Astronomy

What is the role of the mesh on which dipole elements of the MWA antennas are placed?

What is the role of the mesh on which dipole elements of the MWA antennas are placed?


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I was reading about the radio telescope - Murchison Widefield Array (MWA) situated in Western Australia. Antennas of this telescope are quite unique and different from the usual dish radio telescope. In MWA a four by four regular grid of dual-polarisation dipole elements are arranged on a 4m x 4m steel mesh.

So, I was wondering why these dipole elements would have been mounted over the mesh? On searching over internet I found out that this mesh is called a counterpoise (though I am not sure if I am correct). I read more about it and I partially understand that the mesh and ground (Earth) act as two plates of a capacitor but I didn't get why it was done and what was the use of it? In this image mesh can be seen on which dipole are mounted - Image courtesy: SKA

Source click for full size


The mesh acts as a ground plane. An antenna design project can be divided into two theoretical regimes: free space, and ground plane. A free space antenna is an antenna that is located far enough away from anything, that nothing will alter its electrical properties. This is the simplest way of doing a design, but not applicable to this antenna

This antenna is mounted on the Earth. The soil under the antenna has electrical properties that can not be precisely known. The properties may change with the amount of vegetation or rainfall through the year. To avoid having to deal with the exact properties of the soil, we place a metal shield between the antenna and the soil. The shield is call the ground plane. Its purpose is to present known electrical properties to the antenna. Ideally the ground plane is always at zero Volts relative to the antenna.

The reason for using a mesh as opposed to a solid sheet of metal due to economy of materials facilitated by a neat property of electromagnetic waves: a wave cannot pass through an opening that is smaller than its wavelength. Therefore instead of spending all the money, and gaining all the weight of a solid sheet, we can adjust the width of the mesh to block (that is, reflect) the frequencies we are interested in.


What is the role of the mesh on which dipole elements of the MWA antennas are placed? - Astronomy

Corner Reflector Antennas

Corner reflector antennas are probably the most common type of UHF antennas. They are yagis with large reflecting arrays at 45 degree angles from the main boom. These reflectors add considerable gain.

Most corner reflector antennas use traditional rod type elements as directors, while others use wider oval shaped metal elements, instead. The rod type directors are usually used with rod type driven elements, and on some antennas with a bow tie driven element, while the oval shaped elements are only used with bow tie driven elements. Other antennas use X-type directors to increase gain, such as the Terrestrial-Digital 91XG. X-type directors are only used with bow tie, and folded dipole driven elements.

The Corner reflectors are usually made out of a boom with a series of reflecting elements coming off of it. The only exception to this is the Antennas Direct 91XG, which uses screen reflectors, and a short range antenna designed by Burns Digital Consultants and sold under such names as Zenith, which utilizes a solid mesh type reflector.

Corner reflectors usually have gains of about 8-12 DBd, but models such as the Terrestrial-Digital 91XG can achieve as much as 14-15 DBd.

Corner reflector antennas are frequently used on combination VHF/UHF/FM, as they can be designed as such that they do not interfere with the oncoming VHF signal.

Conical antennas have two, four, or eight active elements and some type of flat screen reflector. They are made by just about every antenna manufacturer. We will examine each type individually:

Two bay antennas are only made by Terrestrial-Digital, under the model name, DB-2. This antenna is great for indoor, outdoor, or attic mounting. It is probably the top performing indoor model, as it achieves more than 8 DBd on the high channels. This model is not usually used in an indoor setup, because it looks ugly on top of one's TV, and it doesn't have a stand.

This, however would be really good in an attic mount, or a roof top installation where towers aren't more than 20 miles away.

Four bays are great for reception up to 50 miles from the transmitter. They offer gain comparable to that of a medium-large corner reflector (10-12 DBd). They are relatively cheaper than a competing corner reflector, as well as smaller.

