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Anechoic Chamber

Recommendations for RF Absorber Treatment of Ranges Having a Movable Gantry or Multiple Probes
Vince Rodriguez, Mark Ingerson, October 2023

Absorber treatment for an anechoic range is designed to attenuate the potential reflections from the walls, ceiling, and floor and to keep a certain level below the direct path between the range antenna (or probe) and the quiet zone (or minimum radiated sphere for spherical near-field ranges). There are, however, some antenna measurement systems where the range changes or moves as the data is acquired. In some cases, the probe moves around the antenna-under-test (AUT) along a section of circle supported by an arch or a gantry. In other ranges, the multiple probes are switched on and off; these probes are supported by an arch. Because the direction of the range moves with respect to the walls, ceiling, and floor, it is a bit more complex to arrive to an optimal absorber layout, as well as locating the preferred placements for the instrument rack, door, and vents in the range. In this paper, a higher-order-basis-function method of moments approach is used to model a gantry-supported probe as it moves around the location of the AUT. The power density at the walls as the probe moves is analyzed to arrive to an optimal absorber layout that will provide adequate levels of reflections for measuring an antenna. The paper looks at a gantry that moves from +135° to -135° with the AUT rotating 180° and for a gantry that moves from 0° to +135° with the AUT rotating 360°. The latter will require a smaller range with one of the walls closer to the location of the antenna under test. A series of recommendations based on the electrical size of the absorber at different areas of the range are provided.

Electric-Field Pattern Measurements of Acoustically Driven Piezoelectric Field Emitters
Srinivas Prasad Mysore Nagaraja, Brook Feyissa, Tristan Wilson, Jack Bush, Darmindra Arumugam, October 2023

Piezoelectric transmitters operating at acoustical resonance have been shown to radiate effectively in the Very Low Frequency (3 kHz to 30 kHz) and Low Frequency (30 kHz to 300 kHz) regimes. Such transmitters make use of the inverse piezoelectric effect to couple electrical signals into mechanical vibrations, resulting in near field radiation. This new class of electrically small antennas, known as mechanical antennas or ‘mechtennas’ can provide several orders of magnitude higher efficiency than similarly sized electrically small conventional dipoles. Measuring the dipole-like near field pattern of such piezoelectric field emitters in the Very Low Frequency and Low Frequency range using conventional techniques is not possible. To address this limitation, a simple capacitor plate-based setup is presented that enables the measurement and plotting of the near field patterns of such transmitters. Design and simulation of the capacitor plates to model the fields along with electric field pattern measurements of a Y 36◦ cut Lithium Niobate transmitter having longitudinal mode resonance at 82 kHz are presented.

Novel Application of Compressed Sensing in Cylindrical Mode Filtering for Far-Field Antenna Measurements
Zhong Chen, Stuart Gregson, Yibo Wang, October 2023

Mode filtering has been shown to be very effective in suppressing spurious reflections in antenna measurements. Specifically, it has been well documented that in the quasi-far-field, the two polarizations are decoupled, making it possible to apply standard cylindrical near-field theory on the amplitude and phase data acquired from a single polarization measurement on a great circle cut [1]. The method was further extended to allow data collected from an unequally spaced angular abscissa by formulating the solution as a pseudo-inversion of the Fourier matrix [2]. This formulation, however, can be prone to spectral leakage because of nonorthogonality of the Fourier basis on an irregularly sampled grid, especially when the positions deviate significantly from the regular grid [2]. In this paper, we propose to use Compressed Sensing (CS) to compute the Cylindrical Mode Coefficients (CMCs), which improves the signal to noise ratio, allowing more accurate recovery of the prominent modes. The CS recovery is tenable because with the coordinate translation of the measurement pattern to the rotation center, the Maximum Radial Extent (MRE) of the antenna under test is greatly reduced, making CMCs quite sparse in the mode domain. The novel application of CS presented in this paper further expands the generality of the mode filtering method, which is now applicable to under-sampled data (at below the Nyquist rate) acquired on positions that grossly deviate from the equally-spaced regular grid.

