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Accurate calibration of near-field measurements requires the probe used for the measurement be well characterized. The determination of the absolute gain of rectangular open-ended waveguide probes is difficult due to the broad beamwidth in both the E-plane and H-plane which increase the likelihood of multi-path affecting the accuracy of the measurement. Multi-path may be minimized by reducing the separation distance, but at the price that far-field conditions may no longer apply. A variation of the two matched antenna method is to use a large reflecting plate to form an image of the probe. Use of the entire bandwidth of the probe, and time-gating the results to isolate the signal reflected from the plate allows the gain to be determined. The procedure also allows the determination of the aperture reflection coefficient used by theoretical probe models used for pattern compensation in the near-to-far-field transformation.
The gain-transfer technique is the most commonly used antenna gain measurement method and involves the comparison of the AUT gain to that of another antenna with known gain. At microwave frequencies and above, special pyramidal horn antennas known as standard-gain horns are universally accepted as the gain standard of choice. A design method and gain curves for these horns were developed by the US Naval Research Laboratory in 1954. This paper examines the ability of modern numerical electromagnetic modeling to predict the gain of these horns and possibly achieve greater accuracy than with the NRL approach. Similar computational electromagnetic modeling is applied to predict the gain and pattern of open-ended waveguide probes which are used in near-field antenna measurements. This approach provides data for probes that are not available in the literature.
A broad band interferometer antenna was designed and manufactured by Saab Avitronics. Saab Aerotech has installed a test facility for calibration of the interferometer antenna. The main purpose of the facility is to measure the interferometric function of the antenna. The interferometric function of the antenna can be measured directly but this method puts very high demands on the test range performance. An alternative method where each element is centered on a short far-field range is evaluated and compared by measurement with a large compact range at Saab Microwave Systems. The paper also describes the design aspects when measuring broad band, broad beam interferometer elements together with the actual design of critical components such as positioners, RF-system and absorber treatment.
A new simple approach is presented to calibrate the gain of standard gain horn antennas operating in the millimeter-wave frequency band. In terms of calibration, it is difficult to accurately measure the gain of standard gain horn antennas in the far-field region due to the space limitation. Therefore, near-field measurement methods are generally used to calibrate the gain of standard horn antennas. The extrapolation range method is one of the most accurate measurement methods in the near-field region. In the conventional extrapolation range method, a moving average process is applied to remove multiple reflections between antennas. Moving average can only remove multiple reflections between antennas. Therefore, electromagnetic absorbers are required to remove other reflections increasing measurement uncertainties. The time-domain gating method in extrapolation range allows us to remove all reflection waves, and achieve accurate antenna gain calibration without absorbers. The time-domain gating also reduces the number of measurement positions in the extrapolation ranges and obtains the gain of antennas in wide frequency ranges. In this paper, we compare the theoretical value with the time-domain gating method without absorbers by measuring three W-band standard gain horn antennas.
Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) are the two metrics most commonly used to characterize the over the air (OTA) performance of a wireless device. Measurement of these quantities requires a reference measurement of the loss from the origin of the spherical coordinate system to the power measurement device. Calibrated dipoles are typically used as gain standards for the reference measurement. These narrow bandwidth dipoles can provide low uncertainty reference measurements, but numerous dipoles are required to cover all of the wireless frequency bands. Since typical wireless measurement systems must be calibrated from 700MHz to 6GHz, calibration of the measurement system with narrow bandwidth dipoles becomes a tedious and time-consuming exercise. Broadband gain standards can be used, but due to the uncertainty in their absolute gain and their interaction with the measurement system, these add uncertainty to the reference measurement. This paper reports on a broadband gain standard and a measurement procedure that allow an extremely fast reference measurement while at the same time does not appreciably increase the uncertainty of the reference measurement.
RCS measurements of two larger squat cylinders (with dia. 18” and 15”) have been studied. Numerical extrapolation from the best available MoM-simulation is used to generate the finer oscillations (< 0.1 dB) in RCS-PO at higher frequencies. Though the uncertainties at 0.4 dB would obscure the opportunity for a comparison at this time, a smoothly silver-painted surface did yield error bars at 0.2 dB for the Ku-band.
