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A single reflector CATR exhibits certain depolarizing properties, as any other offset reflector antenna, when illuminated by a linearly polarized feed in its focal point. One way to improve the polarization purity in the Quite Zone (QZ) is to use a feed antenna which aperture fields provide a conjugate match to the electric-field distribution in the reflector’s focal plane when illuminated with a linearly polarized plane wave. MVG developed and built a proof-of-concept demonstrator in the form of a 3x1 array of linearly polarized elements, exited to match the focal plane distribution as predicted by a full-wave simulation of the specific range. This demonstrator has been installed in the CATR at RWTH Aachen University, which is a corner-fed serrated edge reflector system with a 1.2 m diameter QZ and a specified maximum cross polarization level of -30 dB (edge of QZ) due to the offset geometry. In this paper we will show measurement results for planar co- and cross-polar probing of the QZ in X-band, using the demonstrator and compare it to the respective results using the range’s conventional, low cross polarization, corrugated feed horn. The measured data will also be cross-checked against the full-wave simulation results for the fields in the QZ. Furthermore, we will compare 0°, 90°, ±45° pattern cuts of a demanding Antenna Under Test, a 40 cm x 40 cm offset reflector antenna with a wide band dual ridge horn as a feed, again using the demonstrator and the conventional feed. This is to show the potential improvement in measurement quality by using a matched feed in a single reflector CATR.
A Gaussian electromagnetic radiation is always attractive for many scientific research and some practical applications. The importance of the Gaussian microwave beam, especially for high power microwave region of operation, is that its maximum energy density is concentrated on its axis. A two-spiral corrugated Bragg reflector, at the inner surface of a circular waveguide, is a novel way to provide such a radiation. The Bragg reflector has been designed and optimized using the fully electromagnetic HFSS tool. Such a reflector converts the operating TM01-mode of the Cerenkov devices to the forward TE11-mode with a Gaussian microwave beam at the output. The use of the Bragg reflector is not only to reflect the injected TM01-mode but also to convert it to a clean TE11-mode pattern. A cold test structure is fabricated to test the theoretical predictions of the microwave transmission versus frequency and the dispersion characteristics. The dispersion relation is found from the discrete measured resonant frequencies and wave numbers of a cavity containing eight periods of the slow-wave structure. Generally, a slow wave structure has N periods will exhibit N+1 resonant frequencies when shorted at planes of mirror symmetry. The main purpose of this study is to experimentally determine the dispersion relation of the structure. Test results using a vector network analyzer showed a good agreement with the simulations for the excitation of the TM01-mode at 10 GHz.
In military applications, where low sidelobes and high precision in beam pointing are vital, a phased array antenna beamformer requires to be calibrated regarding the cabling that connects the beamformer to the antenna and mutual coupling between antenna elements. To avoid problems associated with mismatched phase transmission lines between the beamformer and the antenna and include the coupling effects, beamforming network characterization must be done with the antenna integrated to the beamformer. In this paper, a method to characterize the beamformer of a slotted waveguide array antenna in the antenna measurement range is introduced. The antenna is a travelling wave slotted waveguide array scanning in the elevation plane. The elevation pattern of the antenna is a shaped beam realized by a phase-only beamformer. The calibration channel includes serial cross-guide couplers fed by a single waveguide line. The channel is integrated to the waveguide lines of the antenna. In the first phase of the characterization, the far field pattern of each antenna element is obtained from the near field measurements at the “zero” states of the phase shifters. In the second stage, all states of the phase shifters are measured automatically using the calibration channel described above. The results of calibration channel measurements are used to determine the changes in phase and magnitude for different states of phase shifters. The phase and magnitude of the peak point of the far field pattern is referenced to the zero state measurement of the calibration channel. Phase only pattern synthesis is carried out using the results of both zero-state near field and calibration channel measurements and the required phase shifter states are determined accordingly. Measured patterns show good agreement with the theoretical patterns obtained in the synthesis phase.
