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New taper chamber feed section was created for numerical analysis. To launch the undisturbed electromagnetic wave into the test zone, newly designed dual polarized aperture-matched blade mode bowtie (ABB) antenna was designed and implemented at the vertex of the feed section of the tapered chamber. For the accurate calculation, wall type absorber samples are obtained and measured. These values are included for realistic configurations. From the simulated time domain result, field distributions at the aperture of the feed sections are investigated. Determination of the usable spaces for different frequencies is discussed. Also, cross-talk levels are presented since the feed antenna designed for dual polarization.
In this paper, the electromagnetic (EM) characteristics of various antennas implanted in both the human head and the human body are analyzed for biomedical applications such as hyperthermia and biotelemetry. The implanted antennas are studied in two ways: the near- and far-field patterns of the antenna are calculated and the potential effects on the human body are observed. To ensure the correctness of the results, we apply two simulation methodologies: dyadic Green’s function (DGF) expansions and finite difference time domain (FDTD). We characterize the performances of the low profile antennas designed for biomedical applications in terms of specific absorption rate (SAR), radiation patterns, maximum available power and safety issues. These results should also provide a good basis for validating the results of experimental data.
Microwave measurement of intrinsic material properties can be performed with transmission-line fixtures such as waveguides or free-space focused beams. However, analyses of measured data usually assume idealized sample geometries. In this paper, Finite Difference Time Domain (FDTD) calculations are used to study the systematic error from non-ideal geometries, in free-space and waveguide measurements of impedance sheets. Analytical models of these errors are developed. FDTD analysis can be used to numerically invert intrinsic material properties from measured freespace transmission coefficients. The focused beam is simulated in FDTD with a sum of weighted plane waves with a Gaussian spectral distribution. The transmission coefficient is predicted by propagating the focused beam through a material slab or sheet; and the dielectric or impedance properties are derived from the transmission coefficient. The focused beam diameter is preferably several wavelengths, which requires large sample size (>1 square meter) at low frequencies. A modified focused beam technique is described that incorporates a finite aperture in a metal groundplane to measure samples with reduced dimensions, even at low frequencies. Calculations are compared to laboratory measurements. FDTD calculations are also applied to study the effect of gaps in waveguide fixtures, since gap and edge effects in both waveguide or free-space aperture fixtures contribute to measurement error.
A technique is presented for determining the insertion phase of array elements directly from time domain measurements. It is shown that the Inverse Discrete Fourier Transform (IDFT) commonly used in swept frequency time delay measurements may yield unreliable phase results. A compensation to the IDFT is proposed which allows the phase of an array element to be accurately estimated from time domain data without gating and without taking a second DFT. The technique is applied to determine the insertion attenuation and phase of the elements in a linear L-band phased array. Compared to conventional array calibrations, the removal of extraneous range reflections implicit to the time domain technique resulted in a significant improvement in measurement accuracy.
The Radar Cross Section (RCS) measurement facility operated by the Stealth Materials Department of BAE SYSTEMS Advanced Technology Centre in the UK is an invaluable tool for the development of low observable (LO) materials and designs. Specifically, it permits the effect of signature control measures, when applied to a design, to be demonstrated empirically in terms of the impact on the RCS. The facility is operated within a 3m by 3m by 12m anechoic chamber where pseudo-monostatic, co-polar, stepped frequency data for a target can be collected in a single measurement run over a frequency range of 2- 18GHz, and for a range of azimuth and elevation angles using a Vector Network Analyser (VNA). The data recorded consists of the complex voltage reflection coefficients (VRC) for the chosen range of aspect angles. This includes data for the target, mount, calibration object, and the associated calibration object mounting where significant. All data processing is conducted offline using a bespoke post processing software routine which implements software time domain gating of the raw data transformed into the time domain prior to calibration. The significant sources of type A (random) and B (systematic) uncertainties for the range are identified, grouped, and an approach to the determination of an uncertainty budget for the complex S21 data is presented. The method is based upon the UKAS M3003 guidelines for the treatment of uncertainties that may be expressed by the use of real, rather than complex numbers. However, a method of assessment of the uncertainties in both real and imaginary parts of the complex data is presented. Finally, the uncertainties estimated for the raw VRC data collected are propagated through the calibration and the uncertainty associated with the complex RCS of a simple target is presented.
A novel technique for mapping stray signal sources in spherical test ranges is presented. The technique is based on near field focusing. However, instead of the phase information, the time of arrival information is used for focusing. Thus, the technique uses field probe data over a frequency band, and provides good down range resolution. The technique is applied to the field probe data of an experimental outdoor spherical test range. The test range uses R-card fences to suppress ground bounce term in the quiet zone. From the stray signal maps obtained using the proposed technique it is clear that the test range is free of the ground bounce term.
