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The RF electromagnetic spectrum, extending from 2 MHz to 94 GHz, can be considered an evolving, but finite resource. This span of spectrum is used for a multitude of purposes including communications, radio and television broadcasting, radio navigation, sensing and radar. The portion of the spectrum from 2-4 GHz has become particularly problematical due to the influx of wireless systems such as WiMAX into a region which has traditionally been associated with radar systems. This paper will discuss the different types of measurements made on radars and other non-radar users of this spectrum. First, the propagation physics of why different types of radars reside in selected frequency bands and why other non-radar systems seek and have gained entrance to these bands is reviewed. The unique spectral characteristics of radars, not generally found in communications systems will be addressed. The trade-offs in using conventional superheterodyne and FFT based spectrum analyzers versus the newer real time spectrum analyzers will be discussed in terms of their capability in evaluating radar spectrum. The spectral characteristics of a variety of communications waveforms will be explored, such as OFDM, and how they can complicate the process of measuring radar type signals in a radiated test. The types of antennas that are used in these radar systems will be discussed and how these antennas influence the reliability of these measurements. Finally, some of the issues associated with separating out antenna effects from waveform effects in legacy & future radars will be discussed. The need for improved measurement techniques and systems will also be addressed.
Large dual reflector compact ranges are typically designed for antenna and payload testing of spacecraft antennas and payload units. Astrium's Compensated Compact Ranges have two major advantages for such measurements: (a) A very small cross-polarization (< -40 dB over the entire test zone) for frequencies = 3 GHz due to the compensating reflector design, (b) A scanning capability of the test zone due to the short effective focal length of the reflector system. The first item is a necessary condition for precise spacecraft antenna measurements at which the cross-polar performance is an important requirement and was subject to multiple publications in the past. The second one, the scanning capability, is an additional feature that was addressed in the past, but has not been analyzed in detail so far. This paper addresses practical implementations, achieved performance figures of the latest installations and inherent limitations by the utilization of the scanned quiet zones at a CCR test facility for antenna and payload testing.
An improved system for antenna gain extrapolation measurements is proposed. The improved method consists of a vector network analyzer, a pair of RF optical links, and a pair of waveguide mixers. This change in hardware equates to a system with better dynamic range and a simplified reference measurement. We present a detailed description of the new extrapolation measurement setup, discuss the advantages and disadvantages, and validate the new setup by measuring the gain of an antenna previously measured with a traditional extrapolation setup. After presenting the comparison, we will discuss applications of this measurement system that extend beyond extrapolation gain measurements (e.g., spherical near- and far-field pattern measurements).
A desired feature of modern field probes is that the useable bandwidth should exceed that of the Antenna Under Test (AUT) [1]. Recent developments in probe and orthomode junctions (OMJ) technology has shown that bandwidths of up to 4:1 are achievable [2-5]. The probes are based on inverted ridge technology capable of maintaining the same high performance standards of traditional probes However, in typical Spherical Near Field (SNF) measurement scenarios, the applicable frequency range of the single probe can also be limited by the content of µ.1 spherical modes in the probe pattern [6-7]. This is because the traditional NFtoFF software applies probe correction under the assumption that the probe pattern is fully specified from knowledge of the E-and H-plane patterns only [8]. While this condition is guaranteed for virtually any type of probe for small illumination angles of the AUT and/or a long probe/AUT distance this assumption may lead to unacceptable errors in special cases. This paper describes the design and experimental verification of a Ka-band probe based on the inverted ridge technology. The probe is intended for high precision SNF measurements in special conditions that require less than -45dB higher order spherical mode content. This performance level has been accomplished through careful design of the probe and meticulous selection of the components used in the external balanced feeding scheme. The paper reports on the electrical and mechanical design considerations and the experimental verification of the modal content.