NIST's Antenna Gain and Polarization Calibration Service Re-instatement
Joshua Gordon, Benjamin Moser, October 2023

After a five-year renovation of the National Institute of Standards and Technology (NIST) Boulder, CO, antenna measurement facility, the Antenna On-Axis Gain and Polarization Measurements Service SKU63100S was reinstated with the Bureau International des Poids et Mesures (BIPM). In addition to an overhaul of the antenna facility, the process of reinstatement involved a comprehensive measurement campaign of multiple international check-standard antennas over multiple frequency bands spanning 8 GHz to 110 GHz. Through the measurement campaign, equivalency with 16 National Metrology Institutes (NMIs) and continuity to several decades of antenna gain values was demonstrated. The renovation process, which included implementing new robotic antenna measurement systems, control software, and data processing tools is discussed. Equivalency results and uncertainties are presented and compared to checkstandard historical values.

A Simple Non-Linear Planar Near-Field Antenna Measurement System
Jason Jerauld, Tarron Teeslink, Felix Yuen, Nathan Landy, Tom Driscoll, October 2023

We describe a planar near-field instrument capable of measuring the non-linear response of an electronically steered antenna (ESA) up to the third harmonic while requiring only a single scan with a single probe. The system performs phase-coherent measurements of the aperture near-field at the fundamental frequency, second harmonic, and third harmonic simultaneously, which are then transformed to the far-field. When system losses are appropriately accounted for, these far-fields are accurate representations of the harmonic patterns relative to the fundamental. A broadband dual-polarized probe combined with a specially-configured network analyzer is used to capture all frequencies and both polarizations within a single scan. Using a ultra-broadband probe introduces some limitations to the measurement, but offers a significant increase in measurement speed. In this paper we disclose various architecture and design aspects of the instrument, discuss its advantages and limitations, and compare non-linear PNF measurements with non-linear array simulations and direct far-field measurements.

Background and clutter removal algorithm for RCS extraction in semi-anechoic chamber
Papa Ousmane Leye, Adamo Banelli, Shaikha Aldhaheri, Chaouki Kasmi, Felix Vega, Islem Yahi, October 2023

The purpose of radar cross-section (RCS) measurement is to determine the amount of scattering that occurs when the radar signal illuminates the target. It is generally performed to prove a design concept. RCS measurement chamber requires a good signal-to-noise ratio during the measurement. When the measurement is performed in a non-controlled environment, coherent background subtraction associated with time gating is commonly used to improve the quality of the RCS data. Although these techniques are usually effective, residual clutter and background level still need to be removed to accurately characterize the target’s RCS in highly cluttered environments, such as semi-anechoic chambers. In this paper, a four-step post-processing technique is presented. In addition to the vector background subtraction and timegating techniques implemented in our previous work, a statistical algorithm called Principal Component Analysis (PCA) is applied to the ISAR image of the target. It is an extension of the PCA technique to RCS measurement. It is shown that residual background and clutter can be reduced by the statistical filtering method through eigenvalue decomposition of the RCS data. The technique is presented and evaluated through measurement of the RCS of a dihedral corner reflector at the X-band in the semi-anechoic chamber of the Directed Energy Research Center.

A Squat Cylinder-Dihedral Dual Calibration Device for Compact Ranges at UHF
Hirsch Chizever, Laura Suzuki, October 2023

The use of squat cylinders as both primary and secondary calibration targets is commonplace within the radar cross section (RCS) measurement community. Secondary calibrations have become a best practice activity for ranges seeking or maintaining certification. The calibration process, often referred to by the measurement community as a “Dual-Cal,” uses two squat cylinders of similar but unequal dimensions that provide range operators with a broadband calibration vector and a measurement uncertainty metric important to range certification. Despite their popularity, the need to ensure resonance scattering occurs below the desired measurement band results in physically large cylinders at UHF. In addition, the need to access the test zone for separate cylinder measurements may add substantial time to the calibration process and require specialized equipment, especially for large ranges. In response to these issues, a 22.5-degree right dihedral has been inserted into a squat cylinder form factor, creating a primary and secondary calibration target within one body, each separated in azimuth by 180 degrees. This two-target calibration device removes the need to access the target zone twice and mitigates errors associated with separate mounting schemes. The cylinder aspect, now truncated by the imposition of a dihedral, has 50% extended lower frequency coverage at UHF due to oblique edge scattering at vertical polarization. At horizontal polarization, the dihedral interruption of the cylinder creeping wave reduces its contribution for ka<4. The dihedral aspect provides a full polarimetric calibration, resulting in co-equal frequency responses for each polarization in the high frequency limit. The design parameters of the squat cylinder-dihedral device, its computed full-wave frequency response, and relevant scattering features are discussed.