Applications using millimeter-wave antennas have taken off in recent years. Examples include wireless HDTV, automotive radar, imaging and space communications. NSI has delivered dozens of antenna measurement systems operating at mm-wave frequencies. These systems are capable of measuring a wide variety of antenna types, including antennas with waveguide inputs, coaxial inputs and wafer antennas that require a probing station. The NSI systems are all based on standard mm-wave modules from vendors such as OML, Rohde & Schwarz and Virginia Diodes. This paper will present considerations for implementation of these systems, including providing the correct RF and LO power levels, the impact of harmonics, and interoperability with coaxial solutions. It will also investigate mechanical aspects such as application of waveguide rotary joints, size and weight reduction, and scanner geometries for spherical near-field and far-field measurements. The paper will also compare the performance of the various mm-wave solutions. Radiation patterns acquired using some of these near-field test systems will be shared, along with some of the challenges encountered when performing mm-wave measurements in the near-field.
Radome enclose antennas to protect them from environmental influences. Radomes are ideally electrically transparent, but in reality, radomes introduce transmission loss, pattern distortion, beam deflection, etc. Radome diagnostics are acquired in the design process, the delivery control, and in performance verification of repaired and newly developed radome. A measured near or far-field may indicate deviations, e.g., increased side-lobe levels or boresight errors, but the origin of the flaws are not revealed. In this presentation, source reconstruction from measured data is used for radome diagnostics. Source reconstruction is a useful tool in applications such as non-destructive diagnostics of antennas and radomes. The radome diagnostics is performed by visualizing the equivalent currents on the surface of the radome. Defects caused by metallic and dielectric patches are imaged from far-field data. The measured far-field is related to the equivalent surface current on the radome surface by using a surface integral representation together with the extinction theorem. The problem is solved by a body of revolution method of moment (MoM) code utilizing a singular value decomposition (SVD) for regularization. Phase shifts, an effective insertion phase delay (IPD), caused by patches of dielectric tape attached to the radome surface, are localized. Imaging results from three different far-field measurement series at 10 GHz are presented. Specifically, patches of various edge sizes (0.5?2.0 wavelengths), and with the smallest thickness corresponding to a phase shift of a couple of degrees are imaged. The IPD of one layer dielectric tape, 0.15 mm, is detected. The dielectric patches model deviations in the electrical thickness of the radome wall. The results from the measurements can be utilized to produce a trimming mask, which is a map of the surface with instructions how the surface should be altered to obtain the desired properties for the radome. Diagnosis of the IPD on the radome surface is also significant in the delivery control to guarantee manufacturing tolerances of radomes.
The Space Communications and Navigation (SCaN) Testbed was developed to investigate the applicability of software defined radios (SDR) to NASA space missions, study the operation of SDRs and their waveform applications in an operational space environment, and reduce cost and risk for future space missions using SDRs. The SCaN Testbed, developed at NASA’s Glenn Research Center, is currently installed on the International Space Station and has line-of-sight connection to NASA’s Space Network and compatible ground stations. To characterize the operation of the SDRs and their waveforms, a new and unique capability was developed at GRC, which uses purposeful antenna off-pointing to provide a wide range of power levels to the input of the SDR. With this capability, a radio can be more fully tested and characterized on-orbit. This paper describes the new antenna off-pointing capability and methodology, and how it was applied to characterize the on-orbit performance of an S-Band radio in the SCaN Testbed. It provides details of the antenna pointing system control algorithm, gimbal articulation limitations, medium gain antenna pattern profile, and phase limitations associated with the medium gain antenna. Finally, the paper presents test results and lessons learned.
The performance of a typical narrowband antenna array is reduced by mutual coupling between radiating elements. The degree to which this inter-element coupling occurs may be correlated with the resonant characteristics and tendency for late time ringing of an individual element. The parabolic reflector impulse radiating antenna (IRA) is an ultra-wideband (UWB) antenna which by virtue of its design and aim to radiate a very short time-domain signal demonstrates significantly decreased late time ringing. Given this quality, the suitability and performance of the reflector IRA in an array configuration is examined. Modeling and simulation of the reflector IRA is accomplished using commercially available software and single antenna results are compared to measured data. Full-wave simulation of arrayed reflector IRAs in varying physical configurations and excitation modes is performed The relative levels of coupling and degradation of radiation pattern and signal quality are discussed.
