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Antennas are essential components in modern communication systems as they allow for the transmission and reception of information in the form of electromagnetic waves. By transforming RF signals into electromagnetic waves and vice versa, antennas serve as our electronic eyes and ears on the world. They play a very important role in mobile networks, satellite communications systems, military communications, radars, and electronic warfare.
One of the most important factors that determine the performance of an antenna is its size and shape. The size of an antenna determines the frequency at which it operates, with smaller antennas operating at higher frequencies and larger antennas operating at lower frequencies. In general, the wavelength of the radio frequency signal is proportional to the size of the antenna. Therefore, smaller antennas are more suitable for high-frequency applications, such as mobile communication, while larger antennas are more appropriate for low-frequency applications, such as broadcasting or satellite communication.
The shape of an antenna also plays a critical role in determining its performance, with different shapes producing different types of radiation patterns that can be used for different applications.
In addition to size and shape, the design and materials used in the construction of an antenna can greatly impact its performance. For example, antennas made from conductive materials such as copper or aluminum can provide better efficiency and performance than those made from less conductive materials.
The first radio antennas were built by Heinrich Hertz, a professor at the Technical institute in Karlsruhe, Germany. Since then many varieties of antennas have proliferated including dipoles/monopoles, loop antennas, slot/horn antennas, reflector antennas, microstrip antennas, log periodic antennas, helical antennas, dielectric/lens antennas and frequency-independent antennas have been . Each category possesses inherent benefits that make them more or less suitable for particular applications.
Wireless electronic systems have been relying on dish antennas to send and receive signals. These systems have been widely used where directivity is important and many of those systems work well at a relatively low cost after years of optimization. These dish antennas having a mechanical arm to rotate the direction of radiation does have some drawbacks, which include being slow to steer, physically large, having poorer long-term reliability, and having only one desired radiation pattern or data stream.
As a result, engineers have pushed toward advanced antenna architecture such as phased array antenna technology to improve these features and add new functionality. A phased array antenna is an array antenna whose single radiators can be fed with different phase shifts. As a result, the common antenna pattern can be steered electronically. Phased arrays were invented for use in military radar systems, to steer a beam of radio waves quickly across the sky to detect planes and missiles. Phased array antennas are electrically steered and offer numerous benefits compared to traditional mechanically steered antenna such as low profile/less volume, improved long-term reliability, fast steering, and multiple beams. With these benefits, the industry is seeing adoption in military applications, satellite communications (satcom), and 5G telecommunications including connected automobiles.
Space Antenna Requirements
Depending on the orbit around the world in general, space vehicles and satellites can be divided into low earth orbiting (LEO) satellites, middle earth orbiting (MEO) satellites, High elliptical orbiting (HEO) satellites, Geostationary (GEO) satellites, Scientific research and exploring for solar system, deep space and others, Manned space flights-for now generally in LEO for example International Space Station (ISS) and space launch vehicles.
In space not only functionality should be taken into consideration but also durability and reliability of antennas should be taken into account. Consequently, in design phase of antennas to be used in space applications, environmental conditions are decisive factors. Materials to be used on space antennas should meet requirements based on space qualifications and factors. These factors can be listed under two main subjects: effects due to the launching activity and space environment.
During launch of spacecraft, acoustic vibrations, shocks, mechanical stress based on static loads, dynamic loads and sudden atmospheric pressure fall occur and those effects should be taken into account in the course of antenna design step. In addition, in commissioning phase pyrotechnical shocks are generated while deploying solar panels and payloads like deployable antennas. All of those may affect objects, for example antennas, detached to surface of spacecraft, adversely.
After LEOP, antennas will be exposed to harsh space environment. Those can be listed as vacuum, high temperature changes regarding nonconductive thermal feature of vacuum typically between −150 and 150°C, outgassing or material sublimation which can create contamination for payloads especially on lens of cameras, ionizing or cosmic radiation (beta, gamma, and X-rays), solar radiation, atomic oxygen oxidation or erosion due to atmospheric effect of low earth orbiting.