There are a number of near-field measurement situations where it is desirable to use a broad band probe to avoid the need to change the probe a number of times during a measurement. But most of the broad band probes do not have low cross polarization patterns over their full operating frequency range and this can cause large uncertainties in the AUT results. Calibration of the probe and the use of probe pattern data to perform probe correction can in principle reduce the uncertainties. This paper reports on a series of measurements that have been performed to demonstrate and quantify the cross polarization levels and associated uncertainties that can be measured with typical log periodic (LP) probes. Two different log periodic antennas were calibrated on a spherical near-field range using open ended waveguides (OEWG) as probes. Since the OEWG has an on-axis cross polarization that is typically at least 50 dB below the main component, and efforts were made to reduce measurement errors, the LP calibration should be very accurate. After the calibration, a series of standard gain horns (SGH) that covered the operating band of the LP probe were then installed on the spherical near-field range in the AUT position and measurements were made using both the LP probes and the OEWG in the probe position. The cross polarization results from measurements using the OEWG probes where then used as the standard to evaluate the results using the LP probes. Principal plane patterns, axial ratio and tilt angles across the full frequency range were compared to establish estimates of uncertainties. Examples of these results over frequency ranges from 300 MHz to 12 GHz will be presented.
The numerical analysis used for efficient processing of spherical near-field data requires that the far-field pattern of the probe can be expressed using only azimuthal modes with indices of µ = ±1. (1) If the probe satisfies this symmetry requirement, near-field data is only required for the two angles of probe rotation about its axis of . = 0 and 90 degrees and numerical integration in . is not required. This reduces both measurement and computation time and so it is desirable to use probes that will satisfy the µ = ±1 criteria. Circularly symmetric probes can be constructed that reduce the higher order modes to very low levels and for probes like open ended rectangular waveguides (OEWG) the effect of the higher order modes can be reduced by using a measurement radius that reduces the subtended angle of the AUT. Some analysis and simulation have been done to estimate the effect of using a probe with the higher order modes (2) – (6) and the following study is another effort to develop guidelines for the properties of the probe and the measurement radius that will reduce the effect of higher order modes to minimal levels. This study is based on the observation that since the higher order probe azimuthal modes are directly related to the probe properties for rotation about its axis, the near-field data that should be most sensitive to these modes is a near-field polarization measurement. This measurement is taken with the probe at a fixed (x,y,z) or (.,f,r) position and the probe is rotated about its axis by the angle .. The amplitude and phase received by the probe is measured as a function of the . rotation angle. A direct measurement using different probes would be desirable, but since the effect of the higher order modes is very small, other measurement errors would likely obscure the desired information. This study uses the plane-wave transmission equation (7) to calculate the received signal for an AUT/probe combination where the probe is at any specified position and orientation in the near-field. The plane wave spectrum for both the AUT and the probe are derived from measured planar or spherical near-field data. The plane wave spectrum for the AUT is the same for all calculations and the receiving spectrum for the probe at each . orientation is determined from the far-field pattern of the probe after it has been rotated by the angle .. The far-field pattern of the probe as derived from spherical near-field measurements can be filtered to include or exclude the higher order spherical modes, and the near-field polarization data can therefore be calculated to show the sensitivity to these higher order modes. This approach focuses on the effect of the higher order spherical modes and completely excludes the effect of measurement errors. The results of these calculations for different AUT/probe/measurement radius combinations will be shown.
Total radiated power (TRP) and total isotropic sensitivity (TIS) are two metrics most commonly used to characterize the performance of a wireless device. These integrated measurement parameters are not as sensitive to the measurement distance as a single point measurement such as an antenna gain measurement, but it is difficult to accurately quantify the effects of measurement distance on these two parameters. This paper presents a simple approach to quantifying the effects of measurement distance using spherical near-field transforms. Data is taken on a typical wireless device at different range lengths and transformed to the far-field using a spherical near-field transform. The total radiated power is then calculated for both the measured data and the transformed data. The difference in the two calculations shows the effect of a finite range length on the measurement. Measured results are presented for three different range lengths. For each of these range lengths the data is transformed to the far-field and the TRP is calculated.
A four arm sinuous antenna able to sustain an order of magnitude higher continuous wave (CW) power than commercially available sinuous articles is presented. The wide bandwidth performance from 550MHz to 2.5GHz of the sinuous was preserved with the use of dielectric lens loading and ferrite-tiled cavity backing. The conducted tests show the antenna capability to radiate at least 200W of input CW power. A high power beamformer network built using commercial of the shelf (COTS) components was also characterized. The integrated antenna and beamformer configuration shows excellent thermal and electromagnetic behavior across the frequency band. Presented results may pave the way for use of the sinuous antennas in relevant wideband high power applications.
