AMTA Paper Archive
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Implementation and Testing of Engineered Anisotropic Dielectric Materials
Several instances in antenna design are known where an anisotropic material is useful ; however, finding a naturally occurring anisotropic material with the required dielectric tensor is often an impossibility. Therefore, artificially engineered anisotropic dielectric materials must be designed, tested, and implemented. In a previous paper by the authors , the design and initial measurement of an anisotropic material in Cartesian coordinates was presented along with predictions of how the material could be used to extend the bandwidth of a simple antenna structure. In this paper we shall present the final implementation of the anisotropic material (with a tensor implemented in cylindrical coordinates) along with data on the material properties, the resulting antenna bandwidth, and radiation pattern. Design considerations for implementation of this approach shall be discussed along with practical limitations. Data shall also be presented on an unexpected result showing that that a reduced volume of anisotropic material produces favorable results. Measured data shall be compared with values predicted using finite difference time domain (FDTD) software and applications of this new broadband antenna for range operations will be discussed. . D. Tonn, S. Safford, M. Lanagan, E. Furman, S. Perini, “DESIGN AND TESTING OF LAYERED ANISOTROPIC DIELECTRIC MATERIALS”, AMTA 2015 Proceedings, Long Beach CA, October 2015.
Correction of Transmission Line Induced Phase and Amplitude Errors in Reflection and Transmission Measurements
Measuring the RF, microwave or millimeter wave reflectivity of materials and components often requires a substantial length of transmission line or cables to connect the microwave source/receiver to the test apparatus. Such cables may be subject to environmental variations (e.g. temperature or pressure) that change the overall phase delay and amplitude of signals that travel through said cables. Furthermore, some testing requires physical motion of the cable, which is another source of phase and amplitude error. When possible, great care is often taken to design a test apparatus or methodology to minimize movement of the test cables so that these position-induced phase errors are small. However, in some measurements, such as those that require scanning sensors or antennas, position-induced phase and amplitude errors cannot be avoided. In some situations, temperature variations that change the cable phase response are also unavoidable. The problem of cable-induced errors has been a concern for many different applications and there have been previous attempts to address it. These previous methods have used specialized microwave circuitry or a separate phase-stable reference device to measure and compensate for phase errors. In this paper, a new correction method is described, which determines and corrects for phase and amplitude errors in transmission line cables. Unlike previous published methods, the present technique does not require any specialized circuitry at the device under test (DUT). Instead it utilizes in-situ reflections that already exist in the measurement apparatus to obtain a reference phase and amplitude signal. The described algorithm combines these reflections with frequency and time-domain signal processing to compensate for erroneous phase and amplitude shifts that occur during a measurement. This paper demonstrates the correction methodology with materials measurements examples. Additionally, this phase and amplitude correction may be applicable for scatter and antenna measurements. It can be applied to either reflection or transmission measurement data.
BIANCHA: A spherical indoor facility for bistatic electromagnetic tests
BIANCHA (BIstatic ANechoic CHAmber) is a singular facility located at the premises of the National Institute for Aerospace Technology (INTA), Spain, and was devised to perform a wide variety of electromagnetic tests and to research into innovative measurement techniques that may need high positioning accuracy. With this facility, both monostatic and bistatic tests can be performed, providing capability for a variety of electromagnetic measurements, such as the electromagnetic characterization of a material, the extraction of the bistatic radar cross section (RCS) of a target, near-field antenna measurements or material absorption measurements by replicating the NRL arch system. BIANCHA consists of two elevated scanning arms holding two antenna probes. While one scanning arm sweeps from one horizon to the other, the second scanning arm is mounted on the azimuth turntable. As a result, BIANCHA provides capability to perform measurements at any combination of angles, establishing a bistatic, spherical field scanner. In this regard, it is worth noting that in the last years, a renewed interest has arisen in bistatic radar. Some of the main reasons behind this renaissance are the recent advances in passive radar systems added to the advantages that bistatic radar can offer to detect stealth platforms. On the other hand, with the aim of developing new aeronautic materials with desired specifications, research on the electromagnetic properties of materials have also attracted much attention, demanding engineers and scientists to assess how these materials may affect the radar response of a target. Consequently, this paper introduces BIANCHA and demonstrates its applicability for these purposes by presenting results of different tests for different applications: a bistatic scattering analysis of scaled aircraft targets and the extraction of the electromagnetic properties of composite materials utilized in an actual aeronautical platform.
