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In this paper we present an optical imaging tool, the Overlay Imaging Aligner (OIA), developed to aid in the mechanical alignment of antenna components in the mm-wave and low-THz frequency regimes (50-500 GHz) where the millimeter and sub-millimeter wavelengths pose significant challenges for alignment. The OIA uses a polarization-selective, machine-vision approach to generate two simultaneous and overlaid real-time digital images along a common axis. This allows for aligning two antenna components to within fractions of a wavelength in the mm-wave and THz frequency regimes. The overall concept, optical design, function, performance characteristics and application examples are presented. Preliminary data at specific frequencies in the WR-2.2 band are presented that compare the alignment achieved with the OIA to an electrical alignment.
Conventional antenna measurement systems use a multiplexer or polarization positioner to sequence polarization and or antenna elements as a function of time, requiring two or more measurement intervals. However, a simpler, more cost effective, and faster technique can be implemented by using frequency diversity to distinguish between polarizations or antenna elements. This paper describes how two slightly different frequencies can be used to make two measurements simultaneously instead of sequentially, cutting the measurement time in half or even more. Additional considerations must be taken into account to achieve good measurements. This paper addresses these issues. Actual measurements are presented.
A method is presented to accurately characterize the backscatter and attenuation properties of vegetation using high resolution measurements with the vegetation placed on a turntable. By this method we obtain a controlled scenario of realistic vegetation. To obtain high cross range resolution, 2D-ISAR technique was used. The full obtainable resolution is then defined by the registered bandwidth (2 GHz) and aspect angle width. 2D-ISAR images were produced from which areas of interest were gated out where the vegetation backscatter coefficient was calculated. This, along with antenna tapering compensation and distance compensation allowed us to accurately normalize the vegetation backscatter coefficient. The received signal power was made independent of range and system parameters by calibration. Hence the received power signal can be written to be only dependent on the backscattering radar cross sections. The resulting values of the volume backscattering and extinction parameters are presented for reeds and birch vegetation at HH and VV polarization.
Multi-layer dielectric rod (MLROD) antenna has been shown to provide wideband, dual-polarization, symmetric patterns, and stationary phase center. The key challenge in designing a MLROD antenna is to choose proper thickness and dielectric constant of each layer, and shape of radiation tip to meet desired VSWR, pattern, and phase center requirements. This becomes even more difficult as the number of layers increases for achieving greater bandwidth. This paper discusses a design optimization procedure of a UWB 3-layer MLROD using Genetic algorithm with novel fitness functions for simultaneously controlling reflection coefficient, phase center, and pattern three key characteristics. The final design exhibited excellent desired performance throughout the desired frequency range.
The proposed method is based on fiber-optic link connected to the antenna under test in order to measure the radiation pattern of electrically small antennas without any metallic cable effects. The experimental radiation characterization of electrically small antennas reveals significant effects of the RF cable strongly disturbing the antenna gain and radiation patterns for both polarization components. With a very simple use and a low cost, the presented systems can characterize radiation of both narrow and UWB compact antennas in the 0.1-4 GHz frequency band. The proposed test-bench is firstly validated then its performances are compared to classical cable measurements and antenna simulation results. Both measured and simulated results are compared and the agreement is excellent (±1 dB) for co polarisation in the different cut planes. Limitations of the proposed method are also addressed. For very low gain levels (for the presented antennas at 950 MHz and 473 MHz, a cross polarization level below -30 dBi) the 42 x 20 x 23 mm3 autonomous optical receiver is too large and alters the antenna under test radiation. In this last case, and for a using at higher frequency, it is recommended to miniaturize the optical receiver whose size is mainly governed by battery performances (6 hours).
Calibration of probes for planer near field range measurements is generally required to obtain accurate cross-polarization (xpol) data; however, probe calibration is costly and time consuming. Using analytical models in place of calibration is generally much more cost effective, but may result in larger measurement errors. In a previous paper [1], we showed that simple models of copol probe patterns with zero xpol can give accurate measured results, provided that the probe xpol is much better, generally 10-15 dB better, than the Antenna Under Test (AUT). The next question is “Can a lower performing (and cheaper) probe be used if both the copol and xpol probe patterns are modeled?” In this paper, we compute AUT xpol measurement errors that result from probe xpol errors, and we compare far field AUT patterns processed using probe models with patterns processed with calibrated probe files.
