AMTA Paper Archive
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Radome Enhanced Antennas
Hybrid radome-antenna designs can enable novel applications and unique benefits that would be difficult to achieve with standalone radomes and antennas. Examples of such designs are provided which use simple antennas and novel radomes to reduce antenna size and weight, to generate and steer antenna beams without use of complex phased arrays and beam forming networks, and to enable precise direction finding with only two antenna elements.
On Convergence of the Upper Bound on the Ratio of Gain to Quality Factor
An antenna’s practical far-field distance can be estimated from the upper bound on the ratio of its gain to quality factor. This upper bound is an infinite series that can be truncated based on the desired accuracy. We investigate the convergence properties of this bounding series. We find that the number of terms required for convergence depends on the antenna’s electrical radius in a way similar to the Wiscombe criterion used in Mie scattering theory. For typical experimental accuracy requirements, such convergence can significantly reduce the effective far-field distance.
Measuring Component Performance in an Integrated Antenna-Receiver System
In this article, a method is presented which describes how to measure the separate performance parameters of an antenna-receiver system after they have been integrated into one system. The integrated receiver may perform different than the cascaded prediction of the pieces that make up the system due to component interaction. This article develops a method that allows the integrated performance of the individual components (an antenna and a receiver for this discussion) to be measured without disassembly. Using the described method, parameters such as, antenna gain, receiver gain, and receiver effective input noise temperature (correspondingly, receiver noise figure) can be measured. Once the receiver effective input noise temperature is measured, then it is possible to determine the remaining parameters. In the past, the difficulty has been separating out the two noise temperature terms (sky noise and receiver effective input noise). The presented method develops multiple equations which essentially separates out the two terms. Once the two terms have been separated, solving for the others is now possible.
Synthesis of Van Atta Array Retrodirective Patterns Using Conventional Array Characterization
Van Atta Arrays are antennas with uniquely configured beamforming networks (BFNs) that allow for innate retrodirection of incident signals. While useful for a range of applications, their characterization has typically necessitated the use of radar crosssection (RCS) ranges. Our work proposes an alternate method that uses conventional array characterization, specifically element patterns and scattering matrix measurements, to synthesize both bistatic and monostatic RCS patterns for Van Atta arrays. This method is demonstrated theoretically and experimentally first with a cross-polarized dipole array followed by a counterwound octafilar helix antenna array. The benefits of the proposed synthesis method include fast design studies and trades of the Van Atta BFN enabling retrodirective operation. Among other things, this allows for broader access to experimental research on this topic. The significance of the structural radar cross-section is also discussed.
Synthesis of a Phased Array with Planar Near-Field Techniques Based on Far-Field Measurements of a Sub-Array in a CATR
Phased array antennas are often built from sub-arrays with identical or symmetrical layout. At an early project stage, performance verification measurements of the sub-array are valuable to proof the single module design. However, the characteristics of the final antenna are questionable without further processing. This work presents a concept that is based on far-field measurements of a sub-array in a Compact Antenna Test Range (CATR) in conjunction with planar near-field (PNF) processing to synthesize the entire phased array antenna characteristics. The procedure is explained with an example of a dual linear polarized L-band planar phased array antenna for an airborne synthetic aperture radar application. It is shown that the measured sub-array can be complemented by the synthesized twin to evaluate the characteristics of a final antenna that is not yet available in this form. The resulting performance of the synthesized entire phased array is presented and compared with simulations. The presented post-processing method would be beneficial to characterizing radiation patterns of large phased arrays by measuring only sub-arrays in a limited test-zone with any measurement principle.
On the Uncertainty Sources of Drone-Based Outdoor Far-Field Antenna Measurements
Unmanned Aerial Systems (UAS), colloquially known as drones, offer unparalleled flexibility and portability for outdoor and in situ antenna measurements, which is especially convenient to assess the performance of systems in their realworld conditions of application. As with any new or emerging measurement technology, it is crucial that the various sources of error must be identified and then estimated. This is especially true here where the sources of error differ from those that are generally encountered with classical antenna measurement systems. This is due to the larger number of mechanical degrees of freedom, and to the potentially less repeatable and controllable environmental conditions. In this paper, the impact of some of these various error terms is estimated as part of an ongoing measurement validation campaign. A mechanically and electrically time invariant reference antenna was characterized at ESAESTEC’s measurement facilities which served here as an independent reference laboratory. The reference results were compared and contrasted with measurements performed outdoors at Quad- SAT’s premises using QuadSAT’s UAS for Antenna Performance Evaluation (UAS-APE). While a direct comparison between the measurement results from ESA-ESTEC and QuadSAT delivers information about the various uncertainties within a UAS-APE system in comparison to classical measurement facilities’ and the validity of such a system for antenna testing, other tests aim at providing an estimation of the impact of each error source on the overall uncertainty budget, thus paving the way towards a standardized uncertainty budget for outdoor UAS-based sites.
