Antennas are our electronic eyes and ears on the world. They play a very important role in mobile networks, satellite communications system, military communications, radars and electronic warfare by transforming a Radiofrequency ( RF) signal, traveling on a conductor, into an electromagnetic wave in free space and vice versa. The RF current flowing through the antenna produce electromagnetic waves which radiate into the atmosphere.
Essentially, all types of antennas and their applications depend on their size and shape. The size, for instance, determines what frequency a single antenna sends and receives. In all cases, antennas create different shaped waves to move electrons between areas. These electrons change direction several times depending on the types of waves being generated. Since all antennas communicate through specific frequencies, the signals you generate have to fit an approximate gap between parts of the electromagnetic spectrum. For instance, radio waves take up an invisible portion of the total number of electromagnetic waves humans can create. This portion is like a keyhole for message-sending. If you have a message that fits, your information will come across.
Those devices that fall under a foot, often communicate microwaves and UHF (Ultra High Frequency) waves. Devices that stand well over a story usually transmit and receive VHF (Very High Frequency) waves. These facts prove a point: changing just a single dimension of an antenna impact the frequency it can communicate with. If you change other aspects of antenna shape you end up with different variables too.
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
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. The gain of an antenna is the ratio of the power radiated (or received) per unit solid angle by the antenna in a given direction to the power radiated (or received) per unit solid angle by an isotropic antenna fed with the same power.The radiation pattern indicates the variations of gain with direction. The 3 dB beamwidth corresponds to the angle between the directions in which the gain falls to half its maximum value.The efficiency of the antenna is the product of several factors which take account of the illumination law, spill-over loss, surface impairments, ohmic and impedance mismatch losses. Consequently, suitable antennas should be designed and utilized according to planned mission for the space launch vehicles similar to the satellites.
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.
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.
LEO satellites orbit between 160 and 1600 km from the Earth’s surface. These satellites are usually small compared to communication GEO satellites, easy to launch and put into orbit. They can be used for different purposes. For ground monitoring purposes, satellite constellation can be placed into orbit and used for voice, fax and data communications. 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.
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.
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.
Antenna types used on spacecrafts
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.
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.
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.
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
In the past, GEO satellites’ main mission was only television broadcasting and voice data transmission. Therefore, there are many communication satellites as geosynchronous. In the last decade, they have started evolving and internet communication mission has begun to take place instead of TV broadcasting. The main reason for this is that the internet goes into all areas of life like business, education, entertainment, etc.
Since GEO satellites are about 36,000 km away from earth, they need high effective isotropic radiated power (EIRP) levels. So usually large aperture reflector antennas are employed. Based on ITU regulations generally these antennas shape their beams according to geographical regions. There is a good example to illustrate evolutionary change of GEO communication satellites. To provide high speed internet data communication JAXA started The “KIZUNA” – Wideband InterNetworking engineering test and Demonstration Satellite (WINDS) project. Its main mission was to enable super high-speed data communications of up to 1.2 Gbps. In this way, everybody can reach high-speed communications, no matter in which geographical region of Japan they live.
KIZUNA was launched and put into Geosynchronous Orbit to acquire the highest-speed data communication of the world in 2008. Its planned operational life was 5 years and failed in February 2019 and started to drift. Therefore, it exceeded its planned operational life successfully.
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
One of the limiting factors preventing CubeSats from venturing into deep space to explore our solar system is the size constraint of each
subsystem, available DC power, and non-availability of sufficiently large RF aperture for communication and science payload. In LEO, CubeSats employ a UHF deployable dipole or S-band patch antenna, as low gain is sufficient to communicate with the large ground stations. 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.
There is tremendous demand to accomplish space research at reasonable prices for universities and commercial entities therefore CubeSat is a practical and functional platform for this objective. Dimensions of a 1 U CubeSat are 100 mm x 100 m and it has aluminum T6061 structure with a total mass of up to 1 kg. However, 1 U can be easily enlarged to larger sizes like 2 U, 3 U, etc. Comparing to other satellite platforms, CubeSats have limited volume therefore submodules and antennas should fit into those tiny platforms.
For GAMALINK1 project for CubeSat antennas , a miniaturized cavity-backed tapered cross-slot antenna has been presented. 38 × 38 mm2 and 30 × 30 mm2 footprints have been obtained on substrates having dielectric permittivity 6 and 9.2, respectively, at operating frequency about 2.44 GHz. Its maximum gain is at boresight and efficiency is small as expected because of miniaturization. However, its tiny dimensions make this antenna beneficial to save space on surfaces of small spacecrafts like CubeSats.
An array antenna uses a large number of radiating elements distributed over the area which constitutes the radiating aperture. The overall radiation pattern results from a combination in amplitude and phase of the waves radiated by the array of elements. The radiating
elements can be horns, dipoles, resonant cavities, printed elements, etc. The distance between the radiating elements is typically of the order of 0:6 times wavelength. The radiation pattern is adjusted by modifying the phase and amplitude of the signal feed to the radiating elements by means of controllable power dividers and phase shifters.
