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In an earlier paper ("System Engineering for a Radome Test System," John R. Jones, et al, AMTA, October 1994) the system level design of a compact range enhancement for the testing of the Triband Radome was presented. This paper will discuss the installation and testing of the radome measurement system in the compact range. The purpose of the radome measurement system is to determine (within close tolerances) boresight shift, transmission loss, antenna pattern changes and polarization effects caused by the radome. Unique features include novel coordinate transformation and correction by means of a laser autocollimator and data reduction algorithms. Also featured is the tracking subsystem which consists of a specially designed two-axis track pedestal, an autotrack controller, and three five-horn compact range feed arrays operating at X, K, and Q-bands. The performance of the triband radome measurement system in the compact range setting will be presented.
Measurement of active array high-power transmit patterns in an indoor near-field facility raises significant issues concerning safe microwave power levels and absorber power-handling capability. An extension of the planar near-field measurement technique for the safe and accurate measurement of active array high power transmit patterns is considered to address these issues. This new technique involves sequentially turning on groups of elements around each probe position while making measurements for each group of activated elements. Simulation results indicate that this technique is potentially feasible for safely and accurately measuring low sidelobe active array transmit patterns.
A technique for estimating measurement errors in near field facilities is presented. Known mechanical and electrical errors can be accounted for in simulation and such results are presented here. Unknown factors like chamber reflection and instrumentation drift can be estimated via selective measurement and the error induced by such anomalies may be combined with the simulated findings to provide error patterns for a particular test antenna and facility. Results are shown where these patterns are used to calculate measurement error limits. The software presented here also allows the generation of parametric curves which show the impact of a parameter of interest.
A method to transform Fresnel field data to far-field data with probe correction, based on a non linear least squares algorithm, is presented. The functional to be considered is the expression of the Fresnel field radiated by an array of isotropic sources located on the antenna aperture, and the complex excitations are the coefficients that minimize the rms error between the measured data and the functional values. The intermediate step of determining the complex excitations can be used as a diagnostic tool. Probe pattern correction has been included in the method, improving the performances of antenna measurement systems placed in small size anechoic chambers.
Far-field patterns obtained from planar near-field measurements of a prototype of the AMSU-B radiometer antenna by phase retrieval at 94 GHz are presented in this paper. Comparison with results from a compact range facility show good agreement within the main beam A modified algorithm takes into account any misalignments of the two intensity data sets so that the RMS near-field error metric comparing retrieved and measured values converges to < -30 dB. Phase retrieval is revealing itself as a useful technique to be applied to electrically large antennas at frequencies extending into the millimetre and sub millimetre bands.
Accurate mechanical-to-electrical axis alignment (boresighting), gain, and pattern testing of radar antennae requires specialized tooling/fixturing. This requirement is often taken for granted and seldom discussed in the EE community. Particularly in a production environment, where rapid change of test configurations to accommodate multiple radar platforms are required, a convenient mounting scheme is mandatory. This paper describes and illustrates a method implemented at the Warner Robins Air Logistics Center to satisfy this demand. Drawings and/or photos of a three-point Universal Adapter fixture and several UUT Specific radar mounting fixtures are discussed. The paper discusses tolerances, materials, manufacturing processes, alignment, and antenna boresight methodologies.
Automobile manufacturers have noticed the proliferation of after market antennas, primarily for cellular phones, defacing their otherwise stylish vehicle designs. Investigations are being made by the manufacturers to include antennas for communications requirements, such as cellular phone, personal communications service (PCS), global positioning system (GPS) and Intelligent Vehicle Highway System (IVHS), within their vehicle This paper presents the initial phase of an investigation undertaken within the Research Branch (ERB) of NASA Research Center (LaRC). The measurements, presented in this paper, were performed using a one-fourth scale model of a currently popular vehicle design. The bands of interest for this investigation include the cellular, GPS and FM broadcast frequencies. Comparisons of measured and computed patterns of commonly used antennas such as wire and microstrip patch antennas are presented.
Due to the revolution in communication technology very sophisticated communicative and navigational tools are becoming a part of automobile electronics. These different applications need antennas that operate at various frequencies and with different polarization requirements. One such antenna is a cavity-backed Circular Archimedean Spiral Microstrip Antenna (CASMA). This pa per will compare radiation pattern measurements of a CASMA with pattern predictions using a hybrid FEM /M oM/GT D technique. The measurements were done at NAS A-Langley Research Center's Low Frequency Antenna Chamber. The predicted and measured patterns are presented and are shown to exhibit a reasonable degree of agreement.
Finite-Difference Time-Domain (FDTD) is prov ing to be a practical and accurate technique for an alyzing and predicting the performance of anten nas mounted on complex structures. As part of an effort to develop and validate an FDTD code, the impedance and radiation patterns of helicopter mounted loop antennas are predicted and compared to full-scale and 1:10 scale measurements. The input impedance and coupling of HF loop an tennas on the scale model helicopter are measured in the ElectroMagnetic Anechoic Chamber facility at Arizona State University. Although made difficult by the large mismatch between the highly reactive HF antennas and the instrumentation, the scaled impedance measurements agree well with the full scale measurements and predictions. In addition, ro tor blade modulation effects on the input impedance are examined.