Antenna alignment for near-field scanning was typically done at NIST with multiple instruments (theodolites, electronic levels, motor encoders) to align multiple stacked motion stages (linear, rotation). Many labs and systems are now using laser trackers to measure ranges and perform periodic compensation across the scan geometry. We are now seeing the use of laser trackers with 3D coordinate metrology to align ranges and take positional data. We present the alignment techniques and positional accuracy and uncertainty results of a mmWave antenna scanning system at 183 GHz. We are using six degree-of-freedom (6DOF) AUT and Probe measurements (x, y, z, yaw, pitch, roll) to align the AUT and then to align the scan geometry to the AUT. We are using a combination of 3DOF laser tracker measurements with a combined 6DOF laser tracker/photogrammetry sensor. We combine these measurements using coordinated spatial metrology to assess the quality of each motion stage in the system, tie the measurements of each individual alignment together, and to assess scan geometry errors for position and pointing. Finally we take in-situ 6DOF position measurements to assess the positional accuracy to allow for positional error correction in the final pattern analysis. The knowledge of the position and errors allow for the correction of position and alignment of the probe at every point in the scan geometry to within the repeatability of the motion components (~30 µm). The in-situ position knowledge will eventually allow us to correct to the uncertainty of the measurement (~15 µm). Our final results show positioning errors on the spherical scan surface have an average error of ~30 µm with peak excursions of ~100 µm. This robust positioning allows for accurate analysis of the RF system stability. Our results show that at 183 GHz, our RF repeatability with movement over 180° orientation change with a 600 mm offset to be less than ±0.05 dB and ±5°.
Antenna and Radar Cross Section (RCS) measurements are often required for orientation sets (cuts) that are difficult or impossible to produce with the positioning instrumentation available in a given lab. This paper describes a general coordinate transform, combined with a general polarization rotation to correct for these orientation differences. The technique is general, and three specific examples from actual test programs are provided. The first is for an RCS measurement of a component mounted in a flat-top test fixture. The component is designed to be mounted in a platform at an orientation not feasible for the flat-top fixture, and the test matrix calls for conic angle cuts of the platform. The transforms result in a coordinated, simultaneous two-axis motion profile and corresponding polarization rotations yielding the same information as if the component had been mounted in the actual platform. The second example is for a pattern measurement of an antenna suite mounted on a cylindrical platform (such as a projectile). In this case, the test matrix calls for a roll-cut, but the range positioning system does not include a roll positioner. The transforms again result in a coordinated, simultaneous two-axis motion profile and corresponding polarization rotations to provide the same information as the required roll-cut but without the use of a roll positioner. Finally, the third example is for an antenna pattern measurement consisting of an extremely large number of cuts consisting of conic yaw cuts, roll cuts and pitch cuts. The chosen method involves the use of the Boeing string suspension system to produce great-circle cuts at various pitch angles combined with the use of the coordinate and polarization transforms to emulate, off-line, any arbitrary cut over any axis or even multiple axes. Keywords: Algorithm, Positioning, Polarization, Coordinates, RCS
A three-step equiangular (120º) phase shifting holography (EPSH) technique is proposed for THz antenna near-field/far-field prediction. The method is attractive from the viewpoint of receiver sensitivity, phase accuracy over the entire complex plane, simplified detector array architecture, as well as reducing planarity requirements of the near-field scanner. Numerical modeling is presented for the holographic receiver performance, using expected phase shift calibrations errors and phase shift noise. The receiver model incorporates responsivity and thermal noise specifications of a commercial Schottky diode detector. Additionally, simulated near-field patterns at 372GHz demonstrate the convenience of the method for accurate and high dynamic range THz near-field/far-field predictions, using a phase-shifter calibrated to ±0.1°.
