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The techniques that are presently available for the measurement of complex permittivities of dielectric substrates are only applicable to single layer substrates. This paper presents a new technique which uses the Finite Element Method (FEM) to estimate the complex permittivities of individual layers from the measurement of the S-parameters of a rectangular waveguide holding a multilayer dielectric substrate sample. In this method, a network analyzer is used to measure reflection and transmission coefficients f a rectangular waveguide loaded with a layered sample. Using FEM, the reflection and transmission coefficients are determined as a function of the complex permittivities of the multilayer substrate. Measured and calculated values of the reflection and transmission coefficients are then matched using the Newton-Raphson Method to estimate the complex permittivities of the layers of the sample.
In this paper we present an improved theoretical modelling for the free space technique for measuring the complex permittivity of materials at microwave frequencies. The theory was developed for a transmission set-up with two identical pyramidal horn antennas. By performing a spectral decomposition of the aperture fields, the new model takes the effect of the non plane wave character into account when the sample is not placed in the far field of the transmitting antenna. With the use of the new theoretical model it becomes possible to place the sample much closer to the antennas without infringing the theoretical assumptions since no plane wave incidence is needed. In this way the transversal dimensions of the sample can be reduced significantly. The validity of the new theoretical model was verified by measurements on many dielectric (Plexiglas, polystyrene,…) and lossy materials. A comparison was made with the values obtained when the usual plane wave theory is used.
Multipactor breakdown is a possible payload failure for communication satellites. The multipactor phenomenon significantly increases noise levels interfering with signals relayed by a satellite; it can even damage or destroy RF components and transmission lines. In order to retire the risk of single point failure in a recent spacecraft C-band antenna suite, certain critical components were tested to determine the threshold of multipaction. The components tested were: waveguide isolators, a waveguide polarization switch, and a waveguide PIM filter. The need to test the components at higher power levels than had been attempted on previous multipaction tests required the development of a new test system. The system utilized ring resonant multiplication to develop pulsed power levels of +77 dBm. The multipaction global detection methods utilized a spectrum analyzer montoring the notched noise level, and peak power meters monitoring the arrier pulse shape for distortion. This paper briefly describes the test system developed.
A technique of measuring the mutual capacitance per unit length of a shielded twisted pair transmission line is developed and used to verify a finite difference model developed previously by the authors. Attempts to measure the very small capacitance values using a capacitance bridge were unsuccessful. The phase technique presented here is easily performed and gives good results.
In this paper we discuss proper RF system design for performing insertion-loss measurements using a microwave receiver and harmonic mixers. Specifically we will deal with problems caused by changing reflection coefficients of the devices which feed the mixer. When broadband mixers and coaxial isolators are used problems may be caused by the changing load seen by the local oscillator. This is due to local oscillator leakage through the mixer and isolator. We will elaborate on this problem, noting its impact on the measurement and suggest a procedure to properly minimize its effect.
To accurately measure static or dynamic Radar Cross Section (RCS), one must use precise measurement equipment and test procedures. Recently, several DoD RCS ranges, including the Advanced Compact RCS Measurement Range at Wright-Patterson AFB, established procedures to estimate measurement error. Working cooperatively with the National Institute of Standards and Technology (NIST), Wright Laboratory established a baseline error budget methodology in 1994. As insight was gained from the error budget process, we noted that many common RCS measurement calibration techniques are subject to a wide variety of potential error sources. This paper examines two common so-polarized calibration devices (sphere and squat cylinder), and discussed techniques for evaluating calibration induced errors. A rigorous “double calibration” methodology is offered to track calibration measurement error. These techniques should offer range owners fairly simple methods to monitor the quality of their primary calibration standards at all times.
The calibration of nonreciprocal radars has been studied extensively. A brief review of known calibration techniques points to the desirability of a simplified calibration procedure. Fourier analysis of scattering data from a rotating dihedral allows rejection of noise and background contributions. Here we derive a simple set of nonlinear equations in terms of the Fourier coefficients of the data that can be solved analytically without approximations or simplifying assumptions. We find that independent scattering data from an additional target such as a sphere is needed to accomplish this. We also derive mathematical conditions that allow us to check calibration data integrity and the correctness of the mathematical model of the scattering matrix of the target.