Dual-polarized Monolithic Leaky Wave Antenna Enabled by Additive Manufacturing
The use of additive manufacturing (AM) techniques to manufacture microwave and mm-wave passive components has recently been demonstrated through various examples . The term AM comprises all techniques based on the successive building of thin layers of material one on top of each other to create a device. When properly implemented, AM offers the possibility to manufacture light-weight and highly complex devices without generating significant costs increase. Among all AM techniques, Stereo-Lithography (SLA) is the most interesting one for the production of mm-wave components. In SLA, the materials are non-metallic epoxy-based polymers, that require a metallic coating to allow them to become RF functional. In contrast to other AM techniques, SLA manufacturing tolerances and surface roughness permit the design of devices up to 300 GHz. SWISSto12 has recently reported the successful performance of metal plated SLA devices, based on a proprietary chemical plating technology enables the processing of monolithic devices. In this contribution, we aim at exploiting the previously described SWISSto12’s AM-SLA technique  to obtain a monolithic directional dual-polarized high-directive Leaky-Wave Antenna (LWA) operating at mm-wave frequencies. The LWA consists of a square cross section waveguide perforated with crossed slots in its top aperture . Moreover, the antenna already includes a side-arm orthomode transducer (OMT) and a smooth waveguide twist, specifically co-designed with the LWA. The squared waveguide supports the propagation of the two first orthogonal modes, which are radiated through the cross-shaped slots. Thus, the vertically (horizontally) polarized mode inside the waveguide produces theta-polarized (phi-polarized) radiation. The pointing angle is approximately 50°, the same for both beams. The simulated cross-polarization values are very low according to the simulations. Moreover, the directivity of each orthogonal beam is controlled by the dimensions of the cross-shaped slot. Weather observation radars are considered as a privileged potential application of this kind of systems. Two different prototypes of this LWA+OMT subsystem (one operating at 30 GHz and the other one at 60 GHz, both achieving gains above 15 dB) are currently being manufactured by SWISSto12. The prototypes and their performance will be included in the final paper.  de Rijk, E.; Silva, J.S.; Capdevila, S.; Favre, M.; Billod, M.; Macor, A.; von Bieren, A.; "Additive Manufactured RF components based of Stereo-Lithography", in Antenna and RF Systems for Space Science 36th ESA Antenna Workshop, 6-9 Oct 2015  M. Garcia-Vigueras, M. Esquius-Morote and J.R.Mosig, "Dual-polarized one-dimensional leaky wave antenna," 9th European Conference on Antennas and Propagation (EuCAP), Lisbon, Portugal, 13-17 April 2015, pp.1-2.
Compact First-Order Probe for Spherical Near-Field Antenna Measurements at P-band
A number of European Space Agency's (ESA) initiatives planned for the current decade require metrology level accuracy antenna measurements at frequencies extending from L-band to as low as 400 MHz. The BIOMASS radar, the Galileo navigation and search and rescue services could be mentioned among others. To address the needs, the Technical University of Denmark (DTU), who operates ESA’s external reference laboratory “DTU-ESA Spherical Near-Field (SNF) Antenna Test Facility”, in years 2009-2011 developed a 0.4-1.2 GHz wide-band higher-order probe. Even though the probe was manufactured of light-weight materials -- aluminium and carbon-fibre-reinforced polymer (CFRP) -- it still weighs 22.5 kg and cannot be handled by a single person without proper lifting tools. Besides that, a higher-order probe correction technique necessary to process the measurement data obtained with such a probe is by far more demanding in terms of the computational complexity as well as in terms of calibration and post- processing time than the first-order probe correction. On the other hand, conventional first-order probes for SNF antenna measurements utilizing open-ended cylindrical waveguides or conical horns fed by cylindrical waveguides operating in the fundamental TE11-mode regime also become excessively bulky and heavy as frequency decreases, and already at 1 GHz an open-ended cylindrical waveguide probe is challengingly large. For example, the largest first-order probe at the DTU-ESA SNF Antenna Test Facility operates in the frequency band 1.4–1.65 GHz and weighs 12 kg. At 400 MHz, a classical first-order probe can easily exceed 1 cubic meter in size and reach 25-30 kg in weight. In this contribution, a compact P-band dual-polarized first-order probe is presented. The probe is based on a concept of a superdirective linear array of electrically small resonant magnetic dipole radiators. The height of the probe is just 365 mm over a 720-mm circular ground plane and it weighs less than 5 kg. The probe covers the bandwidth 421-444 MHz with more than 9 dBi directivity and |µ| ? 1 modes suppressed below -35 dB. The probe design, fabrication, and test results will be discussed.