Modified Thru-Reflect-Match Polarimetric Calibration Technique for Focused Beam Systems
Jeffrey Massman, Michael Havrilla, October 2023

This paper extends the time-domain gated response isolation scheme for full polarimetric calibration with a modified Thru-Reflect-Match procedure for network analyzer selfcalibration where precise knowledge of the metrology standards is not required. Cross-polarization contributions from the measurement setup are neglected to simplify the procedure. A simulated cascade analysis is included to demonstrate the relative scattering parameter error of the sample under test when the measurement setup cross-polarization level is neglected. The featured calibration analysis leverages a 4x4 scattering parameter matrix notation to capture the polarimetric scattering at each cascaded stage and develops a 16-term error correction factor model to account for cross-polarization scattering contributions from the measurement sample. Finally, a wire-grid polarizer is used as a modified Match standard where a series of interrogations at multiples orientations, in combination with Thru and Reflect measurements, enables cross-polarized scattering channels to be characterized. This polarimetric self-calibration approach uses physically realizable metrology standards and accounts for all error terms for precision focus beam system measurements.

Introducing LORENTZ: A novel Low-temperature Near-field Terahertz Chamber for instrument characterisation.
Paul Moseley, Luis Rolo, Andrey Baryshev, Tobias Vos, Alena Belitskaya, Daniele Ronso da Costa Lima, Peter de Maagt, Paul Hartogh, October 2023

The Low-temperature Near-field Terahertz Chamber (LORENTZ) is a novel facility recently installed and commissioned at ESTEC, ESA. This facility has the unique ability to characterize the antenna performance off full submillimeter instruments in operational environments down to 80k. We provide an overview on the various design and commissioning steps that were required to ensure all parts of the test facility would operate reliability in such challenging conditions. We also present how the facility performed during the first full measurement run of flight hardware and a roadmap for future developments.

New Designs for a Feed Fence to reduce the direct coupling to the Quiet Zone on Compact Ranges
Mark Ingerson, Vince Rodriguez, Daniel Janse van Rensburg, Anil Tellakula, October 2023

Absorber fences have been used on compact ranges since their first implementations. The purpose of this fence is to hide the feed positioner and reduce the direct coupling between the feed and the device under test (DUT). A known problem caused by such a fence is that it diffracts the plane wave generated by the reflector, creating an interfering ripple on the illumination of the DUT in the quiet zone. Traditionally, fences have serrated edges to direct this diffracted signal away from the quiet zone. However, this redirection is not always achievable or even repeatable from one facility to the next. Often low frequency requirements drive absorber physical size, leading to very large absorbing surfaces that cannot be optimized to reduce this interfering signal. In this paper, the fence design presented in a recent publication [1] is further optimized by modifying its shape and absorbing material parameters. The performance of this new design is compared with traditional fences.

Diverse Utilization of an Anechoic Chamber for Automated Electronic Warfare (EW) Testing in Support of Explosive Ordnance Disposal (EOD)
Joseph Friedel, David Oyediran, Thomas Higdon, October 2023

The Naval Surface Warfare Center Indian Head Division (NSWC IHD) EOD Technology Center is a United States Navy facility with the urgent mission of supporting the Department of Defense (DoD) warfighter in the detection and neutralization of unexploded ordnance (UXO) and improvised explosive device (IED) threats. The Radio Frequency (RF) Laboratory at NSWC IHD, is centered around its 24’ by 12’ by 12’ anechoic chamber, which was designed mainly for antenna measurement. However, the unique challenges this department was tasked to resolve has resulted in varied and uncommon uses of the chamber. The chamber, RF test equipment and staff of electrical engineers, mechanical engineers and computer scientists, have participated in the automated RF testing of X-ray equipment, bomb suits, radars, electronic jammers and IEDs, to provide just a partial listing of test events. This paper will detail recent unique assignments that required the rapid research, design, development and implementation of automated RF test and measurement systems providing solutions for the EOD community. The anechoic chamber’s system uses, from antenna design and measurements, materials testing, electromagnetic compatibility (EMC) testing to electronic warfare (EW) testing of radars and jammers, will be discussed along with the examination of the software algorithms that enabled fast, repeatable and reliable RF measurements. Focus will be on the roles electromagnetic (EM) measurement has for EOD robotics, EW system development and IED threat understanding. The authors speak from the diverse backgrounds of electrical and mechanical engineering and computer science.

