganged to the amount of defocusing, is needed to cancel the phase variation with defocusing. In addition, amplitude correction is required to keep the error-signal slope constant.     

Theory of double parasitic loop counterpoise antenna radiation patterns

Concepts of ray optics and the results of Sommerfeld's half-plane diffraction theory are applied to obtain theoretical expressions for the elevation plane radiation field produced by double parasitic loop counterpoise antennas. Theoretical results are compared with results obtained from model measurements. Within the range of approximation the agreement between theory and experiment has been found to be satisfactory. A method of designing the antenna for special application to air traffic control systems is also discussed briefly.