In addition, due to the limited surface area and volume available on the satellite, the antenna must be as small as possible in weight and volume. Finally, considering the limited power budget of the satellite, it is important that the antenna may have a passive and conical radiation pattern to direct the electromagnetic energy to low elevation angles.
Antennas used in LEO-type satellites can be divided into three types: payload data transmission (PDT) antennas for downloading high-density data to the ground station or inter satellite link (ISL) communication, payload antennas for special missions like mobile communication, GNSS services or remote sensing operations and TM/TC antennas to control the satellite and receive health parameters to monitor its functionality.
The frequency ranges allocated for LEO satellites vary according to the characteristics of the payload on the satellite, but are determined by International Telecommunication Union (ITU). There are also telemetry/telecommand (TM/TC) communication units in different frequency bands, global positioning systems and other telecommunication modules for transmitting and receiving the RF signals in launch vehicle, respectively.
In recent years, especially antennas used in satellite communication systems are expected to have low volume, lightweight, low cost, high gain and directivity. Since the antennas used here are the last elements of the transmitting and receiving systems, they enable the connection of both sides over the space. They must be therefore suitable to the structure on which they are used, both electrically and physically. In addition, the gain and radiation pattern characteristics must be considered together with the general approaches used in the design of these antennas. The characteristics of the printed circuit antennas in meeting these criteria are more appropriate.
Another important feature is that it is compatible with printed circuit technology and can be produced as a persistence of RF and high frequency circuit topology. Another advantage of printed circuit antennas is that they can be easily mounted on non-planar surfaces or manufactured using flexible printed circuit boards. In order to realize matching circuits, in very small areas inductive, resistive and capacitive surface mount device (SMD) components can be used with the printed circuit technology. Similarly, the frequency tuning of the antennas can be achieved electrically and mechanically in a variety of ways, which makes it particularly advantageous for the printed circuit antennas.
For deeper understanding on Space Antennas please visit: The Complete Guide to Antennas: From Fundamentals to Cutting-Edge Technologies in Space Communications
Satellite antennas
Satellite antennas concentrate the satellite’s transmitting power into a designated geographical region on earth and avoid interference from undesired signals transmitted from outside of the service area. Design parameters and their types are generally defined according to their frequency bands, transmit RF power, mass and volume requirements, mission type and environmental conditions.
Up to date, numerous antennas have been designed and employed for different space missions. Satellites are usually categorized according to their orbits. Those orbits define and affect general characteristics of satellites to be designed and manufactured for power generation from their solar panels, communication period and slot with ground station, radiation endurance, parts to be used because of atmospheric effects like atomic oxygen and their payload specifications.
The reflector antenna is most frequently used in communications satellites because of its simple structure, light weight, and high gain
After frequency definition for subsystems, types of antennas to be used for communication, remote sensing instrument and scientific instruments are selected. For example, circularly polarized antennas are usually preferred for TM/TC antennas not to be affected from polarization mismatch, which can be caused by maneuvers during low earth orbiting phases and atmospheric effects like Faraday rotation.
Besides antennas used on small satellites should be as low profile as possible due to surface and volume restrictions. However, for PDT and remote sensing applications medium and high gain antennas are needed. To use high gain and therefore narrow beamwidth antennas efficiently, they should be steered whether directing whole satellite platform or using additional steering mechanism like electromechanical structures or electronically steerable phased array antenna systems.
The ever increasing demand for more performing, flexible and reconfigurable satellite payloads drives to the adoption of advanced technologies and techniques, such as multi-beam antennas, Software Defined Radio (SDR) and Digital Signal Processing (DSP).
