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This paper presents a 'mid-range' calibration technique, now being developed for a 60 ft. diameter reflector site. With this technique, near-field amplitude and phase is collected at a calibration tower as the reflector scans across it. The mid-range 'near-field' data is then transformed to a far-field pattern using a Fourier transform technique. Information on far-field EIRP, directivity, pointing, axial ratio and tilt, as well as encoder timing is obtained with accuracies comparable to standard measurement techniques. A particular advantage is that the system, once set-up, can be used on a regular basis without impacting site operations.
Calibrated probe complex pattern data is used in planar NFR (near field range) data processing to remove the effects of the probe on the measurement. In a prior paper [1] we proposed a procedure to estimate the measurement error (uncertainty) introduced into a near field antenna radiation pattern measurement due to test frequencies that do not coincide with available calibration frequencies of the range probe. Our prior paper resulted in a “19th term” which was added to the well known NIST NFR 18 Term Error Table used to evaluate the unavoidable uncertainty of far-field radiation patterns derived from a near field scan of a given AUT (antenna under test). A limitation of this procedure, pointed out in our prior paper, is that it was most accurate for a test frequency falling midway between two nearest neighbor probe calibration frequencies. The estimated uncertainty became overly pessimistic as the test frequency of interest moved closer to one of the neighboring calibrated frequencies. The procedure is improved in the present paper by the inclusion of a new term that is a function of the test frequency and the two nearest neighbor probe calibration frequencies. Examples are shown of the use of the new procedure to obtain an improved estimate of this measurement uncertainty and to create the 19th term for use with the standard 18 Term Error Table.
The Range Commanders Council Signature Measurement Standards Group (RCC/SMSG) Performed a Demonstration program with three DOD Radar Cross Section Ranges to evaluate and improve their documentation and evaluation process and criteria documented in what is known as a "Range Book". After a successful Demonstration Program, The RCC/SMSG has embarked on the evaluation of Industry RCS Range Calibration and measurement processes and procedures and compliance with the RCC/SMSG ANSI-Z540 (ISO-25) evaluation criteria. The Lockheed Martin Helendale RCS Range was evaluated by a committee of industry volunteers appointed by the RCC/SMSG after a review of their experience and credentials. The Lockheed Martin Orlando RCS Range requested an evaluation of their "Range Book" shortly after the completion of the Helendale evaluation. Each review committee is made up of three RCC/SMSG approved reviewers, at least one of which has participated in a previous review either as a review requester or a review committee member. This paper will put forth the process used by this review committee and the lessons learned from this and previous reviews. This paper will also discuss the RCC/SMSG process for obtaining an RCC/SMSG review.
NPL has recently commissioned a new indoor test range. This test range has been designed to offer Extrapolation Gain Measurements, Far-Field Probe Calibrations, and eventually, a Spherical Near-Field Test Capability. This paper describes this new range and the results of the initial validation measurements. It also compares the gains of a standard gain horn calibrated in NPL’s old Extrapolation Range with those from the new one.
We discuss progress in developing a new broadband (10 MHz to 40 GHz) RF field standard to be used for calibrating small electromagnetic field probes. The technique generates a well-defined and uniform TEM mode field between the conductors of an air-filled coaxial transmission line of conical geometry (termed a “co-conical” line) that is terminated in a well-matched, high-power, distributed load. We show that generation of higher-order mode fields will be minimal due to the line’s circular and symmetrical crosssection. Internal field levels equivalent to power density levels in excess of 10 mW/cm2 can be generated using broadband power sources of only 20 watts output. The new system will be capable of rapid, automated and accurate calibration of small field probes and can realize significant savings in both equipment/facility expenses and operational costs.
In June 2001, the DoD Range Commanders Council Signature Measurement and Standards Group (RCC/SMSG) certified that the Helendale Measurement Facility (HMF) outdoor radar cross section (RCS) measurement Range Book met the ANSI-Z-540 documentation standards established by the DoD demonstration project. This paper describes how Lockheed Martin Aeronautics (LM Aero) applied the ANSI Z-540 [1,2,3] standard to obtain National Certification of the HMF RCS range. The dual calibration results for Pit #1 and Pit #3 are presented showing upper and lower uncertainty error bounds established by this process. Schedule, cost, range book format, and “lessons learned” from the LM Aero experience are also discussed.
