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This paper describes the Rectangular Coaxial 40’ long measurement system recently designed and installed at AEMI with the primary purpose of measuring the reflectivity of its high performance VHF/UHF absorbing materials in the frequency range 30 – 510 MHz. The basic principles of the system are described in detail in [1] and are based on S11 – measurements of absorbing material reflectivity by a Vector Network Analyzer (VNA). In order to improve the system productivity and measurement accuracy it was enhanced by the time-gating software option – the standard option of ORBIT/FR Spectrum 959 automated measurement software package [2].The measurement system performance was thoroughly evaluated and validated by a number of tests performed in the “empty” coaxial line, and in the line loaded by absorbing materials. The list of RF uncertainties – various measurement error sources - was generated, the main measurement error contributors were identified, the corresponding errors – estimated and the overall RSS measurement errors were calculated for the absorber reflectivity varying in the range of -30dB to – 40dB.
ABSTRACT A new probe compensated near-field – far-field transformation technique with planar spiral scanning is here proposed. It is tailored for quasi planar antennas, since an oblate ellipsoid instead of a sphere is considered as surface enclosing the antenna under test. Such an ellipsoidal modelling is quite general (containing the spherical one as particular case) and allows one to consider measurement planes at a distance smaller than one half the maximum source size, thus reducing the error related to the truncation of the scanning surface. Moreover, it reduces significantly the number of the needed near-field data when dealing with quasi planar antennas. Numerical tests are reported for demonstrating the accuracy of the far-field reconstruction process and its stability with respect to random errors affecting the data.
A novel approach using Artificial Neural network (ANN) is proposed to identify the number of faulty elements present in a uniform linear array consisting faults in multiple elements. The input to the neural network is amplitude of deviation pattern and output is the number of faulty elements. In this work, ANN is implemented with three different algorithms; Radial Basis Function neural network (RBF), Generalised Regression neural network (GRNN) and Probabilistic neural network and their performance is compared. The network is trained with some of the possible faulty deviation patterns and tested with various measurement errors. It is demonstrated that the method gives a success rate of 93.4%.
If an acquired RF field data set captures all the radiated energy, transformations will have minimal errors. However, it is sometimes impractical to capture the complete radiated field. In this case, some sort of edge treatment is required before transforming the data set. Usually, a function such as a cosine taper is added to the edge to minimize transformation errors. Unfortunately, these functions may be discontinuous to the measured data and its’ derivatives. This paper will present a method of truncation which matches the measured data and its’ derivatives. It will then transform the RF field data to the compact range reflector surface and compare the results of several truncation methods.
A new antenna diagnostics technique has been developed for the DTU-ESA Spherical Near-Field Antenna Test Facility at the Technical University of Denmark. The technique is based on the transformation of the Spherical Wave Expansion (SWE) of the radiated field, obtained from a spherical near-field measurement, to the Plane Wave Expansion (PWE), and it allows an accurate reconstruction of the field in the extreme near-field region of the antenna under test (AUT), including the aperture field. While the fundamental properties of the SWE-to-PWE transformation, as well as the influence of finite measurement accuracy, have been reported previously, we validate here the new antenna diagnostics technique through an experimental investigation of a commercially available offset reflector antenna, where a tilt of the feed and surface distortions are intentionally introduced. The effects of these errors will be detected in the antenna far-field pattern, and the accuracy and ability of the diagnostics technique to subsequently identify them will be investigated. Real measurement data will be employed for each test case.