GEO satellite communication antennas
Since GEO satellites are about 36,000 km away from earth, they need high effective isotropic radiated power (EIRP) levels. Antennas used for GEO (geostationary orbit) satellites are typically large aperture reflector antennas that provide high effective isotropic radiated power (EIRP) levels. These antennas are used for various communication purposes such as television broadcasting, voice data transmission, and internet communication. They shape their beams according to geographical regions, in accordance with ITU regulations.
Reflector antennas
A big number of scientist and communication antenna specialists are working on the increase of performance properties of reflector antennas for the widely usage in deep space communication, satellite communication stations, radio astronomy, current microwaves such as radio-links and radars.
Parabolic reflector antennas are preferred to use as main reflector in communication systems due to its high gain and directivity properties. Also, these types of reflector antennas can give the opportunity for usage in multi-band and multi beam applications.
A parabolic reflector antenna consists of many important sections such as main reflector, feed, struts and control units, pedestal or support. Each of these should be carefully analyzed and designed. Additionally, it is possible to use reflector antennas in various forms as:
- Receiver-transformer operation (single earth antenna at the end of down-up links on the same path) form
- Transmitter and receiver (two different earth antennas at the ends of uplink-satellite-downlink paths) form
- Transmitter, satellite control unit and receiver (three different earth antennas at the ends of uplink-satellite-control unit paths and control units–satellite-receiver paths) form
A number of earth reflector antennas depending on coverage areas of satellites. Parabolic reflector can be fed as in axisymmetric, asymmetric and off-focus fed forms. Symmetric feeding causes aperture blockage effects of feed and struts. To avoid this blockage, asymmetric and off-focus fed forms are preferred. For multiple beam generation array type feedings have been used.
Reflector antennas have different shapes such as parabolic, hyperbolic, elliptic, circular and line profiles. Although the shapes are quite different, for mathematical analysis they can be converted to each other by defining a parameter called eccentricity.
Deployable large antennas for tiny satellites
For some specific operations electrically large antennas can be needed on CubeSats. Those antennas are folded, stowed or packed in a CubeSat before and during launch process. After satellite platform is placed into orbit they are deployed to conduct their missions. For this aim, there are deployable antenna examples where cutting edge mechanical technologies are employed.
A stowed 0.5 m Ka-band mesh reflector antenna was installed into RaInCube platform to initiate usage of Ka-Band radar for meteorology on a low-cost and fast applicable 6 U CubeSat platform of NASA. The measured gain and efficiency of this antenna are 42.6 dBi and 52%, respectively, at 35.75 GHz
Small antennas for tiny satellites
There is a tremendous demand to accomplish space research at reasonable prices for universities and commercial entities. CubeSats are a practical and functional platform for this objective, with a 1U CubeSat measuring 100 mm x 100 mm and weighing up to 1 kg. Larger sizes such as 2U, 3U, etc. can also be easily obtained.
The size and power constraints of CubeSats limit their ability to venture into deep space and explore our solar system. For comparison, a LEO spacecraft may have maximum communication range of only 2,000 kilometers; whereas a deep space mission must support at least a 2 million km link back to earth.
In comparison to larger spacecraft, CubeSats have limited volume, and therefore, their subsystems and antennas must be designed to fit within these tiny platforms. For example, in low Earth orbit, CubeSats typically use a UHF deployable dipole or S-band patch antenna, as low gain is sufficient to communicate with large ground stations.
Array antennas
An array antenna is a type of antenna that uses a large number of radiating elements distributed over an area, which together form the radiating aperture. These radiating elements can take many forms, such as horns, dipoles, resonant cavities, or printed elements, and are typically spaced apart by about 0.6 times the wavelength of the signal being transmitted. By adjusting the amplitude and phase of the signals fed to each of the radiating elements, the overall radiation pattern of the antenna can be controlled. The overall radiation pattern results from a combination in amplitude and phase of the waves radiated by the array of elements.
For instance, if all of the radiating elements are fed with the same amplitude and phase, the resulting beam will be similar to that generated by a reflecting antenna with uniform illumination. By reducing the amplitude on the edges of the radiating aperture, the side-lobe level can be decreased and the beamwidth increased, but this will also reduce the on-axis gain. Alternatively, by introducing a linear phase variation from one edge of the array to the other, the orientation of the beam can be changed.