This paper reviews the Helendale Measurement Facility (HMF) ground plane range uncertainty analysis and associated data collection. Range uncertainty analysis is a requirement for ISO-25/ANSI-Z-540 range certification and is a priority one section in the Helendale Range Book. Targets used for the analysis were two sets of right circular “squat” calibration cylinders. These cylinders are the dual calibration cylinders for HMF. Calibration measurement uncertainties are established statistically from a large number of repeated measurements at S, C, X, and Ku bands. Each measurement was taken at two target support locations down range. The field data collected included monostatic scattering from two calibration cylinders, backgrounds with no target and support, and drift data for quality control. I and Q imbalance, frequency stability, range accuracy, linearity, and field uniformity at target locations were considered in the analysis. The uncertainty analysis is based on RSS addition of errors and assumes all errors are additive and that targets are not LO. The statistical approach used to perform the uncertainty analysis reported in this paper was developed cooperatively at AFRL and Mission Research Corporation.
In previous AMTA Symposia, the Air Force Research Laboratory reported on a successful effort to fabricate, measure, and predict the precise radar cross section (RCS) for various cylindrical calibration targets [1]. In this paper, we apply what we have learned about calibration cylinders to the study of a 3.048 meter ogive body of revolution. Recall that an ogive is simply the arc of a circle spun on its axis. The radar signature of this shape is extremely small in the direction of the "point", even at low frequencies. A few years ago, AFRL had the subject ogive built for an RCS inter-range comparison between AFRL and the NRTF bistatic RCS measurement system [2]. In this paper, we utilize this ogive body to assess both the quality and accuracy of VHF RCS measurements and predictions performed using multiple calculation schemes. In the end, reconciling the ogive measurements and predictions led us to reassess how composite objects are "conductively coated" to simulate a perfect electric conductor. This insight resulted in refinements in the process for measuring and predicting the ogive at low frequencies where electrical size and electromagnetic skin depth considerations are important.
In order to better estimate the uncertainties in measured RCS for the Boeing 9-77 Compact Range, we study the responses from three high-quality objects, i.e., two ultraspheres of 14” and 8” in dia., plus the 4.5" squat-cylinder, each supported by strings. When calibrated against each other in pairs, the differences between measured RCS and predicted values are taken as the uncertainties for either object. Two standard-deviations from the target, reference, and background, as computed from repetitive sweeps, are taken as the respective uncertainties for the signals. Using the root-sum-squares (RSS) method, the error bars are found to be between + 0.1 to 0.2 dB for most of the frequency F, from 2 to 17.5 GHz. We also analyze the responses from a thin steel wire (dia. 0.020"), supported by fine fishing strings (dia. 0.012"), at broadside to the radar. When the ‘wire and string’ assembly is oriented vertically, the HH echo from the 3-ft metal wire alone happens to be comparable to the HH from the 30-ft dielectric strings. Varying with F4, the combined RCS in HH for the assembly spans a wide range of 38 dB from 2 to 18 GHz. The error bounds are found to bracket the measured traces even when the signals are barely above the noise floor.
Future scientific and earth observation instruments as MASTER, PLANCK and HERSCHEL of ESA/ESTEC are working in the sub-millimeter wave range. For measurement of the instruments, a study named ADMIRALS was performed, mainly to identify the most suitable test facility, procure transmit and receive modules and perform measurements up to 500 GHz. The CCR 75/60 of Astrium GmbH, Ottobrunn, was selected for the facility calibration and the pattern verification with an Representative Test Object (RTO). The measurements were performed in three different frequency bands between 200 and 500 GHz. The mmwave transmit and receive modules were designed, manufactured and tested by Radiometer Physics GmbH (RPG). A cost efficient design was achieved by a modular concept. Within this paper, the design and realization of the modules as well as most characteristic performance parameter will be presented.