One of the most promising bands for long-range radiocommunications is the Ka-band (25-40 GHz). This is due to the existence of a natural radio transmission window around 30 GHz. Both terrestrial and satellite transmission systems are planned on this frequency band. For satellite applications, circular polarization is needed and the antennas or antenna arrays must frequently exhibit specially tailored radiation patterns. This paper proposes an efficient planar element for Ka-band telecom and remote sensing applications. The element has reduced losses (and hence good efficiency), while providing circular polarization (AR better than 3 dB) and good matching (better than 10 dB) in the 25.5-27.0 GHz frequency band. The element is fed by an integrated low loss transmission line (suspended strip line, SSL). This modular design allows an easy grouping into high efficiency subarrays, which include beam forming networks (BFNs) built in the same SSL technology and an innovative transition to the final standard waveguide feeder.
A spherical near-field measurement range at Nearfield Systems Inc. has recently been used to measure gain, pattern and polarization of a multi-element helix array operating in the UHF band. Verification of gain performance over the operating band was of primary importance and so major efforts were made to obtain the best possible gain results and to quantify the gain uncertainty through a complete error analysis. A single element helix gain standard was first calibrated and the estimated uncertainty in this calibration was 0.35 dB. A double ridged horn was to be used as the probe for the spherical near-field measurements and so the patterns of the horn at all test frequencies were measured on the spherical range using identical horns as the AUT and the probe. From these measurements, probe pattern files were generated that could be used to perform the probe correction in the measurements of the helix gain standard and the multi-element array. The helix gain standard was then installed in a new spherical near-field range at NSI with the double ridged horn as the probe. The range used a specially designed phi-over theta rotator that could support and rotate the array and maintain the required position accuracy. The chamber was lined with 36 inch absorber. Spherical measurements were then performed and the data processed to provide the far-field peak amplitudes at each frequency that were necessary for gain measurements. The far-field peak values are equivalent to the far electric field for the gain standard and are compared to the same parameter for the multi-element array to produce the final gain results. The helix array was then installed in the spherical range and a series of measurements were performed to produce the far-field gain, pattern and polarization results and also to provide the data for the complete 18 term uncertainty analysis. The uncertainty in the gain measurements was 0.45 dB and the axial ratio uncertainty was 0.11 dB.
An Indian Defense Research and Development Organization (DRDO) laboratory has commissioned a state-of-the-art indoor far-field antenna test facility in 2009. This facility supports highly accurate measurement of a wide range of antenna types over 1.12–40 GHz. Owing to the heavy usage of this range, it was decided to enhance the existing facility to include a Hybrid Planar Near-Field facility for high speed accurate antenna measurements with minimal changes to the existing chamber configuration. The scanner is implemented as a highly innovative Hybrid T-type scanner, with a Y-axis that consists of a Linear Multi-Probe array and a traditional single probe configuration. The linear Multi-Probe array consists of two sets of dual polarized probes each one covering a sub set of the full frequency range. In particular one set covers the 1.0-6.0GHz band (operational from 400MHz) and the other set covers the 6.0 to 18.0GHz band. The traditional Single Probe configuration includes a set of Open Ended Waveguide Probes to facilitate an operational frequency range of 1.12 – 40.0GHz, as in the existing Far Field system. The Hybrid scanner is placed along the sidewall opposite the door, on the DUT positioner side. The major benefit of this layout is that there is no need to change the basic design of the chamber and it is built according to the original plans. When the chamber is used in the far-field mode, the tower is moved to the end of the horizontal axis in the direction of the corner of the chamber. The tower sides that face away from the chamber corner are covered with absorbing material to reduce reflections from the tower. Assuming that the chamber is intended to measure directional antennas, the existence of the tower behind and to the side of the AUT is expected to introduce minimal interference. The high speed linear Multi-Probe array has typical measurement speeds ranging between 5 and 15 minutes at 5 frequencies and 2 polarizations. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 6.0 package for both Single Probe & Multi Probe modes. A Real-Time Controller (RTC), accompanied by a 4-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.
With the trend toward energy efficient green technology, the use of metallic films on glass is increasing. These transparent conductive coatings act like a mirror in the infrared range, reflecting heat while keeping the interior of vehicles and buildings cool and reducing energy consumption. The conductivity provides another benefit. When connected to a power supply, it can provide rapid deicing and de-fogging. A number of vehicles are available with heated windshields. However, these coatings have the potential to block the RF signals of the numerous telecommunications devices commonly installed and used inside an automobile. This paper will discuss a method for design and testing of RF apertures in such metal films using modern frequency selective surface (FSS) concepts. Just leaving a "hole" in the metal film creates manufacturing problems. In addition, electromagnetic problems include frequency resonances, side lobe peaks and nulls, polarization nulls and (in the heating application) of interrupting or concentrating the heating current flow. The design and performance measurement of arrays of polarization insensitive slots that do not interrupt heater current flow and also control internal and external side lobes will be shown.
