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It is well known that a microstrip transmission line can radiate if it is excited in its first higher order mode (with the fundamental or dominant mode suppressed). A new microstrip configuration is proposed that supports the first higher order mode while suppressing the fundamental mode. To quantify the leakage constants in the two cases for comparison purposes, several experimental means are considered to determine the source amplitude distribution from which the leakage constants may be deduced. First, an approximation to the source distribution is determined from the far field patterns themselves. Second, the source distribution is determined by carefully probing the near field. This paper uses these techniques to verify the performance of a new leaky wave antenna design.
The Geometrical Optics characteristics of single parabolic reflector compact range systems are presented in rules of thumb for amplitude taper, phase taper and cross polarization. This is illustrated on four different range configurations (two different focal lengths and two different offset angles). Also the influence of the feed system in regard to far field diagram and alignment is discussed for typical low and medium gain corrugated feeds. No diffraction effects are discussed in this paper. With the use of the rules of thumb, a fast and yet precise qualitative and quantitative analysis, optimization and trade off can be made for a compact range optimized for the available space as well as the application.
In this paper, we present an analysis of the impact of positioning errors on the performance of the GDAIS circular image-based near field-to-far field RCS transformation (CNFFFT). The analysis is part of our continuing investigation into the application of near fieldto-far field transformations to ground-based signature diagnostics. In particular, the analysis focuses on the errors associated with ground-to-ground, near-field, whole-body measurements where the radar moves on a nominally circular path around the target. Two types of positioning errors are considered: slowly-varying, long term drift and rapidly-varying, random perturbations about the nominal circular path. The analyses are conducted using simulated data from a target comprised of an array of generalized point scatterers which model both single and multiple interactions on the target. The performance of the CNFFFT was evaluated in terms of the angle sector cumulative RCS statistics. The analyses were performed as a function of frequency for varying amounts of position error, both with and without (approximate) motion compensation. As expected, the results show that the CNFFFT is significantly more sensitive to rapidly-varying position errors, but that acceptable performance can be achieved with motion compensation provided an accurate estimate of the errors is available.
An internal research and development project at the Georgia Tech Research Institute (GTRI) focused on cohering multiple apertures into a single distributed aperture. Cohered distributed aperture antenna patterns were collected on the GTRI far-field range for a 1.5 GHz bandwidth at X-band frequencies. Both 1-way and 2-way antenna patterns were measured, with the 1-way antenna pattern measurement requiring coherence on receive only and the 2-way antenna pattern measurement requiring coherence on transmit and receive. The resulting data were compared with the ideal angular resolution and power-aperture gain product improvements from a perfectly cohered distributed aperture, and the results are presented. As measurement techniques were developed for collecting 1-way and 2-way antenna pattern data, sources of potential errors in measurement collection and aperture coherence were identified, with potential methods of error mitigation outlined.
For enhancing the performance of existing near field antenna test facilities it is quite reasonable to use both conventional (the amplitude and phase measurements) and the phaseless measurements techniques during electrically scanning phased array antennas (PAA) testing. This simple yet critical approach helps to improve the quality of PAA alignment and testing reducing measurement errors and saving costs. In this way many difficulties related to precise phase measurements are overcome. Both simulation and measurement results will be presented to demonstrate the utility of such approach to PAA alignment and determination of its parameters. Comparison will be made between the PAA patterns for electrically scanned beams calculated using traditional near field - far field (NF/FF) transformations, the phaseless methods and the results obtained applying both measurement techniques.
This paper presents a novel variable width time gating technique, which is applied to planar and cylindrical near-field data in impulse time-domain antenna near-field measurements. Due to the changing distance between the probe and the antenna under test (AUT) in planar and cylindrical scans, the conventional fixed time gating technique causes problems to remove multiple reflections from the desired AUT response. It further limits the application of time-domain measurement to planar and cylindrical scans. The new variable width time gating technique provides a flexible way to solve these problems. Test results for both planar and cylindrical near-field measurements are presented. The difference of far-field patterns between time-domain and frequency-domain near-field measurements is noticeable. We also show the effects on the far field patterns due to fixed and variable time gating windows. We further conclude that the time-domain technique also works for planar and cylindrical near-field measurements by using variable width time gating technique.