Scientific-Atlanta has developed a new algorithm for obtaining high accuracy cross-polarization measurements from prime focus, single reflector, compact ranges. The algorithm reduced cross-polarization extraneous signals to levels that rival or exceed much more expensive dual reflector systems, but with the associated cost and simplicity of a single reflector system. This paper provides an overview of the new algorithm. It explains the limitations on conventional polarization measurements in single reflector systems and the methods for overcoming these limitations without error correction for some antennas. A method for determining if error correction is needed for a particular antenna is reviewed and the fundamentals of the error correction algorithm are explained. Preliminary test results are provided.
This paper presents several enhancement tools that were developed to improve the Georgia Tech Spherical Far-Field/ Near-Field Antenna Measurement Range. Measurement amplitude and phase drift was quantified by sampling an antenna measurement signal over long time intervals while leaving the AUT rotation positioners fixed. A return-to-point drift correction tool was implemented to correct for the long-term drift component for spherical surface measurements. Temperature sensitive components of the receiver were moved from an area with severe temperature variations to a temperature stable area to reduce the phase variation. A software tool was developed to display a histogram of the variation in repeated spherical scan measurements. Histogram vales show that drift correction improves the repeatability of an antenna pattern measurement. The shapes of the histograms have been helpful in identifying random and deterministic variations.
We describe an imaging technique which allows the isolation of sources of unwanted radiation on an antenna/RCS range. The necessary data may be collected by using a roll-over azimuth mount to scan a probe over a spherical measurement surface.
A method to generate time and direction of arrival (TADOA) spectra of the quiet zone fields of a RCS/ antenna range is presented. The TADOA spectra is useful for locating the stray signal sources in the RCS/ antenna range. To generate the TADOA spectra, quiet zone fields along a linear scan over the desired frequency band are probed. The probed data are calibrated to remove the magnitude and non-linear phase variation versus frequency. A calibration technique is also proposed in the paper. The TADOA spectra for simulated probed data as well as experimental probed data are shown.
In the antenna measurement field, large investments are typically required for installations; however, these installations soon become obsolete due to advances in technology. In order to recover as much of the original investment as possible, upgrading the original installation becomes an attractive alternative. Here, we detail an ongoing upgrade of a planar near-field system. The original single-beam, single frequency system was installed approximately twenty years ago. By replacing the network analyzer, and installing a faster computer controller and accompanying software, the system was upgraded to a state-of-the-art measurement system. The upgraded system is capable of high-speed beam and frequency switching on the fly. Using the existing scanner, motor control hardware, and laser positioning sensors, the system was delivered at a significantly reduced cost.
The use of global positioning satellite (GPS) signals for automotive navigation and this on-vehicle GPS antennas has become more common recently. As the number of users increases the cost of the highly integrated receiver is predicted to come down to less than $50. It is possible to measure the antenna patterns of GPS antennas as installed on vehicles, but it is important to make sure that the parameters measured are valid for the GPS environment. In this case, sky coverage and polarization are more important than the directive pattern, for example. This paper shows a method of comparing a number of antennas by using the actual GPS satellite signals as test signals.
Dynamic ground-to-air measurement of aircraft RCS has several advantages over static measurements. The target may be measured in flight configuration and the support pylon is eliminated. Although dynamic RCS imagery has been performed since the late 1970s, the cost and complexity of such measurements have limited their utility for routine testing. In this paper, an easily deployable ground-to-air radar imaging system developed by Aeroflex Lintek is presented. This system forms images of aircraft in straight flight, requiring no on-board instrumentation or special pilot training. The radar system, flight profiles, and processing tools required for generating images of aircraft in flight are presented, along with examples of measured target data.
