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The size, cost, and complexity of radio telescopes and their instrumentation have grown enormously during the last few decades. The vast majority of new technology employed by radio astronomy has been developed for the industrial market, and the technology in use by today’s best radio telescopes is 20 to 30 years old. In particular, the antenna technology has changed very slowly, and thus, an important question is whether novel antenna technologies might bring down the cost of collecting area, since efficiency and accuracy compromises become much more costly for large radio telescopes. In this chapter, we discuss the strengths and weaknesses of specific new technologies, and in particular reflectarrays, which have been developed mainly for remote sensing and satellite communications, that might lead to a great leap forward in the design and fabrication of antennas for radio astronomy.
Radio astronomy is currently going through a critical transition, where novel electromagnetic (EM) technologies and approaches are replacing the standard radio astronomical methods for radiation detection. These new technologies are mainly based on the use of arrays, the control and modification of the incident wavefront, the use of “metamaterials,” and the continuous improvement of the performance of both the telescope and its auxiliary instrumentation. Historically, the metrics used to evaluate the performance of a filled-aperture (or single-dish) radio telescope is based on three main parameters: sensitivity, angular resolution, and spectral resolution. The classical approach to improve the first two parameters consists of using a larger diameter for the primary reflector, leading to much increased engineering complexity and construction costs. As a consequence of this “static” process, where the technology in use by today’s best radio telescopes has not changed significantly, the size, cost, and complexity of radio telescopes and their instrumentation have grown enormously during the last few decades.
In particular, while digital technologies applied to radio astronomy have basically followed Moore’s law, antenna technology has changed very slowly, whereas the technology of cryogenic amplifiers has followed a somewhat intermediate improvement rate between the previous two extremes. Therefore, in the development of ever larger and better radio telescopes, with expanded collecting area, field of view (FOV), frequency coverage, instantaneous bandwidth, system temperatures, and spatial resolution, the main bottleneck is constituted by the antenna design and fabrication techniques. In fact, the current paradigm is that, to a first approximation, ten times as much money will buy ten times as much collecting area, all else being equal. In this scenario, it is important to explore the possibilities offered by new technologies and design approaches and analyze how they might be able to improve the capabilities of new and existing telescopes in an engineering and cost-effective way.
The vast majority of new technology employed by radio astronomy has been initially developed for the industrial market, and then, it has been adapted for use with radio telescopes and their instrumentation. During this process, a great deal of design and laboratory work is usually required to learn and incorporate these new technologies. For the antennas, the transfer of new designs and fabrication techniques from the ground and satellite communication industry to radio astronomy has proved to be even slower. This is mainly because technological transfer from one field to another always requires a significant (and costly) development phase and also because of a certain degree of risk associated with this adaptation process. However, new antenna technologies are now sufficiently mature to advocate preliminary studies for their potential applications to radio astronomy, at frequencies higher than a few 100 s MHz.
2. Recent telescope development for radio astronomy
In the late 90s and early 2000s, several trends emerged in the development of filled-aperture radio telescopes, specifically in the case of electrically large (D/λ≳5×104 antennas for the microwave/millimeter wave range (the main components of a dual-reflector radio telescope are schematically indicated in Figure 1). These trends concerned both the mechanical and the optical design of the telescope, which also had to accommodate the newly developed heterodyne and bolometric focal plane arrays (e.g., the review in [1]). In particular, the mechanical design was generally focused on achieving the best possible homology. On the other hand, depending on the scientific programs, the optical designs had different prominent features, such as off-axis (e.g., Green Bank Telescope, GBT), wide-field (e.g., South Pole Telescope, SPT), and shaped optical configurations (e.g., Sardinia Radio Telescope, SRT). If we extend our review into more recent years and also to the submillimeter wave range we can note that three main classes of radio telescopes have been finalized or are currently in the design or construction phase (see also Table 1): (i) large (≳50 m) single-dish instruments (e.g., LMT, AtLAST); (ii) extended synthesis arrays (e.g., ALMA, SKA, ngVLA); and (iii) smaller instruments designed for specific projects or with specific enhanced performance (e.g., CCAT/FYST).
Figure 1.
Example of a classical dual-reflector radio telescope working in the microwave region. Radio waves reflect off the primary and secondary surfaces and focus at the position of the feed horn (or array of feed horns), where the transition from propagation in free space to guided propagation takes place. Receivers and other auxiliary devices then amplify, detect, and analyze the radio signals.
Technical characteristics of some representative radio telescopes currently in operation or in the design/construction phase. For arrays of antennas, the properties of the individual antenna are shown.