Uniaxial Anisotropic Material Measurement using a Single Port Waveguide Probe
Anisotropic material characterization requires versatile sample fixtures in order to provide sufficient measurement diversity for material parameter extraction. However, extensive sample preparation is often required prior to making a measurement, especially for anisotropic materials. An alternative nondestructive material measurement approach using a Single Port Waveguide Probe (SPWP) is proposed to simplify measurement of uniaxial anisotropic media. Instead of cutting a material sample to fit into a given fixture, nondestructively interrogating a sheet of material via the SPWP greatly simplifies sample preparation and measurement. The SPWP system measures a metal-backed sample of a known thickness. A flange with a waveguide aperture cut in the center is placed on the metal backed sample (thus forming a parallel plate region) and a length of rectangular waveguide is connected to the flange aperture. A Vector Network Analyzer port is connected to the end of the rectangular waveguide to collect calibration and sample data. Measurements of two different thicknesses of a sample are performed to provide sufficient data for extracting the sample permittivity tensor. The sample permittivity tensor is computed via comparison of the measured and theoretical S-parameters using a least squares minimization algorithm. The theoretical S-parameters are derived using a magnetic field integral equation which utilizes a uniaxial parallel plate Green’s function to constitute the fields in the parallel plate region. Love’s Equivalence Principle is used to relate the fields in the parallel plate flange region to the fields in the waveguide (assumed to be the dominant TE10 mode only). In this paper, the SPWP theoretical development, measurement and material parameter extraction are discussed. Measurements and simulations of isotropic and uniaxial samples are made to assess the SPWP performance.
A Polynomial Approximation for the Prediction of Reflected Energy from Pyramidal RF Absorbers
Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials, however, the bi-static or off angle performance must also be known. Some manufacturers provide factors at discrete electrical thickness for a discrete range of incident angles. This approximation is based on the curves presented in . In reference , a polynomial approximation was introduced. In this paper, a more accurate approximation is introduced. Pyramidal RF absorber is modeled using CST’s frequency domain solver. The numerical results are compared to results from other numerical methods. The highest reflectivity of the two principal polarizations for a given angle of incidence and thickness of material is calculated. Different physical thickness pyramids are modeled. Once the worst case reflectivity is calculated, a polynomial curve fit is done to get a set of equations that provide the bi-static performance for absorber as a function of angle of incidence and thickness of material. The equations can be used to predict the necessary RF absorber to treat the walls of an indoor range.
Highly accurate fully-polarimetric radar cross section facility for mono- and bistatic measurements at W-band frequencies
New requirements in the field of autonomous driving and large bandwidth telecommunication are currently driving the research in millimeter-wave technologies, which resulted in many novel applications such as automotive radar sensing, vital signs monitoring and security scanners. Experimental data on scattering phenomena is however only scarcely available in this frequency domain. In this work, a new mono- and bistatic radar cross section (RCS) measurement facility is detailed, addressing in particular angular dependent reflection and transmission characterization of special RF material, e.g. radome or absorbing material and complex functional material (frequency selective surfaces, metamaterials), RCS measurements for the system design of novel radar devices and functions or for the benchmark of novel computational electromagnetics methods. This versatile measurement system is fully polarimetric and operates at W-band frequencies (75 to 110 GHz) in an anechoic chamber. Moreover, the mechanical assembly is capable of 360° target rotation and a large variation of the bistatic angle (25° to 335°). The system uses two identical horn lens antennas with an opening angle of 3° placed at a distance of 1 m from the target. The static transceiver is fed through an orthomode transducer (OMT) combining horizontal and vertical polarized waves from standard VNA frequency extenders. A compact and lightweight receiving unit rotating around the target was built from an equal OMT and a pair of frequency down-converters connected to low noise amplifiers increasing the dynamic range. The cross-polarization isolation of the OMTs is better than 23 dB and the signal to noise ratio in the anechoic chamber is 60 dB. In this paper, the facility including the mm-wave system is deeply studied along with exemplary measurements such as the permittivity determination of a thin polyester film through Brewster angle determination. A polarimetric calibration is adapted, relying on canonical targets complemented by a novel highly cross-polarizing wire mesh fabricated in screen printing with highly conductive inks. Using a double slit experiment, the accuracy of the mechanical positioning system was determined to be better than 0.1°. The presented RCS measurements are in good agreement with analytical and numerical simulation.