Comparisons of the far-field results from two different ranges are a useful complement to the detailed 18 term uncertainty analysis procedure. Such comparisons can verify that the individual estimates of uncertainty for each range are reliable or indicate whether they are either too conservative or too optimistic. Such a comparison has recently been completed using planar and spherical near-field ranges at Nearfield Systems Inc. The test antenna was a mechanically and electrically stable slotted waveguide array with relatively low side lobes and cross polarization and a gain of approximately 35 dBi. The accuracies of both ranges were improved by testing for, and where appropriate, applying small corrections to the measured data for some of the individual 18 terms. The corrections reduce, but do not eliminate the errors for the selected terms and do not change the basic near-to-far field transformations or probe correction processes. The corrections considered were for bias error leakage, multiple reflections, rotary joint variations and spherical range alignment. Room scattering for the spherical measurements was evaluated using the MARS processing developed by NSI. The final results showed a peak equivalent error signal level in the side lobe region of approximately -60 dB for both main and cross component patterns for angles of up to 80 degrees off-axis.
Now-a-days, far-field ranges are being used to measure antenna radiation patterns. Two main types of ranges used are used for these measurements: direct and indirect illumination. In either case, the accuracy of the measurement is dependent upon the quality of the range quiet-zone fields. In direct illumination, phase and amplitude taper cause discrepancies in the fields. For indirect illumination, only amplitude taper must be accounted for. Additionally, stray signals and cross-polarization will further distort the quiet-zone fields and lead to measurement errors. This new methodology starts with the measured antenna data and a priori knowledge of the incident fields and estimates an Effective Aperture Distribution (EAD). The EAD compensates for these sources of error and can be used to predict the far-field radiation pattern of the antenna under test. Analytical results are presented for taper and stray signal analysis.
Adaptive antenna has both the amplitude and phase (as weights) which can be adapted optimally to get required multi path arrival estimation or directed beam forming. We had earlier tried to find out errors in adaptive arrays (ULA) and further try to investigate mutual coupling effect in closely spaced antenna elements in rectangular / planar arrangement. It is always desired to place antenna elements closer in order to reduce grating lobes when the main lobe is electrically tilted. In real life when an adaptive array is subjected to multi path and mutual coupling it is necessary to counteract with suitable modeling so as to make it usable for wireless communication. We attempt to study / investigate the mechanism for mutual coupling between antenna elements. In adaptive antenna arrays, mutual coupling can deteriorate the algorithms which try to deal with the direction of arrival (DOA) and beam forming. There is also a need to reduce the size of the antenna aperture and element itself, without degrading the performance and bandwidth of the element. We have simulated in Matlab our planar adaptive array algorithm which mitigates errors and reduces effects of mutual coupling. It was found that Tschebyscheff polynomial distribution was one of the optimum arrangements for antenna synthesis. When aperture length has to be fixed and new antenna elements are introduced we try to find way to deal with this by spacing nulls on unit circle according to Tschebyscheff pattern. We also try to touch issues in implementing the array on FPGA. Key words: ULA, DBF, Tschebyscheff, FPGA.
This paper discusses design aspects related to a tiltable lightweight near-field scanning system for use at sub-millimeter frequencies. It addresses design issues as they relate to accuracy and scanner distortions from multiple causes. Calibration methods to measure and correct for anticipated and unanticipated errors are briefly addressed. Actual test results are presented. The tiltable scanner being discussed was designed for the Atacama Large Millimeter/submillimeter Array (ALMA) [1] and is being used by the National Radio Astronomy Observatory (NRAO) [2]. It has many other applications by virtue of its light weight (approx. 120 lbs) and ability to be oriented at different angles. These include flight-line testing and other in-situ antenna test applications.
This paper will present techniques used to simulate semi-hemispherical spiral antennas with measured VSWR and antenna pattern data for performance verification. Previous work on semi-hemispherical spiral antennas has been done by Lobkova, Protsenko, and Molchanov [1]. GTRI researchers have built on this work by developing a MATLAB computer model to create a general semi-hemispherical spiral antenna pattern model. Parameters that can be adjusted include the radius of the sphere, the number of turns of the spiral, the creation of a 1-arm or 2-arm spiral, and the inclusion of dielectric material between the spiral and ground plane. In creating the MATLAB computer model, GTRI researchers found errors in the notation of the elliptical integral in [1] and added additional details for the calculation of the antenna pattern. The paper will then present the characterization of a specific example of a semi-hemispherical spiral antenna. First, the VSWR of a single antenna was measured using a standard HP8510 Network Analyzer setup. Next, antenna pattern data was measured for a single spiral antenna and a pair of spiral antennas on both the GTRI planar near-field range and the GTRI anechoic chamber. The paper will conclude with the presentation of the modeled and measured antenna pattern data for the single antenna case.
