Fri, 05 Jul 2024 in Ciencia y Poder Aéreo
Comparative study of six antenna designs for receiving APT images from NOAA-19 in urban environments
Abstract:
The objective of this article is to design, build, test, and compare the performance of six antennas (Turnstile, QHF, double-cross, Moxon, V-dipole, and Eggbeater) used for receiving APT meteorological images transmitted from the NOAA-19 satellite in an urban environment. The process follows a four-phase methodology: research, tuning system development, design and simulation, construction, and testing. During the research phase, fundamental concepts such as RTL-SDR, NOAA satellites, and the APT format for image transmission are explored. Subsequently, the development of the tuning system involves the use of programs such as sdRsharp and WXtoImg to receive and decode the APT signal. The next phase encompasses the design, simulation, and construction of the antennas, with the selection of NOAA satellites. Virtual tools are employed to calculate dimensions and parameters, followed by the assembly of the antenna designs. Tests are conducted in open spaces, aligning with the satellite orbits, to receive images. Finally, the results are evaluated in terms of image resolution and audio power to determine the most suitable antenna arrays for this type of communication. The Moxon antenna emerged as the best-performing, recovering images with resolutions of 1.94 megapixels, while the QHF antenna exhibited the highest power reception at 1.9 W. The V-dipole, QHF, and Eggbeater antennas demonstrated the best coupling with the transmission line, achieving low reflection coefficients of 0.16. In conclusion, it is established that in urban environments, the Moxon and QHF antennas effectively receive APT images.
Main Text
Introduction
Receiving images from meteorological satellites has become indispensable for various fields, ranging from weather forecasting to natural disaster management. Satellites operated by the United States National Oceanic and Atmospheric Administration (NOAA) play a crucial role in providing these valuable images globally. NOAA, a United States scientific agency, primarily focuses on the global monitoring of oceans and the atmosphere through its satellites. The satellites used in this study, namely NOAA-15, 18, and 19, are positioned in low polar orbits between 800 and 1500 km above the Earth’s surface. They utilize high-frequency transmitters in the 137 MHz band to transmit collected data. Employing the APT (Automatic Picture Transmission) transmission format, these satellites encode captured images in amplitude modulation (Bosquez,2016). With the RTL-SDR module, the carrier frequencies are tuned, and the WXtoImg software facilitates the decoding of APT signals into Png images (Velasco Tipantuña,2017). SDR technology provides a cost-effective solution for tracking small satellites, thereby reducing ground station development and implementation costs (Peralta et al.,2018).
A critical factor influencing the reception of NOAA satellite signals is the choice of antenna. The selection of an appropriate antenna significantly impacts the quality of received images. While the literature includes publications on antennas utilized for receiving images from NOAA satellites, analyzing their performance compared to one or two other antennas, it does not comprehensively cover a larger number of antennas, as noted by the authors (Bosquez,2016; Velasco Tupantuña, 2017). This article aims to question: What is the most suitable design for a lowcost, quickly constructible antenna that can be easily implemented and operated by university students for effective reception of images from NOAA satellites in urban environments? To address this inquiry, we compare various antennas and evaluate the results to determine the optimal choice for this application.
Methodology
For the Transmission Media course (METX) within the study plan of the Electronic Engineering program, the instructor assigned a laboratory project to the students. This project involved assigning the study, characterization, and construction of a different type of antenna to each group in the course. Ten laboratory groups, each consisting of three members, worked on the design of the following antennas: Turnstile antenna, QHF antenna, double-cross antenna, Moxon antenna, V-dipole, and Eggbeater.
The following flowchart in Figure 1 illustrates the methodological design of the laboratory practice divided into specific phases.
In the research phase (Figure 2), each laboratory group consulted and studied the most relevant concepts related to RTL-SDR and its operation, the set of NOAA satellites, and their utilization for the study of data transmission. Additionally, the definition of the format through which the satellites transmit analog images (APT) was explored.
After acquiring the relevant information and comprehending the concepts, the groups proceeded to configure the receiving system for tuning, as shown in Figure 3.
To enable the tuning system, it is necessary to install and link the SDRsharp and WXtoImg programs for receiving and decoding the audio signal in APT format. Since the receiver is designed on a software-defined radio (RTL-SDR), it is essential to download the required drivers for its operation. This design aims to visualize the carrier signal on a computer using the SDRsharp software, thus confirming the functionality of the signal tuning system. Subsequently, the WXtoImg software is employed to demodulate and recover the received audio or image. Phase three, shown in Figure 4, consisted of the design and simulation of the antennas mentioned.