Eight Bays are the best thing short of a parabolic antenna in terms of gain. They offer up to 15 Db over dipole. They also take up less space than a corner reflector. However, they are heavier (up to 15 LB), and the screen models such as the Channel Master 4228 have more wind load than a corner reflector.

Channel Master: They make the 4228, probably the most popular antenna for DTV reception. It offers the best over all gain of all 8 bays, is commonly available at Lowes, Fry's, and some other local retailers. This antenna is also a favorite of professional installers. There are two variations of this antenna one with continuous screen, and one with two screens. There is virtually no difference in gain between the two variations, but I just thought you should know.

Terrestrial Digital/Antennas Direct: They make the popular DB-8. This antenna is basically just four DB2's ganged together. It is roughly equal in performance to the Channel Master 4228, but all at roughly twice the price. The Channel Master 4228 costs $50-65 depending on the retailer, while the DB-8 is listed at $119 at antennasdirect.com, and sells for $98 on SolidSignal.com.

Winegard: While the gain might not

The lost Zig-Zag/Hoverman 4 Bays.

All the conical antennas we've discussed up to this point have utilized a bow tie element design. However, an alternative Zig-Zag design called the Hooverman was popular during the early 80's. They were manufactured by Winegard, and were also distributed by Antennacraft, and Radio Shack. For more information, please read this article by the WTFDA (World Wide TV and FM DX Association)

Since they only weighed 1.5 Lbs, an 8 bay model would only be a little over 3lb, which is 12 Lb lighter than the Channel Master 4228. An 8 bay Hooverman would also have considerably better gain than a traditional 8 bay. The Hooverman design has a seemingly small wind resistance compared to a screened conical. These two characteristics would make it very suitable for stacking in 16, 32, and dare we say even 64 bay configurations.

The gain curves in the previously mentioned article show that it's gain peaks in the mid 50's, and then drops off to 0db by the mid to high 60's. This is just perfect for DTV, as the spectrum will end at channel 51, and there are presently no DTV stations above channel 51. The gain graph also shows that it has better gain than a traditional 4 bay.


3 Answers 3

A dipole antenna is so named because the electric field around it is an electric dipole.

Derived from work by Geek3, CC BY-SA 4.0, via Wikimedia Commons

This is intuitively obvious: a dipole consists of two wires. At any instant, one has a positive potential and the other has a negative potential, except for the instant where the polarity flips and the potential is zero everywhere.

You will notice that at all points equidistant from the two poles form a plane. The potential at this plane is zero. This is true because these points are balanced between the positive and negative potentials of each pole of the antenna.

A monopole antenna has just one wire, and a ground plane. The ground plane is both physically a conductive plane (or something made to approximate it, like radials), and the imaginary plane just described which is all the points that would be equidistant between the two poles a monopole would have if you considered it to have a imaged second pole.

So while a monopole does have two parts, one of them is not a pole. It's ground: not because it's in the dirt, and not because it's connected to the coax shield (although either of these things may be true), but because its potential is zero.


We are already aware of the fact that this operates at a high range of microwave frequencies. Also, at this frequency, the electromagnetic wave behaves as a light wave. Hence, it gets reflected when strikes a polished surface.

This is the principle of operation of a reflector antenna.

However, a crucial point over here is that a reflector antenna is a combination of feed element and a reflecting surface.

This means in case of reflector antenna a reflecting surface along with an antenna element is required in order to provide excitation to the reflecting element. This means that it is composed of an active and a passive element.

The antenna that is used to provide excitation is known as the active element. While the one that re-radiates the energy emitted by the active element is the passive element which is nothing but the reflecting surface.

So, simply we can infer that the feed is the active element while the reflector is the passive element.

Generally, dipole, horn or slot antennas are used as the active elements for providing excitation to the reflector antenna. Sometimes the active element is also known as the primary antenna whereas the passive element is called the secondary antenna.

  • Reflector antenna plays a very crucial role in radio wave propagation as it modifies the radiation pattern of the radiating elements.

