The Active Compensation Technique for Large Reflector Antennas Based on Quadratic Curve Fitting

Active reflectors are often used to compensate the surface distortion caused by environmental factors that degrade the electromagnetic performance of large high-frequency reflector antennas.This is crucial for maintaining high gain operation in antennas.A distortion compensation method for the active reflector of a large dual-reflector antenna is proposed.A relationship is established between the surface deformation and the optical path difference for the primary reflector by geometric optics.Subsequently,employing finite element analysis,a polynomial fitting approach is used to describe the impact of adjusting points on the reflector surface based on the coordinates of each node.By standardizing the positions of various panels on the reflector,the fitting ns can be applied to the reflector panels of similar shapes.Then,based on the distribution characteristics of the primary reflector panels,the adjustment equation for the actuators is derived by the influence matrix method.It can be used to determine the adjustment amount of actuators to reduce the rms of the optical path difference.And,the least squares method is employed to resolve the matrix equation.The example of a 110 m aperture dual-reflector antenna is carried out by finite element analysis and the proposed method.The results show that the optical path difference is reduced significantly at various elevation cases,which indicates that the proposed method is effective.

Influence and Correction of Wavefront Primary Aberrations for Radio Telescopes using Aberration Theory

Dual-reflector antennas are widely used in astronomical observations and satellite communication.Structural deformations of the reflectors for radio telescopes are inevitable in outside working conditions due to exterior environment loads,which will cause distortion in the surface of the primary reflector and displacement of the subreflector,then lead to gain degradation and misalignment.In this paper,the influence and correction of misalignment in a dual-reflector antenna have been studied.From the perspective of wavefront aberration,a method is proposed to correct the wavefront primary aberration by adjusting the subreflector position.The characteristics of wavefront errors caused by structural deformation of the reflector have been analyzed,and relationships between the position motions of the subreflector and the Seidel wavefront aberrations are derived.The adjustment quantities of the subreflector are also derived.The results show the appropriate positional change of the subreflector in the lateral and axial directions can effectively correct the effects of the tilt and defocus in the primary aberrations caused by antenna structural deformations.

Optimal subreflector position determination of shaped dual-reflector antennas based on the parameters iteration approach

A new method based on the parameters iteration technique has been developed to determine the optimal subreflector position for shaped Cassegrain antennas, that are distorted by gravity, to improve their electromagnetic(EM) performance. Both the features of shaped surface and the relationship between optical path difference(OPD) and far field beam pattern are employed. By describing the shaped dualreflector surface as a standard discrete parabola set, we can utilize the optical features of the standard Cassegrain system in the classical OPD relationship. Then, the actual far field beam pattern is expressed as the synthesis of ideal beam and error beam by decomposing subreflector adjustment parameters using a mechanical-electromagnetic-field-coupling-model(MEFCM). Furthermore, a numerical method for determining optimal subreflector position is presented. The proposed method is based on the iteration technique of subreflector adjustment parameters, and the optimal far field pattern is used for the iteration. The numerical solution of optimal adjustment parameters can be obtained rapidly. Results for a 25 m shaped Cassegrain antenna demonstrate that the adjustment of the subreflector to the optimal position as determined by the proposed method can improve the EM performance effectively.

An adjustment method for active reflector of large high-frequency antennas considering gain and boresight

The design of the Qitai 110 m Radio Telescope（QTT） with large aperture and very high working frequency（115 GHz） was investigated in Xinjiang, China. The results lead to a main reflector with high surface precision and high pointing precision. In this paper, the properties of active surface adjustment in a deformed parabolic reflector antenna are analyzed. To assure the performance of large reflector antennas such as gain and boresight, which can be obtained by utilizing an electromechanical coupling model, and satisfy them simultaneously, research on active surface adjustment applied to a new parabolic reflector as target surface has been done. Based on the initial position of actuators and the relationship between adjustment points and target points, a novel mathematical model and a program thatdirectly calculates the movements of actuators have been developed for guiding the active surface adjustment of large reflector antennas. This adjustment method is applied to an 8 m reflector antenna,in which we only consider gravity deformation. The results show that this method is more efficient in adjusting the surface and improving the working performance.

Modified scaling angular spectrum method for numerical simulation in long-distance propagation

The angular method(AS)cannot be used in long-distance propagation because it produces severe numerical errors due to the sampling problem in the transfer function.Two ways can solve this problem in AS for long-distance propagation.One is zero-padding to make sure that the calculation window is wide enough,but it leads to a huge calculation burden.The other is a method called band-limited angular spectrum(BLAS),in which the transfer function is truncated and results in that the calculation accuracy decreases as the propagation distance increases.In this paper,a new method called modified scaling angular spectrum(MSAS)to solve the problem for long-distance propagation is proposed.A scaling factor is introduced in MSAS so that the sampling interval of the input plane can be adjusted arbitrarily unlike AS whose sampling interval is restricted by the detector’s pixel size.The sampling interval of the input plane is larger than the detector’s pixel size so the size of calculation window suitable for long-distance field propagation in the input plane is smaller than the size of the calculation window required by the zero-padding.Therefore,the method reduces the calculation redundancy and improves the calculation speed.The results from simulations and experiments show that MSAS has a good signal-to-noise ratio(SNR),and the calculation accuracy of MSAS is better than BLAS.