New advances on multi-frequency and multi-beam reflectarrays with application to satellite antennas in ka-band

Resumo

Current high throughput satellites (HTS) in Ka-band are required to provide multiple spot beam coverage based on frequency and polarization reuse, both in transmission (Tx, 19.2-20.2 GHz) and reception (Rx, 29-30 GHz). A four colour scheme with two frequencies and two polarizations is normally used, in which adjacent spots must be generated in a different frequency and/or polarization. The design of multi-beam antennas for Ka-band HTS systems must cope with some challenging requirements: generation of a large number of beams (normally between 50 and 100), very small separation between adjacent spots (a typical value is 0.56º), low spillover, etc. To confront these stringent conditions, most of current HTS systems carry four reflector antennas on board the satellite, each reflector being responsible for generating all the beams in the same frequency and polarization (same colour) in a single feed per beam basis. The problem of this configuration has to do with the accommodation of the four reflectors in the satellite. A reduction in the number of apertures required to provide multi-spot coverage would result in significant savings in the cost, weight and volume of the antenna farm in communication satellites that operate in Ka-band. The motivation of this thesis has been to provide new advances on the design of multi-frequency and multi-beam reflectarray antennas with application to multiple spot beam satellites in Ka-band. In this respect, the thesis can be divided into two main parts: the first part on reflectarrays operating at two different frequencies, and the second for the developing of design techniques to improve the performance of multi-beam antennas. The first part of the thesis contains the description of a novel reflectarray cell to operate in dual-linear polarization (LP) at two separate frequencies (enabling independent phasing in each polarization and frequency), as well as the design of dual-band reflectarrays to provide independent beams in each polarization and frequency band, including the manufacturing and testing of a 25-cm reflectarray demonstrator to operate in dual polarization (linear or circular) in Ku and Ka bands. The reflectarray element proposed for independent operation in dual-LP at two separate frequencies consists of a two-layer configuration with two orthogonal sets of stacked parallel dipoles. Each set, that adjusts the phase in one polarization, is composed of five parallel dipoles on the lower layer and three additional parallel dipoles stacked above the previous ones and printed on top of a second dielectric sheet. The geometrical parameters of the cell have been adjusted to operate, first, at Tx frequencies in Ku and Ka bands (12 and 20 GHz), and then, at Tx and Rx frequencies in Ka-band (20 and 30 GHz). The proposed two-layer configuration allows to perform separate design processes for each reflectarray layer: first, the lengths of the lower dipoles are adjusted to match the required phases at the lower frequency, and then, the lengths of the upper dipoles are adjusted to introduce the required phase-shift at the higher frequency. This step-by-step procedure allows for a simpler and computationally faster design process. Moreover, the design is carried out independently for each linear polarization, by adjusting the set of dipoles associated to each polarization. A Ku/Ka-band reflectarray demonstrator of 25-cm diameter has been designed, manufactured and tested, in order to validate the multi-frequency reflectarray cells and the design technique. The proposed reflectarray permits an independent optimization of the radiation patterns for Ku and Ka bands, as well as a proper accommodation of the feed chains for each frequency band. This concept can be applied to design a satellite transmit antenna which would be able to fulfill independent requirements at each frequency and/or polarization (for example, generation of a contoured beam in Ku-band and multiple spots in Ka-band) by properly designing the elements on each reflectarray layer, using different feed chains for each mission. Moreover, the manufacturing using the technology for multi-layer printed circuits and low profile of the sandwich would lead to significant savings in the costs, weight and volume of the antenna farm for current satellite systems that operate in Ku and Ka bands, thanks to the reuse of the same aperture for two different missions. The second part of the thesis comprises the development of a bifocal design technique for dual reflectarray and dual transmitarray configurations, and its application to the design of multi-beam antennas in Ka-band. The aim of the bifocal technique is twofold, to improve the multi-beam performance of the antenna and to provide a certain degree of reduction in the angular separation between adjacent beams for a multi-spot coverage from a satellite. Two different approaches have been considered: starting from an axially-symmetrical geometry which allows the rotation of a 2D bifocal design around the symmetry axis, and implementing a general 3D bifocal method that directly provides the required phases on both reflectarrays in the selected antenna configuration. First, a bifocal design procedure has been developed for both