The extension of the frequency range for commercial applications of mm-waves to 80 GHz and beyond often requires extended antenna characterization capabilities both at manufacturer and end-user facilities. Presently, most measurements are based on direct measurements using vector network analyzers (VNAs). VNAs that cover a continuous frequency range up to 67 GHz are commercially available. Above 50 GHz, extensions based on external mixers in waveguide technology are typically utilized. They require a tunable local oscillator (LO) that is usually provided by the two additional ports of a 4-port VNA. However, these extensions not only are restricted in bandwidth but also require a significant financial investment especially considering the fact that the expensive 4-port instrumentation is needed. As most laboratories already have conventional 2-port VNAs usable up to 10 GHz or higher and most antenna characterizations are based on transmission measurements, we present a simple extension scheme based on external mixers and a fixed frequency LO that allows for transmission factor measurements. We demonstrate the feasibility of such an extension scheme for transmissions between a pair of horn antennas ranging up to 60 GHz. The measurements include variation of antenna spacing and steering angle and are verified with a computational analysis based on the finite differences time-domain (FDTD) method.
This paper presents the initial field probe characterization results for an RF scattering compact range using a high precision calibration sphere. This approach uses an Inverted Stewart Platform to position the ultra-sphere through the target quite zone. The Inverted Stewert Platform and optical target tracking system provide a fast and efficient for performing a volumetric incident illumination field characterization of the compact range quite zone using a backscatter RF measurement. The Inverted Stewert Platform system uses six small diameter strings attached to the ultra-sphere to provide the ultra-sphere positioning over the entire quiet zone of the compact range. The inverted Stewart platform also offers increased stability of the target by damping out the torsional pendulum motion typically encountered in conventional string support systems. This presentation will discuss an in-house development of the sphere field probe and discuss advantages and disadvantages of the ultra-sphere volumetric field probe.
We used a set of dihedrals to perform polarimetric calibrations on an indoor RCS measurement range. We obtain simultaneously hh, hv, vh, and vv polarimetric data as the calibration dihedrals rotate about the line-of-sight to the radar. We applied Fourier analysis to the data to determine the polarimetric system parameters, which are expected to be very small. We also obtained polarimetric measurements on two cylinders to verify the accuracy of the system parameters. We developed simple criteria to assess the data consistency over the very large dynamic range demanded by the dihedrals. We examined data contamination by system drift, dynamic range nonlinearities, and the presence of background and noise. We propose improved measurement procedures to enhance consistency between the dihedral and cylinder measurements and to minimize the uncertainty in the polarimetric system parameters. The final recommened procedure can be used to calibrate polarimetrically both indoor and outdoor ranges.
Lockheed Martin MS2 has a long history of utilizing antenna ranges for calibration, test and characterization of the phased array antennas. Each range contains an integrated RF receiver subsystem for performing antenna measurements, typically on the full array. For solid-state phased array testing, what is often needed, however, is a test station capable of performing complex S-parameter measurements on a subarray or subset of the full antenna system without incurring the expense of a test chamber. To address this requirement, Lockheed Martin, working with Nearfield Systems, has developed a portable standalone RF measurement system. The standalone system consists of an Agilent PNA, automated transmit/receive unit (TRU) and a waveform generation (WFG) subsystem for interfacing to the phased array beam-steering computer. This paper will discuss the capabilities of the Standalone RF System including the TRU and WFG subsystems. The TRU is used to tailor the RF signal by automated switching of amplifiers and programmable step attenuators for various test scenarios. The WFG is an automated pattern generator used to present many digital waveforms in arbitrary sequences to the phased array beam steering computer. The design features of the standalone RF system will be presented along with the COTS hardware utilized in assembling the station.