Truncation Error Mitigation in Free-Space Automotive Partial Spherical Near Field Measurements
Modern cars are equipped with a large number of antennas which are strongly integrated with the car. A full characterization of the radiating properties of the entire vehicle is thus typically required. In order to characterize the radiating properties of the installed antennas, large measurement systems accommodating the full vehicle are required. As in standard antenna measurements, a full spherical near field (NF) scanning around the car is desirable in order to perform an accurate NF/FF transformation. However, due to size and weight of the Device Under Test (DUT) and/or economic factors a full spherical scan is often unfeasible. For this reason, truncated spherical scanners (such as hemispherical) are typically involved. A classic solution is to combine hemispherical scanning with a metallic ground plane which is assumed to be a Perfect Electric Conductor (PEC) in the NF/FF transformation. However, the PEC ground-plane is less representative of realistic automotive environments such as asphalt that is strongly dielectric. A further drawback is the strong scattering from the large metallic ground-plane which highly compromises the NF measurements at low frequencies. In many situations, it is thus desirable to perform the NF measurements in a condition similar to free-space by using absorber materials on the floor. It is well-known that standard NF/FF transformations applied to partial spherical acquisitions generates the so called truncation errors. Such errors are stronger at lower frequencies due to the lower number of spherical modes for fixed DUT size. Moreover, typical antennas for automotive applications are generally low directive thus, the impact of the truncation on the measured pattern is often non-negligible. In such cases advanced post-processing techniques must be involved to mitigate the effect of the truncation errors. In this paper two truncation error mitigation techniques will be compared when applied to automotive measurements performed in free-space conditions. The first technique is an iterative process which at each iteration applies a modal filtering based on the size of the DUT. The second technique is based on the computation of the equivalent currents of the DUT over an equivalent surface which acts as spatial filter. Both techniques give excellent mitigation performance with different computational effort. The good agreement between two different techniques effectively defining the lower bound for what can be successfully mitigated by post processing techniques.
Comparing Predicted Performance of Anechoic Chambers to Free Space VSWR Measurements
Abstract— Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials; however, the bi-static or off angle performance must also be known. In reference , a polynomial approximation was introduced that gave a prediction of the reflected energy from pyramidal absorber. In this paper, the approximations are used to predict the quiet zone (QZ) performance of several anechoic chambers. These predictions are compared with full wave analysis performed in CST Suite®. A 12 m wide by 22 m long with a height of 12 m chamber was analyzed at 700 MHz. The QZ performance was compared to the polynomial predictions showing a difference of less than 2.2 dB. In addition, comparisons are made with measurements of the QZ performance of anechoic chambers. Measurements performed per the free space VSWR method of three different chambers are compared with the prediction that uses the polynomials presented in . The chambers are: a 18 m long by 11.5 m wide and 11.5 m in height operating from 100M MHz to 12 GHz; a 13.41 m by6.1 m by 6.1 m operating from 800 MHz to 6 GHz; and a 14 m long by 4.12 m by 4.27 m operating in the X band. The results show that the polynomial approximations can be used to give a reasonably accurate and safe prediction of the QZ performance of anechoic chambers.  V. Rodriguez and E. Barry, “A polynomial approximation for the Prediciotn of Reflected Energy from Pyramidal RF Absorbers,” Proceedeings of the 38th annual Symposium of the Antenna Measurement Techniques Association (AMTA 2016), pp. 155–160, October 2016.
Thermal Testing of Small Antennas in Multi-Probe Spherical Near-Field Systems
Temperature change cause thermal expansion of the antenna materials and will have an important impact on antenna performances. In some applications it is sufficient to calculate the antenna deformation due to temperature by mechanical analysis and determine the RF impact by EM analysis tools. However, if the environmental conditions of the final antenna are stringent and considered critical as in some military and civil applications in the space and aeronautics domain, the thermal performance of the antenna must be determined by experiment. Typical temperature testing ranges for civil applications are often between -50°C and +80°C but can be much more extensive for special applications. This paper present a simple and easy method for thermal testing of antennas in a fast spherical near field measurement facilities such as multi-probe system. During the thermal testing, the antenna is maintained inside a RF transparent thermally insulated container including the local heating and cooling equipment. The fast testing provided by the multi-probe system allow to measure the temperature dependence of the antenna at several different temperatures within the investigation range. The method will be illustrated for the cold measurement case but the extension to the full cold-hot temperature range is trivial.