Sardinia Radio Telescope (http://www.srt.inaf.it/)
Despite the great variety in the specific design, overall performance, and scientific applications, all of the telescope projects described above are still based on standard technologies for designing the collecting area of the instruments (in particular at frequencies higher than a few 100 s MHz), with possibly some innovations only in the fabrication of the surfaces themselves, such as hydroforming. Most of the design efforts were instead concentrated in the development of active surfaces and metrology systems, improved optical designs, and the use of novel instruments at the focal plane of the telescope, such as focal plane arrays and phased array feeds. Thus, most of the technological innovations have gone in the front-end and back-end development and have resulted in improved frequency coverage, FOV, instantaneous bandwidth, and system temperature.
We may note that while optical telescopes have indeed been characterized by the use of a novel technology to design and build the primary reflector, specifically, the International Liquid Mirror Telescope (ILMT1), to our knowledge the only novel and funded concept for a radio to submm telescope design has been the Large Balloon Reflector (LBR2), where the main reflector of the telescope is a metallized hemisphere of a smaller spherical balloon, 5 to 20 m in diameter, inflated inside the much larger (100 m) carrier stratospheric balloon. Although the LBR represents a game-changing approach to realizing large aperture telescopes at THz frequencies, this project was specifically designed as a balloon-borne telescope and cannot be easily adapted to build ground-based telescopes.
Even national, or international consortia borne to foster the development of radio astronomy, such as RadioNet3 in the past, and more recently Radioblocks4 that has been specifically proposed and funded to develop next-generation technologies for radio astronomy infrastructures, will mainly focus on the development of new front-ends and back-ends, but not on antenna technology. Given that the cost of large instruments for radio astronomy is largely dominated by the antennas, both during the design/construction phase and later in terms of maintenance costs [2], this leaves the most impacting component in the design and fabrication of a radio telescope without an adequately sustained technology R&D program. Since the current trend in the development of large-scale international radio telescope projects is to explicitly include cost in the design process, it becomes extremely important to explore the parameters space based on the use of new technologies for the design and fabrication of the antennas.
During the last 10–20 years, great efforts have been carried out in the radar and communications industry to control and manipulate the phase (and, in some cases, also the amplitude) of the incident and reflected, or transmitted, waves. The use of these phase controlling devices is likely to continue to expand in the near future, also for photonics applications. At the basis of this technological development lies the concept of the metamaterials, which are three-dimensional (3D), often periodic, artificial materials composed of metals and/or dielectrics, with the ability to modify waves beyond the capabilities of naturally occurring or homogeneous materials (e.g., the review in [3]). Their exceptional ability to manipulate waves is due to their interaction with electric and/or magnetic fields, which can be controlled by the geometry of the individual artificial scatterers, and leads to a wide range of applications such as antenna performance enhancement, perfect absorbers, superlenses, in a variety of wavebands from the microwave to the optical range.
Metasurfaces are two-dimensional (2D) or planar versions of metamaterials with subwavelength thickness, and consist of thin artificial layers with periodic arrangements of small inclusions in a dielectric host medium. They are characterized by light weight and ease of manufacturing, and by engineering the metasurface with different properties to achieve specific functions, numerous applications are exploited in electromagnetics ranging from microwave to terahertz (THz) frequencies and even to optical frequencies, for example, sensors, lenses, absorbers, and reflectarrays. Different functions in these applications are achieved by different unit cell geometries with required reflection or transmission properties. Depending on the specific design and application, and the size of the unit cells, metasurfaces are also sometimes known as phase-shifting surfaces (PSS, [4]), frequency-selective surfaces (FSS), intelligent reflecting surfaces (IRS), or Huyghens’ surfaces [5]. All applications of metasurfaces mentioned above have intrinsic fundamental limitations including narrow bandwidth, linear response, fixed functionality, etc., that will be discussed later. However, recent studies on active and nonlinear metasurfaces have enabled the design, fabrication, and test of metasurfaces with nonlinearity, power dependency, tenability, and switching abilities. This has been achieved by applying active electronics like transistors, diodes, and varactors to traditional passive structures.
Metasurface applications of interest to radio astronomy are mainly those related to wavefront engineering such as beam shaping, including focusing, defocusing, reflection, and refraction [5]. This class of metasurfaces is most commonly known as reflectarrays and transmitarrays (see Figure 2). A reflectarray is a flat, planar, compact structure, typically illuminated by a feed, consisting of an array of isolated radiating elements that are predesigned with a particular phase delay to re-radiate and scatter the incident field to form a desired wavefront. The individual resonant unitary elements are called unit cells and are predesigned with a particular phase delay to scatter the incident field to form the desired wavefront in front of the aperture. Thus, although they are physically flat, reflectarrays behave electrically as a curved reflector. Reflectarrays are manufactured by photo-etching and bonding processes, which are a well-established technology in multi-layer printed circuit boards.
