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This paper introduces a broadband free space material measurement system in Temasek Laboratories at National University of Singapore (TL@NUS). The system is designed by TL@NUS and ST Aerospace for measuring permittivity, permeability, reflection and transmission properties of electromagnetic materials and structures from 1 to 40 GHz. The measurement system includes a pair of double convex spot-focusing lenses, horn antennas, a network analyzer and two arms that can be moved along a circular arc. The two arcs of the arms allow measurement to be done with different incident angles. Each of the double convex lenses is made from two plano-convex dielectric lenses of 77 cm in diameter. The plano-convex lenses can collimate the field from the source horn into uniform plane wave thus also allowing both mono-static and bi-static electromagnetic scattering measurement to be done in very limited space. The system is housed in an anechoic chamber of dimension 6.7 m (D) × 6.6 m (W) × 3.8 m (H) to reduce unwanted reflections and interference signals from the surroundings. Typical measurement results are presented in this paper for dielectric materials, magnetic materials, frequency selective surfaces, and metamaterials.
When specifying a Near Field scanner, intended to measure radiating systems under operational conditions, one of the requirements is the power flux density that the Near Field system and the absorbers on it have to with-stand. Today's trend is to use an EM-solver to calculate field intensities of (aperture) antennas. The advantage of these solvers is that they can handle any geometry but the disadvantages are that they can only handle limited dimensions and use approximations. Analytical solutions are not only more elegant and accurate but they also provide insight in the field behavior. For symmetrical cases, it is clear that the maximum field intensities will appear on the symmetry axis. The only (nearly identical) expressions in the literature are from Rudduck and Chen [1] and Yaghjian [2]. These analytical expressions describe the on-axis electrical field intensity of a circular aperture with uniform illumination. Rudduck and Chen have derived their equation via a Plane Wave Spectrum approach. Unfortunately, Yaghjian provided this version without reference or background about the derivation. It turns out that the expressions of [1,2] need a (minor) correction. Besides that, uniform illumination is not a very realistic case. This paper will also present an analytical expression for a tapered illumination. Graphs will be provided of the equation of [1,2], the corrected formula for the uniform illumination case and the new expression for the tapered illumination case.
Kriging fitting, originally developed for geological exploitation, is here applied for fitting an expected pattern to noisy, irregular in-flight measurements of a satellite antenna. The noise level in in-flight measurements is often so high that only the central part of the main beam appears. By the Kriging method, first a characteristic function, the regression model, is fitted to the measurements. For the main beam this is chosen to be described by a general second order polynomial. To this is added a more detailed correlation model which represents realistic deviations from the regression model but filters out the fast variations of the noise. The method is applied on simulated measurements on the Planck RF telescope and the presented results show a considerable reduction of the noise floor of the Figure 1 – The Planck double reflector antenna system pattern; even beam details invisible in the original with two ellipsoidal mirrors (aplanatic configuration). measurements (a shoulder) are revealed by the pattern From the antenna pattern obtained by the in-flight testing fitting1 .
This paper describes test methods and challenges for performing radio frequency (RF) characterization of a phased array antenna with element-level digital beamforming using planar near-field (PNF) and compact range technologies. The characterization of a digital array requires the synchronization of measurement equipment including positioner controllers, transmitters, and receivers. All hardware and software must remain synchronized with the array clock to achieve accurate amplitude and phase samples and ensure a coherent phase front. This synchronization is achieved through handshake triggers and communication protocols that are managed through external software. The acquisition of element-level data over large PNF scans presents unique challenges in data and post-processing that precipitate the need for optimization of array architecture as well as design of processing software. Advantages of the digital array architecture include being able to generate multiple receive beams from a single near-field scan for each frequency and the ability to compare multiple calibration methods efficiently using off-array processing.
Radar antennas are typically required to operate in transmit and receive modes, and may or may not support both CW and pulsed signal operation. In active antenna applications, these modes may require different operating parameters, which currently dictate testing the antenna independently in transmit and receive using different test system configurations. In testing highly-integrated active arrays, electrical and thermal considerations make it preferable to test the antenna in its nominal Tx/Rx (Transmit/Receive) operating mode as opposed to transmit-only or receive-only. An extension to the NSI Panther 9100 RF measurement system has been developed to support multiplexed transmit and receive, pulse-mode measurements with different measurement parameters during the course of a single data acquisition. This capability allows pulsed transmit and receive tests to be interleaved using a single measurement setup, reducing overall test time and improving the real-world accuracy of the test results.