In this paper application of quasi-planar near-field measurements to characterize the radiation from a test object with the purpose of electromagnetic compatibility (EMC) will be described. First a brief description of an EMC radiated emission Open Area Test Site (OATS) and the setup of a typical EMC quasi-planar table top near-field scanner will be presented. Then the challenges of adapting the near-field scanning technology used for antennas to near-field scanning of EMC test objects will be discussed. Specifically the challenges of 1) Obtaining phase from active equipment under test (EUT) with a radiation caused by quasi stationary random processes. 2) Challenges in probe design and construction that yields satisfactory sensitivity, cross polarization rejection and field disturbance. 3) Measurement in the reactive near-field region and analysis of the probe impact on the measured data. 4) The problems of characterization of non-planar EUT geometry that violates the equivalent surface theorem due to cables leaving the box enclosed by the surface. 5) Near-field measurements on non-directionally radiating test objects 6) Post processing of EMC near-field data: combining of several measurement data sets and taking multiple reflections into account when inserting measured near-field in a CAD model of the full apparatus. 7) Current status in predicting the absolute radiation level in dBuV/m, as measured in OATS, from a near-field measurement
The Air Force Research Laboratory (AFRL), Information Directorate, has served as an Air Force center for antenna measurements for over thirty years. AFRL's Newport Research Facility consists of multiple far field outdoor test ranges with 3-axis positioner towers. The range supports a wide variety of test activities including measurements on simple antennas, complex active phased arrays, avionics, communications and electronic countermeasure systems. The trend towards increasingly complex antenna systems led to a requirement for a faster, more adaptable data acquisition system. To support the changing test requirements, AFRL developed a multi-port 16-RF-switch controller as part of our data acquisition system. In a typical antenna test at Newport, multiple aircraft antennas are multiplexed into a single RF output through a programmable matrix of solid state RF switches. The switch controller is installed inside the aircraft under test which is mounted on a 3-axis positioner. We can then configure and reconfigure each aircraft for a variety of antenna tests at Newport.
Measurement of E and H plane far field patterns for an open-ended rectangular waveguide in the free air operating between the frequencies of 16 and 19 GHz are shown and compared with the simulated patterns derived by several authors. Although the theoretical expressions give a broader pattern for the E-plane than for the H-plane, which is observed, measurements exhibit a sharper decay in the E-plane than the one obtained by simulation. In this work, we calculate the errors associated with the use of the different models that fail to correctly approximate the E-plane. Finally, we introduce a parameter in the best model to adjust the effective aperture dimensions in order to obtain a more realistic representation of the measured far field.
A technique is proposed to measure the permittivity and permeability parameters of a sample of biaxial material placed into a rectangular waveguide. By constructing the material as a cube, only a single sample is required to find all six material parameters. The sample is inserted into the waveguide in three different orientations, and the transmission and reflection coefficients of the sample region are measured using a vector network analyzer. The material parameters are then found by equating the measured S-parameters to those determined theoretically using a mode-matching technique. The theoretical details are outlined and the extraction process is described. Comparison of the mode-matching S-parameters with those obtained using the commercial finite element solver HFSS validates the theory.
Ground to air imaging poses numerous technical challenges, a number of which relate to target motion throughout its inverse synthetic aperture. A well-tracked target benefits not only its illumination, but provides an accurate description of the target’s position as a function of time. Tracking may be accomplished using a monopulse tracking radar or precision GPS/telemetry techniques; either of which are sufficiently accurate for coarse translational motion compensation. However, without target attitude telemetry, the inverse synthetic aperture may still be inferred from the target’s spherical coordinates and their first derivatives, and coarse rotational motion compensation may be performed. Further refinement is available from the in-phase/quadrature data. Residual translational motion may be characterized and corrected by considering both intra-chirp (i.e., within stepped chirps) and inter-chirp phase migration, accommodating fine translational motion compensation. The data become reasonably bandlimited, allowing rotational motion compensation to be performed via bandlimited resampling, yielding a focused ISAR image.