Figure 2.
Generic model of a transmitarray (top panel) and a reflectarray antenna (bottom panel). {ϕ1, ϕ2, ...., ϕn} indicate the phase delays applied to the incident wavefront by each unit cell to scatter the incident field and form the desired wavefront.
A transmitarray (or array lens antenna) is also composed of an array of multiple discrete elements used as a transmitting device. The name is inspired by the reflectarray, but the device is used as a transmitting surface rather than a reflective one. When made out of printed metallic elements, both the reflectarray and the transmitarray are physically flat. Just as a reflectarray can emulate a conventional reflector, a transmitarray can emulate a conventional lens. However, a single layer is generally not enough to realize the needed range of phase shift, and thus, many layers need to be used in cascade. Although the transmitarray does not allow for achieving an optimum thickness reduction, it can be significantly thinner than a standard dielectric lens. Since in reflectarrays the feeding source is on the same side of the radiated field, for similar applications transmitarrays are generally preferred to reflectarrays in all those cases where feed blockage may seriously affect the final performance of the device.
Therefore, reflectarrays and transmitarrays combine the advantages of reflector antennas (or lenses) and phased arrays, and they can replace the traditional solid optical elements, such as parabolic/hyperbolic reflectors and lenses, with easy to handle, low profile, and lightweight planar structures. In addition, since they employ spatial feeding instead of using traditional feed networks, they enable a significant reduction in system complexity and cost, particularly when there is a need to use a large number of unit cells. There are, however, a number of aspects that are specific to metasurfaces in general, such as the need to use low-loss, but low-cost, substrate materials, and the requirement to work and survive a wide range of weather conditions, which may require appropriate environmental protection.
The initial design of reflectarrays and transmitarrays was characterized by a fixed-beam (also known as a passive type). In a fixed-beam array, the beam forming is implemented by properly adjusting the dimensions at each unit cell and cannot be modified. Later, arrays were also developed where the beam could be dynamically reconfigured or scanned by introducing controllable mechanisms at the element level, in order to change the phase-shift and to reconfigure the beam. These are also known as reconfigurable (or active) reflectarrays and transmitarrays, and they are of particular interest for radio astronomical applications since pointing and tracking can in principle be performed electronically, with no or few moving mechanical parts. On the other hand, reconfigurability is achieved at the cost of increased complexity of analysis, synthesis, phase-shifter design and technology, calibration, control, and testing [6, 7].
As mentioned earlier, the main issue that we would like to address in this work is weather the novel technologies described in the previous section are sufficiently mature for applications to radio telescopes. However, we will see that there is not a unique answer to this question, since this depends on a wide range of constraints including (but not limited to) the frequency range, size of the antenna(s), and the specific scientific application. Therefore, our discussion will be limited to some more general aspects of these novel technologies that are likely to affect their application to radio telescopes in the near future, such as:
maximum size of the collecting area;
multi-band applications and bandwidth limitations;
beam-steering capabilities and limitations;
applications as single- or dual-reflector antennas.
Our discussion will also be limited to the microwave and millimeter wave range. In fact, while reflectarray techniques developed for the microwave range can be in general extended also to higher frequencies, several factors emerge that complicate the antenna design, which are often related to material properties and also to cost.
4.1 Collecting area
While certain scientific applications may require telescope designs optimized for specific parameters (e.g., FOV, system temperature), large collecting areas are still desirable for single-dish telescopes that have a wide range of scientific goals. Even for telescope arrays, the size of the individual antenna is a fundamental parameter in the critical trade-off between performance and cost. Therefore, it is important to address the question of how large reflectarrays and transmitarrays can be fabricated. Given that the unit cell has typically a size ≈0.5λ, the number of elements grows quickly with the electrical diameter, D/λ, of the antenna. One of the largest planar reflectarray antennas ever fabricated is a 3-m Ka-band inflatable reflectarray developed at NASA for space applications, which is composed of approximately 2×105 elements. A 3-m dual-band Cassegrain reflectarray (X/Ka-band) with a reflectarray main reflector was demonstrated in Ref. [8]. This reflectarray was composed of 19,000 and 275,000 elements at X-band (8.4 GHz) and Ka-band (32 GHz), respectively.