The efficiency of the array antenna depends on the amplitude weighting at the edge of the array and the ohmic losses in the power splitters and phase shifters, which can range from one to several dB depending on the complexity of the array. One advantage of array antennas is the ability to shape the beam by adjusting the amplitude and phase distribution of the signals fed to the radiating elements. This allows for dynamic control of the beam using controllable power dividers and phase shifters.
Multi-beam array antennas
Multi-beam array antennas find application in communications, remote sensing (e.g. real and synthetic RF instruments such as radars, radiometers, altimeters, bi-static reflectometry and radio occultation receivers for signals-of-opportunity missions, etc.), electronic surveillance and defense systems (e.g. air traffic management and generally moving target indicator radars, electronic support
measure and jamming systems for electronic warfare, RF instruments for interference analysis and geo-location, etc.), science (e.g. multi-beam radio telescopes), satellite navigation systems (where multi-beam antennas can be employed in the user and control segment and could, as well, extend space segment capabilities).
In satellite communication systems, arrays antennas are required to generate multiple spots in a cellular-like configuration, especially for point-to-point services, making available higher gains and thus relaxing user terminals requirements. The development of multiple beams and reconfigurable active arrays is tightly connected to that of Beam Forming Networks (BFNs). Beam forming networks are complex networks used to precisely control the phase and amplitude of RF energy passing through them, which is conveyed to the radiating elements of an antenna array. BFN configurations vary widely from just a few basic building locks up to tens of thousands of them depending on system performance requirements.
More specifically, a Beam Forming Network performs the functions of:
• in an emitting antenna array, focusing the energy radiated by an array along one or more predetermined directions in space by opportunely phasing and weighting the signals feeding the radiating elements of the array; and
• in a receiving antenna array, synthesizing one or more receiving lobes having predetermined directions in space by opportunely phasing and weighting the signals received by the antenna elements of the array.
Two main categories of beam-forming networks can be identified: “fixed” (“static”) BFN’s and “reconfigurable” (“agile”) BFN’s. The main difference between a “reconfigurable” BFN w.r.t. a “fixed” one is the need for variable components. The type of reconfigurability required, whether fast or slow, will drive the selection of the technology.
To offer a certain degree of “smartness” the antenna architectures must include advanced reconfigurable beam-forming networks which make them capable of various kinds of flexible and real time pattern control:
• Beams can be individually formed, steered and shaped.
• Beams can be assigned to individual user.
• Interference can be minimized implementing dynamic or adaptive beam-forming.
The first realizations of BFNs were based on analogue architectures, with networks of transmission lines and power dividers (couplers), working either at IF or at RF frequencies. Together with the development of computing and digital signal processing technologies, digital BFNs are nowadays the baseline for most ground applications (e.g. for radars or for wireless communications) and start being applied also in satellite on-board applications.
Antennas for missile launchers
In order to acquire TM/TC communication, guidance, transmitting and receiving radar signals, sending video and image, communicating with satellite after departing, there are many antennas used on missile launchers. Particularly for TM/TC communication subsystems, missiles need Omni-directional antennas to communicate with ground stations. Since antennas are the final or first component of RF transmitter or receiver, respectively, they must be on outward or just underneath surface of missiles with RF transparent radome.
Nevertheless, they must comply with aerodynamic structure of missile. Otherwise, it will increase air-drag during trip along the atmosphere. Therefore, if antennas will be used over the surface of a missile, they must be compatible with aerodynamic structure. A well-known type of antenna for this goal is transmission line antenna which is also commonly used for other aerospace vehicles. It is known that radiation resistance of a transmission line is quite small. In order to increase the radiated power rather than power dissipated as heat, a transmission line can be terminated with reactive elements like capacitors, conducting bridges or open ends.