Specular Reflectance Measurement of Dielectric Plates in 110-325 GHz Frequency Range
For high speed and high data-rate communications, operating frequency bands of wireless communication systems have been moving to submillimeter frequency range and their bandwidths have been broadening. IEEE 802.15 THz Interest Group (IEEE 802.15 IGthz) has been performing a channel characteristics study for future indoor millimeter and submillimeter wireless communications in the frequency range of 75 - 110 GHz and 270 - 320 GHz. Specular reflectance data of indoor interior materials is a prerequisite to analysis of the channel characteristics of new indoor millimeter and submillimeter wireless communications. Specular reflectiondescribed by the law of reflection states that the direction of the incident wave and the direction of the reflected wave make the same angle with respect to the surface normal, thus theangle of incidence is equal to that of reflection. This paper describes a specular reflectance measurement system and shows measurement result of dielectric plates in the frequency range from 110 GHz to 325 GHz. Specular reflectance measurement system consists of an S-parameters measurement system and a specular reflectance measurement apparatus. The S-parameters measurement system consists of a 67 GHz vector network analyzer used as the main frame and three frequency extenders which are operating at three frequency bands (D-band (110 -170 GHz), G-band (140-220 GHz) and J-band (220-325 GHz)), respectively. The specular reflectance measurement apparatus consists of a transmitting part, a receiving part, and a MUT holder which is positioned in the middle of the transmitting and receiving parts. During the specular reflectance measurement, the transmitting part is fixed while the MUT holder and receiving part are coaxial-rotating with 1:2 speed ratio. The transmitting and receiving frequency extenders are installed on the transmitting and receiving parts, respectively. For the specular reflectance measurement, one measures the transmission coefficient (S21_MUT) corresponding to the specular reflectance of an MUT mounted on the MUT holder. After replacing the MUT with a metal plate, one measures the transmission coefficient (S21_metal) corresponding to the specular reflectance of the metal plate, assumed to be -1. Specular reflectance of the MUT is obtained by taking the ratio (S21_MUT/S21_metal) of the respective transmission coefficients corresponding to the specular reflectance of the MUT and the metal plate. Multiple reflection effects between the transmitting and receiving antennas can be averaged out and minimized by averaging the transmission coefficients measured with changing the separation distances between the two antennas by ?/8 interval (i.e. initial distance + n·?/8, n=0,1,2,3). Specular reflectances of dielectric plates are measured in the 30° to 70° incident angle range with the developed measurement system in the frequency range from 110 GHz to 325 GHz. Description of the detailed measurement system and measurement result will be presented at the symposium.
Low-Cost Pressure/Temperature Measurements of Wideband Antennas
This paper discusses design and fabrication of a low cost, combined pressure / thermal test-bench engineered for environmental tests of UAV mounted antennas. Both test-beds are mainly made of commercial of-the-shelf (COTS) parts and in-house made frames. They occupy small space and do not require specific professional skills for operation or high maintenance cost. Measurement setup is designed to reliably reproduce temperature and pressure corresponding to altitudes from sea level to 6000 m (20000 ft) with dynamic load equivalent for 200 m/s (400 knots) of air speed. Experimental results of radome enclosed wideband antenna are presented.
Aircraft Radome Characterization via Multiphysics Simulation
Altair Engineering Inc. Troy, MI USA-https://www.altairhyperworks.com Figure 1. The electromagnetic, aerodynamic, and structural performance of a nose cone radome can be characterized by computational simulation, allowing for early design concept validation and reducing the dependence on physical testing. Abstract-Radomes protect antennas from structural damage due to wind, precipitation, and bird strikes. In aerospace applications, radomes often double as a nose cone and thus have a significant impact on the aerodynamics of the aircraft. While radomes should be designed not to affect the performance of the underlying antennas, they also must satisfy structural and aerodynamic requirements. In this paper, we demonstrate a multiphysics approach to analysis of airborne radomes not only for electromagnetic (EM) performance, but also for structural, aerodynamic, and bird strike performances, as depicted in figure 1. We consider a radome constructed using composite fiberglass plies and a foam core, and coated with an anti-static coating, paint, and primer. A slotted waveguide array is designed at X-band to represent a weather radar antenna. The transmission loss of the radome walls is analyzed using a planar Green's function approach. An asymptotic technique, Ray-Launching Geometric Optics (RL-GO), is used to accurately simulate the nose cone radome and compute transmission loss, boresight error, and sidelobe performance. In addition to EM analysis, Computational Fluid Dynamics (CFD) analysis is used to predict pressures resulting from high air speeds, which are then mapped to an implicit structural solution to assess structural integrity using the Finite Element Method (FEM). We also demonstrate damage prediction due to a "bird strike" impact using an explicit structural FEM solver. The multiphysics simulation techniques demonstrated in this paper will allow for early design validation and reduce the number of measurement iterations required before a radome is certified for installation.