While even larger reflectarray could certainly be built, it is currently impractical to fabricate antennas for the microwave range with a collecting area of 10s or 100 s of square meters with a single reflectarray (and even less a transmitarray, see Section 4.6). Besides to complications with the technologies used for the fabrication of the reflectarrays, an important limitation to the design of large reflectarrays is the need to compensate for the differential spatial phase delay, which increases with the size of the aperture and also contributes to limiting the bandwidth (see Section 4.3). Another factor that may severely affect the maximum size of a single reflectarray antenna is whether it is designed to have beam-steering capabilities.
In general, a reflectarray antenna mimics the optical behavior of a parabolic reflector, with the advantage of having a flat surface instead of a curved reflecting surface. However, in order to build large reflectarrays it may be necessary to use radiating elements distributed on a curved surface (also known as conformal reflectarrays), or also using multi-facet, or piecewise, configurations that approximate the shape of a parabolic surface using flat reflectarray panels. Both design options introduce a geometric compensation to the feed-element paths delays. In these cases, however, one of the greatest advantages of standard reflectarrays, their flat structure and ease of manufacturing, must be abandoned, at least partially. The added complexity of the design, fabrication process, and mechanical structure must be weighed against the increased collecting area and performance of the reflectarray. Therefore, we may safely assume that the application of reflectarrays and transmitarrays to radio telescopes will be limited, in the near future, to relatively small apertures (with an area ≲10 m 2 in the microwave range), that will be operated as individual antennas of an array, as part of a larger multi-facet antenna or also as secondary reflector in a dual-reflector configuration, as discussed in Section 4.5.
4.2 Beam-scanning capabilities of reflectarray antennas
Pointing and tracking with radio telescopes in the microwave range is achieved by a fully-steerable mechanical structure (or by the movement of the subreflector alone for small angular ranges, up to a few beams), the use of phased array feeds (PAF), or also a combination of both. In fact, this might be the case for the SKA1-Mid antennas, which have been designed to incorporate PAF receivers in the future. The choice between these options is usually driven by factors relating to the mapping speed, FOV, and cost. The electronically scanned PAFs rely on phase shifter technology and although several such receivers have been developed during the last ∼10 years, their main drawbacks still remain the hardware complexity, their broadband sensitivity, and high cost, which may significantly affect their prospect to be built in large numbers for arrays of antennas.
As discussed above, reflectarrays combine key features of standard reflectors and phased array elements to generate a collimated beam as required in high gain antennas, including radio telescopes operating at frequencies higher than a few 100 s MHz. In addition to the various advantages of reflectarrays over standard reflectors for fixed-beam applications, the reflectarray antenna can also establish a new paradigm for beam-scanning applications. The beam of a reflectarray antenna can be scanned by means of the reflector nature or the array nature of the antenna. In the first approach (also called feed-tuning technique), passive reflectarrays are used and it is the feed phase center, which is mechanically moved along a specified path, thus achieving a corresponding angular shift of the beam on the sky, as with a standard reflector. In the second approach (also known as aperture phase-tuning technique), the individual elements on the reflectarray aperture are equipped with a phase-tuning mechanism in order to change the phase-shift and to reconfigure the beam, thus utilizing the array nature of the reflectarray antenna [7, 9]. Clearly, it is also possible to utilize both approaches in a single design to improve scan performance or reduce system cost. Reflectarrays can be fabricated as the subreflector, main reflector, or both in dual-reflector configurations. The choice of single- or dual-reflector configuration is fundamental in determining the beam-scanning performance of the antenna, and it is even more important for the feed-tuning technique since the reflectarray has no phase-tuning capability in this configuration.
Many examples of beam-scanning passive reflectarrays utilizing the feed-tuning technique can be found in the literature (see the review in [9]), with both parabolic and non-parabolic phase distributions over the array, but most of them can achieve a scanning range of a few or several beams. Thus, while this technique has the advantage of the ease of fabrication for the passive reflectarrays, it may not be the optimum choice for radio astronomical applications, unless other beam-steering techniques are also used to increase the FOV. For applications where wide-angle scan coverage is required, including radio telescopes, the aperture phase-tuning approach that utilizes the array nature of the reflectarray antenna is the more promising choice. However, while in passive reflectarray configurations the reflection phase for each element is a fixed quantity, for reconfigurable reflectarrays a tunable phase-shifting mechanism must be incorporated into the individual elements, as already mentioned. Different approaches are available for tuning the phase of reflectarray elements, and each enabling technology has different advantages and limitations, but they all share an increased level of complexity and cost.