Another basic antenna used on missiles is conformal slot array structure. In order to get enhanced coverage for launch vehicles, array antennas are versatile and effective. Almost omnidirectional pattern can be achieved using circumferential or conformal array antennas on the launch vehicles.
Antennas for deep space vehicles
For exploring other planets, comets, moons etc., space vehicles carrying scientific instrumentation are designed and launched. To compensate overmuch free space loss in communication budget, high gain antennas are needed. Therefore, challenging design and manufacturing technologies are employed for those antennas. Moreover, they have to comply with hard space qualification standards to operate in harsh space environment.
High gain antennas for telecommunication applications that produce narrow beamwidths for Earth or Planetary science needs, are crucial for CubeSats. They enable CubeSats to venture into Deep Space and still provide high volume science return. Multiple HGA technologies have been actively developed: reflectarrays, mesh reflectors, and inflatables. Other applicable HGA technologies such as membrane antennas, slot arrays, and metasurface.
Reflectarrays
Reflectarrays are an innovative type of antenna that are increasingly being used in space communications. These antennas consist of flat panels made up of thousands of small, individually controlled elements that can reflect and direct radio signals in specific directions. Reflectarrays offer several advantages over traditional parabolic antennas, including their compact size, ease of deployment, and high efficiency.
They can also be used in combination with solar cells to provide power to spacecraft. Reflectarrays have already been successfully used in several space missions, including the ISARA CubeSat and the Mars Science Laboratory’s high-gain antenna.
ISARA was the first reflectarray in space. It demonstrates a gain of 33.0dBic at 26GHz for Low Earth Orbit communication, which translates into an efficiency of 26%. It suffered from a low efficiency feed and large gaps and hinges, resulting in an increase of the side lobe level and reduced gain.
High gain antenna (HGA) on Mars rover Curiosity of Mars Science Laboratory (MSL) can be given as a pertinent example. HGA was developed by EADS CASA Espacio for NASA/JPL-Caltech. This is circularly polarized microstrip patch array antenna consisting of 48 elements on a gimbal system to send and receive data between Mars and Earth at X-band.
As space technology continues to advance, reflectarrays are expected to play an increasingly important role in enabling high-speed data communication between spacecraft and Earth.
Mesh reflector
Multiple deployable mesh reflector for CubeSats were developed at S-band, X-band, or Ka-band. A Ka-band 0.5m deployable mesh reflector compatible with 6U-class CubeSat was introduced for deep space communication and Earth science mission. Although the antenna fits in a constrained volume of 1.5U (i.e. 10×10×15cm3 ) a gain of 42.4dBi and a 56% efficiency were demonstrated. The antenna was successfully deployed in LEO on July 28, 2018 .
Inflatables
Inflatable antennas were developed and comprehensively tested at S-band and X-band for Deep space communication. Additional work was also reported by another team at W-band . Although the spherical surface aberration can be compensated by adjusting the feed location or
using a corrective lens , it is unlikely that the surface accuracy can be maintained at frequencies above S-band.
Emerging Antennas for spacecraft
Spacecrafts can be divided into four main groups: missile launchers, satellites, radio astronomy and deep space vehicles. High gain antenna (HGA) on Mars rover Curiosity of Mars Science Laboratory (MSL) can be given as a pertinent example. HGA was developed by EADS CASA Espacio for NASA/JPL-Caltech. This is circularly polarized microstrip patch array antenna consisting of 48 elements on a gimbal system to send and receive data between Mars and Earth at X-band.
Metamaterial Antennas Metamaterials are artificially engineered materials that exhibit extraordinary electromagnetic properties. These materials have opened up new possibilities in antenna design by offering improved radiation patterns, bandwidth, and efficiency. Metamaterial antennas have been shown to offer superior performance over traditional antennas and are expected to become a popular choice for space-based communications in the future.