A ground station antenna for Galileosat application operating in right hand circular polarization at P-band has been designed, manufactured, and tested. Other than stringent environmental requirements for typical ground station antennas the specification call for an antenna with very stringent requirements on pattern shape and symmetry and a very severe control on side and back lobes. In order to ease the requirement on the antenna positioner the antenna should have very compact size and low weight. The final antenna consists of an array of 7 medium gain, dual linear polarized yagi elements as shown in Figure 1. This paper describes the antenna design trade-off activity including the selection of the most suited antenna technology and manufacturing details. It also reports on the testing in the SATIMO SG-64 multiprobe spherical near field test range with considerations on the associated measurement uncertainty. The final acceptance of the antenna was based on measurements performed in CNES and SATIMO.
The Mini-RF instrument on NASA’s Lunar Reconnaissance Orbiter is gathering data toward its science goal of probing the permanently shadowed terrain for the presence of water near the lunar poles. The circular polarization ratio is the central parameter used to characterize the lunar surface using Mini-RF radar returns. Accurate use of this parameter requires on-orbit polarimetric characterization of the instrument. This is done with a combination of measurements. Indirect measurements are performed by pointing the radar at the lunar surface to remove any asymmetry in the surface backscatter. Direct characterization of transmit and receive portions of the Mini-RF radar are performed using Earth-based resources. The Earth-based resources include the: Arecibo Radio Telescope, Green Bank Telescope, and Morehead State University Space Tracking Antenna. This paper describes the measurement approach and a sample of results. The primary measurements of interest include: principal plane antenna patterns, transmit polarization ellipse, receiver amplitude and phase balance, and antenna bore sight direction. These measurement values are incorporated into the Mini-RF SAR data processing to produce quality, calibrated CPR measurements
In this paper, a symmetrically feeding structure for linear dual-polarized feeds is put forward. Due to its electrical balance and modes within the circular feeding waveguide are compressed. The polarization purity of feeds is then improved. Thus the level of cross-polarization can be very low, which is the key performance requirement for CATR (Compensated Compact Test Range). Besides, the return loss equation derived from equivalent microwave network for this symmetric structure is different from the ordinary single port feeding structure. In the low frequency range application, reflection at interface between coaxial and circular waveguide can reach a high level, a new transition probe is designed to depress it. Simulation results show that VSWR and cross-polarization performances of symmetric feeding feeds both are better than dual-polarized quadruple-ridged horn. 01TM21TE
This paper discusses design & measurement techniques for two choke horn antennas used as reflector feeds for space applications. Firstly, highly efficient ultra wideband (UWB) choke horn antenna with high beamwidth & linear polarization will be discussed. This choke horn exhibits excellent VSWR characteristics as antenna shows an incredible ultra wide bandwidth (UWB) of at least 5GHz ranging from 7.5GHz – 12.5GHz with an incredible VSWR< 1.2 for the entire frequency range. Wide beamwidth of approximately 45 degrees have been achieved at gain of 13.5dB. Secondly, wideband dual-fed circularly polarized choke horn antenna will be discussed. In this case, 90-degree phase shifter/ power coupler is being used to feed the antenna in order to keep antenna circularly polarized making polarization independent of matching issues. Both designs meet the ruggedness, strength & reliability, durability, weight, corrosion resistance, temperature variation requirements in space environment. Design Simulation & Optimization for horn antenna has been done on Finite Element Method (FEM) based software ‘Ansoft HFSS v11’. Absolute gain measurement techniques have been employed for the measurement of antenna gain.
This paper discusses a measurement technique for accurately characterizing low cross polarization level of antennas, and associated sensitivity and errors. The technique involves two-antenna transmission (S21) measurement that includes an AUT and a reference antenna that has low cross polarization level. This technique needs two far-field transmission data from two different relative roll angles. The cross-polarization sensitivity is determined by SNR of cross-polarization component and cross-polarization of the reference antenna. The cross-polarization error is related to roll angle uncertainty and receiver noise.