The aperture phase-tuning technology is still in its development stage, and many advanced design concepts are only recently being explored. However, its continuous development and diversified applications are likely to lead in the near future to systems that will be able to replace the standard techniques for beam-steering in radio astronomy. Scan performance could also be improved by combining different scanning methodologies, such as multi-facet configurations [10], spherical-phase reflectarray antennas (SPRA, [11]), and the use of dual-reflector configurations with a sub-reflectarray and a spherical primary [12].
The scan capabilities of active reflectarrays are also closely related to their bandwidth performance (see Section 4.3). In fact, the development of new strategies able to improve the bandwidth of active unit cells is an ongoing challenge in the reflectarray antenna design, since most existing active configurations are characterized by only a few percent bandwidths. A review of the bandwidth performance of reconfigurable reflectarrays can be found in Ref. [13].
4.3 Reflectarray bandwidth limitations
Modern radio telescopes are required to operate in a wide range of frequencies, often covering more than an order of magnitude. In addition, state-of-the-art radio astronomical receivers can cover a large instantaneous fractional bandwidth, typically ∼30 % or higher. As described in the previous sections, reflectarrays mimic the standard reflector antennas, but there is an inherent fundamental difference between the two regarding their bandwidths. A reflector antenna is essentially frequency-independent; that is, it has an infinite bandwidth, which is instead limited by the feed and the receiver. On the other hand, reflectarrays suffer from bandwidth limitations due to the intrinsic narrow-band of microstrip radiators and also the differential spatial phase delays from the feed to the reflecting elements. The second effect is dominant for reflectarrays with large aperture sizes D and small focal distances f from the feed (see Figure 3), resulting in f/D≲1. On the other hand, in the case of reflectarrays with f/D≳1, the dominant factor limiting the reflectarray bandwidth is the unit cell operating band [13].
Figure 3.
Reflectarray geometry and the differential spatial phase delay. ΔRxy represents the path length difference between the nominal focal length, f, and the distance from the feed to each element of the reflectarray at position xy.
Both factors introduce phase errors as a function of frequency, which limit the bandwidth of reflectarray antennas. Many solutions have been presented in the literature to enhance the operating band of passive reflectarrays, and improved bandwidths up to ≈20 % have been demonstrated in several designs. However, improving the bandwidth of active unit cells is still an ongoing challenge in the reflectarray antenna design. In fact, most existing active configurations are characterized by few percent bandwidths [13, 14, 15].
Regardless of the choice of the unit cells used for the design, the overall bandwidth will still be limited by the flat nature of the reflectarray aperture. This limiting factor, due to the differential spatial phase delay, can be explained by referring to Figure 3, where for simplicity we consider the case of a broadside pencil-beam reflectarray, consisting of an axial-fed reflectarray designed to have a focal distance f. The reflectarray elements are supposed to be in the far-field region of the feed, and thus, we can assume that a spherical wave is incident on the reflectarray aperture. For this simple case, the reflectarray must be designed to convert the incident spherical wavefront into a reflected plane wave propagating on-axis. This can be achieved by considering the quantity ΔRxy representing the path length difference between the required focal length and the distance from the feed to each element of the reflectarray at position xy. This path length difference generates a frequency-dependent phase difference, or differential spatial phase delay, given by:
ϕxyν∘=2πcν∘ΔRxyE1
For the reflectarray design, the phase shift must be adjusted in each element to compensate for the phases given by ϕxyν. In receiving mode, a plane wave incident on the reflectarray along the boresight direction will be focused on the feed, thus mimicking the response of a paraboloidal antenna. The reflectarray elements are usually selected to satisfy the required phase at the design frequency ν∘ As the frequency changes, the phase delays in the array will change, and the phase response of the unit cell will also change. These two phase variations do not generally match with each other, thus introducing phase errors that will result in pattern deterioration and as a consequence bandwidth limitations of the system.
For beam-scanning applications, the differential spatial phase delay combines the spatial delay from the feed to different elements on the surface (thus increasing with the size of the reflectarray), and the variation of this differential delay due to the incident radiation coming from different angles (thus increasing with the required angular scanning range). The effect on the bandwidth of the compensation mechanism for different phase delays is thus dominant in the case of large reflectarrays and/or wide beam scanning ranges. Therefore, the development of techniques able to exactly compensate the differential phase delay in a large frequency range would be required for radio astronomical applications [16]. An interesting technique that can in principle achieve beam-steering and increase the instantaneous bandwidth is the use of reflectarrays with orbital angular momentum (OAM)-based structures [17].
