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Practical Considerations for Determining Polarization Properties from Measured Linear Components
Polarization properties (e.g. axial ratio, sense, and tilt) of an antenna under test (AUT) are often calculated from measurements with a linear (or dual-linear) polarized range antenna. At first, these calculations appear to be simple and straightforward. However, there are several different conventions used in the literature and some important practical aspects of the measurements are often omitted. Neglect of these small details can easily lead to incorrect results, with the most common error being the reversal of the right-hand-circular and left-hand-circular polarization components. We note the differences in the published polarization conventions and provide practical tips for good polarization measurement practices. We also describe stepby-step procedures for determining AUT polarization properties from two styles of polarization measurements using a linear (or dual-linear) polarized range antenna.
Modeling and Analysis of Anechoic Chamber using CEM Tools
Advances in computational resources facilitate anechoic chamber modeling and analysis at VHF frequencies using full-wave solvers available in commercial software such as FEKO. The measurement community has a substantial and increasing interest in utilizing computational electromagnetic (CEM) tools to minimize the financial and real estate resources required to design and construct a custom anechoic chamber without sacrificing performance. A full-wave simulation analysis provides a more accurate solution than the approximations inherent to asymptotic ray-tracing techniques, which have traditionally been exploited to overcome computational resource limitations. An anechoic chamber is simulated with a rectangular down-range cross-section to utilize the software’s capability to assess polarization performance. The absorber layout within the anechoic chamber can be optimized using FEKO for minimal reflections and an acceptable axial ratio in the quiet zone. Numerical results of quiet zone disturbances and axial ratios are included for both low- and medium-gain source antennas over a broad frequency range.
Performance of Scanned Quiet Zones in Compensated Compact Ranges for Antenna and Payload Testing
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.
MEASURING LOW CROSS POLARIZATION USING A BROAD BAND, LOG PERIODIC PROBE
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.
Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction
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.
Challenges and solutions for quasi-planar near-field measurements in reactive zone applied for EMC purpose
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
Electronically Controlled Tilt Angle Of A Linearly Polarized Signal At Ka-Band
As part of a target simulator , a linearly polarized signal was required with a variable tilt angle that could be controlled electronically and changed at a 1 kHz rate. However, microwave components available in the 33.4 – 36 GHz operating range were inadequate to achieve the desired performance. A novel approach was developed to downconvert the input signal to a lower frequency range and use vector modulators available in this band to produce the appropriate phase and amplitude changes in each path, then upconvert back to the desired operating frequency to drive an orthomode transducer. A calibration and measurement procedure was developed to determine the vector modulator input settings that produced the most accurate tilt angles and best cross-polarization performance. By iteratively measuring cross-polarization and tilt angle, then adjusting the vector modulator controls, a tilt angle accuracy of +/-1 degree was achieved with a crosspolarization of -25 dB, exceeding the required performance. This paper provides an overview of the concept, a block diagram of the design, discussion of the calibration and measurement procedure, and a summary of the results achieved.
Ultrawideband Electromagnetic Polarization Filter (Uwb-Empf) Applications To Conventional Horn Antennas For Substantial Cross-Polarization Level Reduction
This paper addresses the difficulties of achieving lower cross-polarization EM field transmission (or reception) levels by employing wideband electromagnetic polarization filters (EMPF). These EMPFs are applied as add-on screens to reduce the cross-polarization levels of standard gain horns (SGH) and diagonal horns (DH). Cross-polarization level reduction as much as 19 dB is presented for diagonal horns with add-on screens. Similarly, more than 9 dB cross-polarization reduction is shown for standard gain horns across the operational bandwidth. Later, an alteration is done on the add-on screens by making use of Ludwig's 3rd coordinate definition. This modification results in further cross-polarization suppression in the vicinity of boresight direction. For instance, on X-Band SGH, near boresight angular region where cross-polarization level keeps below -60 dB increases by 26% with use of this modified add-on screen.
