Microwave Journal
www.microwavejournal.com/articles/29572-antenna-technologies-for-the-future
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Antenna Technologies for the Future

January 12, 2018

Traditional antenna technology has hit its limits in many demanding commercial and aerospace markets such as 5G, SATCOM, IoT and radar. But there are many companies developing new approaches and materials that could drastically improve antenna performance and enable new applications that were not previously envisioned because of these limitations. In this article, Microwave Journal looks at a sampling of these technologies that have come to our attention in the last couple of years.

3D Printed Antennas

Recent advances in 3D printing or additive manufacturing have enabled complex RF structures to be realized. The characterization of the materials used in 3D printing processes has been shown to be critical in designing and accurately predicting the performance of these structures. Understanding the RF properties of the materials through characterization has led to the development of novel structures that could not ever be realized with traditional manufacturing techniques. 3D printing has also allowed manufacturers to produce traditional antenna shapes with less weight and at a lower cost.

SWISSto12 SA is an offshoot from the Swiss Federal Institute of Technology in Lausanne, Switzerland. The company has developed unique products using 3D printing that are based on polymer materials that are then metal plated or on metallic materials (such as aluminum or titanium) combined with advanced surface treatments and surface plating. Using these processes, SWISSto12 manufactures and tests aerospace qualified advanced RF products such as waveguides, filters, beamforming networks, antenna feed chains or array antennas.

Figure 1

Figure 1 Swissto12 Ku-Band dual-reflector antenna (a) image courtesy of the ESA, and the radiation properties over the design frequency range (b).

Their use of 3D printing allows for increased flexibility in the manufacturing of complex product designs. This freedom can be used to produce higher complexity RF components, which often allows for better RF performance. Traditional machining technologies used to manufacture RF products are limited in their ability to produce products with complex shapes. To circumvent this limitation, complex products are often assembled out of a larger number of simpler sub-components that are produced separately. SWISSto12’s 3D printing technology does not have such constraints, allowing it to produce entire products in one single element that positively impact mass, cost, lead time, assembly quality and RF performance. The use of 3D printing also allows for optimized weight reduction. The technology has been demonstrated on waveguide, filter and antenna components from C- to W-Band (4 to 110 GHz).

As this technology is rapidly gaining maturity and acceptance among the aerospace industry, SWISSto12 has already delivered a variety of prototypes to organizations in the space and SATCOM industries that have been qualified for use in airborne and space environments (the first commercial programs will be flying SWISSto12 products in 2018). More complex and integrated antenna or payload structures are in development, in particular Ku- and Ka-Band products. The company is also targeting emerging market opportunities such as high-altitude platforms, UAVs and Cubesat constellations. An example antenna product is a Ku-Band dual-reflector antenna comprising a conical chocked horn, a sub-reflector, a main reflector and metallic supports. This antenna system only weighs 145 g. The radiation properties of this antenna were found to be in an excellent agreement with simulation results over the design frequency range (see Figure 1). This antenna demonstrator was manufactured and tested in collaboration with the European Space Agency.

Figure 2

Figure 2 Optisys Ka-Band 16-element tracking array with measured and simulated data.

Figure 3

Figure 3 Optisys Ka-Band 64-element tracking array with signal pattern.

Optisys is another company that focuses on the design, fabrication and test of lightweight antennas using metal 3D printing. The specific method that Optisys uses for fabrication is a powder bed fusion process, where thin layers of powder are welded into solid metal by a high-power laser. Through this welding process, a part is built one small layer at a time. This green manufacturing process allows for material to be added only where absolutely necessary to achieve a given mechanical or RF function.

As with any fabrication process, 3D printing has a set of design rules that determines what can and cannot be included in a design. Optisys is reimagining how waveguide is routed to achieve a desired RF function to take full advantage of the strengths of the printing process that allows, in some cases, for over an order-of-magnitude reduction in the size and weight of an antenna. Along with size and weight, part count reductions and absorbing tolerance stack-up can lead to simpler antenna assemblies with improved performance.

