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As an RF cable is moved during data acquisition, its insertion loss will often change [1-3]. Techniques have been published [1-3] that measure and compensate those changes in insertion loss. Each of these techniques, however, requires stable access to both the signal source and the receiver at one end of the cable bundle. This requirement poses a challenge when trying to compensate a moving RF cable between a receive antenna and a mixer where there are additional axes below the mixer. This paper will show measured results of a new technique developed by MI Technologies to do similar compensation where the source and receiver are at opposite ends of the moving (or otherwise changing) cable bundle. The technique was developed for transmission efficiency measurements on radomes, but also has applicability for quiet-zone field probing or any other scenario where a strong signal is always being received. It requires the use of multiple identical RF cables in the cable bundle, and measures multiple cable combinations to determine the cable characteristics.
There is interest in the propagation of EM signals inside jet engine turbines for a number of reasons. Applications include radar scattering phenomenology and jet engine plasma plume formation studies. In our research, we are interested in the communication channel characteristics for micro-size wireless sensors attached to the turbine blades that measure parameters such as strain and temperature. Propagation measurements were performed on both F-16 (F-110) and Boeing 747 (CF6-50) turbines. The frequency band extended from 2 to 20 GHz (wavelengths longer than the turbine blades to wavelengths shorter than the gap between turbine blades). Signals were propagated with both radial and circumferential polarization. Both transmission and scattering measurements were made from both the inlet and the outlet. We also used small probe antennas inserted in boreholes between turbine stages. A range of blade positions were included. We will show the propagation characteristics as a function of polarization, frequency and time (UWB time domain transformations). We will also show the internal radar reflection characteristics of the turbine as a function of various stator blade rotation angles. Comparisons with a hybrid mathematical propagation model will be given.
Double-ridged waveguide horns can provide better than 10:1 relative frequency bandwidth over which they exhibit excellent impedance match and power transfer characteristics. However, the radiation pattern of such an antenna generally becomes more complex at the high end of its operating frequency range. That is, the pattern degenerates from being predominantly single-lobed at lower frequencies to a more complicated pattern exhibiting four gain maxima around the principal axis, all of which are greater than the gain on the principal axis. Here, we present some numerical simulations that appear to indicate that this behavior might not be directly related to higher order modes in the feed region and is not due to manufacturing imperfections, but rather is simply due to the overall taper of the horn itself.
This paper presents a generalized nonlinear interpolation technique for generating 3D antenna radiation patterns from 2D cross sections. The motivation for this work is that most of the patterns provided by antenna manufacturers are only available as vertical and horizontal cross sections. Accurate propagation calculations, however, require gain values at arbitrary orientations, corresponding to points on a 3D gain surface. After reviewing the current methods of generating such a gain surface, we find that linear interpolation algorithms seem the most promising, even though they can often lead to pronounced mathematical artifacts. To overcome these shortcomings a new nonlinear algorithm is proposed. The new approach mitigates, and in most cases eliminates, the artifacts produced by linear interpolation weights. The new method is fast, yields smooth, more realistic surfaces that are consistent with the vertical and horizontal cuts, exhibits diminished mathematical artifacts, and improves the accuracy of propagation calculations of radio frequency signals. Representative examples from the application of the new algorithm to cellular base station antenna patterns will be presented.
A generalized method to diagnose a defective element of an antenna array using neural networks is presented. A defective element with no excitation is classified as on off faults (i.e., total failure) and with current variation from designed values are current magnitude and phase faults. A uniform linear array of 101 isotropic elements with half wave distance between them and 1 amp current excitation is considered. Complex deviation pattern is determined which is the difference between the measured radiation pattern of the array under normal condition and degraded radiation pattern of the array with any one defective element. One radial basis function neural network is trained with all possible angle values of deviation pattern to determine the number of the faulty element. Other radial basis function neural network is trained with all possible absolute value of deviation pattern to determine current in defective element. The trained network showed high success rate. Key words:-Artificial neural networks, Phased array, Radial basis function (RBF), Radiation pattern
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.
This paper presents the design, development and testing of an inverted Stewart platform for suspending and positioning targets during RF antenna and signature testing. Previous string target support systems use multiple string attachment point configurations that do not allow the target roll or pitch to be modified during the azimuthal data collection. This presentation will discuss an in-house development of a scale model target support system that allows for high accuracy simultaneous target roll and pitch positioning. The inverted Stewart platform also offers unique stability of the target by damping out the torsional pendulum motion typically encountered in conventional string support systems. In this paper we will also discuss the advantages and disadvantages of the string support concepts and provide design guidance for a building an inverted Stewart platform support system. If possible, a simple squat calibration standard will be measured to assess the quality and precision of this novel support system.
