Browsing by Author "Türkmen, Ceyhan"

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A Bandwidth Enhanced Circularly Polarized and Omnidirectional Circular Waveguide Antenna for Satellite Telemetry/Telecommand Systems
(2022) Secmen, Mustafa; Türkmen, Ceyhan; Alkın, Esra
In satellite systems, throughout time interval from launching to settlement on proper orbit and possible significant change in aspect angle of the satellite after the settlement on the orbit, it is required continuous communication between satellite and ground station for current position and other information belonging to satellite. However, position and aspect angle of the satellite with respect to earth will be random due to possible jolts, especially while passing through atmosphere. For this reason, telemetry/TM (transmitter) and telecommand/TC (receiver) antennas on satellite platforms should receive electromagnetic waves from all directions and they should send enough electromagnetic waves to all directions to ensure continuous two-way communication, which is called as telemetry/telecommand. The other fundamental problem on signal levels of communication is polarization mismatch, which is caused by variable aspect angle due to satellite jolts/vibrations. The antennas in earth station and telemetry/telecommunication antennas on satellite are generally selected with same circular polarization to reduce these losses as much as possible. Waveguide antennas, which are omnidirectional or hemispherical and having circular polarization, are frequently used as a solution to this problem. However, considering pattern characteristics of radiation in 3-dimensional space, this antenna gives desired performance to cover about half of entire space (at most 60%). For this reason, to cover entire space as radiation pattern, these antennas are located being far away from each other as much as possible, and signals on these two antennas must be connected via very long waveguides in satellite. Also, these antennas work at only telemetry or telecommand frequencies. Therefore, it is needed to use a total of 4 antennas for coverage of overall space as a radiation pattern, when it is considered as telemetry/telecommand (transceiver). In this project, communication, which is ensured by using systems with 4 antennas in literature and real-life applications, is realized with only a waveguide system having circular polarization. It is the first time a circularly polarized transceiver structure fed by one waveguide structure is developed, which operates in both TM (transmitting) and TC (receiving) frequencies. In addition, another important feature developed in this project is the fact that coverage of 3-dimensional space by using 2 antennas placed on satellite being away from each other is provided with only one antenna system. Overall system/antenna in the project has 3 main sections/designs. First of these sections/designs is rectangular TE10 to circular TM01 waveguide mode converter, which provides feeding slotted antenna arrays on circular waveguide with symmetric mode TM01. In literature, there are waveguide mode converters operating at either TM or TC frequencies, a dual-band converter functioning at TX/RX frequencies is designed for the first time in this project. Second part consists of a power divider/combiner design, which connects mode converter and two circular slotted waveguide antennas perpendicular to each other. Third part is the structure of omnidirectional and bi-directional waveguide antennas, which work at TM and TC frequencies, and contain thick inclined slots to increase the frequency band. As for radiation pattern, most of the entire space is covered with circularly polarized by placing omnidirectional and bi-directional antennas in this section perpendicular to each other and putting disks radially away from circular waveguides. The whole system structure/antenna developed and produced for Ku band gives a return loss of about 10 dB within frequency bands of approximately 500 MHz around 11.75 GHz and 13.75 GHz transmitter and receiver center frequencies, respectively, and the measurements revealed minimum gain of -10 dBi and maximum axial ratio of 4-4.5 dB within the average coverage percentage of 65%.
Citation - Scopus: 1
Compact dual-band rectangular TE10 mode to circular TM01 mode converter for telemetry/telecommand applications in satellite communication: design equivalent circuit modeling mode level measurement technique and 3d printed manufacturing
(Turkiye Klinikleri, 2022) Esra Alkın; Ceyhan Türkmen; Mustafa Seçmen; Secmen, Mustafa; Türkmen, Ceyhan; Alkın, Esra
In this study the design of a dual-band mode converter which provides transition from rectangular waveguide TE10 mode to circular waveguide TM01 mode and operates simultaneously in telemetry/telecommand (TT&C) frequencies is presented along with its equivalent circuit and a mode level measurement technique. This dual-band converter is designed to uniformly excite TT&C slot antennas used in satellite communication with symmetric circular TM01 mode. The structure can work as a transceiver due to having one common rectangular waveguide feed. As a Ku-band application a converter giving high purity TM01 mode at circular waveguide at 11.75 GHz/TX and 13.75 GHz/RX center frequencies is designed and manufactured with 3D printing technology. Approximate equivalent circuit models of the designed structure are extracted by using lumped elements from simulated S-parameter results. In addition a measurement technique to detect the normalized power levels of different propagation modes in dual-band mode converter is applied on the manufactured structures to obtain transmission levels of circular TM01 mode and suppression levels of circular TE11 mode. It is revealed from the measurement results that while the designed structure gives more than 17.5 dB return loss and suppression of TE11 mode and less than 0.6 dB insertion loss of TM01 mode within 500 MHz bandwidth at TX band it gives more than 23 dB return loss and suppression of TE11 mode and less than 0.3 dB insertion loss of TM01 mode within 900 MHz bandwidth at RX band. © 2023 Elsevier B.V. All rights reserved.
Uydu haberleşme sistemleri için yönsüz antenler
(2016) Türkmen, Ceyhan; Seçmen, Mustafa
In this thesis, the design, production and measurements of Ku-Band, omnidirectional and circularly polarized antennas, which can be used in satellite communication applications as receiver and/or transmitter, are explained. Ku-Band is a commonly used frequency band in satellite communication applications. These antennas consist of inclined slots (slotted array) placed with a circular symmetry on a circular waveguide. The antenna structures are fed by a special transition from standard rectangular waveguide to this circular waveguide, where the rectangular waveguide is the standard WR75 used frequently in Ku-Band applications. The electromagnetic wave propagates in only dominant TE10 mode inside the rectangular waveguide (main feed of the antenna), while propagating in the second higher order mode of TM01 inside circular waveguide. This TM01 mode is nondominant for circular waveguide; but has circular symmetry in the excitation of the slots on the antenna array, which provides omnidirectional pattern. The mentioned special transition effectively suppresses the dominant TE11 mode and other higher order modes such as TE21 in the circular waveguide, which disturbs symmetry (omnidirectionality) in the radiation pattern. In order to make the antennas circularly polarized, the slotted array elements are designed as inclined way, and parallel metallic plates (disks) are placed around the slotted arrays. Three different versions of the proposed antenna are designed, simulated and implemented. The first one is designed just for transmitting purpose from the satellite to earth, and operates at the frequency about 11.75 GHz. Similarly, the second antenna is realized just for the receiving purpose from the ground segment to satellite. This antenna operates at the frequency about 14.00 GHz. The last antenna design is more sophisticated than the other ones such that it works as a transceiver carrying out transmission at 11.75 GHz and reception at 14.00 GHz simultaneously. According to results obtained for the first two designs, the return loss, which is better than 10 dB, is provided around the frequency regions of both transmitter (11.75 GHz) and receiver (14.00 GHz) within approximately 3 percent frequency bandwidth. In addition, axial ratio is lower than 1 dB in the azimuth plane and lower than 4 dB in a wide beamwidth of elevation plane at the center frequencies. Omnidirectional radiation pattern is provided at azimuth plane such that the variation of gain is about 0.7 dBi at this plane. The transceiver version of the design has frequency bandwidth of about 350 MHz and 500 MHz for the maximum 10 dB return loss around the center frequencies of 11.75 GHz and 14.00 GHz, respectively. The axial ratio values are found to be lower than 2 dB in the azimuth plane for both center frequencies, which corresponds to good circular polarization performance. The values in the radiation patterns change at most 3 dB at the same plane.