The near-field – far-field (NF–FF) transformation with spherical scanning is particularly interesting, since it allows the reconstruction of the complete radiation pattern of the antenna under test (AUT) [1]. In this context, the application of the nonredundant sampling representations of the electromagnetic (EM) fields [2] has allowed the development of efficient spherical NF–FF transformations [3, 4], which usually require a number of NF data remarkably lower than the classical one [1]. In fact, the NF data needed by this last are accurately recovered by interpolating a minimum set of measurements via optimal sampling interpolation (OSI) expansions. A remarkable measurement time saving is so obtained. However, due to an imprecise control of the positioning systems and their finite resolution, it may be impossible to exactly locate the probe at the points fixed by the sampling representation, even though their position can be accurately read by optical devices. As a consequence, it is very important to develop an effective algorithm for an accurate and stable reconstruction of the NF data needed by the NF–FF transformation from the acquired irregularly spaced ones. A viable and convenient strategy [5] is to retrieve the uniform samples from the nonuniform ones and then reconstruct the required NF data via an accurate and stable OSI expansion. In this framework, two different approaches have been proposed. The former is based on an iterative technique, which converges only if there is a biunique correspondence associating at each uniform sampling point the nearest nonuniform one, and has been applied in [5] to the uniform samples reconstruction in the case of cylindrical and spherical surfaces. The latter relies on the singular value decomposition method, does not exhibit the above limitation, but can be conveniently applied only if the uniform samples recovery can be reduced to the solution of two independent one-dimensional problems [6]. Both the approaches have been applied and numerically compared with reference to the positioning errors compensation in the spherical NF–FF transformation for long antennas [7] using a prolate ellipsoidal AUT modelling. The goal of this work is just to validate experimentally the application of these approaches to the NF–FF transformation with spherical scanning for elongated antennas [4], using a cylinder ended in two half-spheres for modelling them. The experimental tests have been performed in the Antenna Characterization Lab of the University of Salerno, provided with a roll over azimuth spherical NF facility supplied by MI Technologies, and have fully assessed the effectiveness of both the approaches. [1] J.E. Hansen, ed., Spherical Near-Field Antenna Measurements , IEE Electromagnetic Waves Series, London, UK, Peter Peregrinus, 1998. [2] O.M. Bucci, C. Gennarelli, C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop. , vol. 46, pp. 351-359, 1998. [3] O.M. Bucci, F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “Data reduction in the NF–FF transformation technique with spherical scanning,” Jour. Electr. Waves Appl ., vol. 15, pp. 755-775, June 2001. [4] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Effective antenna modellings for NFFF transformations with spherical scanning using the minimum number of data,” Int. Jour. Antennas Prop ., vol. 2011, Article ID 936781, 11 pages, 2011 [5] O.M. Bucci, C. Gennarelli, G. Riccio, C. Savarese, “Electromagnetic fields interpolation from nonuniform samples over spherical and cylindrical surfaces,” IEE Proc. Microw. Antennas Prop ., vol. 141, pp. 77-84, April 1994. [6] F. Ferrara, C. Gennarelli, G. Riccio, C. Savarese, “Far field reconstruction from nonuniform plane-polar data: a SVD based approach,” Electromagnetics, vol. 23, pp. 417-429, July 2003 [7] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Two techniques for compensating the probe positioning errors in the spherical NF–FF transformation for elongated antennas,” The Open Electr. Electron. Eng. Jour. , vol. 5, pp. 29-36, 2011.
We propose a technique which achieves highly accurate near-field data as well as far-field patterns despite the positioning inaccuracy of the scanner in the antenna near-field measurements. The method involves position sensing hardware in conjunction with data processing software. The underlying theory is provided by the Field Mapping Algorithm (FMA), which transforms exactly the measured field data on a conventional planar, spherical, or cylindrical surface, indeed on any enclosing surface, to any other surface of interest. In our modified near-field scanning system, a position recording laser device is attached to the probe. The positions of data grid points are thus found and recorded along with the raw RF data. The raw data acquired over an irregular, imperfect surface is subsequently converted exactly to a designated, regular surface of canonical type based on the FMA and its associated position information. Once the near-field data is determined at all required grid points, the far-field pattern per se is obtained via a conventional near-field-to-far-field transformation. Moreover, and perhaps just as importantly, the interplay between our FMA and the free-form position/RF recording methodology just described allows us to bypass entirely the arduous task of strict antenna alignment. The free-form position/RF data are simply propagated by the FMA software to some perfectly aligned reference surface ideally adapted as a springboard for any intended far-field buildup. Our proposed marriage of a standard scanning system and a position recorder, with otherwise imperfect RF/location data restored to ideal status under the guidance of the FMA, clearly offers the advantage of high precision at minimal equipment cost. It is, simply stated, a win-win budget/accuracy RF measurement solution. Two analytic examples and one measurement case are given for demonstration. The first example is a circular aperture within an infinite conducting plane, the second is a 10 lambda x 10 lambda dipole array antenna. The measurement case involves a waveguide slot array antenna. In all three cases, the near-field data were deliberately acquired over imperfectly located grid points. The FMA was then applied to obtain near-field data at the preferred, regularly arranged grid points from these position compromised values. Excellent grid-to-grid near-field comparison and calculated far-field results were obtained.