In this paper, the authors propose an improved Rotatingelement Electric-field Vector (REV) method taking into account amplitude and phase error of phase shifters in order to achieve more precise calibration. The conventional REV method has been used in order to determine and/or adjust amplitude and phase of electrical field radiated from each antenna element -element fieldin phased array antennas. However, amplitude and phase deviations due to phase shifter errors, and so on, reduce the measurement accuracy because the conventional REV method assumes no deviation. On the other hand, the proposed REV method can evaluate element fields without error and error electrical fields -error fields- due to phase shifter errors in each bit, by measuring both amplitude and phase value of array composite electrical field. In a simulation for a 31- element array with 5-bit phase shifter, the evaluated element fields and error fields agree well with the expected values. This result shows that the proposed method allows the phased arrays to be calibrated more accurately as considering phase shifter errors.
A thermal technique for the remote calibration of phased array radar antennas is proposed in this paper. The technique is based on infrared (IR) measurements of the heat patterns produced in a thin planar detector screen placed near the antenna. The magnitude of the field can be measured by capturing an isothermal image (IR thermogram) of the field with an IR imagining camera. The phase of the field can be measured by creating a thermal interference pattern (IR/microwave hologram) between the phased array antenna and a known reference source. This thermal imaging technique has the advantages of speed and portability over existing hard-wired probe methods and can be used in-the-field to remotely measure the magnitude and the phase of the field radiated by the antenna. This information can be used to calibrate the individual elements controlling the radiation pattern of the array.
The level of multiple reflections in near-field antenna measurements is an important issue in a measurement error budget. Traditionally, the interactions between the test antenna and the measuring probe have been reduced by covering the probe mounting structure with absorbing material. In this paper, a novel approach to alleviating the problem is discussed. This implies the use of a skirt to act as a shield against the mounting structure behind the probe, thereby eliminating the need for an absorber, which is a fragile material when exposed to wear and tear. This also has the added advantage that probe calibration data will not depend on a particular absorber that must be considered as an integral part of the probe. With a suitable design of the skirt, the level of multiple reflections can be reduced, whilst at the same time maintaining the pattern of the probe in the boresight direction unchanged. Prototypes of probes for 20 GHz and 30 GHz have been manufactured and tested, and excellent agreement between experimental results and theoretical predictions has been observed.
Compensated Compact Ranges (CCR) represent a high standard of state-of-the-art test facilities with a fast and real time measurement capability up to the submm wave range. Future scientific and earth observation instruments of ESA/ESTEC such as MASTER, PLANCK and HERSCHEL are working within this frequency ranges and require a high measurement accuracy for large antenna apertures. Within the ADMIRALS study for ESA/ESTEC, transmit and receive modules up to 500 GHz and an appropriate large offset reflector antenna with precise surface accuracy in form of a Representative Test Object (RTO) were applied. Related tests in the CCR 75/60 of Astrium were performed in order to qualify the test facility and verify the antenna measurements with theoretical pattern calculations. The present paper shows measurement results with the highly accurate Plane Wave Scanner (PWS) of Astrium GmbH and the RTO. Through the measurements performed, the accuracy of the plane wave field as well as pattern accuracy in the quiet zone of the CCR 75/60 have been qualified up to 500 GHz.
A method is presented for calculating the gain of corrugated conical horns. It is based on basic symmetry conditions of circular or conical waveguide mode fields. This formulation allows to derive the radiation pattern over a complete sphere form two principal polarization patterns (E- and H-plane patterns). This method can be applied for both theoretical or experimental patterns, respectively. The theory has been verified experimentally with measurements carried out on two different ranges. The results agreed within 0.05 dB or less in all situations.