A filter-based approach to software range gating is presented. Conventional approaches to range gating are widely used and include hard gates applied in the time domain and running average filters applied in the frequency domain. The potential problems with those methods are well understood and involve (1) sideloberelated distortion of the frequency-domain data caused by hard clipping in time and (2) the dual problems that arise from finite-duration smoothing kernels in the frequency domain. Herein, range gating is formulated as a digital filter design problem. We employ a type-II Chebyshev design, which has a maximally flat pass-band and a specified stop-band attenuation. User parameters include constraints on the smoothness of the passband and the width of the gate transition. Edge effects are minimized by filtering symmetrically extended copies of the measured data. The results are illustrated on data acquired by the JHU-APL compact range.
A Ground-Based Synthetic Aperture Radar (GB-SAR) interferometer system operating at 17 GHz is used to monitor the movement of an active landslide. The selection of the optimal image formation technique for such an imaging system is addressed. The algorithms considered in this study are those previously developed for spaceborne and airborne SAR. A near-field algorithm that forms the image in the time domain is selected as the optimal solution. Furthermore, example results obtained in a measurement campaign in Schawz (Austria) are shown.
This paper refers to a special type of antenna, called frequency independent antenna, used in Stepped Frequency Continuous Wave (SFCW) radar employed for humanitarian demining. The radar transmits 128 frequencies within the frequency range from 400 MHz to 4845 GHz, in groups of 8 simultaneously transmitted frequencies. It has been built at the International Research Center for Telecommunications transmission and Radar (IRCTR), Delft University of Technology. Two Archimedean spiral antennas with opposite sense of rotation, in order to decrease coupling signal below –55dB, have been chosen. Precise antenna behavior characterization is needed because SFCW radar is phase sensitive. The paper is focused on antenna footprint measurements, translating data from frequency domain to time domain and gating in order to remove any unwanted signals. Some phase and amplitude pattern using gating measurements are presented.
This paper summarizes some results of a recent NIST measurement effort. The purpose of this effort was to use a NIST-developed ultrawideband measurement system to assess the time- and frequency-domain characteristics of selected ultrawideband (UWB) transmitting devices. Brief descriptions of NISTdeveloped measurement systems are provided. Highfidelity time-domain waveforms are shown, along with associated amplitude spectra for two devices. Excellent results are obtained for both conducted and radiated emissions from UWB devices. Keywords: amplitude spectrum, anechoic chamber, conducted emission, frequency domain, radiated emission, time domain, ultrawideband
Time domain processing (TDP) is used to analyze the quiet zone fields of antenna/RCS ranges. To carry out the time domain analysis, the quiet zone fields are probed over a band of frequencies. It is shown that TDP is a very effective tool for analyzing probe data. One can not only estimate the time and direction of arrival of various signals present in the quiet zone, but can also estimate their frequency dependence and quiet zone variations.
Time domain (TD) antenna measurements have been successfully implemented in far field ranges [1,2]. The short acquisition times and the wide-band nature of the measurements make this regime a potential alternative to classical frequency domain measurements. Due to the measurement versatility offered by compact ranges, the implementation of TD measurements becomes especially attractive. This paper presents the modelling of the compact range performance in TD. In addition, a statistical evaluation criterion to assess the quiet zone quality is formulated. The results obtained show that this type of measurements can be successfully implemented in compact ranges.
The growing need of ultra-wide band measurements has promoted the research on real time domain (TD) antenna measurements. Theory has been already established, but practices still under development until the measurement regime becomes fully operational. In the Delft University Chamber for Antenna Measurements (DUCAT) there have been already provided outstanding results in a TD far-field configuration. A TD far field model of the facility has been developed in order to provide a key to improve the range performance and accuracy. This paper presents the model and considerations for establishing TD error correction techniques.
