Analysis and design of antennas and radiometers for radio astronomy applications in microwave, mm-wave, and THZ bands
- Autores: Kerlos Atia Abdalmalak Dawoud
- Directores de la Tesis: Luis Enrique García Muñoz (dir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2022
- Idioma: inglés
- Tribunal Calificador de la Tesis: Raed Shubair (presid.), Adrián Amor Martín (secret.), Jose Manuel Fernandez Gonzalez (voc.)
- Programa de doctorado: Programa de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan Carlos
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
This doctoral thesis is divided into three principal parts where each one is corresponding to one topology of radio astronomy receivers (direct detection, down-conversion, and up-conversion) that operate at various frequency bands ranging from hundreds of MHz up to a few of THz: 1. Ultrawideband (UWB) microwave Circular Polarization (CP) radio telescope feed for Very Long Baseline Interferometry (VLBI) space geodesy (from 2GHz to 14GHz) and Earth observation spaceborne receivers at a low-frequency band from 400MHz to 2GHz.
2. UWB THz antenna arrays for Antenna-based Emitter (AE) THz source for radio astronomy heterodyne receivers, covering various bands from 200GHz to 2THz, 100GHz to 1THz, and 50GHz to 0.5THz.
3. Photonic nonlinear up-conversion radiometer working at room temperature for Cosmic Microwave Background (CMB) spectroscopy at 12GHz and for mm-wave CubeSat satellite climate change forecasting at 183GHz.
Firstly, a receiver based on the direct detection of the Electromagnetic (EM) radiation through a radio telescope working on cryogenic cooling conditions. Here, the focus is on designing conical log-spiral antennas and baluns (balanced to unbalanced transformers) to be used as feeds for VLBI Global Observing System (VGOS) ground-based radio telescopes. The feeds cover the UWB from 2GHz to 14GHz with CP radiation and stable radiation patterns. Two versions are proposed, a single-element spiral for single CP and an array of four spirals for dual CP operations which is called DYQSA (DYson Quad-Spiral Array). The proper designs for the balun and four-baluns system are provided to excite the two proposed antennas respectively. A summary of the designing steps is discussed with the possible procedures to overcome the fabrication challenges, especially at such relatively high frequencies which were one of the main limitations for practical implementation of the conical spiral at higher frequencies.
For each conical spiral, two metallic bow-ties patches are included at the apex to have them ready for connecting to the balun, they can be also used for the direct connection to differential LNAs. A thin dielectric cone is inserted inside the antennas to enhance the mechanical stability specifically at the small top part where the spacing between antenna arms is small. Lastly, a balun (or baluns system) is inserted inside the center of the element (or the center of the array), it is covered by a metallic shielding cone to enhance the electric properties and prevent it from disturbing the antenna performance. The two feeds are analyzed in two scenarios, the isolated antenna performance and after integration to the radio telescope and analyzing the complete performance for the radio telescope including the different components (antenna feed, reflector, and LNA).
Regarding the single-element conical spiral solution, it has an excellent axial ratio below 1.4dB. Its designed balun shows excellent results too with low losses below 0.5dB, common-mode isolation below −30dB, and good matching below −16dB. When it is integrated with the VGOS radio telescope, it provides stable CP radiation patterns over the whole band with Co/Xp-ratio larger than 23dB at the broadside. It enables the reflector to have a high directivity above 45dBi which is comparable to the reflector performance once it is fed by an ideal Gaussian beam. From the same perspective, the DYQSA solution presents a low axial ratio below 2.5dB with small phase center variations. Its balun array excitation system has low losses below 0.5dB with good matching, common-mode rejection, and isolation between the ports of 15dB in most of the band. Besides, when it is used to feed the VGOS radio telescope, the proposed DYQSA solution enables the reflector to have high directivity as in the case of the single spiral antenna with a very small difference below 1.2dB in comparison to the ideal Gaussian beam. The radio telescope fed by DYQSA provides an average spillover and antenna noise temperature of about 6K and 14K respectively when it points near the zenith. The noise temperatures face a small degradation of about 7K when the radio telescope points at further angles (for example, at a zenith angle of 45◦). Including the noise temperature from the LNA designed by Yebes Observatory for VGOS radio telescope and other noise sources (calibration coupler, ohmic losses, and cables), the proposed DYQSA enables the receiver to have a System Equivalent Flux Density (SEFD) of about 1300Jy and 1600Jy at zenith angles of 0◦ and 45◦ respectively.