It operates in a way that energy from the feed is directed towards the reflecting surface placed at an appropriate position. The reflector on gaining the energy further guides it in a particular direction.

The radiation pattern of the feed is referred as a primary pattern but that originating from the reflector is referred as a secondary pattern.

It is to be noted here that high gain antennas operating at microwave frequencies possess such a small physical size that reflector of any suitable shape provides the desired directivity.

Despite offering multiple geometrical configurations, there are some popular shapes in which the reflecting surface of the antenna is formed. And on this basis, the reflector antennas are further classified.

Types of Reflectors

Depending on the geometrical shape possessed by the reflecting surface, the reflector antennas are classified into the following categories:

  • Rod Reflector: As the name indicates this type of reflector possesses the shape of a rod. A rod type of reflector is the one which is majorly used in Yagi-Uda antenna. The reflector is located at a certain distance behind the driven element in that antenna arrangement and has a length generally more than the length of the driven element i.e., half-wave dipole.

Here the reflector offers inductive reactance so guides the field radiated in the backward direction towards the driven element to reduce the losses due to back-reflected wave. Hence, helps to improve the gain.

It does not serve as an active member of the structure but is a parasitic element.

However, it causes variation in the impedance of the driven element. These types of reflectors exhibit frequency sensitive characteristics.

  • Plane Reflector: It is also referred as flat sheet reflector and is regarded as one of the simplest reflectors that direct the electromagnetic wave in the appropriate direction.

It is nothing but a plane metallic sheet that is located at a certain distance from the feed. For the incoming radio waves, it acts as a plane mirror and allows them to undergo reflection through it.

It is to be noted that a plane reflector possesses difficulty in collimating the overall energy in the forward direction.

Thus, to handle the impedance, pattern characteristics, gain and directivity of the system, the polarization of the active element along with its position in reference to the reflecting surface is used.

  • Corner Reflector: It is regarded as a modified version of the plane reflector so as to guide most of the radiation in the forward direction. Basically, the shape of a plane reflector is changed by joining two flat sheets in order to form a corner. There can also be three mutually perpendicular conducting plane surfaces.

These are basically used to enhance the directing ability of electromagnetic energy in the forward direction so as to reduce the percentage of the back-reflected wave.

  • Cylindrical Reflector: This reflector is designed from a cylindrical structure thus is named so. It is another classification of structure in which a reflector is designed. Thus is nothing but a part of the cylindrical structure. Generally, cylinders are present in parabolic shapes however, other shapes are also present that can be used in its construction.
  • Spherical Reflector: Like a cylindrical reflector, a spherical reflector is the one designed from a spherical surface. This means these reflectors are part of spherical surfaces and are used for collimating the energy from the active elements towards the forward direction.
  • Parabolic Reflector: The type of reflector designed in the structure of a paraboloid employing the properties of a parabola is known as a parabolic reflector. The active element is present focusing the main axis, this leads to reflecting the radiated wave in the direction parallel to the main axis.

This gives rise to a small percentage of minor lobes hence the directivity is improved.

In this reflector, the generally pyramidal or conical horn antenna is used as the feed element.

Applications of Reflector Antennas

These antennas are a major part of communication and radar systems. From point to point communication, TV signal broadcasting to satellite communication these antennas are widely used. Along with these, the other applications of reflector antennas involve weather radar and radio astronomy as well as in spacecraft systems.


MeerKAT

MeerKAT is the mid-frequency SKA precursor under construction in South Africa’s Karoo Desert. When complete, that array will contain 64 dish antennas, each 13.5m diameter. Eventually MeerKAT will be integrated into the SKA’s mid-frequency infrastructure and will provide both wide-view and high-resolution images. In a similar configuration to the MWA, 75% of MeerKAT dishes will be within a 1km diameter and the remainder in spiral arms extending out to 8km.