On the Uncertainty Evaluation of Absorber Reflectivity Measurements
Marc Dirix, Amin Enayati, October 2023

The reflectivity of foam absorber materials is governed by the correct loading and mixture of carbon and other supplicants such as fire retardants. In order to assess the reflectivity of the absorbers various measurement setups are applied, each having different advantages and disadvantages in terms of frequency coverage and RF performance. The measurement setups are used both in the quality control (QC) as well as for product development. Especially for the product development case, it is important to understand limits of these setups as the lower the reflectivity gets, the more difficult it becomes to detect minute differences between different variants of the absorbers. For reflectivity measurements of microwave absorbers, the available dynamic range and calibration-quality of the setup plays a vital role in this respect. By determining the uncertainty of the measurement setups, a clear assessment can be made to the quality of the measurement and the product to insure consistent QC, as well as plan for the product development.

Performance Evaluation of RU and RIS Based on OTA Mode Near Field and Bistatic Measurement Systems
Chang-Lun Liao, You-Hua Lin, Ike Lin, Chang-Fa Yang, October 2023

In 5G O-RAN, a radio unit (RU) is connected to an upper-layer network element through an eCPRI interface, relying on digital modulation for data transmission. Therefore, unlike conventional 4G antenna system verification processes, RU radiation pattern testing in this data transmission mode necessitates novel testing approaches. Moreover, millimeter-wave signals in 5G undergo severe transmission losses and lack effective multipath channel characteristics, leading to poor base station coverages in this frequency range. The reconfigurable intelligent surfaces (RIS), an emerging technology that exploits the channel properties for the dynamic manipulation of the propagation environments, is a promising solution to the abovementioned problem. However, evaluating the effectiveness of the dynamic energy transfer for the RIS is a crucial challenge in the development of this technology. This paper presents the novel configuration based on the combined near-field and bistatic measurement systems at Taiwan Tech for RU and RIS performance verifications. We propose a near field measurement system to verify the radiation pattern of the RU in data transmission operation. Also, we integrate a compact antenna test range (CATR) and the planar near field scanner to form the bistatic measurement system and conduct performance evaluation of the scattering characteristics of the object under test. Those testing approaches can more accurately evaluate, verify, and optimize RF coverages of the network deployment for 5G and beyond.

Characterizing Compact Antenna Test Range Using Advanced Computational Techniques
Venkata Bhyrava Murthy Devata, October 2023

Compact Ranges are widely used for antenna measurements across wide frequency ranges spanning frequencies as low as 350MHz to as high as 60GHz and above. Advances in electromagnetic (EM) simulations have significantly improved the design process for compact ranges, resulting in reduced costs. Characterizing compact range including the anechoic chamber is computationally very challenging in terms of computer memory and time. In this paper, we will present the full wave method, MLFMM for characterizing compact range without the chamber and application of asymptotic method, RL-GO to characterize the compact range inside the anechoic chamber.

Progress on the development of IEEE Std 1128 - Recommended Practice on Absorber Evaluation
Zhong Chen, Vince Rodriguez, Lars Foged, October 2023

The existing IEEE-STD 1128 on “Recommended Practice for RF Absorber Evaluation in the Range of 30 MHz to 5 GHz” was published in 1998. The standard has been referenced frequently and used as a guide for RF absorber evaluations. The document has several aspects which need updating, including the frequency range of coverage, requirements for newer test equipment, advances in test methodologies and material property evaluation, measurement uncertainty considerations, and absorber high power handling and fire testing requirements. The working group is divided into task groups and is in the final stage of collecting inputs from these subgroups. The next step is to consolidate the inputs and produce a draft standard for a wider distribution before being submitted for balloting. The subgroup contributions can be found on the IEEE imeetcentral website (https://ieeesa. The sections which have received substantive updates include bulk material measurements, instrumentation, absorber reflectivity measurements, and power handling test. In this paper, we will provide some detailed discussions on the planned updates from these contributions. For areas which did not receive sufficient input, the working group plan to table those topics for future considerations.