4.4 Multi-band operations
Besides wide instantaneous bandwidths, radio telescopes must generally provide the coverage of two or more widely separated frequency bands. Multi-band operation of reflectarray antennas can be implemented by the assembly of several sub-arrays, each operating in a different waveband. These configurations can be generally achieved using two or more stacked layers, with the drawback of an increased cost and complexity [15]. However, single-layer multi-band reflectarrays are more easily manufactured, have a much lower cost, and do not face problems associated with the precise layer alignment. The specific technique to achieve multi-band performance depends on the number of frequency bands and their frequency ratios. Several multi-band reflectarray prototypes, without beam scanning capabilities, have been realized, combining up to six different frequency bands, mostly in the range from C-band to Ka-band [18]. Current research in this specific area is focusing on designing unit cells that minimize the mutual coupling effects between the elements at all frequency bands.
In addition to reducing these coupling effects, radio astronomical applications would ideally require reflectarray unit cells that are designed to simultaneously implement the beam-scanning function (i.e., reconfigurability) with multi-band capabilities. A first proof-of-concept of a multi-band beam-steering reflectarray is reported in Ref. [19], which describes a dual-band cell in K and Ka bands. Another example of a reflectarray cell combining reflection phase tunability (i.e., beam-steering) with frequency reconfigurability is described in Ref. [20]. In this specific case, frequency reconfiguration did not increase the instantaneous bandwidth of the reflectarray, and thus, such a system is useful for narrow to moderate bandwidth applications whose operating band changes over a much wider frequency range.
Ideally, for radio astronomical applications reflectarray cells should be properly designed to have acceptable instantaneous bandwidths and simultaneously implement the beam-scanning function with multi-band operation. The proposed concepts found in the literature are specifically designed for applications in radar and communications systems, while no design exists yet for applications with radio telescopes. The trade-offs and optimization required may also depend on the specific telescope configuration and scientific goals. The parameters space is so wide and complex that the specific reflectarray design cannot be conducted via analytical methods; thus, evolutionary optimization algorithms, or other similar optimization techniques, are likely to be required (see, e.g., [21]). A summary of the main features of reflectarrays (and transmitarrays) is shown in Table 2.
Key features of microwave reflectarrays and transmitarrays as reported in the literature.
The beam-scanning range is currently limited to ≲10 beamwidths with either the feed-tuning or aperture phase-tuning techniques.
4.5 Optical configurations for radio telescopes employing reflectarrays
4.5.1 Single- and dual-reflector configurations
In the literature, it is possible to find several papers showing that the main, the secondary, or both reflectors in a dual-reflector configuration can be effectively replaced by reflectarrays. Various dual-reflector antennas with a reflectarray as the subreflector (or sub-reflectarray) have been proposed in the literature [24, 25, 26]. In this technique, a reflectarray acts as a flat subreflector and by properly adjusting the phase shifts of the unit cells, they can mimic an ellipsoidal- or hyperboloidal-type subreflector for a dual reflector system, or also perform actively phase compensation. If also the main reflector is replaced by a flat reflectarray, a dual-reflectarray antenna is obtained [27, 28, 29]. This configuration provides phase control on both surfaces, which can be used for different purposes such as amplitude and phase synthesis.
In the Cassegrain dual-reflector antenna employing a flat reflectarray subreflector, as analyzed for example in Ref. [24], the antenna beam can be scanned by introducing an appropriate progressive phase distribution across the reflectarray surface. This configuration is attractive for radio telescopes, because it combines the high gain and broad bandwidth properties of the parabolic main reflector with the simplicity of manufacturing a relatively small electronically reconfigurable sub-reflectarray. The achievable angular range with this configuration can be increased by using a more elaborate phase-synthesis technique to obtain the phase distribution required on the sub-reflectarray for each beam pointing [30] (see also the review in Ref. [27]). However, the ultimate scanning range that can be achieved by electronically reconfiguring the sub-reflectarray may not be sufficient for most radio astronomical applications and thus, the antenna would still need a fully steerable mechanical support. The use of a dual-reflectarray antenna, with planar reflectarrays of which only one or both can be reconfigurable, can add additional features to the antenna system. The only dual-reflectarray antennas that can be found in the literature were designed for satellite communications systems, and thus, a preliminary study for radio astronomical applications had not been done until very recently (see Section 4.5.2) [31].
