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Hemispherical Near-Field (NF) antenna measurement technique has been applied for automotive antenna testing within a chamber dedicated to EMC tests. An existing turntable was used for azimuth rotation of a vehicle and a new portable 90°arch was added for elevation scanning of the radiated NF of the Device Under Test (DUT - vehicle with the antenna). Two antenna types were tested during chamber commissioning, one for GPS and another for XM satellite radio applications at frequencies 1.57 and 2.33 GHz respectively. Test results have shown that the EMC chamber can be successfully used for automotive antenna measurements as well, with accuracies acceptable for automotive applications. For higher operating frequencies, the EMC absorbers must be changed to less reflective material. In the paper, the measurement system is described, and the test results are presented, as well as some considerations on far-field pattern restoration based on measured hemispherical NF data.
DSO National Laboratories (DSO) has commissioned a state-of-the-art combined near-field and far-field antenna test facility in 2004. This facility supports highly accurate measurement of a wide range of antenna types over 1–18 GHz. The overall system accuracy allows for future extensions to 40GHz and higher. The 11.0m x 5.5m x 4.0m (L x W x H) shielded facility houses the anechoic chamber and the control room. As the proffered location for this indoor facility is on top of an existing complex instead of the ground floor, antenna pickup is facilitated by a specialized loading platform accompanied by a heavy-duty state of the art fully automated 2.0m x 3.0m (W x H) sliding door, as well as an overhead crane that spans the entire chamber width. Absorber layout comprises 8-inch, 12-inch, 18-inch and 24-inch pyramidal absorbers. The positioning system is a heavy-duty high precision 3.6m x 2.9m (W x H) T-type planar scanner and AUT positioner. The AUT positioner system is configured as roll over upper slide over azimuth over lower slide system. This positioning system configuration allows for planar, cylindrical and spherical near-field measurements. A rapidly rotating roll positioner is mounted on a specialized alignment fixture behind the scanner to facilitate far-field measurements. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 4.0 package. A Real-Time Controller (RTC), accompanied by an 8-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.
wall or ceiling. The resulting motion changes the angle of the The incident field in the test zone can be measured using one-way string with respect to the incident field direction, during which probe antennas or passive two-way reflectors. In most cases time the coherent radar echo is recorded as a function of the string neither the probe antenna nor the passive reflector is very large, angle. The coherent signal-vs-angle data are then transformed to usually only a few wavelengths at best. It must be rolled across the cross-range domain using the fast Fourier transform (FFT), the range on carriages or raised up and down on towers, and the whence we obtain a chart of the incident field amplitude as a distance moved must somehow be measured. If a passive function of cross-range distance. Numerical examples are reflector is used as a probe, the carriage system must be shielded presented that show how variations in the incident field influence with absorbers and the reflector must be mounted on low-the string echo. A sample of experimental data shows that the reflectivity support structures. In addition to the hazard of processed data are readily interpreted. contaminating reflections, the mere assembling of all this equipment can be a significant task.
Reflections in antenna test ranges can often be the largest source of measurement errors, dominating all other error sources. This paper will show the results of a new technique developed by NSI to suppress reflections from the radome and gantry of a large hemi-spherical automotive test range developed for Nippon Antenna in Itzehoe, Germany. The technique, named Mathematical Absorber Reflection Suppression (MARS), is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical near-field test range. It has also been applied to extend the useful frequency range of microwave absorber in a spherical near-field system in an anechoic chamber. The paper will show typical improvements in pattern performance and directivity measurements, and will show validation of the MARS technique using data measured on antennas in a conventional anechoic chamber.
