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Wireless sensors network desires a vertically-polarized, omnidirectional antenna. In the case where the sensor network was deployed close to ground, the communication range of the network degrades. To quantify the communication range near the ground, several field measurements have been reported with monopole and microstrip patch antennas in the literature. In this study, a low-profile antenna design suitable for wireless sensor network is introduced. Its performance near a metallic ground plane is measured and simulated.
This communication addresses the design and implementation of a low-perturbation and high dynamic range near-field (NF) imager with increased measurement speed. The imager is equipped with an array of optically modulated scatterer (OMS) probes, each incorporating a commercial-off-the-shelf photodiode chip and a minimum scattering antenna, i.e. short dipole. In the OMS probes, transmission of modulating optical signals is performed using an optical fiber coupled to the photodiode, which is invisible to microwave signals. The imager measurement speed is also improved as the OMS array eliminates the delays associated with probes translations, in addition to fast switching of modulating light between the probes. Fast switching is accomplished by an array of fiber-pigtailed laser diodes. Improved dynamic range and linearity in the NF imager are achieved by adding a carrier canceller within the imager receiver front-end, eliminating the carrier signal and leaving the sidebands intact. This canceller also improves the isolation between input/output ports of the imager providing a potential for higher signal amplification. The performance assessment of the NF imager, including its linearity and result accuracy is made by comparisons with a known field distribution.
A fully-polarimetric compact radar range operating at 240 GHz has been developed for obtaining Ku-band RCS measurements on 1:16th scale model targets. The transceiver consists of dual fast-switching, stepped, CW, X-band synthesizers driving dual X24 transmit multiplier chains and dual X24 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 45 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 27” FWHM quiet zone. In this paper a description of this 240 GHz compact range is provided along with an ISAR measurement example.
A means of determining Antenna Sky Noise temperature reduction due to the use of choke rings is provided. This critical information is needed to understand the signal-to-noise limitations of a receiver in each of its RF bands. The approach is novel and straight forward, combining measurement and analysis. The methodology will show the S/N contribution due to the antenna and sky noise; and will separate it from the other noise contributors such as the receiver.s RF front end noise figure. Also it will show the specific noise reduction due to the choke ring. The procedure is shown by example for a dual band GPS antenna (L1 & L2 frequencies). The process is also applicable for antennas in other RF bands. This is done thru a process of covering and uncovering the antenna with an electrostatic type sheet covering. The sheet material has high RF absorption/reflection characteristics across broad RF bands including GPS. The covered antenna condition forms a 290 degree Kelvin reference for comparison. Both absolute and relative RF measurements are made during this process, which is performed with and without choke rings. The measured data is then analyzed to derive the total sky noise temperature.
This study demonstrates the operation of 5.8 GHz backscatter radio links in a transient, high-voltage commercial power line environment. The measured results demonstrate that increased RF carrier frequency provides additional resistance to the noise, interference, and corona shielding of communication antennas that operate on high-voltage lines. This study also lists rules to assist in the design and implementation of low-powered wireless sensor applications for the future smart grid.
The Georgia Tech Research Institute (GTRI) analyzed a phased-array antenna for the purpose of testing phase-only defocusing methods. The array is defocused with the objective of broadening its beam at the cost of lower antenna gain. A design for the beam-steering computer is accomplished which adds the capability of focusing a beam, steering in azimuth and elevation, and performing beam defocusing using only element phase. Widening of the beam is accomplished using only 180° phase shifts in the elements, and it is compared with widening accomplished using gradual phase tapers. The antenna is measured in a near-field range to obtain amplitude and phase information as a function of each element in the array. Near-field testing of the antenna is also used to verify the capability of the beam-steering computer; two-dimensional antenna patterns and near-field hologram projections are compiled to prove this functionality. A software model is designed to mimic the behavior of the phased array antenna in its operational modes; it is also used to predict antenna gain and beamwidth prior to near-field testing. Measured and modeled antenna patterns are compared using focused and defocused modes. Metrics are performed on the near-field data to infer statistics of the individual phase shifters and on the computed far-field patterns to characterize the entire antenna. The defocusing methods under analysis are phase-only methods, due to the inability to control amplitude weighting of elements in this antenna. One method discussed uses only 180° shifting of elements in the antenna to achieve a desired beamwidth. This is compared with another method which gradually spoils the beam by applying a phase taper across the aperture. The results from near-field testing compare the defocusing methods and characterize the relationships between gain, beamwidth, and sidelobe levels for both defocusing methods.
In this paper, a Neural Network based methodology is presented to measure the complex permittivity of materials using monopole probes. A multilayered Arti.cial Neural Network, using the Levenberg Marquardt back propagation algorithm is used to back solve the complex permittivity of the medium. The proposed network can be trained using an analytical model, numerical model, or measurement data spread over the complete range of parameters of interest. The input training data for the non linear inverse problem of reconstructing the complex permittivity comprises the complex re.ection coef.cient of the monopole probe. For the results presented in this paper, the network is trained using the analytical model for impedances of monopole antennas in a half space by Gooch et al. [1]. In addition to computational ef.ciency, the proposed approach gives 99% accurate results in the frequency range of 2.55 GHz, with the values of permittivity varying across a range of 3-10 for the real part, and 0 -0.5 for the imaginary part. The accuracy and the effective range of real and imaginary components of the complex permittivity that can be reconstructed using this approach, depends upon the accuracy and robustness of the model / system used to generate the training data. The analytical model used in this paper has a limited range for the values of loss tangent that it can model accurately. However, the performance of the back solving algorithm remains independent from any speci.c model, and the scheme can be successfully applied using any reliable analytical or numerical model, or re.ection coef.cient training data generated through a series of measurements. The methodology is likely to be employed for experimental measurements of complex permittivity of dissipative media.
Very often far field conditions are violated at high frequencies RCS measurements and in real life scenarios. People go to great lengths to carry out these measurements in the far field. They make large investments to build suitable compact ranges, or long outdoor ranges. Others make extensive efforts to correct the near field measurements to the far field values. This paper suggests that those elaborate measures are superfluous, as far as the total RCS is concerned. Although near field measurements clip the high peaks, they broaden their shoulders compensating for the loss. Simulations and actual measurements show that the accumulative distribution of RCS values in the near field is equal or slightly higher than the distribution of these values in the far field, until one looks for very high 90th percentiles. Thus, for detection and survivability estimates the near field measurements provide a close upper bound.
An Indian Defense Research and Development Organization (DRDO) laboratory has commissioned a state-of-the-art indoor far-field antenna test facility in 2009. This facility supports highly accurate measurement of a wide range of antenna types over 1.12–40 GHz. Owing to the heavy usage of this range, it was decided to enhance the existing facility to include a Hybrid Planar Near-Field facility for high speed accurate antenna measurements with minimal changes to the existing chamber configuration. The scanner is implemented as a highly innovative Hybrid T-type scanner, with a Y-axis that consists of a Linear Multi-Probe array and a traditional single probe configuration. The linear Multi-Probe array consists of two sets of dual polarized probes each one covering a sub set of the full frequency range. In particular one set covers the 1.0-6.0GHz band (operational from 400MHz) and the other set covers the 6.0 to 18.0GHz band. The traditional Single Probe configuration includes a set of Open Ended Waveguide Probes to facilitate an operational frequency range of 1.12 – 40.0GHz, as in the existing Far Field system. The Hybrid scanner is placed along the sidewall opposite the door, on the DUT positioner side. The major benefit of this layout is that there is no need to change the basic design of the chamber and it is built according to the original plans. When the chamber is used in the far-field mode, the tower is moved to the end of the horizontal axis in the direction of the corner of the chamber. The tower sides that face away from the chamber corner are covered with absorbing material to reduce reflections from the tower. Assuming that the chamber is intended to measure directional antennas, the existence of the tower behind and to the side of the AUT is expected to introduce minimal interference. The high speed linear Multi-Probe array has typical measurement speeds ranging between 5 and 15 minutes at 5 frequencies and 2 polarizations. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 6.0 package for both Single Probe & Multi Probe modes. A Real-Time Controller (RTC), accompanied by a 4-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.
Obtaining a quantitative accuracy qualification is one of the primary concerns for any measurement technique [1, 2]. This is especially true for the case of near-field antenna measurements as these techniques consist of a significant degree of mathematical analysis. When undertaking this sort of examination, room scattering is typically found to be one of the most significant contributors to the overall error budget [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in quantifying and subsequently suppressing range multi-path effects in first spherical [3, 4] and then, cylindrical near-field antenna measurement systems [5, 6]. This paper details a recent advance that, for the first time, enables the MARS technique to be successfully deployed to correct data taken using planar near-field antenna measurement systems. This paper provides an overview of the measurement and novel data transformation and post-processing chain. Preliminary results of computational electromagnetic simulation and actual range measurements are presented and discussed that illustrate the success of the technique.
Recently, the smaller of the ESTEC CATR’s has been moved to a new location in the ESTEC Test Centre. In the frame of the relocation, the original reflectors of the range were positioned and aligned in a brand new anechoic chamber. The commissioning phase of the new range included a quiet zone field probing in order to verify the range performance in the new situation and to identify direction of arrival of major reflections. During this exercise, it was realized that the criteria for Quiet Zone dimensions are rather arbitrary. The paper addresses a new figure of merit for range comparison in terms of accuracy. Peak to peak values and RMS have been recorded depending on the size of a hypothetic AUT. This analysis resulted in accuracy nomograms that allow ESA staff to easily assess measurement accuracy depending on antenna size and operational frequency. Similar nomograms elaborated for different CATR’s could allow unbiased inter-range comparison. Moreover, a GRASP model of the facility has been developed based on the metrology measurement of the reflectors surfaces, relative position of range feed and AUT positioner.
Far field antenna measurements require specialized chambers, not very commonly available. The measurement process is inherently time consuming. If this time can be reduced it would increase the through put of the test chamber and would decrease the incurred expenses. This paper describes a novel adaptive far field measurement methodology for antennas, by varying the angular resolution and IF bandwidth in an adaptive manner. Different adaptive angular resolution techniques have been proposed and verified. Different type of antennas were measured with conventional antenna measurement methods and compared with proposed technique. It was observed that fine angular resolution can be achieved in main beam and first sidelobe levels with a little compromise on other side lobe and back lobe levels. The comparative results and their analysis are presented. On the average 20 to 40% measurement time is reduced with the proposed methodology. All measurements have been conducted in a CATR.
There are a large number of antennas needed for modern automotive communications systems (AM, FM, FM diversity, TV diversity, remote keyless entry/start, cellular, Bluetooth, automatic toll systems, smart highway information systems, GPS and GPS information systems (traffic information), and radar systems (backup, side impact, lane departure, intelligent cruise control)). Manufacturers are looking for ways to reduce the total number of such antennas by using combinations of a smaller number of antennas. This paper will discuss a software approach that permits the engineer to visualize the antenna performance effects of variations in geometry for a group of antennas based on either test data or simulation. The antennas are typically defined as a set of printed electrically conductive lines on vehicle windows. The number and the location of the lines are parametrically varied and the effect on the RF performance of the total combined received signals for various sets of test data are presented in multi-parametric visualizations. This paper will discuss the development of a tool for multi-antenna testing that can be used by the antenna application engineer to develop an optimized multi-antenna design. Both experimental test data and theoretical simulation data can be used and compared. Examples will be presented.
With the trend toward energy efficient green technology, the use of metallic films on glass is increasing. These transparent conductive coatings act like a mirror in the infrared range, reflecting heat while keeping the interior of vehicles and buildings cool and reducing energy consumption. The conductivity provides another benefit. When connected to a power supply, it can provide rapid deicing and de-fogging. A number of vehicles are available with heated windshields. However, these coatings have the potential to block the RF signals of the numerous telecommunications devices commonly installed and used inside an automobile. This paper will discuss a method for design and testing of RF apertures in such metal films using modern frequency selective surface (FSS) concepts. Just leaving a "hole" in the metal film creates manufacturing problems. In addition, electromagnetic problems include frequency resonances, side lobe peaks and nulls, polarization nulls and (in the heating application) of interrupting or concentrating the heating current flow. The design and performance measurement of arrays of polarization insensitive slots that do not interrupt heater current flow and also control internal and external side lobes will be shown.
Near-field antenna measurements are accurate and common techniques to determine the radiation pattern of an antenna under test. The minimum near-field sampling rate is dictated by the electrical size of the antenna and usually equidistant sampling is applied for planar, cylindrical, and spherical measurements. Certain applications either rely on or benefit from near-field sampling on irregular grids. To handle irregular measurement grids near-field transformation algorithms like equivalent current methods or the multilevel fast multipole accelerated plane wave based technique are required which do not rely on regularly sampled data. In this contribution the plane wave based near-field transformation is applied to spherical, cylindrical, and “combined” near-field measurements employing irregular sampling grids. The performance is assessed by various simulated near-field measurement scenarios.
Wideband dual polarized probes are often used for modern high precision measurement systems. A desired feature of a good probe is that the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign [1]. This paper describes a new field probe taking full advantage of the 1: 4 bandwidth of the Ortho Mode Junction (OMJ) overcoming the aperture size problem by applying different apertures on the same field probe. The apertures are circularly symmetric so the exchange of apertures can be performed rapidly without the need to repeat calibration and alignment procedures for the full probe.
Standard Gain Horns (SGH) are utilized frequently either as measurements antenna or as reference antenna in antenna gain measurements by comparison or substitution method [1]. They also find use as source antennas in anechoic test chambers and for many other purposes such as fixed site antennas. The most widespread SGH geometry has a rectangular cross-section and is pyramidal with optimized geometry to achieve maximum gain [2, 3, 4]. When used as a precision gain reference in antenna measurements the SGH is often calibrated by a reference facility or another third party. When external or internal calibration means are not available the SGH peak gain is often determined directly from the reference tables of the NRL report [2]. The quality of the original work is such that even today the associated uncertainty on these peak gain values are generally accepted to be within +/-0.3dB [1]. In this paper the accuracy of the NRL gain tables are investigated by comparison with a full wave numerical method based on FDTD [7] and measurements in different antenna test ranges. Performance variation of the SATIMO Standard Gain Horns due to the manufacturing and measurement accuracy has been also investigated with conducted and radiated experiments.
A ground station antenna for Galileosat application operating in right hand circular polarization at P-band has been designed, manufactured, and tested. Other than stringent environmental requirements for typical ground station antennas the specification call for an antenna with very stringent requirements on pattern shape and symmetry and a very severe control on side and back lobes. In order to ease the requirement on the antenna positioner the antenna should have very compact size and low weight. The final antenna consists of an array of 7 medium gain, dual linear polarized yagi elements as shown in Figure 1. This paper describes the antenna design trade-off activity including the selection of the most suited antenna technology and manufacturing details. It also reports on the testing in the SATIMO SG-64 multiprobe spherical near field test range with considerations on the associated measurement uncertainty. The final acceptance of the antenna was based on measurements performed in CNES and SATIMO.
The Mini-RF instrument on NASA’s Lunar Reconnaissance Orbiter is gathering data toward its science goal of probing the permanently shadowed terrain for the presence of water near the lunar poles. The circular polarization ratio is the central parameter used to characterize the lunar surface using Mini-RF radar returns. Accurate use of this parameter requires on-orbit polarimetric characterization of the instrument. This is done with a combination of measurements. Indirect measurements are performed by pointing the radar at the lunar surface to remove any asymmetry in the surface backscatter. Direct characterization of transmit and receive portions of the Mini-RF radar are performed using Earth-based resources. The Earth-based resources include the: Arecibo Radio Telescope, Green Bank Telescope, and Morehead State University Space Tracking Antenna. This paper describes the measurement approach and a sample of results. The primary measurements of interest include: principal plane antenna patterns, transmit polarization ellipse, receiver amplitude and phase balance, and antenna bore sight direction. These measurement values are incorporated into the Mini-RF SAR data processing to produce quality, calibrated CPR measurements
This paper shall discuss a method for measuring the current distribution – in both magnitude and phase - along the length of a floating antenna operating on the surface of the ocean. The method makes use of a novel toroidal current sensing device and balun arrangement, with a vector network analyzer serving as the measurement instrument. The current data obtained using this method can then be used to compute the far-field pattern of the antenna, both at the horizon and overhead, in a manner similar to near-field scanning of aperture antennas. This new method has significant advantages over the conventional far-field method of measurement in terms of accuracy, time, and cost, and can also be used to determine the realized gain of the antenna. Measured and theoretical data shall be presented on example antennas to illustrate the process of measuring the current distribution as well as computation of the far-field pattern.