AMTA Paper Archive

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.

This paper addresses some of the practical considerations and numerical consequences of using the Advanced Antenna Pattern Comparison (AAPC) method to improve the accuracy of antenna measurements in compact ranges. Two main issues are of particular importance: 1. Appropriateness of circle-fitting algorithm results to the measured data. 2. Ambiguous circles due to the crowding of data. These issues deal specifically with Kasa’s circle-fitting procedure—an essential part of the AAPC method—and provides useful checks for conditions commonly met with the use of this technique. In addition, we consider the problem of data distribution along the fitted circle, another important element of the AAPC method. Simulation results are submitted in support of the proposed methods.

The development* of a real time electronic system to accurately measure the pattern of high gain, ultralow sidelobe level antennas in the presence of multipath scatterers is described. Antenna test ranges and anechoic chambers contain objects that scatter the signal from the transmitting antenna into the main beam of a receiving antenna under test (AUT), thereby creating a multipath channel. Large measurement errors of low sidelobes can result. The fabrication of a feasibility demonstration model Antimultipath System (AMPS) is complete. This AMPS uses a 10 MHz wide phase-shift-keyed spread spectrum modulated signal to illuminate the rotating AUT to tag each multipath by its delay. The novel receive section of the AMPS sorts out each multipath component by its delay and adaptively synthesizes a composite cancellation waveform (using delay, amplitude, and phase estimates of the scattered components) which is subtracted from the total signal received by the AUT. After subtraction the resultant is the desired direct path signal which produced the free space pattern of the AUT. Laboratory and antenna range test results are presented and show the promise of measuring sidelobe levels 60 dB below the main beam.

Gain calibration of circular horns and radiation pattern integration applying patterns in two principle planes only is accurate and does not require large computational or measurement effort. This technique is thus more practical than the integration over the entire angular domain, required in case of rectangular horns. However, for many types of AUT’s, additional errors may occur due to the differences in aperture size of the AUT and standard gain horn. The AUT will in many cases have physically larger aperture dimensions. Consequently, unknown test-zone field variations across this aperture can result in additional errors in gain determination. The new method uses a flat plate as a reference target. An RCS measurement of the flat plate is used to derive test-zone field characteristics over the same physical area as the AUT. Combined with the accurate gain calibration described above, field information is available over the entire area of interest and the accuracy in gain determination is increased. In this paper, experimental results and practical considerations of the method will be presented.

Residual reflection characteristics should be evaluated for antenna radiation pattern measurements. Authors propose a method for detecting positions of reflection sources by applying the modified far-field antenna radiation pattern measurement scheme described in [1]. In this method, an “accurate” radiation pattern of antenna under test (AUT) and measurement error patterns due to residual reflected waves are separated by changing a range distance and processing Fourier transformation. Also, the positions of reflected sources can be detected from beam directions of patterns due to reflections at each distance. Experiment results confirm that this method is effective for detecting the positions of reflection sources.

Planar near-field antenna measurements have developed into a mature science. Nonetheless, unique difficulties arise when measuring some modern antennas, such as high gain satellite antenna systems. In a typical planar near-field measurement, the antenna under test (AUT) has a collimated beam in the near-field which is perpendicular to the scan plane (i.e. the AUT boresight is parallel to the normal of the scan plane). On the other hand, the scan plane is positioned close to the AUT to maximize the valid angular range in the far-zone patterns. Unfortunately, it is not always possible to place the AUT very close to the scan plane and keep the near-field beam perpendicular to the scan plane. An investigation of the benefits and pitfalls of a planar near-field measurement where the AUT beam is not perpendicular to the scan plane is presented. The measurements of antennas tilted 45 degrees with respect to the scan plane normal are used as examples. With this atypical arrangement, some of the usual errors in a near-field measurement are emphasized. Procedures to identify and reduce these errors will be presented.

This paper reports on a ground penetrating radar (GPR) antenna with an electronically steered beam, currently being developed at South Dakota Tech. The increased power and directivity that result from beam-steered operation have potential utility in deep/lossy GPR environments. The antenna is a transmitting array of up to eight bow-tie dipoles, each driven by a narrow pulse generator connected directly to the dipole. The beam is steered in real time by controlling the timing of the individual element transmitters using digitally-programmed pulse delay units. Reception is through a conventional GPR receiver using a single bow-tie antenna. Modeling the air-ground interface as a lossy half-space, numerical results indicate that, under certain conditions, time-domain beam-forming is possible in such an environment. Antenna patterns and standard antenna measurement parameters, such as beamwidth and directivity, are presented in support of this finding.

A slot spiral antenna and it associated feed are presented for conformal mounting on a variety of land, air, and sea vehicle. The inherent broadband behavior and good pattern coverage of the spiral antenna is exploited for the integration of multiple frequencies, and thus multiple transmitting and receiving apertures, into one compact, planar antenna. The feasibility of the broadband slot spiral antenna relies on the use of an equally broadband, balances, planar, and non-intrusive feed structure. The design of the slot spiral, its feeding structure, and the reflecting cavity are discussed with emphasis on the experimental validation and construction of the antenna.

This paper describes the application of the plane-to-plane (PTP) iterative Fourier processing technique to infrared (IR) thermographic images of microwave fields for the purpose of determining the near-field and far-field patterns of radiating antennas. The PTP technique allows recovery of the phase by combining magnitude-only measurements made on two planes, both in the radiating near field of the antenna under test. We describe the PTP technique and show excellent comparisons between the predicted results and results from measured IR thermograms of the field of a 36 element patch array antenna operating at 4 GHz.

An examination of mutual coupling effects in a linear phased array is presented. The approach derives mutual coupling coefficients from array element patterns measured in the Fresnel region, at R/D=3. The technique allows edge diffraction effects and mutual coupling effects to be identified and separated. The results are compared with conventional mutual coupling measurements and mutual coupling coefficients determined by numerical integration. The technique is used for far-field pattern reconstruction, and for pattern optimization which corrects mutual coupling effects to the maximum extend possible.