All of these systems use the conventional reflectarray antenna design, which is based on the phase compensation of a comparable parabolic reflector antenna (thus, they are also known as parabolic-phase planar reflectarrays), for the main reflector, or of a hyperbolic (ellipsoidal) surface for the subreflector in an equivalent dual-reflector Cassegrain (Gregorian) configuration. These antennas are normally designed for a fully illuminated aperture, leading to high aperture efficiency, but also a limited scan coverage, as we previously mentioned. On the other hand, wide-angle scan coverage with reflector antennas can be achieved by using non-parabolic reflector configurations such as SPRAs, where different portions of the reflector surface are illuminated to collimate the beam in different directions [11, 32]. In this configuration, in order to minimize the adverse effects of the spherical aberration only a small portion of the aperture is illuminated for each pointing position, and thus, the total aperture size will depend on the required scan range. This configuration is attractive for radio astronomical applications, also compared to the use of a fixed real spherical surface with a sub-reflectarray (discussed below), because it only needs planar reflectarrays. However, a detailed comparison should take into consideration also the required frequency range and corresponding bandwidths.
In Section 4.1, we mentioned that multi-facet, or piecewise, configurations introduce a geometric compensation to the feed-elements path delays that allow larger reflectarrays to be designed and in turn can also increase their bandwidth performance. An interesting design of this type is that discussed in Ref. [14] where the authors show that by using a reflectarray with a central rectangular panel and corner trapezoidal panels arranged in a concave shape, it is possible to remove the phase error caused by incident wave variation. The other benefit of their design is a significant increase of the system bandwidth.
4.5.2 Applications to radio telescopes
In Ref. [31], the author analyzed some of the optical configurations incorporating the reflectarray technology described in the previous sections. The purpose of that work was to determine which optical configuration is best suited to exploit the beam-scanning capabilities of these devices, and also discuss the possibility of eliminating the need for any mechanical pointing/tracking system. In this preliminary work, the actual reflectarray networks were not simulated, and only EM simulations of standard antennas were used to evaluate the phase distributions that would be required by ideal reflectarrays.
Both single- and dual-reflector configurations were simulated by the author. Specifically, it was shown that a single-reflector telescope, with a large focal ratio (>1 and especially >>1) and composed by many individual reflectarrays arranged in a piecewise configuration (see Figure 4), similar to that described in Ref. [14], would indeed be able to extend the angular scanning range of the telescope with minimum mechanical movement of the feed support system. If the piecewise configuration is realized in one axis only (e.g., to cover the required elevation range), then the only mechanical movement required would be in azimuth, which is less demanding in terms of engineering requirements. Even if some partial mechanical movement may still be necessary, these solutions would still benefit from the low-profile, low-mass, and low-cost features of the reflectarray technology.
Figure 4.
Example of a reflectarray piecewise configuration using hexagonal panels, each one reproducing the phase distribution of a parabolic reflector with f/D=5. Also shown is the feed at the focus of the antennas, oriented to illuminate the tilted reflectarray panel. The hexagonal profile has been selected because it allows an easy expansion of the total collecting area.
A few simulations with a dual-reflectarray configuration were also carried out in Ref. [31], and the results suggested that the telescope would indeed be able to point and track a target within a small angular field (≲10 beams). On the other hand, a large scanning range for astronomical applications would still require the mechanical movement of the antenna. The advantages of the dual-reflectarray configuration are the compact mechanical structure, as in the classical Cassegrain (or Gregorian) optical configuration, and also the fact that the sub-reflectarray would be composed by a much smaller number of individual phase-changing elements, compared to the larger area of the primary surface. However, the work discussed in Ref. [31] is far from being exhaustive since other promising configurations, such as a dual-reflectarray configuration with a multi-facet primary reflectarray, or alternatively using a non-parabolic illumination, were not considered.
Based on the few models that were simulated in Ref. [31] the author concluded that the most promising solution, providing the largest angular range with minimum mechanical complexity, would be a spherical primary reflector with either a single moving sub-reflectarray/feed support or a system composed by multiple sub-reflectarrays/feeds that would only require an azimuthal movement. In the latter configuration, only the feed needs to be moved, contrary to the antenna azimuthal movement previously mentioned. This solution is therefore well suited for antennas with very large collecting areas, up to several 1000s and 10,000 s square meters (see also [33]). As it was discussed above, SPRAs are another competitive design to achieve a large angular scanning range.
