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The IsoFilterTM technique was originally demonstrated to operate by rejecting secondary signals that derive from reflections off of a nearby metallic object – namely, the ground plane surface supporting a small pyramidal horn.[1,2] The aperture of the horn was located several wavelengths above the ground plane and the sidelobes and backlobes of the horn illuminated the ground plane itself. The success of this demonstration has been sufficient to encourage us to pursue further the question of how well the IsoFilterTM technique will work to suppress other types of secondary signals– such as signals coming from other elements of an array antenna or another individual first-order primary radiator nearby. Here we report on some of the results of that investigation. We have calculated the far-field patterns of a sparsely populated array and applied the IsoFilterTM technique. The goodness of the suppression is judged by how well the “IsoFiltered” result agrees with the calculated pattern of the individual radiator.
A near-field measurement technique for the prediction of asymptotic far-field antenna patterns from data obtained from a modified cylindrical, or plane-polar, near-field measurement system is presented. This technique utilises a simple change in facility alignment to enable near-field data to be taken over the surface of a conceptual right cone [1, 2], or right conic frustum [3, 4] thereby allowing existing facilities to characterise wide-angle antenna performance in situations where hitherto they would perhaps have been limited by truncation. This paper aims to introduce the measurement technique, describe the novel probe-corrected near-field to far-field transform algorithm which is based upon a cylindrical mode expansion of the measured fields before presenting preliminary results of both computational electromagnetic simulations and actual range measurements. As this paper recounts the progress of ongoing research, it concludes with a discussion of the remaining outstanding issues and presents an overview of the planned future work.
In this work an experimental validation of the nearfield – far-field transformation technique with helicoidal scanning tailored for elongated antennas is provided. Such a transformation relies on the theoretical results relevant to the nonredundant sampling representations of the electromagnetic fields and makes use of an optimal sampling interpolation algorithm, which allows the reconstruction of the near-field data needed by the near-field – far-field transformation with cylindrical scan. In such a case, a prolate ellipsoid is employed to model an elongated antenna, instead of the sphere adopted in the previous approach. It is so possible to consider measurement cylinders with a diameter smaller than the source height, thus reducing the error related to the truncation of the scanning surface. The comparison of the reconstructions obtained from the data directly measured on the classical cylindrical grid with those recovered from the nonredundant measurements on the helix assesses the validity of this innovative scanning technique.
The Phaseless techniques have gained considerable attention during the past two decades in the antenna measurements community. The removal of the phase measurements has some immediate advantages over the common vectorial measurements. They are cost effective, well-adapted for higher frequencies and insensitive to phase instabilities. The phaseless techniques have been discussed in the antenna measurements community and the theories behind these techniques are well explained in the literature. Unfortunately the issue of the noise and the presence of measurement errors are not investigated in details to provide strong impetus to the importance of phaseless measurements. In this paper the near field of a number of different types of antennas with high, medium and low side lobes is simulated to create as realistic case as possible. The effects of the probe positioning errors are investigated by injecting random errors in the position of the probe samples along x-, y- and z-axis. It is also illustrated how the positioning errors can distort the phase distributions. Through detailed characterizations of the constructed far field patterns, robustness of the Iterative Fourier technique even at the presence of very high probe positioning errors is demonstrated. It is shown how the utilization of the phaseless techniques will significantly reduce the probe positioning error effects when it is compared to the commonly used amplitude and phase near field measurement techniques.
In this paper a technique is described that allows for the determination and correction of probe translation during polarization rotation in planar near-field measurements. The technique, which relies on the independent translation of coordinate systems for the two orthogonally polarized data sets, has significance for mm-wave testing, where bulky RF components makes probe alignment difficult. Measured data is presented to demonstrate the success of the technique.
Most traditional antenna measurement techniques presume that the antenna under test (AUT) is accurately aligned to the mechanical axes of the test range. Sometimes, however, it is not possible to achieve such a careful antenna alignment [1]. In these cases, standard post processing techniques can be used to accurately correct antenna-to-range misalignment. Alternatively, similar results may be obtained by approximation in the form of piecewise polynomial interpolation. When carefully employed, this method will result in only a small increase in uncertainty, but with a significant reduction in computational effort. This paper describes this far-field alignment correction method, which is closely related to standard active alignment correction methods [2]. This paper then proceeds to use numerical simulation as well as actual range measurements to demonstrate the effectiveness of this method. Finally, the utility of this technique in the presentation of far-field antenna pattern functions is illustrated.
In this paper, it is shown that using the phase center as distance reference point is equivalent to applying the proximity correction in determining gain of antennas with a SGH at finite distances. Clear guidelines for calculating the phase center location of a SGH, for application to antenna gain determination, are presented and explained. The phase pattern necessary for the calculation can be obtained either from measurement data or from computer simulations of the SGH. The region of validity of this approach is outlined and the residual error is quantified.