Design And Measurement Of A 2:45 Ghz On-Body Antenna Optimized For Hearing Instrument Applications
A balanced PIFA-inspired antenna design is presented for use with the 2:45 GHz ear-to-ear radio channel. The antenna is designed such that the radiated electric fields are primarily polarized normal to the surface of the head, in order to obtain a high on-body path gain (jS21 j). The antenna structure can be made conformal to the outer surface of a hearing instrument, such that the bandwidth of the antenna is optimized given the available volume. The radiation patterns, ear-to-ear path gain and available bandwidth is measured and compared to the simulated results. It is found that the antenna obtains a relatively high ear-to-ear on-body path gain, as well as a bandwidth that is large enough to cover the entire 2:45 GHz ISMband.
Understanding The Evolution Of An Ebg Meta-Surface From A Pec Ground Plane
This paper details a systematic procedure of the evolution of an electromagnetic band-gap (EBG) meta-surface from a simple ground plane. The main aspect of the paper is to understand the behavior of these EBG surfaces for low profile antenna design solutions. Reflection phase diagrams are used as a criterion to understand the flat reflection phase response of these surfaces for all angles of incidences and all polarizations. The evolution of EBG from a PEC ground plane to a via-loaded PEC ground plane to a planar patch-type surface and finally to a Mushroom-EBG surface is presented in a novel way. In addition, uniplanar compact-EBG (UC-EBG) properties are also investigated. It is observed that the EBG structures are most robust to different incident angles and polarizations making it a powerful candidate for low profile antenna design solutions. Additionally, the band-gap properties of planar patchtype, Mushroom-EBG and UC-EBG are studied and representative designs are provided.
On The Development Of 18-45 Ghz Antennas For Towed Decoys And Suitability Thereof For Far-Field And Near-Field Measurements
The development of a wideband, high-power capable 18-45 GHz quad-ridge horn antenna for a small towed decoy platform is discussed. Similarity between the system-driven antenna specifications and typical requirements for gain and probe standards in antenna measurements (that is, mechanical rigidity, null-free forward-hemisphere patterns, wide bandwidth, impedance match, polarization purity) is used to assess the quad-ridge horn as an alternative probe antenna to the typical open-ended rectangular waveguide probe for measurements of broadband, broad-beam antennas. Suitability for the spherical near-field measurements is evaluated through the finite element-based full-wave simulations and measurements using the in-house NSI 700S-30 system. Comparison with the near-field measurements using standard rectangular waveguide probes operating in 18-26.5 GHz, 26.5-40 GHz, and 33-50 GHz ranges is used to evaluate the quality of the data obtained (both amplitude and phase) as well as the overall time and labor needed to complete the measurements. It is found that, for AUTs subtending a sufficiently small solid angle of the probe’s field of view, the discussed antenna represents an alternative to typical OEWG probes for 18-45 GHz measurements.
The Software Defined Antenna: MEMS Based Pixel Prototyping
A microstrip feed and patch antenna capable of physical reconfiguration during deployment can provide beam steering and operation at adjustable frequencies and polarizations. This reconfigurable structure uses z-axis deflection of small pixels with a sandwiched ground layer, substrate, and conducting top layer. The Pixel Addressable Reconfigurable Conformal Antenna (PARCA) technology enables millisecond reconfiguration of a microstrip structure on a pixel-by-pixel basis. Pixel fabrication and actuation methods developed using microelectromechanical systems (MEMS) techniques enabled smaller pixels (1 mm square), which allow higher frequencies than past prototypes. Previous pixels relied on a thick metallization to make contact between adjacent pixels and yielded a pixel mass too large for MEMS z-axis deflection. A new pixel with a thin metallization combined with a matrix of dot electrodes on a superstrate has been implemented. Pixels in the up position make DC contact with the dot electrodes and are coupled to adjacent up pixels. This configuration resulted in lighter pixels with better pixel to pixel coupling. A 10x10 matrix of pixels with a set of dot electrodes was measured with results comparable to a continuous patch antenna of similar dimensions. The dot electrode matrix enables DC contact between adjacent pixels and makes a significant difference in the performance.