The R&D testing of antennae today is still an important challenge for many universities. They find it difficult to instrument their antenna labs with equipment that allows the flexibility of re-configuring their test science for their various AUT configurations. Antenna test facilities at educational institutions are typically used sporadically and for a high mix of different antenna types with frequencies ranging up to millimeter wave. Unlike their industry counterparts that build and instrument a production antenna test facility geared to the specifications of the antenna under test. The challenge lies in configuring an antenna test facility to operate within these wide boundaries at a reasonable cost. A flexible RF Sub-System will be discussed that utilizes the Agilent PNA series vector network analyzer and harmonic mixers as the receiver, and a remote PSG series source and multipliers as the stimulus. This paper will examine the steps undertaken to define the requirements necessary to upgrade the existing antenna test facility at the University of Toronto in Toronto Canada. It will also include design considerations necessary to create a power budget in order to estimate the dynamic range of the test system. This paper will also delve into the aspect of selecting and exploring the benefits of the test software requirements.
The increasing numbers of microwave instruments and devices that include IEEE 802.3 interfaces are influencing range design and capabilities, as well as the ability to remotely locate GPIB instruments. The major benefits of LAN based instrumentation systems are increased flexibility in instrument location and increased capabilities over long distances compared to GPIB based ranges. This paper discusses the relative merits of LAN based microwave test instrumentation ranges. Several example range designs are included that demonstrate how LAN based instrumentation can increase range flexibility and reduce costs in range implementation.
NSI has developed a novel technique for automating antenna range configurations. Although automation has shown to dramatically improve range productivity, most of today’s antenna ranges are reconfigured manually. Today’s automated ranges use electromechanical RF switches to control the RF signal path, which is contained primarily in a central rack, thereby limiting automation to ranges that are relatively small in size. Larger ranges, however, tend to locate many of the RF components such as mixers, couplers, amplifiers and multipliers remotely near the probe or AUT, sometimes 100 ft (30 m) or more from the rack, making the remote RF components more difficult to access and control. To address this problem, NSI has developed the Range Transition Manager (RTM) for automating large antenna ranges. The RTM uses modular packaging with a LAN interface and embedded processor to provide commonality and flexibility in automating various range sizes and types. The RTM family of modules provide a full range of automation capability for 0.5 to 18 GHz and higher frequencies. This paper will describe the capabilities of the Range Transition Manager developed for a large near-field scanner and describe how the RTM improves overall range productivity.
Spherical near-field measurements have become a common way to assess performance of a wide variety of antennas. Published reports on range error assessments for spherical near-field ranges however are not very common. This is likely due to the perceived additional complexity of the spherical near-field measurement process as compared to planar or cylindrical measurement techniques. This paper will establish and demonstrate a simple procedure for characterizing the performance of a spherical near-field range. The measurement steps and reporting can be largely automated with careful attention to the test process. We will summarize the process and document the accuracy of a spherical near-field test range at NSI using the same NIST 18 terms commonly used for planar near-field measurements.
The purpose of this paper is to detail the process used to optimize the low frequency performance of 72 inch absorber. The loading optimization was required to provide enhanced performance of a twisted 72 inch absorber which was to be used in the building of a large aircraft test facility. The chamber performance requirements are over a frequency range of 30 MHz to 18 GHz. The chamber dimensions are 30 meters x 30 meters x 20 meters high. This chamber will be used to measure a variety of fighter aircraft for many EW scenarios. The mission of this facility is to “perform radiated immunity testing of aerospace vehicles with high electromagnetic field intensity, radiated emissions measurements, EMC testing, electronic warfare testing, antenna pattern testing”. Due to the broad frequency range and the fact that the chamber is desired to test both in the low frequency EMC domain and high frequency antenna measurements, an extremely broad band absorber material had to be developed and optimized. The use of ferrite hybrids was considered. Due to the roll off at microwave frequencies and the expense of such a high volume of materials, they were eliminated for cost and due to the limited performance in the 1-2 GHz frequency range. The ideal candidate is a 72 inch twisted pyramidal geometry. The standard loading of these materials is ideal for frequencies above 150 MHz.. The performance level in the 30 MHz to 150 MHz range is less than ideal. A design for the chamber was established with specific target performances required of the 72 inch absorbers. This paper describes the effort taken to optimize the loss properties of the dielectric foam to meet the target absorber performance required for the implementation of the design. Key Words: Absorber Measurements, Absorber Performance, Computer Modeling of Absorbers, Dielectric Properties of Absorber
The measurement of the radiation patterns of the PLANCK Radio Frequency Qualification Model (RFQM) is one of the most important elements of the verification of the PLANCK telescope. PLANCK is one of the scientific missions of the European Space Agency and is devoted to observe the Cosmic Microwave Background radiation, with unprecedented accuracy. The satellite payload consists of two state-of-the-art, cryogenically cooled instruments sharing a dual reflector telescope with 1.5 m aperture and covering the frequency range from 27 GHz to 1000 GHz. As a key part of the telescope verification logic, the radiation patterns of the RFQM has been measured in the Alcatel Alenia Space Compact Antenna Test Range (CATR) at four frequencies (30, 70, 100 and 320 GHz) using representative flight feed horns of the focal plane unit. This paper presents the test logic, the measured radiation patterns, the custom-made instrumentation set-up, the correction techniques used and the final link to the Flight Model verification.
