50 research outputs found

    Low-frequency Antennas, Transparent Ground Planes, and Transponders for Communication Enhancement in Unfavorable Environments

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    The communication environment has a major influence on the performance of wireless networks. Unlike antennas, receivers, processors, and other components of a typical wireless system, the designer has almost no control over the communication channel. Therefore, it is imminent that the adverse effects of the communication channel such as path-loss, multi-path, lack of a clear line of sight, and interference are among the most limiting factors in designing and operating wireless networks. Recent investments in infrastructures such as cell-phone towers, communication satellites, routers, and networking devices have been aimed at reducing the aforementioned adverse effects. However, wireless ad hoc networks (WANET) cannot rely on pre-existing infrastructures such as access points or routers. In this thesis, a number of solutions are presented to enhance communication and navigation in harsh environments. 1) At lower frequencies, the defects of the communication channel are less prominent, which has led militaries to use UHF and VHF frequency bands for communication. A number of optically transparent UHF antennas are developed and embedded in the windows of military vehicles to reduce their visual signature. 2) Direction finding at low frequencies using baseline method results in an exorbitantly large array of sensors. However, a vector sensor consisting of three orthogonal two-port loop antennas can be used. A simple and accurate circuit model for the two-port loop antenna is developed for the first time that can be used for direction of arrival estimation over a wide range of frequencies and angles. 3) Using a conventional radio repeater with ad-hoc systems requires a communication protocol and decreases the throughput by a factor of two for every repeater in the chain. A full-duplex repeater, capable of simultaneously transmitting and receiving at the same frequency, is developed for the 2.4 GHz ISM band.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143898/1/manikafa_1.pd

    Finite difference-time domain analysis of electromagnetic interference and radiation problems

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (p. 251-264).by Kevin Li.Ph.D

    Antenna Design for 5G and Beyond

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    With the rapid evolution of the wireless communications, fifth-generation (5G) communication has received much attention from both academia and industry, with many reported efforts and research outputs and significant improvements in different aspects, such as data rate speed and resolution, mobility, latency, etc. In some countries, the commercialization of 5G communication has already started as well as initial research of beyond technologies such as 6G.MIMO technology with multiple antennas is a promising technology to obtain the requirements of 5G/6G communications. It can significantly enhance the system capacity and resist multipath fading, and has become a hot spot in the field of wireless communications. This technology is a key component and probably the most established to truly reach the promised transfer data rates of future communication systems. In MIMO systems, multiple antennas are deployed at both the transmitter and receiver sides. The greater number of antennas can make the system more resistant to intentional jamming and interference. Massive MIMO with an especially high number of antennas can reduce energy consumption by targeting signals to individual users utilizing beamforming.Apart from sub-6 GHz frequency bands, 5G/6G devices are also expected to cover millimeter-wave (mmWave) and terahertz (THz) spectra. However, moving to higher bands will bring new challenges and will certainly require careful consideration of the antenna design for smart devices. Compact antennas arranged as conformal, planar, and linear arrays can be employed at different portions of base stations and user equipment to form phased arrays with high gain and directional radiation beams. The objective of this Special Issue is to cover all aspects of antenna designs used in existing or future wireless communication systems. The aim is to highlight recent advances, current trends, and possible future developments of 5G/6G antennas

    Magnetic properties of individual iron filled carbon nanotubes and their application as probes for magnetic force microscopy