CubeSat Antennas CubeSats are small, lightweight satellites that are becoming increasingly popular for space-based communications. Due to their size, CubeSats require small and lightweight antennas that can provide high performance. Emerging CubeSat antenna technologies are focusing on developing compact, multi-functional, and low-power antennas that can operate in multiple frequency bands.
Phased Array Antennas Phased array antennas are becoming increasingly popular for space-based communications due to their ability to provide directional beams that can be electronically steered. This allows for improved coverage and higher data rates. Phased array antennas can also reduce the overall size and weight of the antenna system while providing improved performance over traditional parabolic antennas.
Optical Antennas Optical antennas are emerging as a potential solution for space-based communications. These antennas use light instead of radio waves to transmit and receive information. Optical antennas offer several advantages over traditional antennas, including higher bandwidth, lower power consumption, and smaller size. However, optical antennas also face several challenges, including alignment and atmospheric interference.
Smart Antennas Smart antennas are antennas that can dynamically adapt to changing environmental conditions to provide improved performance. These antennas use advanced algorithms and signal processing techniques to optimize the radiation pattern, polarization, and gain of the antenna. Smart antennas can improve the performance of space-based communications by reducing interference, improving signal quality, and increasing data rates.
Membrane antennas
Membrane antennas are a promising option for small satellites due to their ability to achieve a large aperture with excellent stowage volume. These antennas can be in the form of patch arrays or reflectarrays, and are well-suited for CubeSats. John Huang, a researcher at the Jet Propulsion Laboratory, has conducted extensive research on membrane antennas. One example is a large patch array operating at S-band, which deploys from a 2U stowage volume to achieve a 1.53m2 aperture. After multiple deployments, a gain of 28.6dBi was measured, which corresponds to an 18% efficiency.
Another example is an X-band reflectarray membrane antenna currently being developed at the Jet Propulsion Laboratory, which deploys into a 1.5m2 aperture from a canister of 20cm diameter and 9cm height. A gain of 39.6dBi was measured using a feed horn located at its focal point, achieving an efficiency of about 40%. However, additional losses may be incurred due to feed deployment inaccuracy, feed efficiency, and feed blockage.
Slot arrays
Slot arrays are a type of antenna that uses a series of slots cut into a conductive surface to radiate or receive electromagnetic waves. They are particularly useful for narrowband applications and can produce linear or circular polarization.
In recent years, the concept of a deployable slot array has been introduced for use in small satellites, such as CubeSats. This involves folding the antenna panels around the spacecraft and deploying them as needed. The design of slot arrays is also constantly evolving, with new developments such as an S-band slot array capable of producing omnidirectional, multibeam, or directive operating modes. These antennas have potential applications in a variety of fields, including satellite communication, remote sensing, and radar systems.
Metasurface antennas
Metasurface antennas are becoming increasingly popular in space communications due to their ability to provide high gain antennas without the need for a deployed feed at a focal distance from the antenna aperture. This eliminates the challenges posed by feed mechanics and geometry, which are often a limitation for deployable antennas as the aperture size increases. The potential for metasurface antennas to achieve large aperture sizes makes them an attractive solution for CubeSats and other small spacecraft where aperture size is limited.
JPL was developing a holographic metasurface antenna based on Si and GaAs semiconductors operating at 94 GHz that achieves beam-forming in a holographic manner through the modulation of a guided-mode reference with a metasurface layer to produce the desired radiation wave-front.
Conclusion
In conclusion, emerging antenna technologies are set to revolutionize space-based communications by providing higher performance, smaller size, and improved reliability. The antenna technologies mentioned above are just a few examples of the many innovations that are expected to drive the future of space-based communications. As the demand for faster, more reliable, and secure communications continues to grow, we can expect to see even more exciting antenna technologies emerge in the years to come.
References and Resources also include:
https://www.intechopen.com/online-first/antennas-for-space-applications-a-review
https://pureadmin.qub.ac.uk/ws/portalfiles/portal/174234474/IEEE_Magazine.pdf