In recent times, planar near-field antenna measurements have largely been performed within fully absorber lined anechoic chambers. However this is a comparatively recent development as, due to the nature of the electromagnetic radiation when measuring medium to high gain antennas, one can often obtain excellent results when testing within only a partially absorber lined chamber [1], or in some cases even when using absorber placed principally behind the acquisition plane. As absorber can be bulky and costly, optimizing its usage often becomes a significant factor when planning a new facility. This situation becomes more pressing when the designated test environment is not exclusively devoted to antenna pattern testing with non-ideal absorber coverage being, in some cases, mandated, c.f. EMC testing. Planar test systems lend themselves to deployment within multipurpose installations as they are routinely constructed so as to be portable [2] thereby allowing partial or perhaps complete removal of the test system between measurement campaigns. This paper will present measured data taken using a number of different planar antenna test systems with and without anechoic chambers to summarize what is achievable and to provide design guidelines for testing within non-ideal anechoic environments. NSI’s Planar Mathematical Absorber Reflection Suppression (MARS) technique [3, 4] will be utilized to show additional improvements in performance that can be achieved through the use of modern sophisticated post processing. Keywords: Planar Near-Field, Reflection Suppression, Scattering, MARS. REFERENCES S.F. Gregson, A.C. Newell, G.E. Hindman, M.J. Carey, “Extension of The Mathematical Absorber Reflection Suppression Technique To The Planar Near-Field Geometry”, AMTA, Atlanta, October 2010. G.E. Hindman, “Applications of Portable Near-Field Antenna Measurement Systems”, AMTA, October, 1991. S.F. Gregson, A.C. Newell, G.E. Hindman, “Advances In Planar Mathematical Absorber Reflection Suppression”, AMTA, Denver, Colorado, October 2011. S.F. Gregson, A.C. Newell, G.E. Hindman, P. Pelland, “Range Multipath Reduction In Plane-Polar Near-Field Antenna Measurements”, AMTA, Seattle, October 2012.
A technique is presented for determining the pattern of an antenna in the focused near-field from cylindrical near-field measurements. Although the same objective could be achieved by conventional near-field to far-field transformation followed by a back projection, the proposed technique has an intuitive appeal and is considerably simpler and faster. The focused near-field antenna pattern is obtained by applying Huygens’ principle, as embodied in the field equivalent principle, directly to near-field measurements and by including an “obliquity factor” to suppress backlobe radiation. The technique was experimentally verified by comparison with far-field patterns obtained by conventional cylindrical near-field to far-field transformation and by EM simulations. Excellent agreement in sidelobe levels and beamwidth was achieved. The technique was applied to the 25 in diameter reflector antenna of a harmonic radar operating at 5.8 GHz and 11.6 GHz. Since the operating range of this radar is less than 40 ft, the reflector is the near-field at both frequencies. By defocusing the reflector at the harmonic frequency the beamwidths and gains at both frequencies can be made the same. The defocusing is accomplished by exploiting the frequency dependent phase center displacement of a log-periodic feed.
Experimental measurement plays a key role for technology maturation in an R&D environment. In this paper we highlight the versatility of a new compact range at the Air Force Research Laboratory (AFRL), Sensors Directorate. In its first year of operation, the OneRY Range supported a wide variety of projects ranging from electrically small antennas to 20’ structures, spanning frequencies of 400 MHz to 45 GHz, and involving applications covering land, airborne, and space-based platforms. Here we present measured results from three different antenna development efforts for the Air Force. The first effort involves a UHF meta-material inspired antenna developed for an airborne application. In addition to successfully demonstrating relatively low frequency capability for a compact range, this effort met the challenge to measure antenna patterns from a physically large target. Results from OneRY are compared to those collected from a tapered chamber. Next we show experimental measurement of digital beam forming (DBF) in a large conformal phased array antenna operating at L and S bands. The DBF experimental testing is part of a follow-on effort to an Advance Technology Demonstration conformal array supporting satellite tracking, telemetry and command (TT&C). Finally, we present results from a “quick look” investigation into the operability of a COTS antenna system matched to a third party radome. The project supports airborne satellite communications at K, Ka, and Q bands. Performance of a high frequency extension (18-50 GHz) to the compact range is examined to include an inter-range comparison to planar near-field measurements. A description of the OneRY Indoor Range is also provided.