Optisys has designed a number of arrays using building blocks developed in-house that are rearranged and optimized for different functions and frequency bands. One of the key antenna designs is a Ka-Band 16-element tracking array shown in Figure 2. Signal patterns show the sum and delta elevation performance with measured and simulated performance overlaid. This part includes a 16-element array of horns that are circularly polarized, with a waveguide combiner network on both left- and right-hand circular polarizations. Additionally, the right- hand combiner network feeds into a dual-axis monopulse comparator. All of this is implemented in a single part that weighs less than 2 oz and fits in the palm of the hand. 

A second array design is the X64 antenna that takes the integration a step further, by including a waveguide dual-axis monopulse comparator on both polarizations and integrating an elevation rotation axis into the design. Fabricating this part in a traditional process would require well over 100 separate parts, compared to the single printed part. Signal patterns are shown for sum, delta elevation and delta azimuth on the right-hand circular polarization (see Figure 3). There are a total of eight simultaneous antenna beams generated in this antenna.

Figure 4

Figure 4 Optisys Integrated Printed Antenna and signal pattern.

Another key area of innovation where Optisys has generated interest is in the design and building of lightweight feeds for parabolic reflectors, where the sub-reflector, feed horn and polarizer are printed as a single metal part without blockage or losses due to struts or a dielectric support. This design has been optimized to the key frequency bands of X-, Ku- and Ka-Band. The company refers to these antenna feeds as the integrated printed antenna family of feeds. The unit and 3D near-field scan pattern at a single frequency and a set of elevation patterns across the frequency band are shown in Figure 4.



Figure 5

Figure 5 A small test coupon of MITRE’s biaxial metamaterial created with a Voxel8 multi-material 3D printer.

As covered in the October 2016 issue of Microwave Journal, The MITRE Corporation is investigating a new generation of 3D printing to realize the complex geometries of wideband phased array and metamaterial designs using commercial, low-cost, compact, desktop printers.1 Samples of the 3D printed plastic and conductive ink printed at room temperature were characterized over frequency. The polylactic acid (PLA) dielectric constant and loss tangent are found to be stable up to 18 GHz. The PLA internal architecture was varied to achieve lower effective dissipation factors, which extends usefulness to high frequency applications. Microstrip line samples were fabricated with simulated and measured insertion loss data validating the high conductivity through mmWave frequencies. A 3D printed monopole Wi-Fi antenna was built and tested, showing good performance and agreement with simulations.

MITRE also has developed a wideband phased array concept that has a complex metamaterial design. It is based on a PCB design that was not physically realizable with traditional manufacturing. The design resembles an egg-crate construction with contiguous electrical connection (interdigitiated fingers) that is embedded within the orthogonal board interface, as illustrated in Figure 5.1 Multi-material additive manufacturing is thought to be the only practical way to realize this design. They successfully printed a sample of the cross in the middle of the array, and a CT-scan showed the details of all of the fingers confirming the construction and working on full scale antenna structures.

Figure 6

Figure 6 Kymeta mTenna™ construction.

Metamaterial Based Antennas

Metamaterials are made by arranging naturally occurring materials in a specific pattern that produces an electromagnetic response that is not found in nature. The periodic structures created are at scales that are smaller than the wavelengths of the phenomena they influence and can create materials with negative indexes that control electromagnetic energy in ways that cannot be done with natural materials. In traditional active electronically scanned arrays (AESA), phase shifters embedded in control circuitry steer the beam direction. Metamaterial-based AESAs can steer the beam without phase shifters, which reduces system complexity, eliminates a source of power loss and simplifies waste-heat dissipation. There are a couple of companies using unique metamaterial structures developed for this application.

Kymeta experimented with these structures for many years and discovered that the metamaterials could be used to form holographic beams that could link to satellites and maintain the link while the antenna is in motion. Kymeta mTenna™ technology (see Figure 6) is manufactured using a completely different process and components than both traditional antennas and phased array antennas.2 The “metamaterial” in mTenna technology is a metasurface in a glass structure. Their glass-on-glass structure is manufactured on the same production lines as LCD flat screen televisions, making it suited for low-cost, high volume manufacturing. They use the thin film transistor liquid crystal as a tunable dielectric. Instead of reflecting microwaves like a traditional dish antenna or creating thousands of separate signals like a phased array, Kymeta uses a thin structure with tunable metamaterial elements to create a holographic beam that can transmit and receive satellite signals.

