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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.
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.
In a previous presentation[1], 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.
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>
The brick-based antenna design is a new concept to the literature. Metals and dielectrics are in brick-form to let the antenna designers connect and disconnect the cells easily. Designing and prototyping an antenna takes only a few minutes with this concept. Antenna engineers directly build their design in front of a network analyzer and iterate to reach their requirements. This hardware-based antenna design solution also creates a design cycle of measure-iterate instead of simulateiterate. This study starts with introducing this new method and continues with a dual-port 3.5 GHz patch antenna design and measurement. After the single antenna reaches the target frequencies, the 16 element 4x4 planar patch antenna array is built and measured.
Modern antenna range design is often a careful balance of several competing objectives. Some of these design parameters are defined by the antenna under test (AUT), i.e. millimeter wave (Ka-band) test frequencies, frequencyconverting and non-converting AUTs, high-power radiation requirements, pulsed RF requirements, and interleaved transmit and receive (T/R) requirements. Other parameters are driven by the AUT’s application, like requirements for providing accurate pattern, gain, EIRP, and G/T predictions based on the measurement data. Yet other parameters are driven by cost and risk considerations, like the need for all-at-once acquisitions incorporating multi-frequency, multi-port, dual-pol, and multistate measurements. Also included in the “cost and risk” category is the need to collect all these measurements in the least amount of time. A planar near-field antenna range designed with all these parameters in mind has been realized and is currently operational. This 1 m x 1 m planar near-field range incorporates several novel electrical and mechanical features, and we illustrate these features in terms of their driving requirements and their limitations. Included in our discussion: modular T/R range “front ends,” reconfigurable probe networks, absorber cooling strategies, near-field probes for high-power measurements, interleaved single-port transmit and multi-port receive measurements, and distributed pulse mode range architectures.
In this paper, an investigation was conducted to find materials that are optically transparent and radio frequency (RF) shielding. Materials were first optically tested, followed by a shielding effectiveness test. The optical test evaluated metal mesh sizes 20, 22, 40 and 100, single layer RF film, double layer RF film, and combinations of film and mesh. Size 22 copper mesh and RF film demonstrated desirable optical properties and were then RF tested from 26 MHz to 40 GHz. The test was conducted using a shielded enclosure featuring an aperture in a wall panel to mount the material under test. Reference field strength measurements of the aperture were compared to measurements taken when material samples were placed within the aperture in order to characterize the shielding effectiveness of each shielding material. Test results for size 22 copper mesh, RF shielding film, and a combination of one layer of size 22 copper mesh with one layer of film demonstrate average shielding effectiveness results of 43 dB for the mesh, 46 dB for the film, and 69 dB for the mesh and film together. This information can be used when there is a requirement for a material to provide optically transparent RF shielding.
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.
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, 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).
A microwave water-cut meter for production fluids applications was designed and a basic test performed. The meter uses a vector network analyzer to measure the reflection (S11) and transmission (S21) spectra of the material under test (MUT), such as production fluids, oil spills, rock cores or soil. The initial design concept consisted of a pair of waveguides whose ends face each other and are placed on the inner surface of the pipe/core holder. The waveguides have a diameter similar to the main pipe and are filled with specific low loss materials with dielectric constant similar to that of the fluid in the pipe. Based on the initial design, a refined water-cut meter design was optimized, via numerical simulations, built and tested. To maximize bandwidth, the improved design adds ridges to the original cylindrical waveguide and optimizes the feed details to maintain an impedance match to the feed connectors. Results show that the ridges in the waveguide significantly improve transmission compared to just the waveguide alone. Initial experimental results show the measuring system is sensitive to the water content of production fluids.
Along with the growth of communication and satellite industry, the importance of satellite antenna evaluation is increasing. Particularly Communication On The Move (COTM) terminal antenna, including the communication between new types of constellations on LEO and MEO, requires tracking accuracy test for the communication on moving vehicles. The conventional test facilities are locally fixed and lack flexibility. To make the antenna measurement more accessible, we are developing a methodology for in-situ measurement by introducing multiple Unmanned-Aerial-Vehicles (UAVs) system with RF payload. Thanks to the dynamic flexibility of UAVs, this system can flexibly change the test configuration on site and make new test scenarios available, such as emulating the orbit of non-GEO satellites during the measurement. However, one of the challenges of the proposed system is the additional uncertainties during the measurement due to the mobility of UAVs. To overcome this challenge, we design recursive stochastic filtering and fusion approaches, and evaluate their estimation performance via numerical simulations. By introducing stochastic filter and fusion algorithms, the effect of error is mitigated, and better accuracy can be achieved compared to an existing method. This project is performed in collaboration with Cranfield University in the UK and QuadSAT in Denmark.
Tapered chambers use the reflections from the surfaces adjacent to the range antenna to illuminate the quiet zone (QZ). Polyurethane substrate is the preferred and most widely used radio frequency (RF) absorber in these chambers, due to its ability to be cut into complex shapes to conform to the tapered sections. Unfortunately, this type of absorber always presents slight differences in permittivity related to the manufacturing process. To analyze the effects of the permittivity of the lossy foam on the QZ illumination in a tapered chamber, a series of numerical experiments using a full wave analysis technique are executed. The results are mainly obtained for frequencies under 1 GHz. The upper frequency of the simulation is limited by the electrical size of the problem and by the available information on the material permittivity. However, frequencies below 1 GHz is where the tapered chambers are superior to other methods for indoor antenna measurements. Magnitude and phase are recorded over a 1.82m diameter spherical QZ to show the effects of the different absorber on the illumination. Results show that a variation on the absorber around the range antenna will deviate the illumination and skew the amplitude taper across the QZ. The amplitude distribution peak can be shifted by as much as 3.5 degrees from boresight. The effect on the phase taper is smaller with a negligible change in phase.