This paper describes the new NIST tabletop millimeter-wave extrapolation range that will provide on-axis gain services up to 110 GHz. A discussion of the extrapolation measurement method, as presented at the Antenna Measurements Techniques Association (AMTA) in 1999, is the basis for much this paper. The extrapolation method for determining gain of directive antennas at quasi-near-field distances is based on a generalized three-antenna approach. It has been used at NIST for more than twenty years to calibrate antenna gain standards up to 20 GHz to within 0.1 dB and up to 50 GHz to within 0.15 dB. The basic theory, description of the measurement system, data acquisition procedure, and measurement results for three antennas at 94 GHz will be presented.
Adaptive array based antenna pattern comparison technique is presented in this paper. In the method, the antenna pattern of the antenna under test (AUT) is measured several times at different positions in the quiet-zone. The corrected antenna pattern is obtained by taking a weighted average of the measured patterns. An array synthesis algorithm is used to obtain averaging weights at the different rotation angles of the AUT. In addition, the weights are adapted specifically for the AUT. The adaptive array correction technique is demonstrated in a hologram based compact antenna test range (CATR) at 310 GHz. The demonstration is based partly on the measurements and partly on the simulations. For verification, the accuracy provided by the method is compared to the accuracy provided by the uniform weighting.
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.
Hand-made scale models in antenna measurements have been used since the late 1940s. Today, aircraft models are fabricated using a stereolithography (SLA) process and the Computer Aid Design (CAD) for manufacturing the full size aircraft. This is the fabrication method used for the V-22 1/15th scale model. Once the SLA machine is programmed, these models are very inexpensive to produce. In this paper, antenna patterns measured on the V-22 scale model are compared with antenna patterns measured on the aircraft in-flight. Comparison of the patterns shows high correlation. Figure 1 V-22 Aircraft
The Phoenix Lander, a NASA Discovery mission which lands on Mars in the spring of 2008, will rely entirely on UHF relay links between it and Mars orbiting assets, (Odyssey and Mars Reconnaissance Orbiter (MRO)), to communicate with the Earth. As with the Mars Exploration Rover (MER) relay system, non directional antennas will be used to provide roughly hemispherical coverage of the Martian sky. Phoenix lander deck object pattern interference and obscuration are significant, and needed to be quantified to answer system level design and operations questions. This paper describes the measurement campaign carried out at the SPAWAR (Space and Naval Warfare Research) Systems Center San Diego (SSC-SD) hemispherical antenna range, using a Phoenix deck mockup and engineering model antennas. One goal of the measurements was to evaluate two analysis tools, the time domain CST, and the moment method WIPL-D software packages. These would subsequently be used to provide pattern analysis for configurations that would be difficult and expensive to model and test on Earth.
Antenna pattern and isolation measurements for the B-1 Fully Integrated Data Link (FIDL) Program have been completed at the US Air Force Research Laboratory (AFRL) Antenna Measurements Facility located near the AFRL Rome Research Site (RRS), Rome, NY. This combined satellite and airborne communications upgrade has been performed under the supervision of the B-1 Systems Group, Wright Patterson AFB, Ohio. One eighth scale antenna patterns were collected on a far field range for new Link-16 antennas, a relocated VHF/UHF2/L-Band antenna and the new Satcom transmit antenna, while on a one eighth scale B-1 model. Antenna to antenna isolation measurements were performed with antennas mounted on a full scale front section of the B-1 airframe. The RF Technology Branch (IFGE) has developed techniques for evaluating the effects of airframe and external stores on the radiation pattern characteristics of antenna systems in a simulated flight environment. Data obtained in this manner is used to evaluate antenna radiation characteristics of antenna/systems without the requirement of an extensive flight test program. Using similar techniques, AFRL has developed procedures whereby precision measurements of isolation between aircraft mounted antennas can be accomplished. This paper will present how the measured data was obtained for the antennas involved in the FIDL upgrade.
Using software and measured receive power data from aircraft during flight test, the pattern of the installed antenna is derived and validated. During the test flight, data packets were continuously transmitted from an aircraft using the antenna under test. The aircraft flew a designated pattern having straight legs oriented at specific angles to the ground station. Throughout each of these legs the aircraft performed appropriate maneuvers to provide elevation pattern data. A ground station recorded the received data packet signal strength with GPS time tags. Position data of aircraft (latitude, longitude, and altitude) and attitude (roll, pitch, and yaw) was recorded with time tags. Satellite Tool Kit, (STK™), by Analytical Graphics Inc. [1] reproduces flight test conditions and calculates the predicted ground station receive power based on vector direction, range, and a theoretical pattern for the antenna under test. The result is a dynamic link budget and a graph plotting predicted received signal strength versus time. Overlaying the recorded ground station received signal strength with the predicted signal strength allows the correlation of the measured data to that calculated using the theoretical antenna pattern. Curves are presented which show correlation sufficient to validate pertinent portions of the theoretical antenna pattern.