This paper presents a novel variable width time gating technique, which is applied to planar and cylindrical near-field data in impulse time-domain antenna near-field measurements. Due to the changing distance between the probe and the antenna under test (AUT) in planar and cylindrical scans, the conventional fixed time gating technique causes problems to remove multiple reflections from the desired AUT response. It further limits the application of time-domain measurement to planar and cylindrical scans. The new variable width time gating technique provides a flexible way to solve these problems. Test results for both planar and cylindrical near-field measurements are presented. The difference of far-field patterns between time-domain and frequency-domain near-field measurements is noticeable. We also show the effects on the far field patterns due to fixed and variable time gating windows. We further conclude that the time-domain technique also works for planar and cylindrical near-field measurements by using variable width time gating technique.
This paper presents an evaluation of the suitability of the 65-foot range for the measurement of the far-field radiation characteristics of antennas located above a sea water half space. The 65-foot range corresponds to the measurement distance of the Overwater Antenna Measurement Facility (Arch) at the Naval Undersea Warfare Center (NUWC) Division, Newport, RI. Four antennas are investigated at the 200- to 400-MHz frequency range for antenna base heights ranging from 0 to 20 feet above sea water. The results presented are based on thin-wire model representations of the antennas using the Numerical Electromagnetics Code (NEC), version 4.1. The radiation parameters investigated are the directive gain, axial ratio, direction of maximum gain, and the location and depth of the first null above the horizon. For each antenna, plots of the differences of the computed radiation parameters at the 65-foot and far-field ranges are given as functions of frequency for various antenna base height above sea water. It is anticipated that the results presented in this paper may be helpful for determining at what frequencies and heights the NUWC Arch will provide accurate far-field measurements for a given type of antenna.
A problem of determination of an antenna phase center (PhC) usually is solved by different ways from a theoretical calculation to the near-field measurements of complex characteristics in the aperture of an antenna or the far-field measurements of the radiation-pattern phase. The present paper is devoted to a general technique of an antenna PhC determination by use of the known (or the measured) distribution of the complex characteristics in the antenna near zone or the phase pattern in the far zone. An algorithm of determination of the phase pattern evolute, based on the lowest moments of distribution, as well as a criterion for PhC existence, which is independent on the observation angle, are offered. A simple expression of PhC for an antenna with a quadratic phase distribution in the aperture is obtained. An error of PhC determination depending on both the error of observation angle and the error of measurement of the phase pattern is considered.
Since the early days of spherical near-field far-field transformations a recommendation for the necessary number of polar modes has been given by , being the wavenumber and or the radius of the minimum sphere. The almost explosive development in computer speed and storage capacity witnessed during the last two decades has made trans-formations of fields from antennas exceeding thou-sands of wavelengths feasible, and a closer investiga-tion of the above expression seems to be appropriate. An improved expression for the number of modes, N, related to the antenna size and the required accuracy will be developed. The impact of truncation of the modal expansion at a given level will be illustrated. This is especially important for measurements where noise is present, or where there is undesirable scatter-ing from objects.
The David Florida Laboratory (DFL) was contacted by the Canadian Department of National Defense (DND) to develop an accurate, reliable, more cost effective method of characterizing existing nose cone mounted radomes for the radar systems aboard aircraft such as CF-18. Traditionally, these measurements have been performed in a far-field (FF) [1] range using conventional positioning and measurement systems and specialized instruments such as a null seeker. Recently, the use of near field methods has been incorporated in radome measurement practices [2]. This paper describes one such adaptation of a cylindrical near-field facility (CNF) for radome measurements.
Accurate near-field calibration of a large 60 ft. diameter reflector can be accomplished with a minimal sampling technique. Near-field amplitude and phase is collected as the reflector scans across a receiving calibration tower. The near-field data is then transformed to a far-field pattern using a Fourier transform technique. Information on far-field EIRP, directivity, pointing, axial ratio and tilt, as well as encoder timing is obtained with accuracies comparable to anechoic chamber measurement techniques. The system was analyzed for sampling and multipath effects, as well as the effects of phase and amplitude stability. A spherical wave expansion technique was compared to a straight-forward summation technique for the Fourier transform.