The electromagnetic field as projected by a 12 ft. prime focus offset fed compact range reflector with r-card edge terminations located in an existing chamber 20 ft. high, 30 ft. wide and 66 ft. long was probed using a broadband antenna to sample the field at 12 inch increments from the center line to the anechoic chamber wall. The purpose of the test was to evaluate the field roll off in dB to see if a narrower room would significantly impact the performance of the existing reflector system. The new chamber is 20 ft. high, 20 ft. wide and 40 ft. long. The probe data at six frequencies from 2.1 to 17.8 GHz indicated that 10 ft. off the center line the measured field level was -20 dB or greater below the level of the test region, which was our maximum acceptable field level goal. It is expected that the sidewall absorber will provide over 20 dB of bistatic attenuation for a total reflected field level of -40 dB, and is sufficient for conducting antenna pattern measurements in an anechoic chamber. Key Words: Compact Range, R-Card Terminations, Absorber Performance
Doing EIRP measurements in a nearfield facility is a known procedure. However, if the transmitted power is relative high, options are limited and care must be taken to prevent damage on equipment and absorbers. This paper describes how EIRP and pattern measurements for high power antennas and transmitters can be done in an indoor facility, and describes various considerations, choices and practical aspects. An example shows that even high power wide-band systems can be measured in near-field facilities.
Techniques for measurement of the phased-array antenna system include ambient temperature measurements in a compact antenna range, thermal vacuum testing, and spacecraft level testing. There have been two novel developments in the characterization of the phased-array system. The first is a “gain envelope” response, which is a measurement of the gain of the array at the intended scan angle as the array is electrically scanned in 1° increments. This response was produced through a combination of hardware and test software to synchronize the gain measurement with the mechanical and electrical scanning. The second is a phase steering verification test that utilizes couplers in each steered element in conjunction with previously measured element patterns to confirm that the antenna beam is steered properly. This method allows functional verification of the phased-array system while radiating into an RF absorber-lined hat during spacecraft-level tests.
A new rectangular anechoic chamber (20’L x 10’W x 9’7”H) has been established at California Polytechnic State University (Cal Poly) through donations and financial support from industry and Cal Poly departments and programs. The chamber was designed and constructed by three graduate students as part of their thesis studies to explore and further their understanding of chamber design and antenna measurements. The chamber project has included RF absorber characterization, overall chamber performance assessment, and software development for the coordination of a positioner with a vector network analyzer. This paper presents absorber characterization as a function of incidence angle and orientation to enable an overall chamber performance analysis. Test data at low incidence angles (< 30o) are compared to manufacturer performance curves at normal incidence. The mean response of the measured data indicates a correlation with manufacturer curves. Through ray tracing analysis, the ripple encountered in the test data is used to identify two effective reflection planes indicative of the foam geometry. The measured data are subsequently used to predict overall anechoic chamber performance to within 1dB for a majority of the actual scan data. Details of this analysis and comparisons to actual chamber performance are presented in a companion paper.
The use of electromagnetic scale models for evaluating the expected performance of a large test facility prior to its construction has been found to be useful in providing insight on how various absorber layouts effect the ultimate performance of the full scale test chamber. This report details the expected and measured performance of a series of absorber sets used in a 1/12 scaled model of a very large anechoic chamber to be used for both EMC and microwave measurements. Electromagnetic scaling of dielectric absorbers involves not just the geometry of the absorbers but also the amount of conductive carbon loading required to achieve a given reflectivity at the scaled frequencies. The goal is to scale performance over a very broad frequency range. It was found that absolute physical scaling is not always possible. Expected and measured performance of the scaled absorbers is detailed over the scaled frequency range of 360 MHz to 12 GHz. Selected measured chamber performance is included for Free Space VSWR, site Attenuation, and Field Uniformity to demonstrate the effectiveness of the scaling.
This paper presents an analysis of the reflectivity performance of the anechoic chamber. Measurements indicating the performance of the chamber-installed foam absorbers (described in a companion paper) are used to complete this analysis. This is followed by a comparison of the analysis results to chamber measurements taken in accordance with the free-space VSWR procedure [1]. Agreement between the analysis results and worst-case VSWR test measurements is within 1dB for a majority of reflection angles. In addition to chamber performance predictions, this paper describes a method of identifying primary reflection paths through interferometer calculations that compare all single bounce reflection path lengths to the direct path length. The angular spacing between interferometer nulls is used to identify the primary reflection direction. This information can be used to improve the overall chamber reflectivity by identifying areas of significant reflections and enhancing absorber treatments in these areas.