Compact Microstrip Defected Ground Structured Antenna for Reduction of Harmonics and Cross Polarized Radiations
Abstract: A novel Inset fed microstrip patch antenna is designed on defected ground structure (DGS) to reduce the higher order harmonics and cross polarized radiations. Square rings, and square shaped slots are placed on the ground plane of the microstrip patch antenna to get the desired operation. These annular ring and arc DGS appears to be highly efficient in terms of suppressing the cross polarization. Relative suppression of radiated field is observed by placing and without placing the defected ground structures. The stop band property exhibited by the DGS is used to filter out the harmonics. The current model successfully reduced the DGS size and by comparing with the well known design, size reduction of 20% is achieved. Instead of normal square patch, a slotted aperture patch model is considered in the current design to reduce the overall size of the antenna.
Advancements in Achieving What is Asked of a Compact Range
Phase variation = +/-10 deg. 18 to 40 GHz Phase variation = +/-20 deg. 40 to 110 GHz Cross Polarization = -30 dB III. MAXIMUM AVAILABLE SPACE Consistency of performance across a waveguide band levies demands on compact range feeds. Because of the constraint of the room size, the design starts with determining the maximum space available for the This paper addresses a recent compact range development by MI reflector. The next step will be to determine the combination of Technologies that achieves desired extended low frequency and reflector body and edge treatment size within that space to millimeter wave performance (1 to 110GHz) while maintaining a deliver the desired performance. To determine the space cost effective reflector size and a small range footprint. The paper available for the reflector a chamber layout analysis is will explore the conventional rule-of-thumb relationships performed. Appropriate absorber is selected and, allowing for between feed, reflector, edge treatments and range geometries an air gap of at least 2 wavelengths at the lowest desired while contrasting them to the resultant design. The paper will frequency between the absorber and the reflector, and allowing highlight an impressive new family of compact range feeds and advancements in cost effectively achieving a superior reflector height for the compact range feed positioner yields the surface. allowable reflector dimensions to be 194 inches high and 222 inches wide as shown in Table 2. The combination of reflector
Scattering of residual field above and beyond the quiet zone of a compact range
Abstract: In order to characterize the Boeing 9-77 compact range, the empty chamber background was measured as a function of frequency, polarization, and the azimuth angle of the upper turn-table (UTT). The results exhibited a near-field diffraction pattern with enlarged hot-spots on a 4-fold symmetry . A 2-D FFT on the diffraction pattern yielded a mapping on the relative arrangement of the absorbers on the UTT . In this paper, we take a closer look at the scattering geometry of the UTT as illuminated by the residual field above and beyond the quiet zone (QZ). The different responses in VV and HH are discussed. The enhanced diffraction due to a “blazed grating” condition is identified and analyzed.
Millimeter Wave Polarization Calibration for Near-Field Measurements
Abstract—In order to optimize accuracy of near field measurements, it is required not only to acquire data for two orthogonal polarizations, but the relative amplitude and phase balance between the two channels must also be accurately matched. This can be difficult at millimeter wave frequencies because of the transmission lines and other components involved. ORBIT/FR has explored multiple methods of achieving optimum vertical and horizontal polarization matching and found a very simple solution to achieve acceptable results. Some of the methods investigated included the use of dual-polarized feeds, dual single-polarized feeds mounted adjacently, waveguide rotary joints with a mechanically rotated feed, and a mechanically-rotated feed using a 1.0 mm coaxial-based cable. Interestingly, the mechanically-rotated feed with coaxial cable provided acceptable results on par with or better than the other methods, which moreover results in a very simple implementation in the measurement system. Measured results are presented for the chosen implementation demonstrating the near field data quality is adequate for a variety of antennas.