The proposed DYQSA array solution and the two main state-of-the-art UWB feed solutions have been compared from different points of view. These two state-of-the-art feeds are Quadruple-Ridged Flared Horn (QRFH) developed at California Institute of Technology and the Eleven feed developed at Chalmers University of Technology. From the critical total feed efficiencies point of view, this comparison demonstrates the frequency-independent behavior of DYQSA with an efficiency of 65±5%, contrary to the linear reduction for the other two solutions at high frequencies. In a conclusion, the proposed feeds provide three new attractive characteristics compared to the state-of-the-art feeds: a direct single/dual circular polarization in contrast to single/dual linear polarization which avoids the use of additional analog hardware or digital software for linear-to-circular polarization converting. Secondly, stable radiation patterns and beamwidths over a UWB with the easy adaptation to different radio telescopes. Thirdly, the constant with an almost real input impedance of the antenna due to its self-complementary geometry (which is beneficial for the other receiver components such as LNA and/or balun). These characteristics make the proposed antennas beneficial not only as radio astronomy feeds but for many other UWB applications that require such tricky performance.
The two UWB antennas (single-element and four-elements array) are 3D manufactured in titanium with a thin Nylon supporter cone inside each conical element with a detailed explanation of the assembly process. The measurement results have a good level of agreement with the simulated ones. Based on the measured radiation patterns of the isolated antennas, VGOS radio telescope has good CP radiation patterns over the whole frequency range with a Co/Xp-ratio above 15dB. The average aperture efficiencies are 70% for the frequencies ranging from 7GHz to 14GHz for the single-element with a small reduction in the DYQSA efficiency results to be 50% averaged over the entire band from 2GHz to 14GHz.
Also, in the same first part, the proposed single-element feed (antenna + balun + metallic cone) is readjusted for being used for CryoRad spaceborne Earth observations missions from 400MHz to 2GHz. This feed has a single pure CP which has an axial ratio below 1dB over low-frequency UWB. It provides also stable radiation patterns and a flat high gain of 10.5dBi with low return and isolation losses. Recalling that even though only a single CP is required, other standard feed solutions such as QRFH still need dual linear polarization antennas for obtaining the CP performance, this adds more important advantages to the proposed feed such as compact size and low weight which all make it suitable for satellite installation.
The second part of the thesis is dedicated to mm-wave/THz sources to be used as local oscillators for heterodyne radio astronomy THz receivers in which the down-conversion of the THz radiation to a lower frequency occurs. The source is based on an array of 3 × 3 self-complementary bow-tie antennas and photomixers that lies on a dielectric lens forming an AE THz source. This source can be scaled easily to cover different UWB ranges, three versions are analyzed from 200GHz to 2THz, 100GHz to 1THz, and 50GHz to 0.5THz. The arrays elements are electrically small around 0.038λmin × 0.038λmin (λmin corresponds to the minimum frequency of the UWB band) and in contact with each other which purposefully increases the mutual coupling between them to cause constructive interference. This forms a high-density array with a compact size that does not have the problem of out-of-focus when the AE array is placed over Si lens. They are called Chessboard arrays following the resulting shape of this high-density element distribution. The electrically large extended-hemispherical lens has the role of increasing the radiated power compared to the extracted power and having a single beam directed downwards below the Si-lens with very high directivity. The effect of different commercially-available Si lenses (with a maximum diameter of 25mm) is studied depicting, in some cases, a considerable change in the radiation patterns that can be reflected with about 10dB difference in antenna-directivity even with the use of the same antenna array. These proposed arrays provide stable radiation patterns in their corresponding UWB THz ranges with average directivity of 35, 33, and 27dBi for the three arrays respectively.