ORBIT/FR is presently under contract to provide Alenia Aeronautics with the HIRF – EW test facility to perform radiated field immunity testing of aerospace vehicles with high electromagnetic field intensity: radiated emission measurements, which belong to EMC testing; electronic warfare and antenna pattern tests. This unique facility will combine specific EMC, EME, EW measurements as well as specific antenna measurements. An anechoic-shielded chamber therefore, represents the ideal solution to perform these tests, because it provides the electromagnetic shielding and protection against the internal and external electromagnetic environments. While in many cases as little as -10 dB of round trip reflection may be adequate for EMC testing applications, in the EW tests to be performed at frequencies higher than 500 MHz, is required a fairly lower level of reflectivity. The facility will include an anechoic-shielded chamber (ASC) where the System under Test (SUT) is installed and operated in its functional modes to perform susceptibility tests and emission tests. The ASC will be equipped with a turntable having the capability of arranging the System Under Test (SUT) in front of the radiating antennas at different aspect angles. The ASC will provide internal size of 30 x 30 x 20 (H) m. The pyramidal absorber material shall be permanently installed on ASC ceiling, vertical walls and doors. As far as the floor is concerned two configurations are possible: proposed facility. The model will be described and the effort to scale the performance of the full size absorbers. The development and fabrication of scale model antennas. The establishment of measurement techniques, which will allow the correlation of the scale model measurement to the computer model performance predictions and the potential performance of the completed full size chamber.
The optimum source antenna in an anechoic chamber provides adequate uniform amplitude illumination of the antenna under test, but it minimizes the level of energy reflected from the walls of the chamber. The selection is a function of the range length (R), test aperture (D), source antenna gain (G), and the chamber’s aspect ratio (AR) (range length/width). The latter sets the angle of incidence seen by the absorber on the chamber walls. Adequate phase uniformity is assumed.
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.
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.
A single tapered resistive strip (R-Card) has been used in the past in several applications related to antenna designs and ground bounce reduction for far-field ranges. Several antenna designs use single tapered R-Card to significantly reduce the diffracted fields from the antenna to achieve low side lobe performance and also maintain stable phase center location across wide frequency bandwidth. Single layer R-Card fences have also been successfully designed and used to reduce the ground bounce stray signal in far field ranges. Recently, a multilayer tapered R-Card concept has been investigated and implemented in two different applications for interaction reduction due to performance requirements. One of the applications is to use multilayer R-Card fences to reduce the groundbounce effect between two antennas for GPS applications. The second application is to embed the multilayer R-Card with the Styrofoam target support column used in RCS measurements to reduce the interaction between the target-under-test and the metallic azimuth rotator underneath the Styrofoam column. In both applications, the multilayer R-Card concept, with different resistance distributions and proper spacing, has been designed and evaluated such that it behaves as an absorber to reduce the interference/interaction between two antennas or two scattering objects. The design and evaluation of this new multilayer R-Card concept will be presented in this paper.