dual reflectarray and dual transmitarray antennas by starting from an axially-symmetrical geometry with the two reflectarrays/transmitarrays placed in parallel planes. A 2D bifocal design performed in the offset plane by means of an iterative ray-tracing routine is rotated around the symmetry axis, and then, both centered and offset configurations are possible by choosing specific portions of the revolution surfaces. In the case of offset dual reflectarray configurations, both reflectarrays can be tilted a certain angle to obtain smoother phase distributions. For this purpose, a novel phase adjustment routine has been implemented to compensate the tilting and maintain the bifocal characteristic of the original design. On the other hand, the design with transmitarrays provides some advantages, such as lower sensitivity to surface deformations, absence of blockage and use of centered geometries with a focal ring. These advantages are achieved at the cost of a larger antenna volume. Hence, different dual transmitarray configurations have been studied to try to reduce the antenna volume, such as placing the feeds close to the first transmitarray (to integrate both elements on the same sub-system), or reducing the distance between the transmitarrays (to hold them with the same supporting structure). Secondly, a general tridimensional bifocal technique for dual reflectarray antennas has been developed, which makes it possible the direct synthesis of the required phase distributions on each reflectarray without imposing geometrical restrictions in the antenna configuration. The proposed 3D bifocal method is based on an iterative 3D ray-tracing routine that provides a grid of points on the surface of each reflectarray and the values of the partial derivatives of the phase associated to those points. The partial phase derivatives are interpolated, and then, properly integrated to obtain the bifocal phase functions required on each reflectarray. A preliminary study on the bifocal technique for the design of multi-beam satellite antennas in Ka-band has been carried out, considering three different degrees of reduction in the beam spacing with respect to the equivalent monofocal antenna: high beam spacing reduction (by a factor of 2, in order to provide adjacent beams with 0.56º separation), low beam spacing reduction (by a factor of 1.1 or 1.2), and no beam spacing reduction. The results show that the bifocal technique allows to provide the required 0.56º spacing by using non-overlapping feeds, but at the cost of a lower radiation efficiency of the bifocal antenna (the main reflectarray should be significantly oversized). The most interesting case is that for low beam spacing reduction, which allows to obtain closer beams with non-overlapping feeds, at the same time as improving the performance of the extreme beams and providing reasonable values of gain and radiation efficiency. A bifocal dual reflectarray antenna demonstrator with a main reflectarray of 57-cm has been designed, manufactured and tested in order to validate the proposed 3D bifocal technique. The demonstrator has been designed to operate in dual-LP in the 19.2-20.2 GHz band, but the technique can be also used to generate adjacent beams in dual-circular polarization by using a sequential rotation method. The results of the measurements show the capability of the bifocal technique to reduce the beam spacing and provide a better multi-beam performance than the equivalent single-focus antenna (particularly, the gain and side lobes are improved for the most external beams). The first factor will allow to reduce the antenna size with respect to conventional reflectors to provide the same beam spacing. Moreover, the fabrication of the bifocal dual reflectarray antenna involves the same conventional processes used for printed reflectarrays, without any need of custom moulds, allowing a significant reduction of manufacturing time and cost, particularly when compared with bifocal dual reflectors that require expensive custom moulds for the two shaped reflectors. Finally, a bifocal dual reflectarray antenna with an elliptical main reflectarray has been proposed to provide all the required spots (four colours) for transmission from a geostationary satellite in Ka-band, in order to substitute the four conventional antennas (one for each colour). The bifocal technique has been applied with a high degree of beam spacing reduction to produce adjacent beams with 0.56º separation in the offset plane, while using a monofocal phase condition in the orthogonal plane (beam spacing around 1.1º). The interleaved beams required for providing full multi-spot coverage are generated in the orthogonal polarization. This solution presents some advantages with respect to other configurations that use a single oversized reflector to provide multi-spot coverage, as it requires a smaller aperture size and a lower number of feeds. The use of flat reflectarray panels, which can be fabricated by the same conventional and relatively inexpensive processes used for printed circuits, allows for a more efficient packaging and deployment on the satellite. Moreover, the design of a Tx/Rx multiple spot beam satellite antenna can be addressed by the use of appropriate dual-frequency reflectarray cells that will enable independent phasing at Tx and Rx frequencies in Ka-band.