The development of the Configurable Robotic Millimeter-Wave Antenna facility (CROMMA) by the antenna metrology lab at the National Institute of Standards and Technology in Boulder Colorado has brought together several important aspects of 6-degree-of-freedom robotic motion, positioning and spatial metrology useful for high frequency antenna characterization. In particular, the ability to define a unified coordinated metrology space, which includes all the motion components of the system is at the heart of this facility. We present the details of integrating robotics that have well defined kinematic models, advanced spatial metrology techniques, and millimeter wave components which make up the CROMMA facility. From this, a high level of precision, accuracy, and traceability that is requisite for performing high frequency near-field antenna pattern measurements can be achieved. Emphasis is placed on the ability to precisely characterize and model the movement patterns of the robot positioners, and probe and test antenna apertures using state-of-the-art full 6-degree-of-freedom spatial metrology, while being able to manipulate this information in a unified measurement space. The advantages of using a unified coordinated metrology space as they pertain to complex antenna alignments, scan geometry, repeatability analysis, traceability, and uncertainty analysis will be discussed. In addition we will also discuss how the high level of positioning, and orientation knowledge obtainable with the CROMMA facility can enable the implementation of sophisticated near-field position correction algorithms and precisely configurable scan geometries.
It is well known that a field probe acts as a filter for the measured antenna under test (AUT) fields, whose influence can be either described in spatial or in spectral domain. Directive probes, for instance, serve to filter out signals that originate far away from the boresight axis. However, there are several drawbacks to the use of such directive probes including the possibility of multiple reflections and probe nulls. This contribution discusses the application of beamforming techniques to suppress unwanted echo signals in planar near-field antenna measurements. The AUT is measured with a small probe antenna such as is normally used for such measurements. Neighboring measurement signals are thereafter combined in a moving average manner in order to generate the signal as would be measured by a probe array. Successive filter lengths, such as 3x3, 5x5, etc., are utilized such that the valid angle is preserved without extending the measurement plane. The generated near-field signals are then transformed using a flexible plane wave based near-field far-field transformation algorithm. Probe correction does not reverse the reduction in multipath signals achieved by the use of a directive probe or beamforming since sources are assumed only within the minimum sphere enclosing the AUT. Results are presented for simulated data with substantially improved results of the far-field pattern of the AUT.
Fieldprobing is often the tool of choice for validating the characteristics of a quiet zone (QZ). Some of the main disadvantageous of fieldprobing are the expense and stability of the setup, e.g. a stable non-reflective linear axis has to be build. Furthermore regular 1-dimensional fieldprobing is not very suited for detecting extraneous reflections in the measurement chamber. Former work has shown that using a second linear axis below the AUT positioner (which is sometimes present for Antenna Pattern Comparison (APC) measurements) can improve the detection, but further increases the cost factor. Using Spherical Near-Field scanning [FRANCIS,WITTMANN,BLACK,JOY] most of these disadvantageous are solved, only a rather simple, although sturdy, beam is built on top of the roll-over-azimuth positioner, placing the antenna on a sphere surrounding the QZ. Using only one measurement, for each frequency, a complete analysis of the measurement chamber can be performed. It can be used for both looking inside the QZ, i.e. chamber reflectivity and outside on extraneous reflections. This paper will show both actual spherical near-field and fieldprobing measurements of the CATR at the Institute of High Frequency Technology (IHF) of the RWTH Aachen, and compare both results.