The ongoing rapid introduction of new and enhanced electronic products, especially in the area of wireless communications, put an increasing burden on manufacturers in regard to demonstration of product compliance with applicable EMC standards. Wireless communication systems use higher operational frequencies than ever before and PCs, a commodity item in many regions of the world, use clock frequencies in excess of 2 GHz. These technical innovations require emissions measurements above 1 GHz to minimize interference of products with communication systems. The accelerated technical innovation presents a real challenge for national and international standardization bodies which have to determine suitable limits, reflecting the necessary protection, along with the specifications for test measurement equipment and test procedures. At this point in time (May 2002), only very few general EMI standards do contain requirements for emissions measurements above 1 GHz. For instance CFR 47 Part 15 calls out emissions limits up to 40 GHz; the accompanying measurement standard, ANSI C63.4-2000, includes a generic procedure to perform the measurements. However, currently there is no criterion available for the validation of the test site above 1 GHz, similar to the normalized site criterion for the 30 MHz to 1000 MHz frequency range. Therefore, the test environment, which has a significant impact on the test results, cannot be qualified against an independent reference. The international standardization community, i.e., CISPR/A/WG1, is actively working on a verification criteria and procedure for test sites above 1 GHz to address this shortcoming. The following paper presents an alternative method for evaluating a test site above 1 GHz. Test data is presented and discussed which resulted from measurements, conducted to determine the suitability of an existing site for measuring emissions above 1 GHz.
Tapered chambers have long been used for far-field antenna and RCS measurements. Conventional taper chambers used commercial antennas such as horns or log-period dipoles as wave launchers. One problem of this approach is the movement of the phase center associated with the antenna design. The positioning of the antenna inside the chamber is also critical. Undesired target-zone amplitude and phase distortion are caused by the scattering from the absorber walls. A novel feed antenna design for a tapered chamber is proposed here to provide broadband and dual polarization capabilities. This design integrates the absorber and the conducting walls behind the absorbers into to ensure a stationary phase center over a wider frequency range. In such a design, the dielectric constant of the absorber is utilized to maintain a clean phase front and a single incident wave at high frequencies. The conductivity of the absorber is also utilized to shape the field distribution at low frequencies. As a result, a wider frequency range can achievable for a given chamber size. One trade-off of this design is its reduced efficiency could be associated with the absorber absorption. Some simulation results from a 3-D FDTD model of a prototype design will be presented.
A compact range was recently constructed at the Applied Physics Laboratory to measure broad-beam, fan-beam, and pencil-beam antennas (max aperture: 1 meters). Chebyshev absorber treatments, lightweight composite reflector, foam column mount for light-weight antennas, automated measurement software, and a novel feed spillover rejection algorithm are the technology elements implemented in this compact range measurement facility. This paper will describe a trade study that APL performed before the compact range antenna facility was built. Solutions to some of problems that were encountered during the construction will be discussed as well as the overall performance of the facility. The measurement of a broad-beam antenna will be compared to calculated pattern. This measurement will highlight the advantages of using a software range gate that was recently developed.
In this paper, techniques are presented for the measurement of element radiation patterns of a belt-like C-band conformal array of microstrip patch elements, which wraps completely around the cross-section of an aircraft wing. The element patterns were measured, in situ, then analyzed in terms of phase and amplitude ripple versus element location around the wing. These results indicated trends in interference due to the experimental environment and the geometry of the wing itself. Experiments were conducted which minimized interference effects due to the environment, resulting in the true element patterns in the presence of wing platform interference. In an effort to identify platform-induced interference, anechoic absorber was used to minimize pattern ripple attributed to the edges of the wing, enabling validation of the measured element patterns against simulated data, which did not include platform interference. Thus, determining whether to include the platform effects in the measured data is dependent on the intended use of the results.
In some antenna applications, it is desirable to introduce an interior surface that is absorptive at one frequency, and reflective at an adjacent frequency. Even a narrow band absorber, such as iron loaded Magnetic RAM, has absorption qualities far outside its optimal absorption band. The concept is to use a conductive-backed Radar Absorber Material (RAM) covered by a band pass Frequency Selective Surface. The FSS allows the frequencies to be absorbed to pass through to the absorber while reflecting frequencies away from the pass band. The example shown in this paper was designed to absorb energy in the 2-4 GHz band, and to be reflective below 500 MHz. Design considerations include: Overall thickness; Coupling between the FSS and RAM, and Size of the FSS elements relative to the internal antenna structure. Potential applications include: broad band antennas, scatter control, and cosite interference mitigation.