Recently ORBIT/FR Inc. has introduced a far – field antenna measurement anechoic chamber design method called “ Two Level GTD “ , which combines shaped chamber walls with a specific absorber layout intended to achieve a better level of reflectivity in the test zone [1-3]. The sidewalls may have the shape of an “inverted open book “, while the back wall may be a pyramidal shape with a small subtended angle at the base. A wedge type foam absorber with a variable orientation of the wedge tips can treat the sidewalls in a “fishbone” layout, while the back wall may be treated by using conventional foam based pyramids. The’ fishbone’ like layout is intended to adverting the reflected waves by the sidewalls out of the test zone, while the back wall pyramid layout is applied to utilize both: the optimum pyramid reflectivity at almost normal incidence; the back wall shape diverting the reflected incident plane or quasi - plane wave out of the test zone. It well known that GO and GTD principles are widely applicable to electrically large structures, delivering a high quality simulation accuracy and good correlation with measurement results. Therefore, the application of the “Two –Level GTD “ is expected to deliver well predicted improvement in the reflectivity of anechoic chambers operating at relatively high frequencies , where the chamber sidewall characteristic dimensions may reach 30. where . is the wavelength at lowest operating frequency. The key question to be answered is - Can the method be successfully applied to cases where the chamber sidewall characteristic dimensions are only – 2-3.? This represents a typical situation in anechoic chambers designed for operation at VHF/UHF frequency bands. In order to answer the question, a full wave 3D simulation has been performed on two anechoic chambers having similar dimensions: 20’ x 20’ x 33’ (L). The two cases are a conventional anechoic chamber and a shaped wall chamber designed based on the “Two – Level GTD” principle. The simulation results were compared, and the “Two - Level GTD” has shown superior performance. Based on these encouraging results, the anechoic chamber was constructed and measurements were performed in the tests zone at a number of frequencies down to 100 MHz. The chamber construction, simulation and measurement results are discussed in the paper below.
It is no easy task to determine electromagnetic fields close to scattering objects. In some cases, it is required to measure the fields in shadow regions, or to find the effects of unwanted obstacles in the vicinity of an RF equipment system. We introduce a unique and rather simple way to obtain fields in areas inaccessible to measurement. The technique is based on the Field Mapping Algorithm (FMA), which determines fields in those regions where direct measurement is not feasible, from near-field measurements that are taken on an easily accessible surface. Verification of our FMA was carried out by analytic examples, by FDTD simulations, as well as by hardware measurements. For determining the fields close to scattering objects, three measurement test cases are given, with excellent results evident throughout.
In recent years, personal area networks (PAN), which use human bodies as transmission channels, have received considerable attention. During system design, interactions between a PAN device and a human body must be considered. Therefore, it is essential to establish a measuring method for PAN devices by using a network analyzer (NA). Few studies have focused on the problem of connecting the NA directly to a signal and ground electrodes of wearable devices. The ground level of PAN devices is not shunted to the Earth but is floating, because they are portable. A closed circuit is formed from the transmitter to the receiver through the ground line of the NA by connecting the ground electrode of the wearable device to the stable ground level of the NA. Therefore, under this condition, the result does not precisely reflect the transmission coefficient of portable wearable devices that have a floating ground level. This paper clarifies that the received signal level strongly depends on the stability of the ground by using the PAN devices attached to a biological tissue-equivalent phantom. In addition, it discusses a valid measuring method for PAN devices.