We use a rotating dihedral to determine the cross-polarization ratios of radar cross section measurement systems. Even a small amplitude drift can severely degrade the calibration accuracy, since the calibration relies on accurate determination of polarimetric data over a large dynamic range. We show analytically how drift introduces errors into the system parameters, and outline an analytic procedure to minimize the in.uence of drift to estimate system parameters with greater accuracy. We show that only very limited information about the drift is needed to provide measured system parameters accurate to second order in the error-free parameters. Higher-order accuracies can be achieved by using more detailed information about the drift. We use simulations to explain and illustrate the analytic development of this theory. We also show that, using cross-polarimetric measurements on a cylinder, we can recover the exact system parameters. These .ndings show that we can now calibrate polarimetric radar cross section systems without the large uncertainties that can be introduced by drift.
This paper discusses the process of optimization of a spherical near-field range for measurement of large UHF antennas used in space applications. Results of a study undertaken to understand and optimize range performance in presence of multi-path errors and mutual coupling are presented. Data is presented showing variation in measured patterns of a generic UHF antenna as a function various parameters such as a) use of probes of different gains, b) separation distance between the probe and the antenna and c) absorber rearrangement. Use and effectiveness of software post processing approaches such as spherical mode filtering, time domain gating and use of proprietary algorithms (e.g. “MARS processing” developed by NSI Inc.) is illustrated. Practical implementation of these approaches and corresponding impact on data density, test duration and computational effort are also discussed.
A large rolled edge compact range system featuring a 12’H x 16’W quiet zone has been designed, fabricated, installed, and tested in a large aerospace test facility. During the program, a high precision alignment methodology was utilized in conjunction with electromagnetic prediction capability to verify both mechanical and electrical performance while still under trial assembly conditions at the factory. A coherent laser radar (CLR) was utilized to measure the reflector surface on a very fine grid, and the electromagnetic (EM) quiet zone performance was calculated from the raw CLR data using a Physical Optics (PO) model. Despite extremely high surface accuracy of the panels, this evaluation methodology highlighted systematic alignment errors in the reflector system, and guided the process of correcting these errors to achieve a final factory verification assembly for the entire 20’H x 24’W reflector system of better than 0.001” over the quiet zone section of the reflector, and 0.004” rms over the entire reflector. This procedure was also utilized for the on-site installation to achieve alignment of the reflector to an AUT positioning system using the CLR, as the positioning system and chamber were already existing and operational. Thus, it was required to align the reflector to the positioning system, and not the positioning system to the reflector as is usually the case. A unique vertical carousel feed system was also aligned using this procedure. Predicted EM results were again used to finalize alignment on site prior to quiet zone field probe evaluation. This paper summarizes the overall alignment and EM evaluation process, and presents results for the installed compact range reflector system.
A probe station based antenna measurement setup is presented. The setup allows for measurement of complex impedance and radiation patterns of an on-wafer planar antenna, henceforth referred to as the device under test (DUT), radiating at broadside and fed by a coplanar waveguide (CPW). The setup eliminates the need for wafer dicing and custom-built test fixtures with coaxial connectors or waveguide flanges by contacting the DUT with a coplanar RF probe. In addition, the DUT is probed exactly where it will be connected to a transceiver IC later on, such that no de-embedding of the measured data is required. The primary sources of measurement errors are related to calibration, insufficient dynamic range (DR), misalignment, scattering from nearby objects and vibrations. The performance of the setup will be demonstrated through measurement of an on-wafer electrically short slot antenna (.0/35 × .0/35, 5 mm2) radiating at 2.45 GHz.