A proof-of-concept array is manufactured showing the details of the complete assembly of the AE and measurements setup. A microlens array that is held by a 5-axis micro-positioner is used to direct the optical beam towards the photomixers and a PCB circuit is utilized for the DC biasing for the photomixers. The measured THz radiated power shows a peak CW power of about 60μW at 150GHz which outperforms other integrated THz sources working at the same range and are based on the same concept of combining photomixers and self-complementary antennas. The proposed antenna sources have several advantages such as high directivity, compact, ease of integration, relatively high radiated power, tunability, and low cost, in addition to operating at room temperature. This makes it a good candidate as a local oscillator in radio astronomy heterodyne receivers as well as a directed Continuous-Wave (CW) source for several THz applications transmitters that require these advantages.
Additionally, in this part, studies for the effects of metal losses on such THz planar antennas are performed which are not well-investigated in literature yet, this is based on the actual measured metal resistivity for the metal alloy used to manufacture a THz spiral AE. The studies are performed for wide mm-wave and THz bands, from 100GHz to 800GHz in which most of the THz applications exist. These studies demonstrate that, unlike the conclusion that can be derived from the gold characteristic assumption, the metal losses have dominant and very strong effects on the antenna gain (which in turn would affect its radiated power), they can reach 30dB at 100GHz compared to less than 5dB if an ideal gold is considered in the losses estimation. This especially occurs at lower frequencies where the skin depth of the manufactured metal alloy goes greater than its thickness. The metal surface roughness effect is studied too, however in such case of larger skin depths (of the lossy metal), it seems that its contribution to the metal losses is negligible over the whole studied band.
Generally, the exact amount of such losses depends on several parameters such as metal deposition, metal thickness, surface roughness, operating frequency, and antenna topology. Hence, with fixing the same fabrication process and frequency range, two alternatives are provided to overcome such significant losses. First, increasing the metal thickness from 160nm to 500nm, which would decrease the losses by around ten times at lower frequencies and to its half at higher frequencies. But on other hand, this would increase the fabrication complexity and its cost too. Alternatively, by changing the antenna topology using log-spiral with the same relatively small metal thickness. This achieves better performance as it decreases the losses to be an almost flat value less than 4dB in the whole band. However, this comes with the cost of a significant increase in the antenna size. The proper method can be selected depending on the required specific application. Such studies are interesting for avoiding the huge losses from the metal which if it is correctly considered, can significantly enhance the radiated THz of the antennas. Although these proposed THz sources themselves can work at room temperature, the receiver probably still needs the cooling for the other receiver components (such as the mixer) to work efficiently at such high frequencies. This is the motivation for the third part of this thesis which presents a different type of radio astronomy receiver that is completely able to work without cooling.
The third part is dedicated to a radiometer that is based on the nonlinear up-converting of the microwave radiation into the optical domain using Whispering Gallery Mode (WGM) resonators which can work at room temperature efficiently. For such advantage and since this concept is naturally narrow band, it can be a proper candidate for CMB spectroscopy and space applications. A study of the photonic efficiency estimation for the system is briefly discussed which is a key parameter for such receivers. This is followed by possible techniques for maximizing it which is mainly achieved by enhancing the modes overlapping while satisfying the phase-matching condition between the two input signals. This enables the development of a novel metal-covered WGM resonator with an enhancement of the photonic efficiency around 5000 times compared to the corresponding standard dielectric ones. The system design and its performance are analyzed for Ku band at 12GHz with proposing a novel microwave coupling scheme for enhancing the up-conversion photonic efficiency which is the main limitation for such upconversion systems. Knowing that the radiometer can be easily scaled to be used in different frequency bands (microwave, mm-wave, or THz).