They use software to steer the antenna, eliminating the need for mechanical gimbals to point the antenna toward a satellite. The antenna does not require active phase shifters or amplifiers. Key features of the approach:3

  1. Transmit and receive via a single aperture
  2. Wide angle scanning and excellent beam performance
  3. Electronically controlled pointing and polarization
  4. Extremely low power consumption
  5. First electronically scanned antenna designed for mass production.


Figure 7

Figure 7 Echodyne’s radar vision unit next to an iPhone.

Traditional satellite dishes are heavy, large, expensive, consume a lot of power and have mechanical gimbals for steering, which have prevented or limited their adoption on most mobile platforms. Kymeta’s mTenna technology provides software-enabled, metamaterials-based, electronic beamforming satellite solutions that are flat, lightweight, small and use software to steer instead of mechanical parts.

This technology is being used to deliver internet connectivity to industries that have historically been inaccessible or difficult for the satellite industry to address, such as rail, bus and automotive. Also, the maritime and aviation markets have struggled to implement satellite technology broadly across smaller vessels and aircraft.

A second company, Echodyne, has developed metamaterial arrays for radar using similar antenna technology to Kymeta but optimized for radar applications. Echodyne’s radar vision platform represents a unique sensor technology that combines the all-weather, long range and ground-truth measurements of radar with high resolution imaging capabilities (see Figure 7).4 Radar vision consists of high performance agile imaging radar hardware combined with computer vision-like software for classification, recognition and perception.

Their metamaterial based, electronically steered array radars operate in the same way as traditional designs, providing high resolution data at any time and in any weather. Like Kymeta’s approach, they can be produced in high volume, at commercial price points and in small lightweight form factors. Their technology can switch in less than 1 μs, has beam shaping and multi-beam capabilities and can steer in both directions, providing near full hemisphere coverage. It operates at 24 GHz and has an operational range of 3.4 km with a field of view ≥120 degrees azimuth and 80 degrees elelvation with a range resolution of 3.25 m and velocity resolution 0.9 m/s.4

LiDAR and cameras have limited range and do not operate reliably in adverse weather, while traditional radar in this sector has inadequate resolution. Echodyne’s radar vision platform represents a new category of sensor technology to enable many autonomous vehicles from drones to cars. Their high performance imaging radar is viable and affordable on commercial and small platforms, including all types of autonomous and unmanned vehicles and machines.

Figure 8

Figure 8 Fractal Antenna’s RF invisibility cloak and measured data.

Fractal Based Antennas

A fractal is “self similar” complex pattern built from the repetition of a simple shape. A fractal element antenna is shaped using fractal geometry. The inherent properties of fractals can enable high performance antennas that can be 50 to 75 percent smaller than traditional antennas. Typical advantages are increased bandwidth, better multi-band performance and higher gain. Fractal antennas can be more reliable and lower cost than traditional antennas because antenna performance is attained through the geometry of the conductor, rather than with the accumulation of separate components or separate elements that can increase the complexity, potential points of failure and cost.

Fractal Antenna is a small company that produces fractal versions of many existing antenna types, including dipole, monopole, patch, conformal, biconical, discone, spiral and helical, as well as compact variants of each that is made possible through fractal technology. They were the first to demonstrate wideband RF invisibility cloaking and used fractal shaped metal patterns on a mylar sheet. In their demonstration, a signal from 750 to 1250 MHz was attenuated by only a fraction of a dB over the same 50 percent bandwidth that would normally be attenuated by 6 to 15 dB without the cloak (see Figure 8).5,6



Figure 9

Figure 9 The CUBE mXTEND™ antenna booster from Fractus Antennas (5 mm3).

At EDI CON USA 2016, Dr. Nathan Cohen of Fractal Antenna gave a session and demonstration of their unique RF/microwave cloaking and deflection technology using fractal structures. Over a broad band, 2.5 to 3 GHz, he created a Waldo (window around a wall) that channeled the RF energy around a barrier (the “wall”) using an array of closely packed fractal-shaped resonators that was wrapped around the barrier, creating a “window.” The bandwidth is an impressive 500 percent for front scatter and 100 percent for backscatter with about 1 dB insertion loss.7

Fractus Antennas was featured in the October 2017 issue of Microwave Journal and has a new “antenna-less technology” that is based on replacing a complex and usually customized antenna design with an off-the-shelf, standardized, miniature component called an antenna booster.8 Being a surface-mount, chip-like device, the antenna booster can be picked and placed like other surface-mount components onto a PCB for low cost assembly (see Figure 9). Aimed at mobile devices and IoT applications, it is made with metalized ceramic layers that use fractal shapes designed to meet different design requirements.