Parameter Extraction Algorithm for Conductor Backed, Bi-Layered Uniaxial Materials
An algorithm is developed for the extraction of constitutive parameters from bi-layered uniaxial anisotropic materials backed by a conductive layer. A method of moments-based approach is used in conjunction with a previously-determined Green function. Possible challenges related to measurement diversity are highlighted and a possible mitigation path is proposed.
Specular Reflectance and Antenna Property Measurements in 325-500 GHz Frequency Range
Specular reflectance data of indoor interior materials is a prerequisite to analysis of the channel characteristics for new millimeter and submillimeter indoor wireless communications. Antenna property such as gain and radiation pattern is one of the key measurement quantities in electromagnetic wave metrology. This paper describes a specular reflectance and antenna property measurement system and shows measurement results of the specular reflectance of an Acetal plate and the antenna property of a 24 dB horn antenna in 325-500 GHz frequency range.
Equivalent Sources Based Near-Field Far-Field Transformation Above Dielectric Half Space
In order to support near-field measurements of automobile antennas in as realistic as possible environments, an equivalent sources based near-field far-field transformation approach for near-field measurements above a possibly lossy dielectric half-space is presented and evaluated. Different possibilities for considering the half-space influence are discussed, where an approach with an appropriate half-space Green's function is found to be most accurate, as expected. The formulation of the equivalent sources transformation approach with the half-space Green's function and a formulation with free-space Green's function together with equivalent sources representation of the half-space influence are discussed and a variety of results are presented in order to corroborate the feasibility of the various approaches.
Reflection-Based Inverse Scattering Image Reconstruction for Non-Destructive Testing
Non-destructive testing (NDT) is a fundamental step in the production chain of aircraft structural components since it can save both money and time in product evaluation and troubleshooting. This paper presents a reflection-based imaging technique for electromagnetic (EM) testing of composite panels, with the device under test (DUT) being metal backed and both the transmitting and receiving components of the NDT system situated on the same side of the DUT. One of the key properties of the presented technique is the complete redundancy of a reference measurement, thereby making it feasible to retrieve a high quality image of the DUT with only a single measurement. Data for both a proof-of-concept DUT and an industrially manufactured composite panel is provided, and the retrieved images show the applicability of both the measurement technique and the imaging algorithms.
Spot-Probe Reflectometer Measurements of Geological Core Slab Samples
Rock core specimens collected during surveys for oil drilling have, in a standard form, a 4" diameter. Cores are cut in half or in 1/3-2/3 sections to provide core slab. We developed a measurement procedure based on spot probe illumination to characterize geological and/or geochemical properties of core slab specimens via their complex permittivity for frequencies between 2.5 GHz and 20 GHz. Conventional reflectometer methods are based on illumination of a thin slab of air-or metal-backed material. However, in this case only the front surface is flat and the back surface is semicircular. A measurement method was developed based on time-domain gating to separate the back-surface reflection from that of the front. Material inversion is then based on the amplitude and phase of the reflection just from the front surface. This paper presents details of the calibration for this reflectometer measurement method, along with example measurements of core slab materials. Two different inversion methods are applied to these measured data. The first is a more conventional frequency-by-frequency method for inverting complex permittivity from the amplitude and phase of the reflection. The second method applies a physical model, the Debye relaxation model, to the data. This model-based approach minimizes the errors from edge diffraction from the small sample size.
A Novel S-band Two-Layer Dielectric Rod Antenna with High Gain and Very Low Cross-polarization
In this paper, the concept of a new S-band dual-polarized dielectric rod antenna is discussed. The antenna is composed of two concentric dielectric cylinders. The inner dielectric presents high dielectric constant, while the outer has a lower dielectric constant. Given this configuration, the resulting antenna provides high gain, narrow beamwidth, large bandwidth, and very low cross-polarization. In addition, the antenna is lower size in the transversal dimensions, and is predicted to be lighter than other antennas that provide equivalent performance, especially at low frequencies (S-band). An antenna with such an architecture can be 3D-printed, and therefore, the cost for the fabrication are considerable low. Numerical results of the antenna performance are presented and discussed.
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