Huffman Radar Site: Far Field Calibration and Testing Range
Tomas Kendo, Ryan Thompson Thompson, Thomas Corigliano, Chad Shaffer, Thomas Steffen, October 2022

This paper will describe the Huffman Radar Site (HRS), a unique in-situ remote radio frequency calibration and characterization capability located at the Air Force Research Laboratory Sensors Directorate, Wright Patterson Air Force Base (WPAFB), OH. HRS is a part of the OneRY Range complex which consists of Indoor and Outdoor Ranges used to conduct test, evaluation, integration, and demonstration of novel sensing systems and technologies. The Outdoor Range has diverse capabilities at several sites distributed across the local area. Within the Sensors Directorate complex there are three 100 foot antenna towers: the South Tower holds a dish-based S-Band Radar, the East Tower holds a large digital phased array radar, and the West Tower is reconfigurable as needed based on customer requirements. The Huffman Radar site is used to validate the proper functionality of systems on these towers, conduct experiment witness testing, and provide calibration signals for phased-array antennas. The site is primarily used as a Direct Illumination Far Field Range source standing approximately 2 miles away with direct line of sight to the South, East, and West towers. The capability includes full polarimetric transmit from 2.9 to 3.5 GHz and receive from 800 MHz to 6 GHz with future plans to expand the frequency range. This paper will include the design, link budget, hardware implementation, test, and validation of the site. Preliminary far-field antenna pattern data and calibration results for the S-Band Radar system and digital phased-array radar system will be presented. The discussion will include challenges and successes in standing up a multi-function outdoor remote testing capability.

Design and Measurements of a Small End-fed Normal Mode Helical Antenna with Integrated Microstrip Structure
Sri Lekha Srimat Kilambi, Herbert Aumann, Mauricio Pereira da Cunha, October 2022

Compact omnidirectional antennas are highly sought for a multitude of present-day wireless applications such as smart car keys, radio frequency identification (RFID) tags, tire pressure monitoring system, hand-held communication devices, and high-temperature harsh-environment wireless sensors. This paper discusses the performance and the unique challenges in measuring the radiation performance of a compact (~1/25th to 1/10th of a wavelength) helical and microstrip combined structure operating as a normal mode helical antenna (NMHA) around 300MHz. The helical wire structure (27-turn, 37mm high and 6.2 mm wide) is connected to the end of a 50 Ωmicrostrip line fabricated on 1.5 mm thick FR4 substrate. The microstrip line provides a ground plane to the helical structure, serving as an integral part of the radiating element. A tiny 1:1 balun transformer was used to partially decouple the integrated NMHA from the external sheath of the coaxial cable connected to a vector network analyzer, thus allowing proper NMHA impedance measurement. The NMHA S-parameters were simulated on two different platforms, ANSYS-HFSS and WIPL-D Pro, and compared to the frequency of the measured structures, with all simulations and measurements agreeing within 3.5%. Varying the length of the ground plane associated with the microstrip line from 13 mm to 76 mm resulted in the decrease of the measured NMHA operational frequency by 3.2%. The measured impedance of the fabricated NMHA (including the balun) was close to 50 Ω for the 51 mm long line without the need of additional matching circuit. The measured transmission loss for two identical antennas (each 26 cm3) placed about 1 m apart was 22 dB. This performance is comparable or better than the coupling between much larger antennas currently used in harsh environment power plant applications, such as suspended plate antennas (42,500 cm3) or planar inverted F-antennas (11,800 cm3) operating around the same frequency. In addition, the proposed NMHA structure can be implemented using substrates and wires capable of operation at temperature above 300 °C, which constitutes an appealing solution for high-temperature harsh-environment applications such as those found in industrial machinery, metallurgic industry, power plant boilers, and turbine engines.