A unique wideband, dual-beamwidth, X-Band antenna has been developed by STAR Dynamics Corporation in support of a Ground-to-Air Radar Signature (GTARS) measurement system. The GTARS radar system provides precision dynamic RCS measurement and radar imaging capabilities for maneuvering in-flight aircraft. This specialized antenna and radar system provides the capability to track and measure dynamic radar target signatures and parameters that are not practical to measure on a static ground-based RCS measurement facility. The GTARS radar requirements posed significant challenges for the antenna design, among which are the capabilities to measure and track targets in wide and narrow fields of view (FOV) with simultaneous linear co- and cross- polarizations. Precision target tracking is required during dynamic measurements to maintain an accurate beam centered on the target during its flight. Consequently, STAR Dynamics has developed an offset reflector antenna with dual polarized five-aperture eight-port feed network that maintains the antenna beam precisely centered on the maneuvering target. The dual beamwidth functionality is achieved by two separate reflectors while each reflector provides multiple channels for simultaneous radar signature measurement and monopulse tracking using the eight-port feed array.
The design of a specialized reflector antenna set that supports dual polarization, dual beam widths, and an integrated wideband monopulse tracking capability in the X-band range is described in this paper. The reflector antenna code available at The Ohio State University has been used as the design tool. The design of such an antenna has posed several challenges in the feed and reflector assemblies. The requirement for an integrated wideband monopulse has resulted in a feed array that contains 5 rectangular feed elements with a center-to-center spacing of 1" and a diamond configuration. The 5 feed design has been selected to enable a shared feed array and reflector surface for both transmit and receive beams that eliminates the need for a high-power wideband receiver protector in the radar system. The center feed element is used for transmit waveform and the 4 outer elements are used as receive elements only. Each feed element operates with horizontally and vertically polarized waveforms, requiring a total of 8 waveguide input ports. In this paper, the challenges of the dual beam widths, dual polarized, integrated RCS and tracking antenna are delineated and the tradeoffs among several design configurations are shown. The final design is selected based on the performance predictions using The Ohio State University Reflector Antenna Code. The performance of the antenna has been validated at the OSU compact range for pattern and gain. Both the design and measurement data are presented in this paper.
Instruments for Earth observation working from W-Band up to mm-wave frequencies mainly use quasi-optical feed feeds (QOF) to illuminate the corresponding reflector antennas. The final design of the QOF for the Cloud Profiling Radar System (CPR) for the EarthCARE satellite has been finalized. The QOF is designed to perform polarization and frequency tuning, as well as the separation of transmit and receive channels. The final design verification was performed theoretically by Astrium with QUAST, a new add-on to the GRASP software, especially developed by TICRA for a fast and accurate set-up and analysis of quasi-optical networks. Within the paper, at first a short description of the QOF working at 94.05 GHz will be given. Secondly, the modeling of the QOF will be explained. At last the RF test setup will be described and comparisons between resulting calculated and measured antenna pattern will be shown.
This paper presents a new approach to the inversion of boundary value (BV) problems in an infinite conductive, homogeneous media. Our interest is to investigate the possibility of imaging underwater electromagnetic sources from remote electromagnetic sensor data. Specifically, given two polarizations of the electric/magnetic fields on a cylindrical surface exterior to the electric and magnetic sources, we develop a frequency domain, back-projection technique that allows for the complete determination of the electric and magnetic fields in the region between the BV surface and the sources. This is an inverse problem and Tikhonov regularization is used to obtain an accurate filtered, back-propagated solution. In this approach two components of the measured field yield the six components of the field closer to the source. Of particular interest is the active part of the Poynting vector that is constructed from the back-projected fields, providing the power per unit area radiated from the sources. We believe it may be of immense practical use in diagnosis of electromagnetic sources underwater. We test the theory with a numerical experiment using a linear array of either magnetic or electric dipole sources excited in a frequency range of 1 to 1000 Hz in seawater that generate two cylindrical holograms (30m radius) of the axial polarization of the magnetic and electric fields, respectively. The complete (all polarizations) electric and magnetic fields are predicted along with the real and imaginary parts of the Poynting vector on a cylindrical back-propagation surface (20m radius). These simulations show that very accurate results are obtained even with low signal-to-noise levels. Work supported by the Office of Naval Research.
This paper examines the inter-relatedness between the polarization vector of the linearly polarized feed/probe, the analysis coordinate system used for the DL-to-CP transformation, and how the test antenna generates its circular polarization response. The measurement of the performance characteristics of circularly polarized antennas is often accomplished using a linearly polarized feed/probe to measure horizontal and vertical polarization components. The measured orthogonal linearly polarized components are combined using a mathematical technique to transform them to the orthogonal right-hand and left-hand circular polarization components of the test antenna. One of the difficulties in using this technique is insuring the proper orientation of the feed/probe for the measurement, the analysis coordinate systems used for the DL-to-CP transformation algorithm. Another is understanding the manner in which the circular polarization is being generated by the antenna under test. These factors are inherently related, and without proper care the wrong answer can easily be calculated.