Active phased array antennas are often considered for many applications in radar and communications, particularly in millimeter wavelengths. The ability of active phased array antennas to be reconfigured with different beam shapes and pointing directions makes them attractive to increase the flexibility of the next generation of communications satellites so that they can adapt to the needs of a fast changing communications market. The antenna noise temperature of an active array is an important performance parameter, which is difficult to measure compared to a classical passive antenna. Moreover, for a satellite antenna, which has to be evaluated in an anechoic chamber before integration on the spacecraft, the ability to characterise the noise contribution of the antenna itself independent of the environment noise would be very interesting as it would allow better prediction of the antenna performance when it is deployed in orbit on the spacecraft. The paper describes the results of a study undertaken for the French Space Agency (CNES) to devise a new method for the measurement of the noise temperature of a Ka band active phased array antenna when mounted in a Compact Antenna Test Chamber (CATR). An important objective of the study was to find a method which did not rely on the substitution of the antenna under test with a reference antenna, which is the method often used in practice. The method of measurement of noise was based on digital processing of signal to noise ratio rather than analogue detection of noise level, which improves the measurement precision.
This paper describes joint studies of Wroclaw University of Technology and Denmark Technical University on optimizing placement and performance of low-profile antennas on small satellite, such as ESMO Moon orbiter. After comprehensive electromagnetic studies with use of numerical analysis, a spacecraft mockup modeling its conductive surfaces was developed. Two to four antennas were mounted and several placement configurations were investigated. For verification purpose of numerical analysis and formulating design guidelines to an actual Moon probe, precise measurements of combined radiation pattern were performed at the Near-Field Antenna Test Facility, Denmark Technical University.
Lockheed Martin MS2 has a long history of utilizing antenna ranges for calibration, test and characterization of the phased array antennas. Each range contains an integrated RF receiver subsystem for performing antenna measurements, typically on the full array. For solid-state phased array testing, what is often needed, however, is a test station capable of performing complex S-parameter measurements on a subarray or subset of the full antenna system without incurring the expense of a test chamber. To address this requirement, Lockheed Martin, working with Nearfield Systems, has developed a portable standalone RF measurement system. The standalone system consists of an Agilent PNA, automated transmit/receive unit (TRU) and a waveform generation (WFG) subsystem for interfacing to the phased array beam-steering computer. This paper will discuss the capabilities of the Standalone RF System including the TRU and WFG subsystems. The TRU is used to tailor the RF signal by automated switching of amplifiers and programmable step attenuators for various test scenarios. The WFG is an automated pattern generator used to present many digital waveforms in arbitrary sequences to the phased array beam steering computer. The design features of the standalone RF system will be presented along with the COTS hardware utilized in assembling the station.
Ultra Wide Band systems show promises in high data rate radio communications. However, these systems interfere with other communication protocols like the WLAN. To solve this problem, current works propose the insertion of thin half wavelength slots in the antenna structures, thus creating filtering antennas. Many criteria were used to characterize the filtering of these antennas, like the measurement of the VSWR, the return loss, or the maximum gain of the antenna versus frequency. In this paper, we show by studying the 3D gain pattern of these antennas that their filtering is not just dependent on the frequency but it also depends on the radiation direction, due to the radiation of the slots. Then we propose efficiency measurement as the best way to quantify the filtering of these omnidirectional antennas independently of the radiation direction. Two filtering antennas were characterized using a modified wideband Wheeler Cap efficiency measurement method.
Full-wave modeling and far-field measurements are utilized to study the effects of a microstrip feeder on the band rejection and the far-field purity of planar log-periodic antennas. Three different configurations are investigated. Specifically, band rejection by relevant teeth removal (near/far-field), integration of the band-stop filter (near-field only), and the combination of the two are studied. Far-field contamination effects due to a microstrip feed line, and coupling to the antenna radiator, are evaluated for both radiating and band rejection regions. Important guidelines regarding the position and distancing of the feed to the radiator, as well as the trade-offs between substrate and superstrate configurations are derived. Antennas are developed to have a VSWR better than 2.5:1 in the 2-4 GHz and 7-11 GHz bands, and band rejection centered at 6 GHz. It is clearly shown that log-periodic antennas can be readily designed to have arbitrary, even reconfigurable, band rejection regions where overall realized gain is notched for more than 20 dB. A computer aided analysis was performed using commercial finite element and method of moments software tools. The measurements were conducted at Lockheed Martin in Denver, Colorado.
Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) are the two metrics most commonly used to characterize the over the air (OTA) performance of a wireless device. The minimum range length for these measurements has usually been determined using the arbitrary far-field criteria of R>2D2/?. This paper quantifies the changes in measured TRP as the range length is increased from D2/? to infinity (or thereabouts). TRP measurements on a UMTS dipole combined with a phantom head have been made at different range lengths. Additionally, numerical simulations of TRP measurements on an array of point sources have been made at different range lengths. The result is a theoretical determination of TRP measurement errors versus range length supported by actual measurement results.