Antennas characteristics can be measured in two ways. lfrequency Domain Method (FDM) is more widely known. The main measuring instruments: Microwave Generator and Receiver. In Time Domain Method (TDM) measurements are fulfilled by using superwide band pulses. The main measuring instruments: Pulse Generator and Sampling Oscilloscope. TDM shows a number of advantages but for narrow-band antennas TDM is difficult to apply and FDM is required. At the testing polygons aimed to measure various antennas we set equipment allowing to use both measurement methods. For TDM we used a two channel sampling converter SD200 of Geozondas production with bandwidth 0-18 GHz. To unify measurements we developed a 3-channel sampling converter SD303 allowing besides pulse to measure sine wave amplitude and phase difference in dynamic range 100 dB. The third channel is used for synchronization. Thus the same instrument assures antenna measurements both in TDM and FDM. At 100 m distance the following characteristics are obtained in Time and Frequency Domains Measurements: Bandwidth 1- 18 GHz. Antenna pattern dynamic range 60 dB Gain measurement accuracy 0.5 dB Phase difference between 2 antennas error 0.5 - 3° (depends on frequency). Hardware, software and digital signal processing algorithms are considered.
In this paper, a near-field time domain scattering measurement technique is described. Near-field measurements are typically performed for radiation applications but not scattering applications. This time domain measurement approach borrows from many of the principles developed in the frequency domain and is ideally suited for broadband scattering characterization. The goal of determining the scattered far-fields of a structure is accomplished by the transformation of near-field data collected over a planar sampling surface. The scattered near-fields were generated with a probe excited by a fast rise time step. In particular, the near-fields were sampled with a second probe and digitized using a digital sampling oscilloscope. The bandwidth of the excitation pulse was approximately 15 GHz. The overall accuracy of this approach is examined through a comparison of the transformed far-field pattern to a numerical calculation.
Probe correction is required to accurately determine the far-field pattern of an antenna from near-field measurements. At Raytheon Primary Standards Laboratory (PSL) in El Segundo, CA, data acquisition hardware, instrument control software, and a mechanical positioning system have been developed and used with an HP Network Analyzer/Receiver system to perform these measurements. Using a three antenna technique, the on-axis and polarization parameters of a linearly (or circularly) polarized probe are calibrated. The relative far-field pattern of the probe is then measured utilizing the two nominal, orthogonal polarizations of the source antenna. All measurements are stepped in frequency and use a time domain gating technique. The probe and the source antenna are optically aligned to the interface and unique, kinematic designed interface flanges allow repeatable mounting of the antennas to the test station.
The backtransformation in (planar) Near Field processing is often claimed to be a very powerful tool for antenna diagnostics. Less known is a kind of defocusing effect which is introduced by the processing. Selecting the visible space in the Far-Field domain has a similar effect as a bandfilter in the frequency domain of an electric signal. In that analogous case it is better known that after the transform to the time domain, one has to deal with sin(x)/x behavior, limiting the resolution. The mathematics and convolution effects of both the onedimensional time-frequency transform as the two dimensional Near-Field Far-Field transform will be explained. Some measurement procedures are proposed, including S/N requirements. It turns out that the back transformation technique has some nasty properties which limit the use for alignment purposes. Some alternatives are discussed.
A development work of a 2.4 m x 2.0 m hologram for testing the 1.1 m offset reflector of the Odin satellite at 119 GHz is reported. The analysis of the hologram is based on physical optics (PO) and finite difference time domain method (FDTD). The hologram is fabricated with an etching process. A comparison between the theoretical and measured quiet-zone fields of the hologram type of compact antenna test range (CATR) is made.
Multipath on far-field ranges causes distortion of pattern measurements. The multipath components can be removed by illuminating the antenna under test with short-duration pulses and applying a time domain gate. Equivalently, the measurements can be made in the frequency domain and transformed to the time domain with the Fourier transform. After gating, the time-domain data are transformed back to the frequency domain, yielding improved CW patterns at discrete frequencies. Virginia Tech has recently added time-domain gating capability to its far-field antenna range. The data acquisition and processing software is implemented using the LabVIEW language, which makes the data acquisition and time-domain processing very easy to control. Practical guidelines for selecting a gate are given. Results are presented for an open-ended waveguide and conical dipole. With wideband antennas, gated patterns show significantly improved symmetry and null depth.
This paper presents the implementation of the time domain measurement technique in IRCTR's antenna measurement facility. Using electromagnetic pulses with duration and rise times in the order of 50 ps, the antenna characteristics are determined. From the pulse response, the behavior of the antenna is characterized in the frequency range 1-18 GHz. An X-band Standard Gain Horn (SGH) has been used to verify the overall performance of the measurement system. An excellent agreement between time domain and standard frequency domain measurements has been observed. This novel antenna measurement technique offers advantages over the traditional techniques for wideband measurements in frequency domain in reducing the measurement time and has great potentials. The paper describes the Antenna Time Domain Measurement (ATDM) system, including measurement results and summarizes the basic principles, error sources, advantages and disadvantages of such a measurement technique.