MeerKAT will support eight key science objectives, in addition to enhancing the sensitivity of the global Very Long Baseline Interferometer (VLBI) operations. The two priority MeerKAT science objective are:


Interference into Circuits

5.3 The Linear Dipole

Consider the linear dipole antenna shown in Figure 5.3 . In the transmitting antenna case the source voltage Vo is applied at the gap ℓ =0, which excites the currents in the dipole. As the current propagates along the dipole, the current does attenuate, because at the end caps of the dipole the current experiences a discontinuity which causes the radiation of the antenna. Though the current distribution could be calculated using the integral Equation (5.16) by the method of moments, an approximate expression for this current can be stated as

Figure 5.3 . Transmitter and receiver dipoles.

where γ = 0.577 and k = 2 π λ . This expression gives an accurate value for the real part of the input current.

When the initial current arrives at the end of the wire antenna dipole, some of it is reflected. The current distribution of the reflected wave is given by

where E(j2kℓ) is the exponential integral of the first kind. Because there are reflections from both ends of the dipole wire antenna, two reflected currents are present. This process continues, leading to two sets of infinite, though summable, series of multiple reflected currents. The total current distribution can be defined as

Using the current distribution in Equation (5.19) , the far-field pattern of a thin wire antenna is given by

The current distribution given by Equation (5.19) , though not as exact as the one that could be obtained using numerical methods, is a very good analytical approximation for the current distribution in a wire antenna. Let us now consider some more simple approximations of the current distribution, and then derive the resultant field.


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6 CONCLUSION

We have described the design and measured performance of the EDA. It is a station of 256 antennas, spread over a diameter of 35 m, formed by 16 groups of 16 pseudo-randomly placed MWA dipoles (Figure 3). The EDA re-uses as much MWA hardware and software as possible. It uses standard MWA analogue beamformers to perform first-stage beamforming (hence, the EDA is comprised of 16 sub-arrays) and standard MWA dipoles with slightly modified LNAs to receive signals down to 50 MHz.

The EDA was conceived as a rapidly deployable test and verification system to support the development, test, and verification programmes for both SKA Low and MWA. In its initial incarnation, the EDA uses two-stage analogue beamforming to form an array beam, however work is underway to upgrade to digital second-stage beamforming.

Using drift scans and a model for the sky brightness temperature at low frequencies, we have derived the EDA’s receiver temperature as a function of frequency. The results show that the EDA is sky-noise limited over most of the frequency range measured, with the exception being below 60 MHz where the receiver temperature is comparable to the sky temperature at high Galactic latitude.

Using the derived receiver temperature, we have measured the sensitivity of the array by measuring the noise variance in calibrated visibilities. The measured sensitivity agrees very well with the predicted sensitivity of the array taking into account losses due to uncorrected gain errors in the second-stage beamformer. The results demonstrate the practicality and feasibility of using MWA-style precursor technology for SKA-scale stations and highlight the benefits of rapid prototyping and verification for array development.


Antenna requirements for Space Applications and emerging Antenna technologies

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.

Space environment

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.

Satellite antennas

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.

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

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.

Array antennas

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.

Reflectarrays

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.

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.

Membrane antennas

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.

Slot arrays

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

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.


What is the role of the mesh on which dipole elements of the MWA antennas are placed? - Astronomy

Bow-Tie antennas aren't really Log Periodic antennas however, the bow tie (or butterfly) antenna makes a good starting point to begin talking about Log-Periodic antennas. So we will first discuss properties of wideband antennas, and then discuss the infinite bow-tie antenna, and then measurements and properties of the real bow-tie antenna.

Wideband Antennas

If you think about the Half-Wavelength Dipole Antenna, the antenna design is specified by the length - the length should be equal to a half-wavelength at the frequency of interest. Hence, if you want your antenna to radiate at 300 MHz (1 wavelength at 300 MHz = 1 meter), you would make the antenna 0.5 meters long. Now, this is fine for 300 MHz - but what if you also want the antenna to radiate well at 200 and 400MHz? Because at 200 MHz the 0.5 meter antenna is too short (wavlength at 200 MHz = 1.5 meters) and at 400 MHz the 0.5 meter antenna is too long (wavelength at 400 MHz = 0.75 meters), we won't get efficient radiation at these frequencies.