In the paper [1] the principles of operation of high performance absorbing materials were described and the criterion for absorber performance optimization at UHF/VHF frequency bands was proposed and confirmed experimentally on a number of absorber components optimized for operation at low frequencies such as the VHF/UHF bands. C:\Publishing 2011\AMTA 2011\Papers\Absorbing Material Performance\freq dom 18 24 36 60.jpg The experimentally verified optimization criterion is intended to determine the optimum carbon loading of the absorber components, thus delivering optimal reflectivity of the full absorbing assembly (absorber components on a metallic backing plate) at the lowest possible operating frequency. The optimization is based on equalization of reflections in the time-domain from the front face surface of the absorbing component and from the backing metallic plate. Validity of the criteria was confirmed by measurements of the reflectivity of pyramidal absorbing components of various heights, (3’, 5’, 6’ and 8’ [3]) in a 40’ long coaxial line terminated in a metallic back wall [2,4]. In this paper, more details are highlighted explaining how the criterion is delivering the best absorber reflectivity at low frequencies. This is accomplished by implementing time gating post-processing to isolate two primary concurrent peaks corresponding to the reflections from the front surface and metallic backing substrate. It is shown that the improved reflectivity is achieved by a self-cancellation of the two signals delivering the “null” in the frequency domain, which, in turn determines the lowest operating frequency attributed to an absorber of a given height. It is shown that the “null” property of the reflectivity pattern, as well as the properties of the peaks in between “nulls”, can be scaled and, therefore, predicted based on the height of the absorber almost everywhere in the UHF band. Thus, it is possible to optimally choose the grade of the absorber necessary to meet or exceed given reflectivity specifications, or to manufacture the appropriate absorber grade which can deliver the optimum reflectivity at the specified frequency.
Virginia Diodes has recently demonstrated a pair of network analyzer extension heads with very high dynamic range for the WR10 waveguide band (70110 GHz). Designed for the Agilent PNAX and similar vector network analyzers, these heads have high signal to noise ratio even in the presence of large path loss, as might be encountered in an antenna test range. Specialized heads are used with source power boosted by 3 dB across the band, while loss in the measurement receiver has been minimized. Careful optimization of system parameters, particularly IF levels, allows the complete system to have an effective dynamic range exceeding that of the PNAX receivers. An effective 150 dB signal to noise ratio has been demonstrated on a PNAX 5245A operating at an IF of 279 MHz with an IF Bandwidth of 10 Hz. The details of the system and the use of source power leveling to improve dynamic range performance across the band is described. With proper optimization, high dynamic range network analyzer measurements can now be extended to even higher frequency bands, and the discussion concludes with a very brief review of the dynamic range achieved in bands as high as WR1.0 (7501,100GHz).
We derive here a simple function describing antenna performance in the time domain. This single function describes antenna performance in both transmission and reception, and in both the time and frequency domains. The resulting equations are as simple as one could hope for. The function isolates the performance of the antenna from the impedances of the source, load, and feed lines. From this function one can simply derive such conventional frequency domain quantities as gain, realized gain, and antenna factor. It is hoped that this function will be adopted as an IEEE standard for time domain antenna performance.
A novel probe design for measuring complex permittivity of soils in-situ from 10 to 1000 MHz without taking soil samples is presented. The dielectric constant and conductivity of soil is derived from step-frequency reflection taken inside a small freshly bored hole. As a result, permittivity at various depths with in-situ moisture content and soil texture can be obtained in the fields. A novel calibration method was developed to account for the frequency- and material-dependent geometrical factor which causes bias errors in conventional calibration methods. Experimental measurement results and simulation results are used to prove the efficiency and accuracy of this method.
Electromagnetic Interference (EMI) in enclosed areas such as the inside of an automobile is often hard to deterministically measure and to geometrically characterize. EMI can be created by motors, actuators, power systems, computer-based control systems, etc. The non-sinusoidal signals can propagate to the radio system or data system by RF radiation, by coupling to power or data lines, or by direct conduction through the metallic structure of the vehicle or system suffering from the EMI issues. The technique to be described here uses direct A/D conversion at rates in excess of 2 GHz to collect two channels of data with up to 15,000 data points. One channel is captured from the potential source of the EMI. The other channel is captured along wiring harness points or over a region of space inside the vehicle. Transforms such as cross correlation or cross spectral analysis are used to characterize and/or map the relationship between the reference channel and the data channel. The algorithms will be discussed, and results will be shown for specific examples of EMI in an automotive body.