Miniature chip antennas are not new, so what is unique here is the multiband capability with a single device. While conventional miniature chip antennas were based on high-permittivity ceramics and delivered good performance for narrowband, single frequency applications, these new boosters can deliver full mobile performance within a broad range of frequency bands (e.g., 698 to 2690 MHz) with a single device. The integration requires a matching circuit that allows the device to operate at the desired bands of interest. Based on conventional low-cost materials and assembly processes, the boosters can be made in high volume at very low cost.

Figure 10

Figure 10 VSWR and efficiency for 5 band mobile antenna from Fractus Antenna.

An example booster is 5 mm3 in size and operates from 824 to 960 MHz and 1710 to 2170 MHz simultaneously. With a matching network on the PCB, a VSWR ≤ 3:1 across the operating bands and an average total efficiency of 56.7 and 75.8 percent in the 824 to 960 MHz and 1710 to 2170 MHz frequency regions, respectively, is achieved (see Figure 10).8

Other Unique Technologies

Plasma Antennas (PSiAn) offers a range of innovative plasma-silicon devices (PSiD) to form the compact RF core of future smart antennas. The PSiDs provide fast, electronic beamforming and beam selection functions. A PSiD can be regarded as a multi-port, wideband switch that replaces RF switches, phase shifters and attenuators with one compact, low loss device. Due to their silicon IC construction, PSiDs can be reproduced with high precision for the mass market at low cost. They have high power handling and, unlike RF MEMS, can be “hot” switched.



Figure 11

Figure 11 Plasma Antenna 360° field of view beamforming and steering 28 GHz, 5 W, 16 dBi gain PSiAn.

PSiAn uses either single or multiple PSiDs to perform azimuth and elevation beam steering. The PSiDs are mounted on RF PCBs and use transmission lines to link the device ports to traditional RF and antenna technologies, such as LNAs, PAs, printed feeds, lenses and reflectors to produce efficient smart antennas with steerable narrow beams. Potential applications of PSiAn plasma antennas include: small cell backhaul at V-Band (60 GHz), gigabit wireless LAN (e.g. WiGig), intelligent transport systems (ITS) at 63 GHz and vehicle radar (77 GHz).

The company recently introduced an antenna that reduces the cost of a 5G base station by up to 50 percent by eliminating phase shifters, reducing and consolidating amplification and reducing computation. The technology does not need calibration and can handle high-power, having been tested up to 40 W. The company has shown the technology in a variety of scenarios, including a 360 degree field of view, beamforming and steering, 28 GHz, 5 W, 16 dBi gain PSiAN, useful for pole mounted small cells, indoor small cells—also on a vehicle and a high-power, long range, low loss small cell base station antenna for standalone and MIMO 5G, fixed wireless access (FWA) and connected vehicle applications (see Figure 11).9 These devices can also be stacked to form and steer beams in two dimensions (azimuth and elevation) or to form multiple beams and MIMO applications.

They also announced their mmWave PSiAn for use in smartphones and other consumer electronics, delivering high throughput with low latency and utilizing directional beams that generate less interference and maximize energy efficiency.10 The introduction of mmWave connectivity for smartphones and other mobile devices faces significant problems as the signals are easily blocked by fingers, hands, heads and bodies. When used in combination with distributed radiating elements, PSiDs can be used as a switch and beam former to utilize only elements that are able to receive and transmit line-of-sight or reflected signals resolving this issue. Plasma Antennas recently modeled plasma silicon corner antennas as replacements for array modules for device manufacturers and silicon suppliers. This approach closely represented the publicly available solutions from Qualcomm and Samsung, for which there are many handling scenarios that would block the antennas. The array Plasma Antennas proposes now solves these problems and brings the intrinsic qualities of plasma silicon.

Figure 12

Figure 12 Gap waveguide structure.