For this phase, the students selected the NOAA satellite to tune, as each satellite operates on a specific frequency for data transmission, forming the basis for antenna design. Due to the course’s nature, students possessed prior theoretical knowledge of antenna design. Consequently, they conducted a comprehensive review of the theory for each antenna type, identifying and determining the corresponding parameters for the design. Utilizing virtual design tools, the students calculated the dimensions and specifications of the antenna. In the case of simpler antennas, some students proceeded directly to construction. They verified the radiation pattern, directivity, and impedance of their designed antennas using the collected data through simulation.
Figure 5 shows the construction and testing phase of the receiving antenna for NOAA signal tuning using an RTL-SDR module.
Using their theoretical designs as a basis, the students classified and identified the recyclable materials intended for constructing their radiant system. They utilized recycled low-gauge copper pipes, wood, PVC (polyvinyl chloride) pipes, and joints for the antenna. The transmission line and tuning system were constructed using coaxial cable, SMA (Subminiature version A) connectors, a computer, and an RTL-SDR module. The selection of these materials adhered to specific criteria: recyclability, easy accessibility, and low cost.
Then, they consulted the schedules in which the satellites performed their respective orbits, and which coincided with the coordinates of Bogota (4°46’56”N 74°02’38”O / 4.78222, -74.0439), where the tests were carried out. The laboratory groups looked for open and clear spaces, with acceptable weather conditions to obtain better results.
In this phase, researchers identify both successes and failures resulting from the precision or lack thereof in antenna construction. Deviating too far from the theoretical measurements directly impacts the antenna’s resonance frequency, leading to inadequate reception of NOAA signals. Additionally, poor impedance coupling between the antenna and the transmission line negatively influences data reception.
To ensure rigor and quality in the design and manufacturing of the antennas, uniform specifications and recyclable materials were employed for all antennas, utilizing the same manufacturing elements (aluminum, PVC, and 75 Ω coaxial cable). Likewise, all antennas were specifically designed for reception at the 137.1 MHz frequency of the NOAA-19 satellite. To enhance data comparability, tests were conducted simultaneously at a single geographic site, under the same environmental conditions.
Each working group compared the data received based on criteria such as image resolution and peak audio power. This comparison aimed to inform a decision regarding the most suitable type of antenna for receiving this type of communication.
Results
As mentioned above, the evaluation of the images was performed taking into account the resolution in megapixels, which was calculated using the number of pixels in the vertical and horizontal dimensions. A higher number of megapixels and pixels was interpreted as better image reception.
On the other hand, the audio of the carrier signal, which contained the transmitted image information, was analyzed through SDRsharp and Python COLAB software. The Fourier function was studied and the power spectrum of the audio was examined. The peak power of the audio was used as an indicator of the quality of the received signal. Higher peak power was considered as a better quality signal. The captured images are shown in Figure 6.
Figure 6a shows the image taken by the Turnstile antenna; Figure 6b shows the image corresponding to the QHF antenna, while Figure 6c shows the image captured by the double-cross antenna. The image obtained by the Moxon antenna is shown in Figure 6d, with the V-dipole antenna the image presented in Figure 6e. Finally, Figure 6f shows the image received by the Eggbeater antenna. The information on the resolution and peak power of the audio corresponding to the six antennas is shown in Table 1.
Analysis of results
NOAA satellites orbit in a polar orbit, so they appear in azimuth and elevation directions from any terrestrial location. Consequently, to receive APT signals originating from these satellites, it is recommended to use receiving antennas with right-hand circular polarization (RHCP) and terrestrial radiation patterns with a minimum of nulls within the hemisphere, and a maximum in all directions in the horizontal plane. For this reason, antennas such as double-cross, QHF, Turnstile in axial mode, Moxon, V-dipole, and Eggbeater were selected, as shown in the following figures.
Delving into the topic, the Turnstile antenna in axial mode consists of a set of two half-wavelength dipoles aligned at right angles to each other; the currents in the dipoles have equal magnitude and are phase-shifted by 90°. The antenna is supported by a mast. The radiation pattern is omnidirectional with a null along the X-axis. To further enhance the shape of the horizontal pattern, additional elements are placed in a vertical array; with this type of antenna, the bandwidth can be expanded, gain improved, and polarization controlled; in addition to its ease of installation. See Figure 7 to learn about its radiation pattern.
In contrast to the Turnstile, the QHF antenna has circularity in its entire reception area; it is formed by two linear conductors forming a helix, this shape is made to achieve circular polarization. The quality of the antenna reception depended largely on the quality of the joints and the symmetry that this antenna maintained in its final arrangement. The diameter-to-height ratio of the antenna was 0.44; however, if it is built taller and narrower, the directivity of the antenna can be improved, thus receiving better signals from satellites near the horizon. The radiation pattern can be appreciated in the following image Figure 8.