Abstract — A technique for near-field measurements over a digital PCB is presented. Phase measurement with a vectorial network analyzer (VNA) is possible with antennas but it is in principle not possible when the DUT has its own free-running oscillator. In order to get around this problem, a two-probe approach is proposed. While one of the probes is mobile the other one is fixed and collect the reference signal. An analogue circuit must be used to obtain the specifications in power and phase of the reference signal of the VNA. The collected near field above the test circuit allows us to clearly identify the hot spots and the constant phase areas. These results could be used to find problematic spots on a board or to extrapolate the far-field. This is of practical interest in EMC testing of digital devices.
We propose a simple method for estimating uncertainties due to multiple reflections between the test antenna and probe in near- field spherical-scanning measurements. To estimate uncertainties in far-field parameters, we measure the test antenna by scanning the probe over two spheres whose radii differ by a quarter wavelength (?/4). We compare this estimate to that obtained with a reduced data set (containing all values of ? but only a few values of f). In our example, we find that measuring only two f cuts suffices to obtain RMS uncertainties within 1 dB of those obtained using full-sphere data.
The gain patterns of VHF/UHF antennas on ground structures and vehicles are influenced by the characteristics of the ground. The measurement of the performance of such antennas is more accurate with a test chamber that incorporates a realistic ground surface. This paper will discuss the near field to far field transformation process for the case where there are reflections from a ground surface outside the probing hemisphere. We will show that the ground reflection term in the transformation must be based on the characterization of the ground outside the probe region.
A general approach is introduced for estimating uncertainties in far-field parameters obtained from spherical near-field measurements. Although the analysis is incomplete at present, we expect that as the measurement radius increases, our results will transform smoothly into the far-field case, where uncertainties depend on the on-axis gain and polarization of the probe and on the measurements in the far-field direction of interest.
A prototype design of the dielectric rod antenna is discussed. This novel design is suitable for nearfield probing application in that it provides broad bandwidth, dual-polarization and low RCS. The design details are provided in this document along with measurement data associated with important antenna characteristics such as VSWR and far-field radiation pattern
The Institute for Integrated Circuits of the Fraunhofer Gesellschaft recently acquired a combined spherical nearfield / far-field (SNF/FF) antenna measurement range with a shielded anechoic chamber for verifying passive and active antenna design concepts. A single 9-pin digital control connector allows the range to remain sealed from external RF, while maintaining full motion and data acquisition control. This set-up uses two different illuminators, separated 180° as seen from the AUT. This combined SNF/FF configuration gives the opportunity to perform intra-range measurement comparisons (SNF vs. FF) with not only the distance between AUT and illuminator being varied, but also with the measurement zone being reversed. In this manner, a comparison between SNF and FF measurements also compares the quality of two sides of the measurement chamber.
An indoor far-field range consists of the appropriate instrumentation and an anechoic chamber. In most of cases, the construction of the anechoic chamber is a laboring task and costs at a great expense. To save the money and labor, efforts for the range design are needed before the chamber been constructed. In this paper, the finite-difference time-domain (FDTD) method is employed to establish the design criteria for the far-field ranges. The commercial package named “FIDELITYTM”, based on FDTD algorithm released by Zeland Software, Inc., is used for the numerical calculations. To emulate the test procedure of the free-space VSWR technique, the electric fields of the points on the scanning axis are recorded during the simulation. And then, by plotting the amplitude ripples calculated from the recorded data, the range performance can be evaluated. The criteria of chamber layout, absorber arrangement, and source antenna selection and placement will be presented and discussed.
Probe position errors, specifically the uncertainty in the theta and phi position of the probe on the measurement sphere, are one of the sources of error in the calculated far-field and hologram patterns derived from spherical near-field measurements. Until recently, we have relied on analytical results for planar position errors to provide a guideline for specifying the required accuracy of a spherical measurement system. This guideline is that the angular error should not result in translation along the arc of the minimum sphere of more than ?/100. As a result of recent simulation and analysis, expressions have been derived that relate more specifically to spherical near-field measurements. Using the dimensions of the Antenna Under Test (AUT), its directivity, the radius of the sphere (the minimum sphere) enclosing all radiating surfaces and the frequency we can estimate the errors that will result from a given position error. These results can be used to specify and design a measurement system for a desired level of accuracy and to estimate the measurement uncertainty in a measurement system.