There is a desire for antenna technologies that will support surveillance needs in a complex Radio Frequency (RF) environment. There are many current technologies that support these needs, including individual components such as broadband phased array antennas, broadband RF components, and miniaturized digital receivers. A testbed has been established to develop systems combining these elements, resulting in wideband phased arrays encompassing multiple receiver channels and capable of forming multiple independent beams through digital beamforming. This effort revolves around phased array calibration and testing, RF component characterization, system integration, system testing, and digital beamforming. The Transformational Element Level Arrays (TELA) Testbed allows for the integration of these technologies so that they can be tested and verified as a system. What will be described here is recent and current work taking place in this testbed. Some of this work includes system integration and testing and subsequent digital beamforming of a four-channel recieve system. Also included is the calibration process of an 8:1 bandwidth, 256-element phased array, and integration and testing of the 16-channel recieve system corresponding to this array.
Currently there is a lack of facilities capable of measuring the full upper hemisphere radiation patterns of antennas mounted on an infinite ground plane. Measurements performed with a finite ground plane suffer diffraction interference from the truncated edges. To circumvent this problem, a new measurement setup was developed at the Ohio State University ElectroScience Laboratory (ESL) for fully characterizing upper hemisphere radiation gain patterns and polarization for antennas up to 4” in diameter from 1-18 GHz. A probe antenna is positioned 46” away from the antenna under test (AUT). The ground plane end diffractions are removed using time-domain gating. The key design consideration is to position the probe antenna in the far-field region and yet shorter than the radius of the ground plane. This paper will present the calibration procedure necessary for the measurement system and it’s limitations due to ground plane probe antenna coupling at low elevation angles. In addition, the complete radiation pattern of a 4” monopole measured from 1-5.5GHz to demonstrate the systems capability for the lower third of the systems operating frequency range.
Calibration of planar near field probes is generally required to obtain accurate cross-polarization measurements of satellite antennas; however, probe calibration is costly and time consuming. One way to avoid probe calibration is to ignore the probe cross-polarization and use the probe co-polarized patterns alone for probe correction. Then the probe can be easily characterized by standard, in-house measurements or by analytical models. Of course, if the probe cross-polarization is ignored, additional errors are introduced in the co- and cross-polarized pattern measurements, but the errors can be manageable, depending on the probe and Antenna-Under-Test (AUT) polarization properties. Complete formulas and/or tables for near field measurement errors for three popular measurement configurations are presented, along with experimental verification of the error estimates for one case.
The 2004 AMTA paper entitled “The “Cam” RCS Dual-Cal Standard” introduced the theoretical concept of the “cam,” a new calibration standard geometry for use in a static RCS measurement system that could simultaneously offer multiple “exact” RCS values based on simple azimuth rotation of the object. Since that publication, we have constructed a “cam” to further explore its utility. The device was fabricated to strict tolerances and its as-built physical geometry meticulously measured. Utilizing these characteristics and moment-method analysis, a high-accuracy computational electromagnetic (CEM) “exact” file required for calibration was produced. Finally, the “cam” was evaluated for its efficacy as a single device that could be utilized as a dual-cal standard. This development was conducted with a particular focus on the hypothesized improvements offered by the new standard, such as the elimination of frequency nulls exhibited by other resonant-sized calibration devices, and improved operational efficiency. In this follow-on paper, we present the advantages to and challenges involved in making the “cam” a viable RCS dual-cal standard by describing the fabrication, modeling and performance characterization.
Modern day remote sensing spacecraft often feature multiple payloads sharing a common bus (spacecraft platform). RE02 emission testing (1, 2) characterizes the emission signature of a given payload in order to assess electromagnetic compatibility with respect to other payloads (i.e. “victims”) on the bus. Typically, a simple path loss model based on 1/r2 power variance (ref: Friis path loss equation) is used to account for the distance between the emitting and victim payloads using measured amplitudes taken during RE02 measurements. RE02 measurement technique (2) dictates that emissions testing take place at a fixed radial distance of one meter from the radiating instrument. At certain frequencies, however, this measurement takes place in the near field of the emitter. In general, power density amplitudes are greater in the near field than its far field counterpart. This paper investigates any potential error incurred by not accounting for this effect. A simple math model for a common mode radiator is developed to estimate this error and attempt to better understand the field relationships at lower frequencies where the near field predominates.