Reduced-Order Model for Antenna Pattern Characterization from a Small Number of Samples
The characterization of the radiation performances is a necessary step in the conception of any wireless system. These systems require always more demanding radiation performances that calls for time consuming characterizations. This duration can be reduced by the decrease of the number of field samples. By enclosing the antenna in a Huygens’ surface, we can build a radiation matrix that maps equivalent surface currents to the radiated field. A singular value decomposition of this matrix enables to build a compressed representation of the antenna measurement and more specifically a reduced basis of the radiated fields. By harnessing the outer dimensions of the antenna, the number of field samples can be reduced as compared to spherical wave expansion techniques. This number is shown to be connected to the area of the convex equivalent surface enclosing the AUT, as hinted by previous analytical works for canonical enclosing surfaces. The whole antenna characterization procedure is validated by simulations and experiments.
Genetic Evolution of the Reflector Edge Treatment of a Single Offset-Fed Compact Antenna Test Range for 5G New Radio Applications
While the size of the parabolic reflector in general determines the usable area of the quiet zone of a compact antenna test range (CATR) inside which a pseudo plane-wave condition is produced, the reflector edge treatment also plays a significant role in terms of overall quality and electromagnetic field distribution & uniformity, and especially so at mm-wave frequencies. Using modern powerful digital computational simulation technology in combination with genetic optimization, the edge treatment can be evolved for a specific CATR application as part of the design process. This is crucial as it attempts to maximize the performance of a given solution while ensuring efficient use of the available space which correspondingly provides an economical implementation. This is particularly important in 5G production test applications where, in many instances, multiple systems are required to be collocated within a given host building and in which case, the savings become multiplicative. In this paper the novel design methodology is introduced for the genetic optimization (GO) of blended rolled edge single offset reflector CATRs. Several edge blends and treatments are considered with the genetically optimized design parameter. For each variation the quiet-zone performances are compared and contrasted.
Bi-static RCS variations of pedal and wheel movements on bicycles between 1 and 10 GHz
One benefit of cooperative automated and connected driving lies in the fusion of multiple mobile wireless sensor and data transmission nodes, covering complementary technologies like radar, cellular and ad-hoc communications, and alike. Current developments indicate enormous potential to increase the environmental awareness through joint communication and radar sensing. In this respect, future channel models require knowledge of bi-static reflectivities of road users over a range of illumination and observation angles, both in the nearfield and in the far-field. To establish reference data and model such angle-dependent RCS variations, this paper deals with realistic pedal and wheel rotations of a bicycle based on electromagnetic simulations. In the simulation setup, idealized far-field conditions with plane-wave illumination and observation were assumed, while the angles covered the entire azimuth with 201 variations of the pedal and wheel positions. The fluctuation of the RCS is analyzed and discussed in terms of its probability density and cumulative distribution functions. Depending on the angular constellation, the range of the fluctuation varied between 1 dB and 14 dB, while the specular reflection and forward-scattering showed almost no fluctuation.
On the Challenge of Over-The-Air Measurements of High-Power Massive MIMO Radio Base Stations
Using beam-steering technologies, 5G massive MIMO base stations are capable to radiate typical equivalent isotropic radiated powers as high as 80 dBm (100 kW). Such levels create challenges in Over-The-Air (OTA) testing, both for the RF test system hardware, and the anechoic chamber / absorber layout designs. In this paper, a calculation tool is introduced which allows evaluations of the Poynting vector at ”mid range” distances, from given base station models. This code is used to deduce conservative power density distribution estimates and identify possible critical exposure areas in the test facility. General design criteria for the chamber, absorber layout and choice of material are derived. The specific case of a plane-wave synthesis OTA test site is investigated, where an experimental setup is used to demonstrate the power tolerance of the solution and its compatibility with base station testing requirements.
Consideration of the Feeding Networks for Measurement of mmWave/Sub-THz SoP/SoC/SoD Antennas in 5G and 6G
This paper presents a reliable design and measurement methodology of using various feeding networks for mmWave/Sub-THz SoP/SoC/SoD antennas in 5G and 6G communication. In order to achieve reliable and precison testing results, the electrical, mechanical, and thermal consideration have been precendently investigated and discussed through various examples of feeding network based on lots of the advanced materials and fabrication process. First, for a realization of the minimized discrepancy between simulation and measurement without any calibration kit and resistive films for 50-Ω termination load, two examples have been presented. In other words, a symmetrical power divider with back-to-back transition structures and a leaky wave antenna design topology featuring high attenuation constant have been demonstrated. Finally, despite challenging fabrication condition resulting in performance degradation, a low-loss transition structure in mmWave SoD antenna and its design methodology is also presented and discussed.