Gapwaves AB was founded in 2011 by Professor Per-Simon Kildal at Chalmers University of Technology in Gothenburg, Sweden, with the aim of enabling efficient wireless communication through the patented GAP waveguide technology. GAP waveguides provide a unique packaging technology for mmWave and terahertz circuits and components, with advantages compared to existing transmission line and waveguide technology. The technology is based on an artificial magnetic conductor that enables contactless propagation of electromagnetic waves, significantly reducing transmission losses. The GAP waveguide is built up of two parts: a structured metal surface and a flat metal surface placed close to one another, allowing for an air gap between the two part (see Figure 12).11 The structured surface is characterized by pins forming a barrier, preventing the electromagnetic waves from propagating in undesired directions. In this way, the pins replace the walls in traditional rectangular waveguides without requiring perfect metallic contact. The waves are guided by ridges or grooves within the pin structure and propagate in air, resulting in low power losses. Antennas based on the GAP technology have more than 10x lower losses than micro strip lines, more than 3x lower losses than substrate integrated waveguide (SIW) and approximately the same losses as rectangular waveguides.11

As no metallic contact is required between the layers, the assembly of multilayered, closely spaced waveguide structures is simplified. The antenna layers can be glued together with no screws, pressure or heat. Production of the antenna parts is accomplished using plastic injection molding in combination with metallization or using die-casting in metal, resulting in high volume capabilities and a cost-efficient manufacturing process. The low power losses enable broadband antenna arrays with gains up to 38 dBi to be achieved with above 80 percent efficiency. The design flexibility that comes from using multilayered waveguide structures enables tailoring of the radiation pattern, as demonstrated by a recently developed 38 dBi E-Band antenna that has achieved ETSI class 3 radiation pattern performance.



Figure 13

Figure 13 Active antenna system construction using Gap waveguide technology.

The properties of GAP waveguides make it suitable for designing active antenna systems. A schematic view of an active antenna based on the GAP waveguide technology is shown in Figure 13. Due to its contactless nature, GAP waveguide based antennas can be more easily integrated with PCB, as RF interconnects between the PCB and the antenna layers can be made without electrical contact. The pin structure also acts as a shield for the active components, protecting them from interference and preventing propagation within the substrate of the PCB. Removing the need for shielding walls and via holes frees up valuable circuit board space, which becomes available for the placement of active circuits and routing lines. The die-cast antenna layers also provide effective cooling for the active circuits from two sides. This is useful when integrating high-power amplifiers and CMOS based control circuits onto the same circuit board, which often require cooling from different sides.

Summary

There are many exciting new technologies such as 3D printing, metamaterials and fractal antennas, that promise to revolutionize antenna technology in the next few years. They will solve many challenges that traditional antenna technology has not been able to overcome and enable new antenna shapes and applications that are not even possible with traditional antenna technology. The unique new approaches will address many of the challenges faced today in 5G, IoT, SATCOM and radar applications.

References

  1. M. W. Elsallal, J. Hood and I. McMichael; T. Busbee, “3D Printed Material Characterization for Complex Phased Arrays and Metamaterials,” Microwave Journal, Vol. 59, No. 10, October 2016.
  2. www.kymetacorp.com/kymeta-products/, November 25, 2017.
  3. www.kymetacorp.com/why-kymeta-connectivity/, November 25, 2017.
  4. https://echodyne.com/products/, November 26, 2017.
  5. N. Cohen, “Fractals,” World Scientific Publishing, Vol. 20, Nos. 3 & 4, 2012, pp. 227–232.
  6. N. Cohen, “Wideband Omnidirectional Microwave Cloaking,” Microwave Journal, Vol. 15, No. 1, January 2015.
  7. www.microwavejournal.com/articles/27219, November 20, 2017.
  8. J. Anguera, A. Andújar and C. Puente, “Antenna-Less Wireless: A Marriage Between Antenna and Microwave Engineering,” Microwave Journal, Vol. 60, No. 10, October 2017.
  9. www.microwavejournal.com/articles/29472, November 22, 2017.
  10. www.microwavejournal.com/articles/29249, November 22, 2017.
  11. http://blog.gapwaves.com/what-is-a-gap-waveguide, November 26, 2017.