4.6 Transmitarray antennas
As previously mentioned, transmitarrays can modify the original radiation pattern of a directional antenna source, for example, a horn antenna, after transmission through the array (see Figure 2). A transmitarray thus acts like a lens, allowing to pass-through the incident wave while modifying its direction of propagation. The direction to which the incident wave is being re-radiated depends on the design of the structure. These structures are composed of several resonant unitary elements with a spatial periodicity forming a planar array, in a way very similar to reflectarrays. The fundamental difference between a transmitarray and reflectarray is that in the former both magnitude and phase control of the element are required. In fact, while in a reflectarray all power is re-radiated, independent of the frequency or cell design (and thus only phase control of the unit cell is required), and in transmitarrays the magnitude of the transmission coefficient needs to be close to 1, to ensure a high efficiency over the entire operational bandwidth(s). In fact, if the unit cells are not adapted to the frequency of operation the incident, EM wave will be totally reflected back, resulting in no transmission through the structure. Therefore, a transmitarray is desirable to be as transparent as possible, introducing very low loss so the EM field of the propagating wave is not severely attenuated [22].
The challenges for designing electronically reconfigurable beam-forming transmitarrays are similar to those described earlier for reflectarrays. However, the designs currently found in the literature may be appropriate only for small transmitarrays (from a few 10s up to several 100 s elements) or applications where only a small scanning range is required. Thus, they look less promising for radio astronomical applications than reflectarrays, except maybe for those specific applications where feed blockage is not desirable, such as possibly in multi-feed, multi-beam applications [34]. A review of the characteristics of various beam-steering high gain reconfigurable transmitarrays and their bandwidths can be found in Ref. [23]. An interesting example is a 200-mm-diameter refractive telescope composed of metamaterial Gradient Index (GrIn) lenses based on photolithographic meshes, which has been developed for space applications at sub-THz frequencies [35]. This prototype has a wide FOV (±10o) and can operate at frequencies between 55 and 183 GHz. The drawback of this specific device was the low optical efficiency and the inadequacy of the design to build a much larger diameter telescope composed solely of metamaterials. However, this result is of scientific interest for its potential applications as an individual optical component of a telescope with a much larger collecting area.
The size, cost, and complexity of radio telescopes and their auxiliary instrumentation have grown enormously in the recent decades. Most of the newest design and fabrication methods applied in radio astronomy were originally developed for industry, and some technologies used today in the best radio telescopes date back to 20–30 years ago. In particular, the methods and technologies used in the design and manufacture of the reflecting surfaces of radio telescopes have remained almost unchanged during all this time. A fundamental question for the future of radio astronomy is therefore whether the antenna technologies currently under development could reduce the cost of the collecting area and almost completely eliminate complex mechanical support and moving structures. In this work, we have analyzed some emerging technologies, developed mainly for remote sensing and satellite communications, which could lead to a paradigm shift in the design and manufacture of antennas for radio astronomy.
One of the most promising new technologies is reflectarrays. The reflectarray concept is not new, but the rapid development of microstrip antenna technology, coupled with the need for low-cost and high-gain antennas for commercial applications, has led to the use of microstrip elements in a variety of reflectarray configurations. Microstrip reflectarrays combine the performance versatility of antenna arrays with the simplicity of reflectors. They are low-profile, low-mass, inexpensive, easy-to-build, passive devices with phase controlling capability. By properly adjusting the reflection phase of each sub-reflectarray element, a uniform phase distribution is achieved at the reflector aperture. The use of microstrip reflectarrays is currently largely diffused in many application fields, such as remote sensing and satellite communications. With new emerging applications, advanced features are being continuously developed in terms of broadband, multi-band, dual-polarization, and beam-scanning capabilities.
Radio astronomy has so far seen no application of reflectarrays. Besides to the relative newness of this technology, the lack of applications of reflectarrays to the design of radio telescopes is undoubtedly due to the specific requirements in this field, which usually includes large collecting areas (compared to applications in remote sensing and satellite communications), multi-band and broadband operations, and fully-steerable antennas. In this work, we have seen that reflectarrays can be successfully used from microwave to millimeter wave frequencies, and they also offer the possibility to perform the pointing and tracking operations electronically, which is of great interest for radio astronomical applications. However, we have also shown that their current limitations in terms of maximum size, bandwidth, and beam-scanning range may restrict their application to the design of radio telescopes.
The examples described in this chapter demonstrate the results that can be obtained from microstrip reflectarrays, even with their current limitations. Given the great variety of radio astronomical projects, and the telescopes designed for specific applications, it is not possible to give a unique recipe for the use of reflectarrays in the design of radio telescopes. However, it would be desirable that for any future project careful engineering studies will be carried out in order to determine which design and fabrication methods, with or without reflectarrays, represent the optimum trade-off given the required scientific goals, telescope performance matrix, and of course the projected final cost.
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