On the other hand, the double-cross antenna consists of two sets of two half-wavelength dipoles aligned at right angles to each other. The omnidirectional pattern, which can be seen in Figure 9, has characteristics of circularly symmetric radiation (almost isotropic: 0 dBi is maintained in almost all directions in the entire 3-D space) and with greater horizontal directivity. This behavior is attributed to the fact that the two dipoles of each cross perform an omnidirectional pattern in the horizontal plane; while the set of the two crosses leads to a radiation pattern expanded to the vertical plane, due to their separation by a quarter wavelength. Field tests with this antenna indicate that it produces very little radiation pattern nulling within the hemisphere.
The Moxon antenna consists of two squares crossed at right angles and electrically phased 90° to obtain circular polarization. This antenna differs from the Turnstile in the shape of its radiation lobe; generally it is presented that for elevation angles between 90° and 70° the Turnstile outperforms the Moxon, between 70° and 40° they perform similarly, and for elevation angles less than 40° the Moxon outperforms the Turnstile. The radiation pattern of the same can be seen in Figure 10.
The V-dipole antenna is formed by two linear conductors with a half-wavelength length and separated by 120°. It has a low gain of 2.15 dBi and an omnidirectional radiation pattern, which can be seen represented in Figure 11, its polarization is linear, so it presents losses due to polarization since the APT signal has RHCP polarization.
Finally, the Eggbeater antenna consists of two loops phased 90° and the reflector elements that form the ground plane; its radiation pattern is omnidirectional, and can be appreciate in Figure 12, and its performance is good for receiving lEo satellite signals, although it does not reach the gain of the QFH; below 20° elevation, better performance is achieved than with the QFH. From 20° elevation, a better signal is usually obtained with the QFH.
In comparison, the Moxon antenna demonstrated the best performance, recovering images with resolutions of 1.94 megapixels. The QHF antenna exhibited the highest power reception at 1.9 W. The V-dipole, QHF, and Eggbeater antennas displayed the best coupling with the transmission line, achieving low reflection coefficients of 0.16. All antennas adhered to the transmission line length of λ/4.
All antennas, except the Moxon, evidenced circular polarization, which generally has an extra 3 dB loss due to polarization mismatch. Some disadvantages of the Turnstile antenna are: poor performance near the horizon, high image quality in a +/-30° approximately vertical field of view, and its radiation power is about 3 dB less compared to the maximum of a half-wave dipole radiating the same power.
When using reception antennas that are sensitive only to RHCP fields, the reception of electromagnetic waves reflected on the ground and suggesting left-hand circular polarization (lHCP) is not taken into account, particularly in the reception of all electromagnetic waves coming from the horizontal direction.
The receiving antennas studied were all mounted in the same fixed azimuthal position and with the same pointing (at the satellite) to be able to track the satellite during its pass. However, the appearance of the satellite in the azimuthal and elevation direction did not always coincide with what was budgeted. In one of the tests, it was observed that when the satellite was located at the highest and closest point, where the antennas should have a minimum of 12 dB towards the zenith, since theoretically the satellites are 12 dB stronger when they are high compared to their strength when they are on the horizon due to path loss, the Moxon antenna performed the best during the satellite pass, because it had lower gain at the most favorable moment (high and close satellite) and higher gain at the most unfavorable moment (low and distant satellite).
In addition, NOAA satellites orbit at an average altitude of 800 km, which provides between 12 and 15 minutes of signal reception on the ground; however, the range of best signal reception is approximately 8 minutes.
Conclusions
In urban environments, the Moxon and QHF antennas excel in the reception of APT images sent from the NOAA-19 satellite. The Moxon antenna exhibits adequate performance in the resolution of the received signals, managing to recover images with a quality of 1.94 megapixels, while the QHF antenna tunes the highest power of 1.9 W.
For optimal reception of polar satellites, nondirectional antennas with large capture area and circular polarization are required, such as the Turnstile, QHF and double-cross antennas. Although the Turnstile has good performance, it has limitations at the horizon and elevations outside +/-30°. Most are omnidirectional in azimuth, but some like the Turnstile perform better at low elevations. In general, the non-directional ones like QHF and double-cross are better choices for polar satellite reception without the need to adjust angles.
Although the antennas utilized in this study were not constructed to the highest standards, they offer a clear comparison under the same criteria of the performance of each antenna. It is acknowledged that each antenna has the potential for improvement to minimize noise, distortions, etc.
After conducting a thorough literature review, it is evident that our study presents a novel approach and scope not previously addressed. While some similar information exists in other articles, these works focus on different aspects or use different antenna configurations. For instance, one study developed a meteorological monitoring prototype using a different antenna and Raspberry Pi 3, while our study compares six types of antennas for receiving NOAA signals. Similarly, another study focused on a ground station prototype using different technology. Therefore, our study makes a significant contribution to the field by evaluating antennas under specific conditions. This highlights the originality and relevance of our work, offering new insights into antenna effectiveness in urban environments for receiving NOAA signals. Further research in this area is warranted
Abstract:
Main Text
Introduction
Methodology
Results
Analysis of results
Conclusions