All of us involved with antenna measurements or radar cross section measurements are familiar with the absorber seen on the walls, ceiling, and floor of anechoic chambers. It helps simulate free-space conditions. It comes in various shapes and lengths, and it reduces the reflections, or unwanted energy, from encroaching on the quiet zone. But what makes one absorber better than another? Further, what advances in composition have been made over the last 50 years to improve the simulation of free space? This paper will address differences in geometry and differences in materials and “ingredients” for optimizing performance. Also, it will discuss the advantages in using different materials to create stronger absorber to help maintain performance and for creating clean and safe environments, for such endeavors as measurements involving flight hardware.
The use of adaptive acquisition techniques to reduce the overall test time in planar near-field antenna measurements was presented in [1] & [2]. In those publications the concept of a decision function to track the uncertainty of a measurement as the data acquisition proceeds and also to adapt the acquisition region dynamically, was introduced. In this publication we build upon that work and present the concept of near-field array initialization. This is tested on different antennas and simulation results are presented. We also present actual measurement results to validate simulations that have to date been used to demonstrate advantages of the adaptive techniques.
A Plane Wave Synthesis Approach for mitigating errors in antenna measurements caused by stray signals and imperfections in the measurement range illuminating fields has been demonstrated previously for a 2D scalar source [1]. This paper presents algorithms developed for the Range Transfer Function (RTF) method for a 2D vector source. Vector basis functions for both the field representation and the AUT representation are implemented to provide a robust numerical solution. The new algorithms are more stable because the plane wave angles and the antenna measurement angles may be completely general, provided that Nyquist rules of sampling are observed during both the field probing (to obtain the plane wave coefficients) and the antenna measurement (to obtain the raw pattern data).
This paper addresses the difficulties in measuring the broadband complex permeability of thin films using conventional stripline or microstrip permeameters and outlines a novel methodology to solve them. It is shown using full-wave simulations that several of the conventional assumptions made for extracting permeability from a permeameter are not justified. In particular, the proportionality factor used to relate the measured effective permeability to the actual film permeability is shown not to be a constant. Another typical drawback is the need for a known reference sample for calibration. By exploiting the analyticity of the function relating effective to true permeability we have come up with a general methodology to derive this proportionality function for permeameters free of the problems mentioned before. The validity of the method is confirmed with fullwave simulations. Moreover, this general approach can be applied to other similar test devices. A key issue in measuring a thin film’s permeability over a broadband frequency range is assuming that its permittivity is known. More often than not, this data is not available. We show a method to extract a film’s permeability without the need to assume or know its permittivity value. This is done measuring two identical films of equal widths.
As a novel concept for field probes, RCS measurements on long rigid objects rotated within a small angular range about the broadside condition are reported. The rotation was maintained either in a horizontal (H) plane or in a vertical (V) plane containing the center of the quiet-zone (QZ). Processing the RCS data by DFT yields a spectrum which is recognized as the field distribution along that object. This spectrum compares extremely well to traditional field-probes taken earlier by translating a sphere across the QZ in H- or V-direction. Preliminary results at several S-band frequencies are presented and discussed.
Traditionally Taper chambers are constructed using a square based pyramidal shaped taper. The taper is then shaped into an octagon and finally transformed into a cylindrical launch section. This approach is related to the manufacturability of different absorber cuts. This presentation introduces a chamber where the conical shape of the launch in continued through the entire length of the taper chamber. The results are of the free space VSWR test over a 1.5m diameter quiet zone are presented at different frequencies. The conical taper appears to have a better illumination wave front and better QZ levels even at frequencies above 2GHz than the standard traditional approach. The implementation of this design was done in Singapore and the actual chamber was designed to have a secondary Near Field range for Planar and spherical scans.
An adaptive approach for optimized sampling in cylindrical and spherical near-field antenna measurements is described. The presented technique applies higher sampling density in rapidly varying near-field regions and skips data points in the smoother regions. Abrupt changes in the near field are detected by comparing the extrapolated and the measured near-field values at coarser sampling points during the measurements. A decision function based on signal-to-noise ratio of the measured value is used to determine the threshold difference between the extrapolated and the measured near-field value for skipping the sampling point. The reduced near-field data collected is processed using the fast irregular antenna field transformation algorithm (FIAFTA). FIAFTA is computationally efficient and capable of handling data on irregular grids with full probe correction. Several test cases are then presented on the applicability of the given approach and significant reduction in the number of measurement points is observed, thereby reducing measurement time and the computational effort.