Likewise, several high gain 3D-printed Dielectric Resonator Antenna (DRA)s are proposed in both isolated and array configurations to have a direct coupling of the microwave radiation to the proposed scheme without using any extra interfaces. Higher gain DRAs are achieved by either a single antenna working at higher-order modes or by changing dielectric shape or by configuring a DRA array fed by a novel feeding technique based on standing-wave excitation. This novel feeding technique eliminates the use of power dividers or quarter-wave transformers to keep the low losses feature of the DRA and have a high radiation efficiency. All three antenna alternatives provide high gain of about 9dBi, 11dBi, and 15dBi for the single rectangle, single-pyramidal horn, and a 3 × 3 rectangle array respectively. The rectangle element and array antennas are 3D manufactured and the measured results show a good matching compared to the simulated ones with a measured peak gain of about 15dBi for the array.
Another practical application for the proposed nonlinear receiver is presented for CubeSat missions at the mm-wave band (183GHz) for water vapor profile measurements and weather forecasting. A complete integrated optoelectronic up-conversion radiometer is proposed by engineering a micro-machined mm-wave cavity that maximizes the mode overlapping between mm-wave and optic signals with fulfilling the phase-matching and energy conservation conditions. The system has a predicted high photonic efficiency η ≈ 10−2 which is surpassing the state-of-the-art by around three orders of magnitude in this frequency range. The microwave coupling is performed using a microstrip line printed in a Si substrate which has two transitions to standard WR5 metallic waveguides which makes the whole system ready to integrate to any mm-wave antennas with a metallic waveguide end. A CVD diamond prism and two GRIN lenses are used for the optical coupling.
The tolerance study for each of the dominant system parameters is done which shows that the microwave frequency resonance is shifted around 2GHz in case a tolerance of about 5μm or 2μm occurs for the resonator radius or thickness respectively. A metallic rod with a ring-ended connected to a piezo is designed to shift up or down the resonance by moving it upwards or downwards to the resonator respectively. this enables the system to be tuned over a very wide band (183GHz ± 22GHz) with fine-tuning on the order of tens of MHz. With this mechanism, the complexity of the manufacturing process is reduced and both fabrication and alignment tolerances are completely compensated. The resonator, diamond prism, micro-machined Si cavities, microstrip substrate with transitions, and CNC-machined metallic covers are manufactured and under assembling process to be ready for the measurements. So, this proposed radiometer has four main attractive features, firstly, it operates without cryogenic cooling which decreases the satellite cost, weight, and volume with a significant increase in the satellite mission lifetime. Secondly, it has a very high photonic efficiency that outperforms other mm-wave up-conversion schemes. Thirdly, it is fully integrated which makes it convenient for the satellite environment. Fourthly, it can have a refined tuning over a wide frequency range which completely compensates both fabrication and alignment tolerances.
Finally, it is worth noting that besides the radio astronomy applications, the proposed receivers (and/or their antenna/components) can be used for many other applications. For example, the UWB antennas in the first part can be used as wideband scalable probes for EM compatibility testing or other wireless systems that require single or dual CP such as radar and military applications. This is because the solutions provide constant beam characteristics with good CP polarization purity and stable performance over the operating UWB. In the same way, the proposed THz source in the second part can be used in several THz applications such as very high-speed wireless communications, high-resolution imaging for medical and security purposes. This is because of its key benefits as decade bandwidth, compact size, low noise, low power demand, high tunability, and the ability to work at room temperature. For the up-conversion scheme proposed in the third part, due to its high photonic efficiency, low noise level which enables it to work at room temperature, and its scalability from a few GHz up to several THz, it is suitable for low-cost and high sensitivity applications. Specifically, the ones that need to get rid of the hard-cryogenic cooling conditions, or at least, relax them and allow the system to work efficiently at higher temperatures. For instance, portable mm-wave and THz systems for quality control, security, and biochemistry. Furthermore, in this same third part, the proposed DRA elements and arrays, due to their low cost, high gain, and low losses, can be used for sensing applications and 5G base station antennas.