The Microwaves and Radar Institute regularly performs calibration campaigns for spaceborne synthetic aperture radar (SAR) systems, among which have been X-SAR, SRTM, and ASAR. Tight performance specifications for future spaceborne SAR systems like TerraSAR-X and TanDEM-X demand an absolute radiometric accuracy of better than 1 dB. The relative and absolute radiometric calibration of SAR systems depends on reference point targets (i. e. passive corner reflectors and active transponders), which are deployed on ground, with precisely known radar cross section (RCS). An outdoor far-field RCS measurement facility has been designed and an experimental test range has been implemented in Oberpfaffenhofen to precisely measure the RCS of reference targets used in future X-band SAR calibration campaigns. Special attention has been given to the fact that the active calibration targets should be measured under the most realistic conditions, i. e. utilizing chirp impulses (bandwidth up to 500 MHz, pulse duration of 2 µs for a 300 m test range). Tests have been performed to characterize the test range parameters. They include transmit/receive decoupling, background estimation, and two different amplitude calibrations: both direct (calibration with accurately known reference target) and indirect (based on the radar range equation and individual characteristics). Based on an uncertainty analysis, a good agreement between both methods could be found. In this paper, the design details of the RCS measurement facility and the characterizing tests including amplitude calibration will be presented.
This paper discusses design aspects related to a tiltable lightweight near-field scanning system for use at sub-millimeter frequencies. It addresses design issues as they relate to accuracy and scanner distortions from multiple causes. Calibration methods to measure and correct for anticipated and unanticipated errors are briefly addressed. Actual test results are presented. The tiltable scanner being discussed was designed for the Atacama Large Millimeter/submillimeter Array (ALMA) [1] and is being used by the National Radio Astronomy Observatory (NRAO) [2]. It has many other applications by virtue of its light weight (approx. 120 lbs) and ability to be oriented at different angles. These include flight-line testing and other in-situ antenna test applications.
Focused-beam measurement systems are commonly employed in material characterization measurements due to their inherent broadband capability. Calibration of this system is typically performed using a simple response calibration in conjunction with gating techniques to eliminate unwanted reflections. An undesirable artifact of this calibration technique is the extracted permittivity and permeability measurements can be highly dependent on the width and shape of the gate. This paper explores a three-short full two-port calibration technique which eliminates the need for gating. The two-port calibration consists of three independent short measurements in both the forward and reverse directions (i.e., S11 and S22), two isolation measurements (S21 and S12) and four empty measurements (S11, S21, S12, S22). Material parameter extraction measurements based upon this calibration technique were performed using both a low-frequency (0.5 - 2 GHz) and high-frequency (4 - 18 GHz) focused-beam system. Initial results show the technique’s viability and the dependency on accurate positioning of the shorts used in the calibration process and possible interaction between the sample and the sample holder.
While using squat cylinders for calibrations, we study the MoM-simulated data in terms of surface waves. We have found that the fine structures in both the amplitude and the phase are related to the target geometry. Key Words: RCS calibration, simulation, polarization
We use a rotating dihedral to determine the cross-polarization ratios of radar cross section measurement systems. Even a small amplitude drift can severely degrade the calibration accuracy, since the calibration relies on accurate determination of polarimetric data over a large dynamic range. We show analytically how drift introduces errors into the system parameters, and outline an analytic procedure to minimize the in.uence of drift to estimate system parameters with greater accuracy. We show that only very limited information about the drift is needed to provide measured system parameters accurate to second order in the error-free parameters. Higher-order accuracies can be achieved by using more detailed information about the drift. We use simulations to explain and illustrate the analytic development of this theory. We also show that, using cross-polarimetric measurements on a cylinder, we can recover the exact system parameters. These .ndings show that we can now calibrate polarimetric radar cross section systems without the large uncertainties that can be introduced by drift.