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    Iron filled carbon nanotubes (FeCNT) can be described as carbon nanotubes which contain an iron nanowire of several micrometers length and a diameter of approximately 10-100 nm. The carbon shells protect the iron core from oxidation and mechanical damage thus enabling a wide range of applications that require a long-term stability. The magnetic properties of the enclosed nanowire are in part determined by its small size and elongated shape. Magnetic force microscopy (MFM) measurements show that the iron wire exhibits a single domain behavior. Due to the large shape anisotropy it is magnetized along the long wire axis in the remanent state. Two magnetic monopoles of opposing polarity are located at the wire extremities. Depending on the structure and geometry of the individual nanowire, switching fields in the range of 100-400 mT can be found when the external field is applied along the FeCNT’s easy axis. Cantilever magnetometry shows that the switching can be attributed to a thermally assisted magnetization reversal mechanism with the nucleation and propagation of a domain wall. The defined magnetic properties of individual FeCNT combined with their mechanical strength make them ideal candidates for an application as high resolution high stability MFM probes. The fabrication of such probes can be achieved with the help of a micromanipulation setup in a scanning electron microscope. FeCNT MFM probes achieve a sub 25 nm lateral magnetic resolution. MFM measurements with FeCNT MFM probes in external fields show that the magnetization of these probes is exceptionally stable compared to conventional coated MFM probes. This greatly simplifies the data evaluation of such applied field MFM measurements. The emphasis of this work was put on the calibration of FeCNT probes to enable straightforward quantitative MFM measurements. The defined shape of the magnetically active iron nanowire allows an application of a point monopole description. Microscale parallel current carrying lines that produce a defined magnetic field are used as calibration structures to determine the effective magnetic moment of different MFM probes. The line geometry is varied in order to produce multiple magnetic field decay lengths and investigate the influence on the effective probe moment. The results show that while the effective magnetic monopole moment of a conventional MFM probe increases with an increasing sample stray field decay length, the effective moment of a FeCNT MFM probe remains constant. This enables a MFM probe calibration that stays valid for a large variety of magnetic samples. Furthermore, the fitted monopole moment of a FeCNT probe (in the order of 10E-9 Am) is consistent with the moment calculated from the nanowire geometry and the saturation magnetization of iron

    High efficiency planar microwave antennas assembled using millimetre thick micromachine polymer structures

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    Communication systems at microwave and millimetre wave regimes require compact broadband high gain antenna devices for a variety of applications, ranging from simple telemetry antennas to sophisticated radar systems. High performance can usually be achieved by fabricating the antenna device onto a substrate with low dielectric constant or recently through micromachining techniques. This thesis presents the design, fabrication, assembly and characterisation of microstrip and CPW fed micromachined aperture coupled single and stacked patch antenna devices. It was found that the micromachining approach can be employed to achieve a low dielectric constant region under the patch which results in suppression of surface waves and hence increasing radiation efficiency and bandwidth. A micromachining method that employs photolithography and metal deposition techniques was developed to produce high efficiency antenna devices. The method is compatible with integration of CMOS chips and filters onto a common substrate. Micromachined polymer rims (SU8 photoresist) was used to create millimetre thick air gaps between the patch and the substrate. The effect of the substrate materials and the dimensions of the SU8 polymer rims on the performance of the antenna devices were studied by numerical simulation using Ansoft HFSS electromagnetic field simulation package. The antenna structures were fabricated in layers and assembled by bonding the micromachined polymer spacers together. Low cost materials like SU8, polyimide and liquid crystal polymer films were used for fabrication and assembly of the antenna devices. A perfect patch antenna device is introduced by replacing the substrate of a conventional patch antenna device with air in order to compare with the micromachined antenna devices. The best antenna parameters for a perfect patch antenna device with air as a substrate medium are ~20% for bandwidth and 9.75 dBi for antenna gain with a radiation efficiency of 99.8%. In comparison, the best antenna gain for the simple micromachined patch antenna device was determined to be ~8.6 dBi. The bandwidth was ~20 % for a microstrip fed device with a single patch; it was ~40 % for stacked patch devices. The best bandwidth and gain of 6.58 GHz (50.5%) and 11.2 dBi were obtained for a micromachined sub-array antenna device. The simulation results show that the efficiency of the antenna devices is above 95 %. Finally, a novel high gain planar antenna using a frequency selective surface (FSS) was studied for operation at ~60 GHz frequency. The simulation results show that the novel antenna device has a substantial directivity of around 25 dBi that is required for the emerging WLAN communications at the 60 GHz frequency band
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