A New Handheld Sensor for Measuring Intrinsic Dielectric Properties at 100 to 1000 MHz
John Schultz, Ren Geryak, October 2022

Electromagnetic materials characterization at UHF and VHF frequencies is typically done with laboratory fixtures such as the coaxial airline or rectangular waveguide. These conventional methods require material specimens to be cut or machined to precision tolerances for insertion within the transmission line fixture. Measurement accuracy dictates there should be little or no air gaps between the specimen and the transmission line walls. Transmission line methods also require significant handling and multi-step calibration procedures to characterize a material specimen. This paper describes a new handheld measurement device that overcomes these limitations with a simple calibration and non-destructive measurement procedures. This new method applies an open-ended stripline sensor tuned to maximize measurement sensitivity in the 100 to 1000 MHz range. The sensor footprint is approximately 100 mm square and utilizes an integrated one-port vector network analyzer. It operates by measuring the amplitude and phase of the reflection coefficient when placed adjacent to a material specimen. While traditional transmission line methods employ analytical expressions to relate scattering parameters to intrinsic properties, The open-ended stripline sensor geometry and its interaction with the material cannot be easily modeled with an analytical approximation. Instead, it is modeled with a full-wave Finite Difference Time Domain (FDTD) code to develop the relationship between measured reflection and complex permittivity. This inversion method precomputes a translation table by iteratively modeling the measurement fixture across a range of complex permittivities and specimen thicknesses. From this inversion database, interpolation is then used to calculate the frequency dependent complex permittivity or sheet impedance of a given specimen. This paper provides details about the calibration and use of this new device as well as the material property inversion algorithm. Measurement examples of low-loss and lossy materials as well as resistive sheets are also presented and compared to more conventional transmission line results, and are discussed in relationship to measurement uncertainties.

Co-Site Interference Analysis on Aerospace and Naval Platforms using Advanced Simulation Tools
V B Murthy D, CJ Reddy, October 2022

Modern Aerospace, Naval and Defense platforms are overwhelmed with radiofrequency (RF) signals competing for spectrum. RF co-site interference has become a major problem due to the RF interference from jamming, radio stations nearby, or even from civilian communications such as mobile phones. It can be a problem on military platforms like surface warships, land vehicles, and aircraft where many different RF transmit and receive antennas must share a relatively small space. This can be a communications nightmare in which separate RF systems inadvertently step on each other's signals, causing an RF communications fratricide problem that can also be compounded by intentional or accidental RF jamming. It has become more important than ever to address these issues that arise due to RF co-site interference. In this paper, we present advanced simulation tools for antenna placement, antenna coupling and cosite interference on electrically large naval and air platforms using Altair Feko and Wrap softwares. S-parameter coupling matrix of various antennas (both in-band and out-of-band) are computed using either full-wave solutions such as MoM, MLFMM or using asymptotic methods such as PO, RL-GO and UTD. Alternatively Coupling Loss Matrix, defined as the power ratio between powers at the terminals of the transmitting and receiving antennas can be computed using equivalent sources. S-Parameter matrix or Coupling Loss values are then used to study the parameters of co-site/collocation interference such as inter-modulation products, adjacent channel interference and harmonic interference. Furthermore, we will also discuss the options to mitigate collocation interference by adding appropriate filters.

Radiation pattern measurements using an active radar module
Anna Granich, Roland Moch, Amar Al-Bassam, Dirk Heberling, October 2022

In the automotive sector, driver assistance systems are playing an increasingly important role in automated driving, with radar sensors being a critical component for environmental perception. The implementation of safety-relevant functions places ever higher demands on sensor technologies in order to provide high-quality and reliable data. For radar sensors the radiation pattern of the antennas is a crucial factor for the performance of the overall system. As the technology moves towards highly integrated systems, the antennas are integrated directly on the circuit board or even the radar chip package. This complicates or even eliminates the possibility for classical antenna measurements, as there is no access to the antenna feed line. Here the integrated receiver and transmitter module of the radar system are used to measure the two-way radiation pattern with a reflector. However, this leads to a lot of unknown factors and influences that differ from classical antenna measurements. Within this study the formal built up radar measurement setup for the robot-based antenna measurement system of the Institute of High Frequency Technology of RWTH Aachen University is used for accurate two-way pattern measurements based on the sampled raw data of the radar system. A modular frequency modulated continuous wave radar setup with high configurability is used to build up measurements on well-known antennas. The flexibility of the modular radar allows for measurements on different antenna types as scalar feed horn and travelling wave antennas. Different parameters like the choice of the reflector, the measurement distance and repeatability of the measurements are examined on their influence on the measured two-way-patterns of these antennas. The radiation patterns are resolved over the frequency bandwidth of the chirps using the intermediate frequency signal of the radar sensor to investigate the influences on their frequency dependence. The possibility on measuring the co and cross polarization components of the patterns is studied.
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