Transformation of Monostatic Near-Field Scattering Data By Fast Irregular Field Transformation Algorithms
Abstract—It is well-known that a complete bistatic set of near-.eld scattering data is required to compute far-.eld radar cross section (RCS) quantities. In many practical applications, however, only monostatic scattering data is available. Almost all algorithms for the transformation of monostatic near-.eld data are based on the synthetic aperture radar (SAR) image representation.Since these algorithms are often acceleratedbythe fastFouriertransform(FFT),they usuallypose manylimitations on the measurement procedure such as regularly spaced grids and separate treatment of the different polarizations due to scalar processing. In this paper, a novel and .exible algorithm is presented which is not based on the FFT but on multi-level fast multipole method (MLFMM) principles. Therefore, it is similar to the fast irregular antenna .eld transformation algorithm (FIAFTA) which has been designed for the transformation of antenna .elds and measurements. Numerical results of different scenarios show that these principles can also be successfully applied to monostatic scattering data. In summary, this approach is superior to existing algorithms, because it provides more .exibility while it is still very ef.cient.
Four-Arm Wideband Log-Periodic Antenna and its High Power Measurements
Abstract—Four arm Log-Periodic (LP) antennas are frequency independent antennas that are capable of producing dual circular polarizations from the same aperture and over the same bandwidth making them more versatile than commonly used spiral antennas. In this paper we present a four arm LP that is capable of being a high power radiator. Each pair of arms of the LP is fed with a microstrip line that functions as both an impedance transformer and a 180° balun, thereby greatly simplifying the required beamformer. The antenna is tested successfully up to 500W of input CW power. Post high power characterizations of the antenna (far-field gain, radiation patterns, and VSWR) for linear polarization are presented and the stable high power performance of the antenna is demonstrated. With an appropriate beamformer, good quality circular polarization can be expected. Presented results should pave the way for use of the LP in relevant wideband high power applications.
On the Use of Phase Data When Conducting an Extrapolation Measurement
Abstract— The extrapolation measurement technique has been used with the three antenna method for more than 40 years, to determine absolute antenna gain and polarization. The critical part of the extrapolation technique is an insertion loss measurement that is done repeatedly as two antennas are physically separated. When the two antennas are close together, they may unintentionally interact and reflect signals between the antennas. Part of the data processing procedure requires a subjective determination about the distance at which the antennas are far enough apart that they are no longer interacting with each other. In this paper, we present an alternative method with the goal of providing a more objective way to determine when the antennas are no longer interacting with each other. The proposed method relies on using the phase data obtained from the insertion loss measurement.
Surface Electromagnetic Wave Characterization Using Non-invasive Photonic Electric Field Sensors
Abstract— Electromagnetic properties of aircraft and missile skins have a large effect on radar cross sections and determine the level of stealth that is achieved over the various RF bands currently in use. RF absorption, reflection, and propagation along the skin surface all serve as important measures of the electromagnetic performance of the coated surfaces. Non-invasive probing of the electromagnetic field just above the propagating wave at multiple spots along the propagation direction can be used to determine and measure wave propagation parameters, including effective RF index, loss per length, wave impedance, and frequency dependent material properties of the coatings. Wide-band photonic electric field sensors have been demonstrated for probing of dielectric layers by measuring the traveling waves along the coated aircraft surface. The photonic E-field sensors are extremely linear and produce an exact real time analog RF representation of the electric field, including phase information. These ultra-wideband (UWB) photonic RF sensors are very small and contain negligible metal content, allowing them to be placed at close proximity without perturbing the RF surface waves. This is very important in accurately characterizing highly damped surface waves on absorber layers. This paper discusses the linearity, bandwidth, polarization, and sensitivity of the unique UWB photonic E-field sensor design. Experimental results are presented on surface-wave characterization measurements using these sensors.
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