This paper investigates on the postprocessing of spherical near-field measurements in phaseless scenarios. Traditionally, iterative algorithms have been used to propagate between two measurement surfaces to retrieve the near-field phase. In the last years, advanced phase retrieval techniques have been developed formulating the phaseless problem in matrix form. Both approaches are introduced and investigated, comparing its performance with numerical and measurement data. Preliminary results indicate that iterative propagation techniques offer superior performance, yet with a more irregular and nonlinear behavior. The matrix approach, however, offers much more flexibility on its formulation leaving room for more potential improvements.
We present the synthesis, prototyping and measurement of a transmitarray antenna for the generation of circularly-polarized (CP) orbital angular momentum (OAM) beams. A novel ”S-ring” transmitarray element is designed to sustain CP operation with only three metal patterned layers. The unit cell provides arbitrary CP phase compensation by merely changing the rotation angle of the element, while the transmission magnitude remains greater than 0.9. In previous work, transmitarrays that support circular polarization were designed based on the assumption of normal plane wave incidence; thus, the feed source of the transmitarray must be placed far from the transmitarray. In this work, an aperture phase synthesis methodology that accounts for the spherical phase of the feed is presented such that the feed can be placed near the aperture at about F=D = 1. A proof-of-concept prototype transmitarray antenna with a thickness of 0.3 cm operating at 19 GHz is constructed and measured for far-field performance. The measurements agree well with predictions obtained by full-wave simulations and demonstrate that the proposed transmitarray antenna can be a unique apparatus that generates OAM CP cone-shaped patterns.
A probe-compensated near-field-far-field (NF-FF) transformation with planar spiral scan, particularly suitable for flat antennas under test (AUTs), is proposed in this communication. It relies on the nonredundant sampling representations of electromagnetic fields and has been achieved by properly applying the unified theory of spiral scannings for nonvolumetric antennas, when such a kind of AUT is considered as enclosed in a dish with diameter equal to its maximum dimension, thus better shaping its geometry. An efficient two-dimensional optimal sampling interpolation (OSI) algorithm is then developed to recover the NF data required by the standard NF-FF transformation with plane-rectangular scan from those collected along the spiral. Since the number of NF data and spiral turns is related to the area of the modeling surface, the here proposed NF-FF transformation technique allows one to further reduce the measurement time with respect to those based on the modelings for quasi-planar AUTs, which instead involve, in such a case, a residual volumetric redundancy. Some numerical simulations, assessing the accuracy of the OSI algorithm and of the so developed NF-FF transformation, are shown.
We present a fast source reconstruction method suitable for antenna diagnostic applications of radiating structures on electrically large platforms. The method is based on a novel implementation of a recent reformulation of the inverse electromagnetic scattering problem, and is solved using a Higher Order Method of Moments (MoM) discretization. The novel implementation achieves asymptotically better scaling the previously possible, and in particular the memory use is substantially lower than was previously possible. Results from two example cases are presented where the new method is compared to the current commercial state-of-the-art solver in DIATOOL 1.1, and significant improvements are observed in terms of computation times and memory requirements.
The main goal of this work is to present a real-time system to evaluate the impact of multi-sines in Multiple Signal Classification (MUSIC) algorithm. The MUSIC algorithm is applied in several localization applications, where the accuracy of this algorithm is required. For this reason, a specific real-time system was developed to characterize the impact of multi-sine parameters in the MUSIC algorithm in order to improve system efficiency. From several experiments, the impact multi-signal number of tones and space between tones is analyzed.
Electrical properties of materials are requisite to analyze and design electromagnetic (EM) devices and systems. Free-space material measurement method, where the measurand is the free-space scattering parameters of an MUT (material under test) located at the middle of transmit (Tx)/receive (Rx) antennas, is suitable for non-destructively testing the MUT without prior machining and physical contact in high frequency ranges. This paper proposes a free-space two-tier one-port calibration method using three planar offset shorts with the respective offset of ???????? for the measurement of the full scattering parameters of a reciprocal planar MUT from two successive oneport calibrations. Measurement results of a glass plate of 4.775 mm thickness are shown in W-band (75-110 GHz).
We present a new method for measuring thin, polymer sheets using a slotted rectangular coaxial transmission line (RCoax). This method allows a sheet of material to be inserted into the R-Coax slot, greatly simplifying the measurement procedure over traditional waveguide methods. The permittivity inversion is performed with the aid of computational simulations of the RCoax conducted across a range of expected dielectric properties. In particular, the slotted R-Coax device was optimized to enhance signal strength but has no simple analytical solutions for inversion. This new measurement technique is demonstrated on several thicknesses of commercial polyethylene terephthalate (PET) films, with a maximum thickness of 10 mils (0.254 mm). Due to the coaxial geometry, this technique does not have an intrinsic lower frequency cutoff and has an upper frequency cutoff near 3 GHz from over-modeing within the transmission line, though this frequency range could be extended by shrinking the fixture. However, the signal strength and calibration stability limit the useful range of permittivity measurement to 0.5-3 GHz for 10 mil thick specimens (and a range of ~1 GHz-3 GHz for 0.5 mil thick specimens). Repeatability for the real part of the permittivity ranged between 2-5% and loss tangents of ~0.006 were measured. Thus, this paper demonstrates the R-Coax measurement technique as a potential QA tool for microwave frequency electrical properties of thin polymer films.