For example, by feeding all the radiating elements in phase with the same amplitude, the beam obtained has characteristics similar to those of a beam generated by a reflecting antenna with uniform illumination. By attenuating the amplitude on the periphery of the radiating aperture, the side-lobe level is reduced and the beamwidth increased. On the other hand, the on-axis gain decreases. By feeding the elements with a phase which varies linearly from one element to the next from one edge of the array to the other, an inclination of the phase plane with respect to the surface of the array can be introduced and this modifies the orientation of the beam.
The antenna efficiency is determined by the amplitude weighting at the edge of the array and the ohmic losses in the power splitters and phase shifters (from one to several dB depending on complexity). The ohmic losses in the power distribution constitute a critical parameter.
A shaped beam is obtained by feeding the radiating elements with a particular amplitude and phase distribution of the power available at the antenna input. Dynamic control of the beam is obtained by 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 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.
In 1996, John Huang introduced the idea of using deployable reflectarray composed of flat panels that could also potentially be combined with solar cells in the back of the reflectarray. This concept takes advantage of flat reflecting surface relying on a simple mechanical deployment with spring loaded hinges. His concept was implemented for the first time for the technology demonstration CubeSat ISARA
(Integrated Solar Array & Reflectarray Antenna).
ISARA is 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 suffers from a low efficiency feed and large gaps and hinges, resulting in an increase of the side lobe level and reduced gain. The antenna was successfully deployed in orbit as witnessed by and measurement from the ground. The project demonstrated
on-orbit operation of the combined solar arrays and reflectarray.
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.
To achieve a smaller beamwidth for remote sensing science applications, a deployable reflectarray antenna compatible with 6U-class CubeSat was developed; it is currently the largest Ka-band cubesat-compatible antenna. While this antenna was designed primarily for Earth Science remote sensing, it can easily be redesigned for Deep Space communication.
Another challenging antenna design and application for deep space mission is Mars Cube One (MarCO) project of NASA/JPL-Caltech. NASA launched a Mars lander whose name is Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) to Mars on 5 May 2018. The main task of this antenna with X-Band transponder is to support the communication of NASA’s Mars Reconnaissance Orbiter (MRO) for downlink of the telemetries during InSight Rover’s entry, descent and landing phases. This reflectarray antenna has 29.2dBic gain at X-Band.
Since reflectarray antennas have low stowage volume, manufacturing easiness using printed circuit board technology and lightweight mass, they became attractive in space industry. TUBITAK2 Space Technologies Research Institute started a project named as YADAS in 2015 to develop X-Band reflectarray antenna to be used on LEO satellites. Through this project many reflectarray prototypes in different element arrangements were designed, manufactured and measured.
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 .
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.
Membrane antennas were highly investigated by John Huang at the Jet propulsion Laboratory for small satellites as they allow achieving large aperture with excellent stowage volume. Membrane antennas can be patch arrays or reflectarrays and are a natural option for CubeSats. A large patch array operating at S-band was recently introduced for 6U-class CubeSat. A 1.53m2 linearlypolarized patch array deploys from a 2U stowage volume. After multiple deployments, a 28.6dBi gain was measured which translates into an 18% efficiency.
A X-band reflectarray membrane antenna is under development at the Jet Propulsion Laborator. It deploys into a 1.5m2 aperture with a 0.5mm surface rms 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. Although this is not the complete antenna, the efficiency achieved is about 40%. The feed deployment inaccuracy, feed efficiency, and feed blockage will incur additional losses.
The concept of a deployable slot array was presented for 100kg small satellites. It consists of six deployable panels folding around the spacecraft . Slot arrays are good solutions for single-band and narrow-band applications with linear or circular polarization. The concept introduced in can be implemented for CubeSats at Ka-band or above.
Reference presents the development of an S-band slot array able to produce three operating modes: omnidirectional, multibeam, or directive.
Metasurface antennas could potentially also be a good solution for high gain antennas. They provide the ability to deploy a large aperture antenna without deploying a feed at a focal distance from the antenna aperture. Feed mechanics and geometry is often the biggest challenge as antenna aperture increases and in particular for deployable antennas. Similar deployment approaches for deployable reflectarrays
can be applied. From 6U- or 12U-class CubeSats, the maximum aperture achievable is about 1m2. The effect of small gaps between the panels remains to be assessed.
A silicon (Si) and gallium arsenide (GaAs) semiconductor based holographic metasurface antenna operating at 94 GHz is under development at JPL . The metasurface antenna achieves beam-forming in a holographic manner; involving the modulation of a guided-mode reference with a metasurface layer to produce the desired radiation wave-front.
References and Resources also include: