AMTA Paper Archive
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Antenna Miniaturization Using Artificial Transmission Line Concept
Antenna miniaturization will continue to be a key issue in wireless communications, navigation, sensors, and RFIDs. For instance, each cellular tower is often populated with many antennas to cover different angular sectors and different frequency bands. Each modern notebook computer is likely embedded with multiple antennas to provide service in WWAN (824 MHz to 2170 MHz) and WLAN (2.4 GHz and 5.5 GHz), Bluetooth, etc. Also automobiles, vessels, and aircrafts will require more antennas to compete for very limited real estate. This dire situation is changing antenna designer worldwide with a goal to develop a new generation of physically small antennas that multi-bands or wideband. This paper presents several generic miniaturize antenna design examples that applies the concept of artificial transmission line concept for artificially control phase velocity and impedance. This miniaturization approach can be applied to reduce the size of both narrowband and wideband antennas using minimal amount of materials. Thus improves antenna’s efficiency, and reduces its cost and weight.
MEASUREMENTS OF POWER HANDLING OF RF ABSORBER MATERIALS: CREATION OF A MEDIUM POWER ABSORBER BY MECHANICAL MEANS
The rise in the number of active antennas used in radar applications calls for changes to the common absorber treatment used in chambers. The electronic circuits that are imbedded into these scanning arrays are non-linear in nature so they must be tested at the correct power outputs to get the correct pattern behavior. The combination of higher power and narrow beams means that areas of the anechoic treatment in a chamber can be subjected to high power densities. High power absorber has been used in the industry for many years. The substrate used in these absorbers makes the material very expensive. While in the past it was common to use this material only in regions where the main beam was going to be illuminating the absorber treatment the new electronically scanned arrays will have main beams that can illuminate several areas of the chamber. In cases where Near Field systems are used the absorber material will be in a radiation region where the main beam has not been formed, but the Absorber is closer to an array that is radiating high power so a large area of higher power absorber is needed to treat the chamber. In the present paper the authors present a medium power absorber 3kW per square meter (versus 775w per square meter) using the same material used in common RF absorber. Mechanical changes to the absorber are performed to increase the thermal dissipation of the EM energy absorbed. A series of measurement of the absorber is performed with a without additional air flow for cooling. The result is an absorber that can handle higher power densities without the need for exotic substrates.
Back Wall Design Trade â€“ Offs in High Performance VHF/UHF Chambers
The back wall is an important element in a high performance tapered or compact range anechoic chamber operating at VHF/UHF frequencies, as by design it is intended to absorb the non-intercepted portion of the incident plane wave containing the majority of the power transmitted by the chamber illuminator. Back wall reflections may interfere with the direct illumination signal and thus influence the test zone performance. Consequently, in order to ensure that the overall test zone reflectivity specification is met, the reflectivity produced by the back wall should be better than the reflectivity specified for the test zone. The conventional approach used to achieve good reflectivity is to apply high performance, high quality absorbing materials to the back wall. Further improvement of up to 10 dB can be achieved if a Chebyshev absorber layout is implemented [1, 2]. This layout consists of high performance absorbing pyramids of different heights, and assumes that the performance does not depend on a metallic backing plate. This approach is expensive, and presents technical challenges due to the complexity involved in the design and manufacturing of the absorbing material. In addition, installation and maintenance is an issue for such large absorbers. In this paper an alternative approach is presented which is based on an implementation of a shaped back wall as, for example, suggested in [3-5], and use of lighter, lower grade absorbing materials whose performance essentially depends on reflections from the metallic backing wall. This type of design can be optimized at the lowest operating frequency, if the back wall and absorber front face reflections cancel each other. Different back wall shapes are considered for a tapered chamber configuration, and the test zone reflectivity produced by a flat, inverted “open book” and a pyramidal back wall are evaluated and compared at VHF frequencies using a 3D EM transient solver .
Determination of Complex Permittivity of Molded Interconnect Device Materials at Microwave Frequencies
The laser based Molded Interconnect Device technology offers the potential of designing electrical and mechanical components on three-dimensional surfaces to increase functionality,level ofintegration and to reduce costs.To utilize this technology especiallyfor thedesign ofRFdevices, the electromagnetic parameters of the substrate materials as the complex permeability and permittivity have to be known precisely, as these quantities strongly in.uence the device performance. This paper therefore presents a broadband characterization of the dielectric properties of two Molded Interconnect Device materials. Based on this parameter characterization, two simple antennas are designed and their simulated and measured input re.ection coef.cients are compared.
Noise Radar Correlation Patterns of Human and Non-Human Objects at Various Look Angles
In a search and rescue effort following a natural disaster, rubble and debris within the search environment can obscure human victims. A digital noise radar operating at UHF can penetrate the types of materials typically found in these situations with relatively little loss. This paper compares and contrasts measured correlation values of human and non-human objects taken from a digital noise radar. A noise radar works by cross correlating the received signal with a replica of the transmit signal. A high correlation indicates range to the target. Measured results with the noise radar show that patterns of peaks and valleys exist near the range of the targeted object. This paper looks at the correlation patterns generated by two different sized hollow metal tubes and a human at various look angles. The results show that the correlation patterns of the tubes are similar, but the correlation patterns of the human differ. Full characterization of human and non-human noise radar correlation patterns could confirm the presence of a human victim and information about his location within the rubble.
PERFORMANCE IMPROVEMENT WITH AN R-CARD FENCE
Hardware-In-The-Loop chambers provide the chamber designer with many difficult obstacles to overcome in order to establish a high performance environment for the measurement of missile seeker systems. One of the most difficult challenges is to overcome the low performance of absorbing materials at low grazing angles. To solve this problem Tapered R-Card Fences have been used in conjunction with Chebyshev absorbers. Last year we reported on the ATK chamber built in Woodland Hills which showed preliminary test results well within the system requirements. This paper will make a direct comparison of chamber performance with and without Tapered R-Card Fences. The establishment of a sister chamber built in the ATK Alliant Techsystems Inc. ABL facility has provided us with the unique opportunity to test the chamber prior to the installation of the R-Cards and then to test it again with the installation of the R-Cards. This unique opportunity has allowed us a direct comparison of an advanced chamber deign with Chebyshev absorbers as would be utilized in a conventional chamber and the performance increase directly attributable to the introduction of the Tapered R-Cards in the anechoic chamber. The chamber evaluation is carried out utilizing The Ohio State developed TDOA measurement method utilizing their proprietary measurement and analysis software.
Assessment of a Candidate Metallic Waveguide Standard, Based on S-parameter Uncertainty Due to Dimensional Manufacturing Errors.
An effort to ascertain the accuracy of the rectangular waveguide measurement technique for permittivity and permeability characterization of materials, has led to the development and application of a waveguide notch filter as a scattering parameter (S-parameter) reference standard. The S-parameters of this reference can be determined accurately using simulations that implement a full wave model of the waveguide measurement technique. The notch frequency response characteristic allows testing over the dynamic range of the measurement system. When fabricated in metal, the filter provides a predictable frequency response, has mechanical and temporal stability, and is reproducible using standard machining techniques. However, manufacturing errors introduce uncertainty in the measured S-parameters. Determining the sensitivity of S-parameter uncertainty as a function of manufacturing errors is important in assessing the appropriateness of the notch filter as a metallic standard for use throughout the material measurements community. This paper presents the characteristics of the filter, showing both calculated and measured S-parameter values, and provides an analysis that demonstrates the relationship between dimensional manufacturing tolerances and the resulting S-parameter uncertainty.
Computational Analysis of a Permeameter Materials Measurement Fixture
High frequency (up through X-band) magnetic materials are gaining in importance across a wide range of applications such as microwave components, electromagnetic shielding, and antenna substrates. Development of new magnetic materials and alloys requires convenient and accurate measurement methods with well-understood uncertainties. For this reason, a finite difference time domain (FDTD) model was developed of a shorted microstrip (single coil) permeameter, appropriate for measuring small samples or thin films. Simulating the response to various magnetic materials, this model was used to analyze the prevailing semi-empirical inversion methods and a new, more accurate inversion method was developed to correct deficiencies in existing techniques.
Effect of Moisture for Backscatter Communication
This paper describes how Radio Frequency Identification (RFID) utilizes typically backscattering communication between reader unit and transponder. In passive UHF RFID system the transmitted electromagnetic wave must propagate two times thru the medium. The level of moisture changes electromagnetic properties of medium and the electromagnetic properties of medium have effects on quality of backscattered signal . The well-known fact is that the demand for item level RFID tag will face challenge of different packing materials. However, environment where some parameter such as humidity and temperature changes a lot, will be a great operational challenge for future RFID systems as well. In this paper we have used one application and present measured result how different moisture levels effects for passive UHF RFID operational main parameters such as threshold power level and level of backscattered signal.
TEST ZONE PERFORMANCE IN LOW FREQUENCY ANECHOIC CHAMBERS
Advantages of Far-Field (FF) anechoic chambers utilized for antenna measurements, as compared to conventional outdoor ranges, such as security, interference-free radiation, and immunity to weather conditions allowing broadband antenna measurements on a 24/7 basis, are well known. The dimensions of an anechoic chamber are primarily determined by the lowest operating frequency and are, therefore, significantly increased if operation is required down to VHF and UHF frequency bands. As a result, the advantages of indoor chambers are often disputed when considering low frequency applications. The main counter-argument is the real estate required for chamber construction. In addition, such chambers require the use of high performance absorbing materials, and consequently, chamber certification is always a challenging task. Therefore, rigorous and accurate 3D EM analysis of the chamber is an important procedure to increase confidence, reduce the risk associated with achieving the required test zone performance, and to make the design more efficient. Thus, an accurate simulation of the chamber is even more important these days due to a dramatically growing number of antenna manufacturers supplying products at VHF and UHF bands. Such analysis is a standard procedure at ORBIT/FR, and is described below for the example of a chamber with dimensions of 6m (W) x 6m (H) x 10m (L), operating down to 150 MHz.
VHF/UHF High Performance Absorbing Material Measurements in a Coaxial Line Using Time-Gating Techniques: Validation & Error Analysis
This paper describes the Rectangular Coaxial 40’ long measurement system recently designed and installed at AEMI with the primary purpose of measuring the reflectivity of its high performance VHF/UHF absorbing materials in the frequency range 30 – 510 MHz. The basic principles of the system are described in detail in  and are based on S11 – measurements of absorbing material reflectivity by a Vector Network Analyzer (VNA). In order to improve the system productivity and measurement accuracy it was enhanced by the time-gating software option – the standard option of ORBIT/FR Spectrum 959 automated measurement software package .The measurement system performance was thoroughly evaluated and validated by a number of tests performed in the “empty” coaxial line, and in the line loaded by absorbing materials. The list of RF uncertainties – various measurement error sources - was generated, the main measurement error contributors were identified, the corresponding errors – estimated and the overall RSS measurement errors were calculated for the absorber reflectivity varying in the range of -30dB to – 40dB.
A Technique for Materials Characterization from Backscatter Measurements
Method of moments (MoM) codes have become have become increasingly capable and accurate for predicting the radiation and scattering from structures with dimensions up to several tens of wavelengths. In particular, for simple structures like canonical shapes or antenna / RCS test fixtures, especially those with material treatments, the primary source of disagreement between measurements and predictions is often due to differences between the “as-designed” and “as-built” material parameters rather than to the underlying MoM code itself. This paper describes an algorithm that uses a MoM model combined with backscatter measurements to estimate the “as-built” materials parameters for the case where the treatments can be modeled using an equivalent boundary condition. The algorithm is a variant of the network model technique described in -. The paper presents a brief formulation of the network model materials characterization algorithm, along with numerical simulations of its performance for a simple canonical RCS shape using the CARLOS-3D™ MoM code . The convergence properties of the algorithm are also discussed.
FSS-Loaded Pyramidal Absorber
This paper describes a new approach to improving the low frequency reflectivity performance of geometric transition radar absorbent materials through the use of impedance loading in the form of one or more included FSS layers. The discussion includes theoretical predictions and measured data on modified commercially available RAM which confirm the validity of the concept.
A Dual-Linear Polarization UWB Dielectric Rod Probe Design
A dual-polarization ultrawide bandwidth (UWB) dielectric rod antenna containing two concentric dielectric cylinders was developed for near field probing applications. This antenna features more than 4:1 bandwidth, dual-linear polarization, stable radiation center and symmetric patterns. The antenna begins with a tapered wave-launching section consisting of shaped conducting plates and resistive films. This launcher section is followed by a guided section where the excited HE11 modes are transported to the radiation section. The radiation section contains specially shaped dimensions and materials to generate similar E and H plane patterns with 3-dB beamwidths greater than 55° over 4:1 bandwidth (2 to 8 GHz).
Electromagnetic Material Characterization Using a Rectangular Waveguide to Excite a Homogeneous Media in a Parallel-Plate Waveguide Through a Slot
A material measurement technique is developed to simultaneously characterize electric and magnetic properties for homogeneous lossy material in a parallel-plate region. The material is excited by a rectangular waveguide which interacts with the parallel-plate region through a slot. In order to extract the complex constitutive parameters from the material two independent measurements are required. If the material is attached to a PEC surface and is unable to be removed, the most obvious manner to characterize the material is a parallel-plate region. This paper demonstrates through the use of a magnetic field integral equation (MFIE) how a rectangular waveguide interacting through a slot with a parallel-plate region can be used to obtain two independent measurements, which are necessary for characterizing the homogeneous lossy material. Experimental results for various lossy materials are compared to stripline and waveguide measurements to verify the MFIE
Sub-Wavelength Measurement of Electromagnetic Inhomogeneities in Materials
Microwave microscopes that measure surface impedance or roughness have been demonstrated with fine spatial resolutions of less than a micron. These microwave probes are practical only for samples less than a few inches in size. However, composite materials in applications such as multi-layer radomes, embedded frequency selective surfaces, or integrated EMI shielding, have larger length-scale features embedded within a multilayer laminate. Diagnosing larger-scale, subsurface features such as joints/seams, periodic elements, imperfections, or damage is driving a need for methods to characterize embedded electromagnetic properties at mm or cm length-scales. In this research, finite difference time domain (FDTD) simulations and experimental measurements were used to investigate a probe technique for measuring sub-wavelength sized features embedded within a dielectric composite. For these applications, the probe interacted with the sample material via both evanescent and radiating fields. A dielectrically loaded, reduced size, X-band waveguide probe was designed in a resonant configuration for improved sensitivity. Experimental measurements demonstrated that the probe could characterize small gaps in ground planes embedded within a dielectric laminate. Simulations also demonstrated the possibility of detecting more subtle imperfections such as air voids.
Simultaneous Extraction of Permittivity and Permeability from a Conductor-backed Lossy Media
Nondestructive evaluation (NDE) of a conductor-backed lossy material plays an important role in electromagnetic shielding applications. Current NDE techniques limit such evaluation to non-magnetic materials and/or use of approximate solutions. In this paper a technique employing a flanged opened-ended rectangular waveguide and a rigorous integral equation model is presented allowing simultaneous extraction of both permittivity and permeability from a lossy magnetic shielding material. The technique reduces sample preparation time leading to rapid measurement. Calculation of both material parameters is accomplished using reflection measurements from two different thicknesses of a given material sample. A theoretical solution to the reflection coefficients is developed using a magnetic field integral equation (MFIE) formulation. The theoretical solution, along with experimental results of the reflection measurements, allows extraction of the material parameters using a two-dimensional Newton root search. Results using single-mode calculations are presented and compared to those obtained using traditional waveguide material characterization techniques. Future work will also be discussed.
Novel Method for Antenna Material Characterization
This paper describes the method and hardware implementation of a test bed that was designed and built to characterize the reflection characteristics of various types of reflector materials. The system described measures reflection amplitude and phase from flat test panels relative to a metal panel standard at normal incidence and for dual linear polarizations simultaneously. The measurement’s theoretical concept is based on a focused free space technique with time domain gating to remove the effect of multi-path coupling between the test panel and the feed assembly. The system as a whole demonstrates a novel method for measuring the reflection from reflector materials and characterizing their potential impact on polarization purity. The measurement system consists of: 1) A fixed reflector, 2) An alignment fixture accommodating feed assemblies, which include corrugated horns that operate over a 40% bandwidth that may be swapped out in order to cover a continuous frequency band from 18 to 75 GHz and Orthomode Transducers (OMT) in order to measure dual linear polarizations simultaneously, 3) An additional alignment fixture for mounting the flat panels under test, and 4) A Vector Network Analyzer (VNA) and computer for data collection and processing. The system is assembled on a bench top and aligned utilizing a Coordinate Measurement Machine (CMM). Sample results demonstrating the measurement of various types of reflector materials including composite reflector lay-ups with graphite face sheets and mesh samples for deployable reflectors are presented.
Low Frequency Optimization of 72 Inch Absorbers
The purpose of this paper is to detail the process used to optimize the low frequency performance of 72 inch absorber. The loading optimization was required to provide enhanced performance of a twisted 72 inch absorber which was to be used in the building of a large aircraft test facility. The chamber performance requirements are over a frequency range of 30 MHz to 18 GHz. The chamber dimensions are 30 meters x 30 meters x 20 meters high. This chamber will be used to measure a variety of fighter aircraft for many EW scenarios. The mission of this facility is to “perform radiated immunity testing of aerospace vehicles with high electromagnetic field intensity, radiated emissions measurements, EMC testing, electronic warfare testing, antenna pattern testing”. Due to the broad frequency range and the fact that the chamber is desired to test both in the low frequency EMC domain and high frequency antenna measurements, an extremely broad band absorber material had to be developed and optimized. The use of ferrite hybrids was considered. Due to the roll off at microwave frequencies and the expense of such a high volume of materials, they were eliminated for cost and due to the limited performance in the 1-2 GHz frequency range. The ideal candidate is a 72 inch twisted pyramidal geometry. The standard loading of these materials is ideal for frequencies above 150 MHz.. The performance level in the 30 MHz to 150 MHz range is less than ideal. A design for the chamber was established with specific target performances required of the 72 inch absorbers. This paper describes the effort taken to optimize the loss properties of the dielectric foam to meet the target absorber performance required for the implementation of the design. Key Words: Absorber Measurements, Absorber Performance, Computer Modeling of Absorbers, Dielectric Properties of Absorber
Investigation of Several Miniature Antenna Design for Tri-Band GPS Applications
Abstract The undergoing development of GPS system requires the integration of all three GPS bands: L1 (1575MHz), L2 (1227MHz), and L5 (1176MHz) into one miniature antenna. The objective of work is to develop a small GPS antenna (less than 1/10 wavelength) to cover all the three GPS bands with a minimum bandwidth of 24MHz in each band and a minimum gain of 0 dBic (RHCP). Three novel miniature antenna designs: Quad-F antenna (QF), proximity-fed stacked patch (PFSP) antenna and channel fed ring (CFR) antenna were investigated. These antennas utilize high-index inhomogeneous dielectric materials to achieve antenna miniaturization, mode excitation and mode control. The full-wave simulation (HFSS) and measurement results are presented and discussed in this paper.
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