NSI recently completed installation of two large 7m x 7m horizontal planar scanners to support the Globalstar satellite program test activity. These systems were installed at Alcatel in France, and Alenia in Italy. These two systems are similar to the NSI system installed at Space Systems/ Loral in Palo Alto, CA. described in previous AMTA papers. The companies are part of the Globalstar satellite consortium, committed to launching a constellation of satellites for mobile telephone communications. The paper will summarize the hardware configuration and the unique features of the two new test systems including high power phased array testing and the interface to the Globalstar payload for active antenna control and payload testing. In addition, range data comparing all 3 test ranges will be shown.
A cost-effective, versatile instrumentation system for measuring antennas and radomes is described. The system features the use of high load capacity, high accuracy stepper motor based positioners as the primary system axes. The system is capable of being easily reconfigured to perform tests on antenna/radome systems with antennas fixed relative to the radome, or with the antenna and radome capable of movement relative to one another. Measurements may be performed at RF, IF or baseband, depending on the portions of the seeker or monopulse assembly to be included in the test. The system also contains analysis capabilities that simulated mode forming and beam forming functions to isolate antenna effects.
A new 1-50 GHz Near-Field measurement system is now in operation at Matra Marconi Space, Portsmouth, UK. The system has the largest vertical planar scanner installed so far. The planar scanner is constructed of steel and has four moving axes: 22 meter horizontal X axis, 8 meter vertical Y axis, 25 cm Z axis for probe alignment and a 540o Roll axis for polarization. Precision bearings are used to ensure straightness over the full length of the X-Y travel. The vertical Y axis is exceptionally fast, 500 mm/sec, to minimize acquisition time. The scanner has extremely high positioning accuracy and planarity - ±0.2 mm over the entire 22m x 8m range – allowing uncorrected operation (without laser) up to 26.5 GHz. To achieve higher accuracy and a higher frequency range an advanced 3-axis (X, Y, Z) laser correction system automatically creates correction tables for use by the transformation routines. The scanner’s exceptional repeatability allows the use of correction tables created off-line, without need for an on-line laser correction system, considerably reducing measurement time. To create these correction tables, the scanner is fitted with laser interferometers for X and Y axes and with a spinning-diode laser to calibrate for planarity. Additional features include a shielded constant-radius cable carrier, giving minimal phase errors due to cable flexing.
We discuss how RCS target depolariza tion enhances cross-polarization contamination, and we present a graphical study of measurement error due to depolarization by an inclined dihedral reflector. Error correction requires complete polarimetric RCS measure ments. We present a simple polarimetric calibration scheme that is applicable to reciprocal antenna radars. This method uses a dihedral calibration target mounted on a rotator. Because the calibration standard can be ro tated, there is no need to mount and align multiple sepa rate standards, and clutter and noise may be rejected by averaging over rotation angle.
We discuss how RCS target depolariza tion enhances cross-polarization contamination, and we present a graphical study of measurement error due to depolarization by an inclined dihedral reflector. Error correction requires complete polarimetric RCS measure ments. We present a simple polarimetric calibration scheme that is applicable to reciprocal antenna radars. This method uses a dihedral calibration target mounted on a rotator. Because the calibration standard can be ro tated, there is no need to mount and align multiple sepa rate standards, and clutter and noise may be rejected by averaging over rotation angle.
Historically, radar imaging sensors have been divided into two categories, SAR and ISAR systems. Even though they are solving the same imaging prob lems the data collection environment is dramatically dif ferent between the two. Consequently, the particular waveforms selected for the two have been different. The primary waveform for ISAR RCS measurement systems is stepped frequency, while the FM-chirp (linear-FM) waveform has been used much more often in SAR applications. However, recently this boundary has been blurred, in that stepped frequency radars are being applied to long range dynamic measurements, long the domain of chirped waveforms, and conversely the chirped waveform has been applied to target RCS mea surements of both static and dynamic targets. This paper will address the system parameter tradeoffs involved in selecting between the two waveforms for two different applications; (i) near range static target imaging, and (ii) far range dynamic target imaging. The system parameter tradeoffs involve RF bandwidth, PRF, scene size, trans mitter power, doppler frequency spread of target, etc. The advantages, disadvantages, and inherent limitations of each waveform will be analyzed to yield a better understanding of the tradeoffs involved, and the data collection examples will further illustrate these tradeoffs for the two specific applications.