This measurement paper presents the methodology used in the characterization of a short, broadband, High Frequency (HF) mobile antenna operating in the frequency range from 1.8 to 30 MHz. The antenna patterns were measured while the antenna was mounted on a High Mobility Multipurpose Wheeled Vehicle (HMMWV). Pattern and performance predictions were made using the Numeric Electromagnetic Code (NEC) Pro 2 Version 5 before the field tests. Stimulation of the AUT was done with the use of a large magnetic loop to minimize AUT pattern perturbations. An attempt was made to measure gain profiles by comparisons to a monopole, ground based, resonating at the center of HF band (15 MHz). Measurement results provided the performance of the antenna in a mounted profile and the pattern distortions produced by the host vehicle.
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.
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.
A recent project to develop and optimize the RF reflectivity of a Composite/nano-material, X-band reflector was pursued by a team lead at AFRL – RF Technology Branch. This was accomplished by iterative testing and signal pattern measurements performed at the AFRL-Rome Laboratory that provided critical feedback affecting the laminate design and configuration of the composite reflector. Testing of multiple configurations of composite reflectors provided data that was key to successful progression throughout the product engineering cycle, and accomplished the following milestones: . Initial performance verification . Characterization of influential material constituents . Optimization of design Eclipse Composites Engineering, along with metallic nano-material supplier Metal Matrix Composites worked to develop a composite dish with reflective properties identical to the baseline metallic reflector model. Through various methods of testing, the reflective influence of each material constituent was characterized and the relative effect within the overall composite laminate was modeled. Based on initial measurements, prototype articles were fabricated for testing and comparative evaluation. This project demonstrates the critical feedback loop between measurement/testing and design/development leading to the successful production of a segmented composite reflector that is lighter weight, more durable, with increased performance in the field to support today’s military and commercial operations.
Decomposing a signal of unknown polarization into combinations of linear and circular components can be very confusing, especially when one wants to relate them to a known phase and amplitude reference. Many publications have addressed parts of this issue, some employing the square root of two and some not. There does not seem to be a substantial consensus on this is-sue, experts being somewhat evenly divided. The prob-lem relates to both mathematical analysis and meas-urement analysis, which must be in agreement when comparing measurement to theory. This paper presents an abbreviated mathematical analysis involving depolarization of a wave incident upon a radome, yielding the magnitude and phase of both resultant circular components. The result is com-pared to well-established published formulas. However, derivations of the same components from measure-ments may reach a different conclusion depending on the procedure used. The difference is a factor of the square root of two. These two conflicting results are compared and a resolution proposed.
Highlights of work at the Naval Research Laboratory in evanescent near-field electromagnetic holography (ENEH) will be presented. This work grew out of extensive experimental work in near-field acoustical holography at our laboratory that has been recognized formally by the Laboratory as one of the 75 most innovative technologies over the past 75 years. This new electromagnetic approach differs from the usual nearfield imaging in that it provides much better than halfwavelength resolution due to the inclusion of evanescent waves. Furthermore ”imaging” to a source surface provides a reconstruction of the surface currents, Poynting vector as well as the E and H field vectors. These quantities are derived from two measured holograms (phase and amplitude) of two polarizations of the electric and/or magnetic fields over a 2-D surface (the hologram). Experimental work in both low (100 Hz) and high frequencies (10GHz) is of interest, although we present here results of the latter along with the theory. Two approaches will be discussed for backtracking the measured fields: one that uses wave function expansions in plane, cylindrical or spherical geometries, highlighting the cylindrical geometry in this paper, and a second more general formulation that uses the field expanded using an array of equivalent dipole sources especially useful in arbitrary geometries. Both approaches represent inverse methods and are ill-posed and require regularization to stabilize the reconstructions. We hope that these methods will provide high resolution new diagnostic tools for antenna analysis, as well as diagnostics for applications in EMC and EMI among others. Currently we are seeking partnership with other laboratories and universities to direct this technology towards problems that could benefit from its unique diagnostic capabilities. Work supported by the Office of Naval Research.
We propose a novel method for measuring the total radiated power (TRP) of small radio terminals, such as mobile phones, active RFID tags, and UWB devices, using a spheroidal cavity coupler in which an EUT and a receiving antenna are displaced symmetrically around the focal points of the spheroid. The proposed method provides a compact, low-cost TRP measurement system with high sensitivity and high speed, supporting TRP measurement for both in-band signals and higher-order spurious radiation. Although the behavior of electromagnetic waves in the coupler is complex due to multiple reflections, we can evaluate the maximum TRP from the EUT by using the displacement technique and comparison with a reference system using a reference transmitting antenna and signal generator. Computer simulation verifies that the method measures TRP with high accuracy.