Chapter IV: Large Scale Flexible Arrays
4.1 Introduction
Increased aperture is one of the fundamental advantages of phased arrays over many other antennas.1 More aperture allows the array to concentrate more power in a narrower area. With sufficient imagination any surface area could potentially be uti- lized as aperture for an array. Array architectures that unlock new surfaces are a step towards a future which arrays are ubiquitous. Flexible surfaces or surfaces otherwise undergoing mechanical change are a promising area for array development.
Mechanical flexibility introduces considerable challenges for phased array designers.
Radio frequency designs involve conductors whose size is comparable to that of the signal wavelength, almost by definition [127]. When these conductors are deformed their field profile deforms as well, causing path length changes, impedance changes, reflections, pattern deformation, and a variety of other potential issues.
Given the complexity inherent in phased array systems, it is understandable that RF designers almost always avoid flexibility altogether and opt for flat, rigid profiles with 𝜆/2 separated radiators. Historically, when low aerodynamic profile, non- planar shapes are needed for aerospace antennas, rigid and statically conformal non-flexible antennas are designed which are suited for a single application and use scenario [11] [39] [82] [51] [86].
While flexible phased arrays are still in their infancy, the broader field of flexible electronics is highly mature.Today’s flexible systems use off-the-shelf short-range low-data-rate radio modules (e.g., Bluetooth) and rigid ceramic chip antennas [180]
[139] [162] [182] [71] [179]. These RF transceivers lack high gain electronically steerable beams with spatial selectivity, which have become the central feature of
1Florian Bohn, Behrooz Abiri, Matan Gal-Katziri, Mohammed Reza Hashemi, Ailec Wu, Oren Mizrahi, Alex Ayling, and Mohith Manohara worked on the Caltech Space Solar Power Project and contributed to the concepts, prototypes, and measurements that appear in the Caltech Space Solar Power Project section. System level concepts and prototypes were developed with the Pellegrino Lab and the Atwater Lab. Richard Madonna and Damon Russell also contributed as advisors to the project. The Caltech Space Solar Power section is partially adapted from the material in [54], [88], [62], [105], and [48]. The co-cure fabrication of the pop-up dipole antennas was developed and implemented with Alan Truong, Fabian WiesemĂĽller, and Eleftherios Gdoutos of the Pellegrino Lab. The material presented in the Flexible Array Shape Reconstruction Section was performed in collaboration with Oren Mizrahi. This chapter is adapted from [42].
emerging communication systems, such as 5G. Future flexible systems could benefit from the orders of magnitude higher data rates and incorporation of microwave ranging, sensing, and power transfer functions. A new design paradigm is needed in order to close the functionality gaps between flexible and rigid RF communication, sensing and ranging systems. Lightweight, dynamically flexible radiating arrays are a promising candidate to answer this challenge. 4.1 presents a variety of flexible phased array concepts and prototypes.
Figure 4.1: Flexible Phased Array Concepts and Prototypes.
Lightweight, dynamically flexible arrays enable significant increases in effective aperture, as they can be used on surfaces and in applications that would not be con- sidered for conventional arrays. Many flexible array applications require deploya- bility and/or conformality. Deployable arrays are sufficiently flexible or jointed such that they can be compactly stored when not in use and then unrolled or unfolded to offer a large aperture when in use. Deployable arrays are under consideration for ultra-large arrays for wireless power transfer in space [62] [134], but could also find use as rapid set-up/take-down communication relays for large events (concerts, conventions, etc) or disaster relief/emergency situations when existing infrastructure is damaged. Alternatively, flexible conformal arrays can create useful apertures on surfaces undergoing constant shape change such as the human body (wearables) or
fit on varyingly shaped surfaces. A single flexible array design can also be used on several different rigid non-planar surfaces without requiring an additional design cycle to account for specific curvature. For many airborne and space-borne appli- cations, the low mass of flexible arrays is also a boon. Commercial, mechanically steered, low profile antennas for aerospace offer a 30 cm aperture at 5 kg (5.5 g/cm2) [165], while flexible phased arrays have been reported at 0.1 g/cm2[62]. For emerg- ing high altitude platforms [30] [106] such as Airbus Zephyr [75], which aspire to provide internet access during months-long stratospheric flights, even a few kg is a significant fraction of total vehicle weight (75 kg) and any reduction in weight increases flight time and available electrical power.
In this chapter we will discuss the design and operation of flexible phased arrays.
First, we will describe the large-scale ultralight, wireless power transfer arrays designed for the Caltech Space Solar Power Project. Second, we present a method for determining the shape of flexible arrays, a critical step in their proper use. Prior to the discussion of our work on large-scale space arrays, we will take a brief moment to discuss typical, existing microwave antennas for space applications.
Existing Space Antennas
Microwaves have long been the go to medium for spacecraft guidance, telemetry, and communication. While optical communication with satellites is receiving increased attention [140], microwaves are unlikely to be unseated as the go-to choice. Low atmospheric absorption, wide availability, low cost, and highly durable generation and reception circuitry, options for both omni-directional and directional radiation, and substantial existing infrastructure in ground stations and flight heritage are compelling reasons for the past, present, and future use of microwaves in space.
Large scale, flexible, deployable arrays represent a significant deviation from pre- vious and existing space microwave antennas. In order to present a complete story, existing systems will be explored in this section.2
The simplest antennas for space systems are low-gain, omni-directional radiators [13] [92]. These antennas allow for transmission and reception without control of the spacecraft’s orientation. Some designs achieve omni-directionality using multiple antennas to cover directions blocked by the body of the spacecraft [27] [32]. Higher gain antennas ease requirements on other components in the link and can allow higher
2Several of the example antennas described in this subsection as well as a survey of other existing satellite antennas can be found in [130].
bandwidth channels but must be pointed in the intended direction of radiation. A conventional high-gain antenna requires use of flywheels or other inertial systems to turn the entire spacecraft. Gimballed antennas have been used [50] but also require use of inertial systems to compensate for their angular momentum during steering.
High gain antennas require large apertures. At a point the aperture may exceed what can easily be mounted onto a spacecraft. To overcome this practical limitation, a variety of deployable antennas have been demonstrated [2]. These include mesh dish antennas [24], inflatable arrays [79], and unfolding reflectarrays [66].These de- ployable apertures provide high-gain patterns but still require mechanical steering.
Electronically steered arrays (ESAs) can rapidly steer beams without effecting satel- lite pointing. ESAs are also highly desirable for scientific missions as they allow other concerns (such as the primary scientific instrument) to determine spacecraft orientation. They have seen prior use in communication networks but are expected to see a dramatic increase in usage as satellites become integrally tied into planned 5G/6G networks [112] [95] [115]. Published space ESAs are not deployable, mean- ing they are limited to aperture sizes less than the dimensions of the spacecrafts un-used surfaces.
The flexible phased arrays described in this chapter have the potential to combine the benefits of a deployable aperture and ESAs. An array would be compactly stored in the spacecraft until the payload is delivered to its desired orbit, where it would deploy and begin operation. In space, there is essentially no limit on the volume a flexible or multi-faceted arrays could deploy into. Provided the nuances of reliable mechanical deployment as well as element drive and synchronization are adequately handled, array apertures can be expanded until the beam is too narrow to be properly steered or until time delay correction of data coherence needed. Since arrays are typically single-sided they are also prime candidates for integration with other area intensive subsystems such as photovoltaics or sensing instruments. While large scale flexible phased arrays have a variety of terrestrial uses, the emptiness of space is especially fertile ground.
4.2 Caltech Space Solar Power Project