Base Station Specific Absorption Rate Assessment Based on a Combination of Over-The-Air Measurements and Full-Wave Electromagnetic Simulations
Radio Base Stations (RBS) must comply with applicable radio frequency electromagnetic field exposure regulations. Although compliance evaluation is typically carried out using field strength acquisitions or computations, Specific Absorption Rate (SAR) measurement is the reference method for low-power RBS, such as those used for indoor coverage. As classical robotbased probing is extremely time-consuming, especially when the whole-body SAR in a large phantom is to be assessed, faster alternative techniques are of high interest. Such solutions are becoming even more crucial, as the number of test modes is multiplying with modern communication technologies. This paper introduces an alternative, based on the convergence of Over- The-Air (OTA) measurements, equivalent current reconstruction and full-wave electromagnetic simulation. A first set of results demonstrates the relevance of this combination, by comparing actual dosimetric measurements to OTA-based reconstructed SAR values in a flat body mannequin, for a commercial lowpower RBS. A test system is realized which enables OTA electric field phase evaluations for a self-powered device under test, using digitally modulated signals. This proof of concept establishes the applicability of the technique to actual regulatory testing conditions.
Simultaneous Measurement of Analog Phased Array Elements Using Orthogonal Coding
Evaluation and calibration of individual elements of a phased array is a time-consuming process that involves not only the radiation pattern and RF circuitry of each element, but the interaction of each element with all of the other elements within the array. Iterating through each element in order to test them one at a time is extremely time consuming, and in some cases, depending on the design of the array, this approach may not work reliably at all. In cases where the impedance of the “off” elements differs from their impedance when actively transmitting or receiving, they can distort the resulting single element pattern due to mutual coupling. Even in the case where the elements themselves are well behaved, the driving circuitry can exhibit non-linearities due to the differences in signal levels or device heating present when all elements are active vs. only a single element. Thus, it would be ideal to be able to extract individual element performance from the combined pattern of the array with all elements active. This paper will investigate the use of orthogonal coding applied to each element of the array through the onboard gain/phase control circuitry using different modulation coding schemes in order to extract the average performance of each element from the measured total result.
Measurements and Simulations of a 2.4 GHz Circular Waveguide Antenna for a Portable Radar Kit
A custom radar kit that integrates with a portable computer (laptop) for assembly and operation by students and researchers has been developed at MIT Lincoln Laboratory. The assembled radar kit uses two low-cost cylindrical metal cans that serve as the antennas, one for transmitting and one for receiving radar signals. The antennas operate as linearly polarized openended circular waveguides (10.5 cm diameter) fed with a thinwire monopole probe. Over the 2.4 to 2.5 GHz band, the measured reflection coefficient is less than −10 dB, the peak realized gain is greater than 7 dBi, and the half-power beamwidth is approximately 70 degrees in both the E- and Hplanes. FEKO method of moments simulations of the antenna are compared with the measured data and good agreement is demonstrated.
Validation of Over-The-Air Testing Accuracy at Mid-Range Distance for Massive MIMO Base Stations
5G base stations are gradually evolving into Active Antenna Systems, improving the link budget with beamsteering capabilities. As such antenna arrays are typically eight wavelength large or more, the question of reducing the footprint of far-field testing facilities has experienced a growing interest. Recent research results have established that it is possible to conduct accurate Over-The-Air measurements around the peak radiation, at an effective far-field distance which can be as low as 20% of the Fraunhofer distance, depending on the electrical size of the antenna aperture. This paper complements the published validations of this finding, with an application to commercial massive MIMO base stations. The previously identified midrange far-field distance is even shown to be conservative for such devices. A mathematical analysis based on plane-wave expansion is proposed and allows for a general interpretation of this result.