Far field measurements of ground vehicle antennas in anechoic chambers often require the creation of a plane wave by near field hemispherical probing with associated mathematical transformations to the far field/plane wave result. Direct far field measurements can be done to save time when the frequency is low enough. This paper discusses a method of extending the frequency band where direct measurements can be done by synthesizing a plane wave using a small array of antennas. The use of an array to create a plane wave in an anechoic chamber usually results in errors due to the reflections from the walls of the chamber. The technique to be described in this paper is to model the wall reflections and the array antenna characteristics and to use optimization techniques to derive an antenna placement and power distribution scheme to optimize the plane wave. Several optimization techniques will be described and results from testing in a 1.2 meter long sub-scale chamber model will be shown. Improvements in the far field measurements will be discussed.
Collecting accurate gain measurements on antennas is one of the primary tasks for our community. One of the primary concerns in making gain measurements is choosing one of the well known gain measurement techniques to make the measurement. Each gain measurement technique has an inherent accuracy limit based on the measurements made, the measurement environment and the equipment required. The frequency band of interest may also have an impact on the gain measurement scheme employed. In addition, each technique can affect the throughput of the range in question. Balancing the cost of obtaining the gain versus the required accuracy of the gain measurement is a difficult task. This paper will discuss the basic accuracy limitations for several of the standard gain measurement techniques and will catalog the accuracy limitations of the various gain measurement techniques versus the cost associated with obtaining that quality of measurement.
Highly accurate spherical near-field measurement systems require precise alignment of the probe antenna to the measurement surface. MI Technologies has designed and constructed a new spherical near field arch positioner with a 1.5 meter radius to support measurements requiring accurate knowledge of the probe phase center to within .0064 cm throughout its range of travel. To achieve this level of accuracy, several key design elements were considered. First, a highly robust mechanical design was considered and implemented. Second, a tracking laser interferometer system was included in the system for characterization of residual errors in the position of the probe. Third, a position control system was implemented that would automatically correct for the residual errors. The scanner includes a two position automated probe changer for automated measurements of multi-band antennas and a high accuracy azimuth axis. The azimuth axis includes an algorithm for correcting residual, repeatable positioning errors. This paper defines the spherical near-field system and relation of each axis to the global coordinate system, discusses their associated error sources and the effect on global positioning and presents achieved highly accurate results.
Since the early days of antenna pattern recorders, advances in instrumentation and computers have enabled measurement systems to become highly automated and much more capable. Automated systems have provided higher productivity, more efficient use of test facilities, and the ability to acquire more data in less time. In recent years, measurement speeds of microwave receivers and vector network analyzers have advanced considerably. However, to take full advantage of these speed improvements, the measurement system architecture must be carefully considered. Small differences in instrument timing that are repeated many times can make large differences in system measurement time. This paper describes a general method of calculating system measurement time based on the primary factors that affect system timing, including position trigger detection, frequency switching time, multiplexer switching time, receiver measurement time, and timing overhead associated with triggers, sweeps, and measurements. It also shows how key features of instruments available today can be used along with improved antenna measurement system architectures to optimize system throughput.
Conventional antenna measurement systems use a multiplexer or polarization positioner to sequence polarization and or antenna elements as a function of time, requiring two or more measurement intervals. However, a simpler, more cost effective, and faster technique can be implemented by using frequency diversity to distinguish between polarizations or antenna elements. This paper describes how two slightly different frequencies can be used to make two measurements simultaneously instead of sequentially, cutting the measurement time in half or even more. Additional considerations must be taken into account to achieve good measurements. This paper addresses these issues. Actual measurements are presented.
A free space transmission line measurement method for dielectric constant and loss tangent determination in low-loss dielectric materials has been analyzed and implemented. This method utilizes dielectric materials with thicknesses greater than half the wavelength in the material to obtain greater sensitivity for determining intrinsic dielectric properties. An analysis of the process sensitivities and experimental measurements has been utilized to estimate the accuracy and lower limits of the dielectric property extractions from the reflection loss magnitude.
Recently, a significant effort from different research groups have been aimed at the problem of reconstructing the extreme near field radiated by an antenna from measured field data [1]–[8]. Among the different proposed solutions, the inverse source or equivalent current/source method (EQC) based on discretization of integral equations has attracted considerable attention due to a host of promising applications in antenna design and diagnostics. The integral equation approach constitutes a complement to more established tools such as plane-wave or spherical-wave expansion. At the expense of heavier computational burden this method offers a greater generality and flexibility since it allows reconstructing sources on arbitrary 3-D surfaces enclosing the antenna under test (AUT).