Accurate gain measurements using any measurement technique require a calibrated gain standard, and the uncertainty in the gain of the standard is usually the largest term in the error analysis. To reduce the uncertainty, gain standards are often calibrated using a three- antenna measurement technique and the resulting gain values are generally certified to have an uncertainty of approximately 0.10 dB1-11. For near-field measurements, the gain standard may be the probe that is used to obtain the near-field data or it may be a Standard Gain Horn (SGH). Since the calibration of the gain standard is time consuming and often costly, it is desirable to verify that the gain of the standard is stable over long periods of time. This paper will describe tests to verify the gain stability of the standard and will also illustrate the terms in the error analysis that have the major effect on the uncertainty of any near-field gain measurement. With proper attention to the major error terms, the stability of the gain standard can be verified to approximate the original calibration uncertainty.
A fundamental part of the work of the BAE SYSTEMS Advanced Technology Centres Materials Group at Towcester (UK) is the microwave characterisation of the electromagnetic parameters of lossy materials. This paper describes a Quasi-optical microwave system for the free space measurement of material parameters in the frequency range 5 GHz to 18 GHz. The system employs two spherical reflectors which are illuminated from the side by gausian beam forming antennas. This produces a well defined parallel beam between the reflectors. The 5 GHz ro 18 GHz frequency range is covered in three bands with three pairs of corrugated feed antennas. An advantage of this system is that the beamwaist diameter (or illumination area) is essentially the same for each of the three frequency bands The measurements are taken using a vector network analyser under computer control. The parallel beam enables a “Through,Reflect,Line” calibration technique to be used. After calibration the sample under test is placed in the beam (mid way between the reflectors) and the four microwave ‘S’ parameters are recorded automatically in complex form. The permittivity, permeabilty or lumped admittance (if the sample is very thin <ë/50) for the material are then determined from the ‘S’ parameters. The operation and performance of the system is discussed and some material parameter measurement results are given.
Microwave holography is a popular method for diagnosis and alignment of phased array antennas. Holography, commonly known in the near-field measurement community as "back transformation", is a method that allows computation of the primary (aperture) fields from the secondary (far-zone) fields. This technique requires the far-zone fields to be known over a complete hemisphere and adequately sampled on a regular spaced grid in K-space. The holography technique, while known to be mathematically valid, is subject to errors just as all measurements are. Surprisingly, very little work has been done to quantify the accuracy of the procedure in the presence of known measurement errors. It is unreasonable to think that the amplitude and phase of the array elements can be trimmed to better than the uncertainty of the back-transformed amplitude and phase. This makes it difficult for an antenna engineer to determine the achievable resolution in the measurement and calibration of a phased array antenna. This study reports the results of an empirical characterization of known errors in the holography process. A numerical model of the near-field measurement and holography process has been developed and many test cases examined in an effort to isolate and characterize individual errors commonly found in planar microwave holography. From this work, an error budget can be developed for the measurement of a specific antenna.
Undesired antenna motion can significantly degrade SAR and ISAR image quality on an instrumentation radar operating in an outdoor or uncontrolled environment. Antenna vibration on the order of only a few hundredths of an inch at X-band frequencies can degrade performance to the point that one cannot reliably differentiate between the true and false peaks in the radar image. This paper describes a motion compensation technique that utilizes the measurements from an auxiliary antenna pointing at a stationary target. This "Phase Monitoring Subsystem" accurately records the linear antenna motion profile, which can then be used for compensation. Data collected at the US Naval Undersea Warfare Center (NUWC) Fisher Island Test Facility on a calibration target demonstrate that this compensation technique can reduce image artifacts by more than 20 dB.
Recently there has been a large effort to improve RCS range performance. Reducing errors associated with an RCS measurement requires the identification of stray signal sources, highly accurate calibration, and an understanding of the target mount interactions. This paper will illustrate the potential errors resulting from target mount interaction. A complex RCS target of generic shapes was designed to illustrate target support interactions. Target features include a front wedge shape, a rear circular shape and a vertical fin. All the target features are separable in time using a 2-18 Ghz measurement system. The target features were designed to strongly interact with the ogival pylon. Measurements using the metal ogival support show strong interactions resulting from the shadowing effect produced by the metal ogival pylon. The measurements were repeated using a foam column mount. Since the foam column interacts much less strongly than the metal ogive, the foam column results are much more accurate.