Analysis of Far-field Condition of Broadband Quad Ridge Horn Antennas
The most common antennas used for antenna pattern or gain measurements are Standard Gain Horn Antennas, Circular Horn Antennas, Dual Ridge Horn Antennas or Quad Ridge Horn Antennas. In addition, the far-field criteria for the antennas is currently revised as per the latest draft of IEEE 149 standard, based on the largest dimension, D, of the antenna and wavelength, , of interest. Conventionally, the largest aperture dimension of the antenna is considered as the dimension, D. One could question, if considering the same aperture dimension to compute the far-field distances over entire frequency range is accurate. It could lead to longer test range distances at higher frequencies for broadband horn antennas, which in turn will lead to much larger chamber sizes. Thus, it is imperative to investigate the electrical dimension, D, as a function of frequency for the broadband horn antennas to accurately yield the far-field distances needed to characterize the different antenna parameters like half-power beam width, first null level, side lobe level, etc. This paper explores the utilization of the spherical modes and underlying Minimum Radial Extend (MRE) from Nearfield to Far-field transformation theory to extract the electrical dimension, D, so as to accurately characterize the HPBW across the frequency range. Firstly, the near fields are transformed to far-fields by incorporating spherical modes. The transformed farfields are compared to the ideal far-field pattern for standard gain horns, with respect to the equivalent noise level parameter over the HPBW solid angle, to compute the acceptance criteria. Based on the acceptance criteria of the equivalent noise level for standard gain horns, the same exercise is repeated for a broadband quad-ridge horn over the HPBW solid angle across the frequency range. The MRE is computed from the number of spherical modes across the frequency range and the electrical dimension, D, is calculated to be twice of the MRE value. The far-field distance is calculated based on the computed electrical dimension and compared to the far-field distance calculated per the physical dimension of the antenna structure.
A Novel Complex Image Expansion for Antenna Measurements Above a Lossy Half Space
In a previous presentation, the author has reported that nearfield antenna measurements taken in the presence of a lossy half space such as the ocean can be accomplished by the use of Complex Image Theory. The approach allows the user to collect data over the upper hemisphere of space and employ Complex Image Theory to “fill in” the field information in the lower hemisphere. The approach has been limited, though, in application due to the limited applicability of the Complex Image approach. In this paper, a fresh look is taken at Complex Image Theory and a new field expansion proposed that allows the fields due to a source operating above a lossy half space to be expressed in terms of the fields due to an infinite sequence of equivalent Huygens sources located in complex space. The new expansion has advantages over previous work in that it properly predicts the formation of a surface wave along the interface between the two half-spaces in addition to properly accounting for the space wave field.
Accurate Retrieval of Antenna Patterns in Reflective Environments Via the Test-Zone Field-Compensation Technique
Countless degrees of freedom in the design of antenna test ranges are enabled if the measurement errors caused by the environment can be precisely compensated. Measurements in highly reflective measurement chambers and with broadband feeds or probes are possible since the quality of the test zone is not essential anymore. To create a reflective environment, a metal plate is installed in an anechoic chamber and a base transceiver station antenna is characterized with and without the additional scattering source at a frequency of 2:11GHz. To compensate for the undesired signals, the field of the test zone is measured on a spherical surface using a scanning arm. Via a wave expansion of the field and the antenna under test into spherical mode coefficients, the undesired signals are compensated. It is shown that the error of the compensated pattern compared with the undistorted measurements is mainly below 30dB and that the directivity is retrieved with a difference of only 0:011dB>
A Near-Field to Far-Field Transformation with Noncanonical Plane-Rectangular Scan Using an Effective AUT Modeling
This communication provides the experimental validation of an effective probe-compensated near-field to far-field (NFFF) transformation with a nonconventional plane-rectangular scan suitable for flat antennas under test (AUTs). It is based on the nonredundant sampling representations of the electromagnetic fields, on the use of optimal sampling interpolation expansions, and assumes a flat AUT as enclosed in a dish having diameter equal to its maximum dimension. This source modeling results to be very effective from the NF data reduction viewpoint, since, by fitting very well the geometry of such a kind of AUT, it is able to reduce as much as possible the residual volumetric redundancy related to the use of the other modelings suitable for quasi-planar AUTs (an oblate spheroid or a double bowl). Experimental results, assessing the practical feasibility of the proposed NF-FF transformation technique, are shown.
Closed Form Expressions for Gain Curves of Standard Gain Horns
Closed form expressions, based on curve fitting have been derived for the gain characteristics of Standard Gain Horns from three brands (Scientific Atlanta/MI Technologies, Narda and Custom Microwave). These polynomials are not more accurate than the original data but since the maximum deviation is 0.01 dB (in most cases a rounding off error), they also do not add inaccuracy. They just replace the Look-Up data sheets, provided as tables or graphs. These polynomials can easily be implemented in a tool (nowadays called “app”) that provides the gain value based on model number and frequency of interest. Curve fitting coefficients have been derived for 12 Scientific Atlanta horns (0.4 – 26.5 GHz), 10 Narda Horns (1.12-40 GHz) and 12 Custom Microwave Horns (18-325 GHz).
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