In classical probe-corrected spherical near-field measurements, one source of measurement errors, not often given sufficient consideration is the probe [1-3]. Standard near-field to far-field (NFFF) transformation software applies probe correction with the assumption that the probe pattern behaves with a µ=±1 azimuthal dependence. In reality, any physically-realizable probe is just an approximation to this ideal case. Probe excitation errors, finite manufacturing tolerances, and probe interaction with the mounting interface and absorbers are examples of errors that can lead to presence of higher-order spherical modes in the probe pattern [4-5]. This in turn leads to errors in the measurements. Although probe correction techniques for higher-order probes are feasible [6], they are highly demanding in terms of implementation complexity as well as in terms of calibration and post-processing time. Thus, probes with high azimuthal mode purity are generally preferred. Dual polarized probes for modern high-accuracy measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port isolation. As a desired feature of modern probes the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign. Consequently, the probe design is a trade-off between performance requirements and usable bandwidth. High performance, dual polarized probe rely on balanced feeding in the orthomode junction (OMJ) to achieve good performance on a wide, more than octave, bandwidth [5-7]. Excitation errors of the balanced feeding must be minimized to reduce the excitation of higher order spherical modes. Balanced feeding on a wide bandwidth has been mainly realized with external feeding network and the finite accuracy of the external components constitutes the upper limits on the achievable performance. In this paper, a new OMJ designed entirely in waveguide and capable of covering more than an octave bandwidth will be presented. The excitation purity of the balanced feeding is limited only by the manufacturing accuracy of the waveguide. The paper presents the waveguide based OMJ concept including probe design covering the bandwidth from 18-40GHz using a single and dual apertures. The experimental validation is completed with measurements on the dual aperture probe in the DTU-ESA Spherical Near-Field facility in Denmark. References: [1]Standard Test Procedures for Antennas, IEEE Std.149-1979 [2]Recommended Practice for Near-Field Antenna Measurements, IEEE 1720-2012 [3]J. E. Hansen (ed.), Spherical Near-Field Antenna Measurements, Peter Peregrinus Ltd., on behalf of IEE, London, UK, 1988 [4]L. J. Foged, A. Giacomini, R. Morbidini, J. Estrada, S. Pivnenko, “Design and experimental verification of Ka-band Near Field probe based on wideband OMJ with minimum higher order spherical mode content”, 34th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2012, Seattle, Washington, USA [5]L. J. Foged, A. Giacomini, R. Morbidini, “Probe performance limitation due to excitation errors in external beam forming network”, 33rd Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2011, Englewood, Colorado, USA [6]T. Laitinen, S. Pivnenko, J. M. Nielsen, and O. Breinbjerg, “Theory and practice of the FFT/matrix inversion technique for probe-corrected spherical near- eld antenna measurements with high-order probes,” IEEE Trans. Antennas Propag., vol. 58, no. 8, pp. 2623–2631, Aug. 2010. [7]L. J. Foged, A. Giacomini, R. Morbidini, "Wideband dual polarised open-ended waveguide probe", AMTA 2010 Symposium, October, Atlanta, Georgia, USA. [8]L. J. Foged, A. Giacomini, R. Morbidini, “ “Wideband Field Probes for Advanced Measurement Applications”, IEEE COMCAS 2011, 3rd International Conference on Microwaves, Communications, Antennas and Electronic Systems, Tel-Aviv, Israel, November 7-9, 2011.
Traditional rectangular waveguide measurements are operated in the frequency regime of the dominant TE10 mode. The general guideline for determining the dominant mode frequency regime is to operate 25% above the TE10 mode cutoff to avoid high dispersion and 5% below the TE20 cutoff to avoid higher-order mode excitation. The X-band waveguide for example, with cross-sectional dimensions 0.9 inches by 0.4 inches, has a TE10 and TE20 cutoff frequency of 6.56 and 13.12 GHz, respectively. Using the above guideline, the approximate bandwidth of operation is 8.2-12.4 GHz. In addition, coax-to-waveguide adapters must be employed in order to connect the network analyzer coaxial cables to the rectangular waveguide sections. In modern (i.e., commercially off the shelf - COTS) microwave coax-to-waveguide adapters, tuning stubs are employed to minimize voltage standing wave ratio and thus maximize energy coupling into the waveguide sections. Unfortunately, these tuning stubs are placed in asymmetric patterns that can cause coupling into the TE20 mode, which is the very reason why one must operate at a frequency of at least 5% below this mode to safely avoid higher-order mode contamination. The goal here is to show that, by designing symmetric coax-to-waveguide adapters, excitation of the TE20 mode can be avoided for operational frequencies above the TE20 cutoff. Thus, the frequency of operation may be extended to the TE11 mode (the next higher-order mode that can exist) having a cutoff frequency of 16.16 GHz. Consequently, the operational frequency band is enhanced from 8.2-12.4 GHz to 8.2-15.4 GHz, representing a 70% improvement in operational bandwidth. A comparison of newly-designed symmetric and COTS asymmetric coax-to-waveguide adapters for material characterization measurements will be provided and advantages/limitations will be discussed.