An indoor localization and communication project is described that proposes to use RFID (radio-frequency identification) tags, placed in the building beforehand, as navigation waypoints for an inertial navigation system carried by a first responder. RFID devices commonly are attached to persons or to moveable objects so that the objects can be tracked by using fixed readers (special-purpose radio receivers) at different locations. In this project, we explore the “flip side” of this practice. Our concept is that detection of RFID devices in known, fixed locations by a moving reader provides a precise indication of location for tracking the person or moving object that is carrying the reader. This information can then be used to correct for any errors of an inertial tracking system.
Adaptive antenna has both the amplitude as well as phase (as weights) can be adapted optimally to get required Direction of Arrival (DOA) estimation or directed beam forming. This paper tries to analyze state of the art criteria for Adaptive antenna, suppressing the interference in directions other than desired. We model the Uniform Linear array (ULA) based on simulations of various adaptive and non-adaptive algorithms. We list possible types of errors in brief. Element spacing and mutual coupling influence each other and affect the antenna element pattern. We formulate the array antenna that tries to reduce the error by optimally adjusting the weights. We make an attempt to model mutual coupling. A high precision array antenna can be designed keeping in mind error factors, optimum adjustment of the element interval and mutual coupling. An adaptive antenna optimal weight adjustment is discussed here. Key words: ULA, DOA, DBF.
The potential transmit power, and hence dynamic range of monostatic millimeter wave RCS measurements may be limited by the feed coupling of the antenna. Time domain gating can be used to reduce the measurement errors caused by this signal, as well as other undesired signals from scattering sources in the range, but does not protect the receiver from compression. Hardware gating can allow increases in transmit power by protecting the receiver from the effects of the feed coupling return. Unfortunately, equipment capable of hardware gating at millimeter wave frequencies is difficult to obtain. In addition, the usefulness of hardware gating is limited by the duty cycle loss in the measured signal. We describe a practical system using gating of the low frequency intermediate frequency (IF) signal in the receiver and a microwave pulse modulator prior to the millimeter wave multiplier in a mono-static millimeter wave RCS measurement system. We also describe methods to minimize the loss of measurement dynamic range due to duty cycle losses in this system. We demonstrate the use of this system for RCS measurements of simple targets, and compare the results with those obtained using software gating alone.
Abstract. We present new RCS measurements from an 8-foot square flat plate for frequencies from 0.15 to 5.5 GHz. Guided by the theory, we study the peak RCS at normal incidence, the principal plane pattern, and the 3-dB beam-width in detail. The broadside echo from the plate is found to be extremely narrow at higher frequencies. From the errors, we estimate that the wave-field experienced by the plate is reasonably uniform to within +0.3 dB, over a wide dynamic range of 60 dB.
This paper presents results of planar near-field measurements at 16, 35 and 94 GHz using probe position correction algorithms. The algorithms correct for position errors of the probe near the scan plane. The probe’s actual position is measured using a laser tracker integrated into the planar-near-field scanning system at NIST. The laser tracker simultaneously obtains probe-position information at each point where amplitude and phase data are acquired during planar near-field antenna measurements.
A method is presented to measure the antenna pattern of an AUT where the antenna port is inaccessible. That means that it is not possible to connect a test cable, nor can the termination be changed physically. In some cases there is no test port at all. The only variation possible is to change the input impedance of the first receiver or LNA by switching it on and off. An RCS-technique can be used to retrieve the radiation pattern. By experimental comparison between the conventional pattern measurement technique and the RCS-technique it is shown that pattern determination via RCS-measurements is feasible. In addition, the measurement method offers the advantage of directly reducing the influence of systematic measurement errors. On the other hand, the penalty is put on power efficiency and a subsequent limited dynamic range.