Due to the limited size of modern helicopters, airborne antennas must be physically small and lightweight. Slot antennas have been widely used by the aerospace community to meet the size, weight, and aerodynamic requirements when flush-mounted to a platform surface. Having these characteristics, a ferrite-loaded cavity-backed slot (CBS) antenna is an excellent choice for as a tunable low-frequency antenna. Excitation of a magnetostatic mode in the ferrite results in resonances at frequencies below those of the dynamic modes of dielectric-loaded CBS antennas. Frequency agility is achieved by varying the applied DC magnetic bias. Two ferrite-loaded CBS antennas were built and their impedances and radiation patterns were measured. Reasonable (0-6 dBi) with dynamic 3 dB bandwidths in excess of 20% were measured in the UHF band. Air-filled versions of these antennas agree well with Method of Moments (MoM) predictions, but non-uniformity of the magnetic field in the ferrite violates assumptions made in the theoretical model, resulting in discrepancies.
High performance antennas require very accurate measurements which are difficult to meet in the conventional compact antenna test ranges. This measurement errors are produced by the non perfect plane wave synthesized by the compact range system. By the application of the reaction between the antenna under test true pattern and the compact range incident field, a closed form relation is found for the measured radiation pattern. Under certain conditions, this measured pattern can be approximated by the convolution of the two diagrams. In this paper it is presented the inverse procedure: the deconvolution to numerically calculate either the true radiation pattern of the antenna under test or the plane wave spectrum of the compact range incident field . The effectiveness and limitations of the method are discussed by numerical simulations and tested by measurements.
Earlier measurement results are reviewed to understand the result that cross -polarized patterns agree well when compared between a point-source compact range and spherical near-field scanning. By taking account of the symmetry of the aperture distribution, one can see how the cross-polarized pattern can be affected only moderately by the classic polarization feature of an offset reflector geometry.
A novel computer simulation methodology to properly characterize the role of probe directivity/pattern compensation in cylindrical near field scanning geometry is presented. The methodology is applied to a linear test array antenna and the JPIJNASA scatterometer (NSCA1) radar antenna. In addition, error analysis techniques of computer simulation and measured have been developed to determine the achievable accuracy in pattern measurements of the NSCAT antenna in cylindrical near field.
The radiation characteristics for an active phased array receive antenna operating at K Band were measured at the Ipswich Research Facility. On-pole and cross-pole radiation patterns were measured for several scan angles. In this paper we'll discuss the general design of the antenna and the instrumentation ensemble used to perform the far field and near field characterization of this antenna. Measurements taken on a 2600 foot far field range vs. a near field planer scanner are compared.
Norden Inc. has developed and instrumented its JSTARS 1000' Outdoor Antenna Range with a multi-port antenna measurement system designed to acquire antenna data (patterns and other related signals) at the antenna's respective radar system's intermediate frequency (IF). The measurement system utilizes the JSTARS RF microwave receivers attached to the multiple channels of the JSTARS antenna. These receivers obtain the RF signal from these multiple channels and provide the IF signals to the measurement system.
This paper demonstrates a high speed antenna measurement capability recently developed in the MDTI Radar Measurement Center. Originally constructed as a Radar Cross Section facility, the RMC has added the capability to measure antenna patterns on apertures up to 40-feet in length in the far field. Data will be presented to demonstrate system performance through the use of modern output formats, such as global plots and videotape presentations.
Details of a public domain Microsoft Windows-based database program to organize and distribute measured antenna pattern and RCS data are described. Tools for editing, viewing and plotting data are also included.
This paper presents the various aspects involved in the design, development and establishment of Cylindrical Near-Field Measurement(cnfm) facility. A brief description of the hardware and the method of data acquisition are outlined. The capabilities of the CNFM system are brought into focus. The effects of alignment errors are presented. The patterns of various test antennas are presented over different frequency bands.
Accurate multiwavelength radiometric remote sensing of the ocean and the atmosphere from an aircraft requires antennas with the same beamwidth at the various frequencies of operation. Scientists at the National Oceanic and Atmospheric Administration designed an offset antenna with a pressure-compensating corrugated feed horn to meet this criterion. A specially designed fairing was incorporated into the antenna to optimize the aerodynamics and minimize the liquid buildup on the antenna surfaces. The antenna has two positions: the zenith (up) position and the nadir (down) position. The planar near-field facility at the National Institute of Standards and Technology was used to determine the far-field pattern of the antenna. The results show that the antenna beamwidths at 23.87 and 31.65 GHz are nearly the same as expected from the design criterion. This antenna was recently used in an ocean remote-sending experiment and performed according to expectations.
This paper will describe a new type of automotive AM/FM conformal antenna. The slot formed between the body of the automobile and a metal solar heat reduction film imbedded in the front windshield of the automobile is used to form an annular slot. Such partially conducting (4 to 12 ohms per square) metal films represent an opportunity to incorporate an antenna in the overall design at only marginal costs. The characteristic impedance and gain patterns will be described and techniques for improving the impedance match will be shown. A mobile measurement system will be described along with an on-road system to characterize the performance of a number of difference vehicle antenna systems in urban and suburban environments. The application of this system to the measurement of calibrated fain patters will be demonstrated.