If you think about that last paragraph for a while, you may note that one problem with the above Half-Wavelength antenna design is that the design depends solely on length, which will mean much different things in terms of wavelengths at different frequencies. What if instead, we could design an antenna that was completely specified by Angles instead of Lengths? Angles do not depend on distance - and hence don't depend on wavelength, so if we could design such an antenna it would be frequency independent.

The Bow-Tie Antenna

As a simple (and non-manufacturable) infinitely wideband antenna, let's look at an infinite bow-tie antenna:

Figure 1. Infinitely Long Bow-Tie Antenna.

In Figure 1, we have an antenna that is specified solely by the angle between the two metal pieces, D. The antenna feed (where the radio positive and negative terminals connect to the antenna) is at the center of the antenna. Our antenna here is infinitely long in both directions, so that wavelength never comes into the equation. As a result, this antenna would theoretically have an infinite bandwidth, because if it works at one frequency (any frequency), it must work at ALL frequencies, because the antenna looks the same at all wavelengths. This is a nice antenna.

In terms of making a real antenna, we can take the simple approach and just clip it after some distance and seeing what happens. The result is the bow-tie antenna (also known as a butterfly antenna, or a biconical antenna):

Figure 2. The Bow Tie Antenna.

This antenna will have a similar radiation pattern to the dipole antenna, and will have vertical polarization. A L=76.5mm Bow Tie antenna with width W =36mm (so that the angle D=2*atan( 76.5/36 )= 130 degrees). This antenna was mocked up as shown in Figure 3:

Figure 3. A 76.5mm Bow Tie Antenna.

The real bow tie antenna of Figure 3 is fed with a coaxial cable. The coaxial cable is soldered along the lower arm of the antenna - the purpose of this is to minimize the impact of the antenna feed cable on the antenna. This is similar to using a balun.

The VSWR of the Bow Tie antenna of Figure 3 is shown in Figure 4:

Figure 4. VSWR of the Bow Tie Antenna of Figure 3.

Our bowtie antenna of Figure 3 is L=76.5mm long, which would be a half-wavelength at f=c/2/L=1.96 GHz. We see the first resonance occurs at about 1.5 GHz. The reason this occurs lower than 1.96 GHz is because the antenna is very wide at the top - this essentially makes the antenna behave as if it is longer than it really is. This will be true of non-thin-wire antennas in general.

From Figure 4, we see that the bow-tie antenna has much better bandwidth than a thin-wire dipole antenna. In general, antennas with more volume have wider bandwidth (and I've said this many times on this site, as it is one of the fundamental antenna rules). More radiation modes can fit on the structure when the current is less constrained. The VSWR=3:1 bandwidth for the 1.5GHz mode is from fLow=1.18GHz to fHigh=1.65 GHz. This produces a Fractional Bandwidth of 33%. Note that the fractional bandwidth of a thin wire dipole antenna is about 8%. Hence, we see the bow tie has a much larger bandwidth than the dipole antenna. Since this antenna is easy to construct, it is very popular for this reason.

Note that a low VSWR does not necessarily imply radiation - the power could be absorbed or lost. Low VSWR means power is being delivered to the antenna and not being reflected. However, in this case, where we have no real non-metal materials (no lossy dielectrics, and all metals are good conductors) then it is reasonable to assume most of the energy is being radiated (which it is).

Now that we have discussed the bow tie antenna, we can extend some of the wideband concepts here to a very popular and useful extension, the Log-Periodic tooth antenna. The bow tie can be considered a simplified version of the LP tooth.


Watch the video: Ο Χρήστος Χατζησάββας για την εγκατάσταση κεραιών κινητής τηλεφωνίας muHOF5EDujE (May 2022).