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Standard Gain Horns (SGH) are utilized frequently either as measurements antenna or as reference antenna in antenna gain measurements by comparison or substitution method [1]. They also find use as source antennas in anechoic test chambers and for many other purposes such as fixed site antennas. The most widespread SGH geometry has a rectangular cross-section and is pyramidal with optimized geometry to achieve maximum gain [2, 3, 4]. When used as a precision gain reference in antenna measurements the SGH is often calibrated by a reference facility or another third party. When external or internal calibration means are not available the SGH peak gain is often determined directly from the reference tables of the NRL report [2]. The quality of the original work is such that even today the associated uncertainty on these peak gain values are generally accepted to be within +/-0.3dB [1]. In this paper the accuracy of the NRL gain tables are investigated by comparison with a full wave numerical method based on FDTD [7] and measurements in different antenna test ranges. Performance variation of the SATIMO Standard Gain Horns due to the manufacturing and measurement accuracy has been also investigated with conducted and radiated experiments.
The Mini-RF instrument on NASA’s Lunar Reconnaissance Orbiter is gathering data toward its science goal of probing the permanently shadowed terrain for the presence of water near the lunar poles. The circular polarization ratio is the central parameter used to characterize the lunar surface using Mini-RF radar returns. Accurate use of this parameter requires on-orbit polarimetric characterization of the instrument. This is done with a combination of measurements. Indirect measurements are performed by pointing the radar at the lunar surface to remove any asymmetry in the surface backscatter. Direct characterization of transmit and receive portions of the Mini-RF radar are performed using Earth-based resources. The Earth-based resources include the: Arecibo Radio Telescope, Green Bank Telescope, and Morehead State University Space Tracking Antenna. This paper describes the measurement approach and a sample of results. The primary measurements of interest include: principal plane antenna patterns, transmit polarization ellipse, receiver amplitude and phase balance, and antenna bore sight direction. These measurement values are incorporated into the Mini-RF SAR data processing to produce quality, calibrated CPR measurements
The calibration and measurement of bistatic reflectivity at short range (3.3 m) presents challenges that are significantly different from the usual test range measurements (typically monostatic at 100 to 150 m). In order to overcome this an object-free calibration procedure has been applied, eliminating crosstalk, reducing other interferences and removing errors associated with the RCS and alignment of calibration objects. It is based on calibrating the transmitter and receiver antennas as a pair by directing the antennas toward each other. The method thus requires that the antennas can be separated. Furthermore the signal level needs to be handled e.g. by the separation distance or attenuators. The bistatic reflectivity measurements were performed by placing a clutter sample on a turntable which is located at the centre of a bistatic arc. This configuration enables us to do ISAR images. Background contributions were discriminated using a combination of synthetic resolution and zero-doppler filtration. The sensitivity variation across the antenna footprint was handled by calculating an equivalent area from measured off-axis antenna sensitivities. Reflectivities have been measured for a metallic test surface and for grass. The metallic test surface had been manufactured to correspond to typical theoretical bistatic clutter models.
There are only a handful of commercially available antenna calibration laboratories in the US that are accredited to ISO-17025. Satimo has been operating an accredited laboratory in Atlanta since 2005 and an accurate and documented evaluation of measurement uncertainty has been a key element of the accreditation process. In order to develop the budgets, the parameters affecting the accuracy of the antenna measurement must be well understood. There are several references [2-9] that outline the method for preparing a measurement uncertainty budget, but few encompass the unique attributes associated with antenna measurements. Many of the papers published on the subject of the measurement uncertainty for antenna measurements address the characterization of a specific error term associated with an uncertainty budget, but few describe all of the terms contributing to the error budget nor how to practically determine their values. The intent of this paper is to outline and briefly describe the derivation of the uncertainty terms that contribute to the overall error budget for antenna measurements. Topics that will be discussed include: the uncertainty types, how to obtain or derive the error term for each uncertainty type, the distributions associated with each uncertainty type, the determination of the confidence level and coverage factor, how to combine the error terms as the references listed above are not in agreement on the method for combining the terms.
We report on the initial phase of our study to assess the electromagnetic interference and electromagnetic compatibility measurement and calibration procedures at the National Institute of Standards and Technology. We are developing a measurement-based uncertainty analysis of calibrations and measurements in the anechoic chamber. We intend to characterize all sources of uncertainty, which include power and probe-response measurements, noise, nonlinearity, polarization e.ects, multiple re.ections in the chamber, drift, and probe-position and probe-orientation errors. We present simple and repeatable measurement procedures that can be used to determine each individual source of uncertainty, which then are combined by means of root-sum-squares to state the overall measurement or calibration uncertainty in the anechoic chamber. We report on work in progress and future plans to characterize other EMI/EMC facilities at NIST.
Over-the-air (OTA) performance testing of wireless devices has become a major component of product qualification for today's wireless networks. The methodologies used currently expand on traditional passive antenna pattern measurement techniques and operate under the fundamental assumption that the radiation pattern of the DUT does not change for the duration of the test. Emerging wireless technologies such as MIMO that use multiple antennas and adaptive algorithms for communication violate that assumption. In addition, the enhanced feature set and higher complexity of these newer technologies means that there are other performance criteria to be evaluated beyond just radiated power and sensitivity. A new OTA test system has been developed that can simulate complex multipath systems in a fully anechoic environment. This technology allows evaluating the performance of the DUT in a host of simulated real-world environments. Existing spatial channel models used for conducted testing of these radios can be easily adapted to OTA testing. This paper describes the details of such a system, including requirements for calibration and validation, as well as showing typical test results and available metrics.
Polarimetric radar cross section systems are characterized by polarimetric system parameters Eh and Ev. These parameters can be measured with the use of rotating dihedrals. The full polarimetric dataset as a function of the angle of rotation can be analyzed with a nonlinear set of calibration equations to yield the system-parameter complex constants and the four polarimetric calibration amplitudes. These amplitudes appropriately reproduce the system drift and satisfy a drift-free system con.guration criterion very accurately. The results indicate that the nonlinear approach is better than the previously studied linear approach, which yielded system parameters that are seriously distorted by drift.
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.