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?Radiation efficiency is an important attribute of an antenna that can be calculated from its gain and directivity. This paper focuses on investigating the effects of influence factors for antenna radiation efficiency measurement in an anechoic chamber (AC). The gain transfer method (GTM) is used widely during the gain measurement, but the results can be influenced by many factors. A comparison of gain measurement performed by GTM and the three-antenna method (TAM) is presented. All measurements were carried out between 1 GHz and 8 GHz in an anechoic chamber with a double-ridged waveguide horn antenna as the antenna under test (AUT), which has a relatively broad half-power beamwidth. The results show that the maximum difference between the two methods is about 1.5 dB and the GTM may bring greater measurement uncertainty. To evaluate the influence of directivity and its repeatability, two sets of directivity measurements were performed using four different antenna mounting brackets, namely: Rohacell foam, Tufnol, metal, and metal covered by radio frequency absorber. Amongst the antenna mounting brackets, the Tufnol bracket gives the best repeatability performance. The antenna axial symmetrical properties were also assessed for each antenna mounting bracket except for Rohacell foam. The results shows that the gain measurement has more influence over characterization of antenna radiation efficiency as compared with the directivity measurement. To improve accuracy for radiation efficiency measurement, one suggests to use TAM for the antenna gain measurement.
Antenna measurement accuracies or error budgets are related to the signal-to-noise ratio of the measurement receiver. Signal-to-noise can be modelled taking two vectors, the intended signal and an interfering signal (i.e. from unwanted multipath propagation or simple noise), which superpose to the actual measured quantity. Although this approach is widely used, it is rarely discussed to its full extent. Instead, the error of the measured quantity is often estimated for high signal-to-noise ratios by applying worst case assumptions to the unwanted part. This excludes not only any statistical nature of the interfering signal but also the probability of the error appearance represented by its standard deviation. Especially when considering several error contributions in a total budget the adequate combination of different error probabilities yields a much more realistic result than adding single worst cases. Within this paper probability functions are analytically derived for measurement errors depending on the signal-to-noise ratio according to the aforementioned model. This yields a more sophisticated analysis of amplitude and phase errors having standard deviations for measured quantities. The confidence intervals of measurement errors are given with respect to varying signal-to-noise ratios. Limitations of the white noise synthesis with uniform phase and amplitude distributions are explained. Further, the applicability of worst case assumptions to the analytical solutions is discussed in the presence of high and low dynamic ranges. The derived expressions are statistically tested using Matlab calculations and compared to measurements with a vector network analyzer. The results are interpreted with respect to practical applications.
Crystallographic sample design of complex media influences material tensor properties. These properties offer amplitude, phase and polarization control of the electromagnetic (EM) fields. Previous works have evaluated crystallographic sample designs for isotropic, uniaxial and biaxial anisotropic media, each respective design offering more ways to control the fields. The tensor elements for these designs are all aligned along the main diagonal of the permittivity and/or permeability tensors. Additional field control can be obtained by producing materials that have off-diagonal tensor elements in addition to the aforementioned main diagonal elements. A monoclinic crystal sample design supports the existence of two off-diagonal elements and offers more field control than biaxial anisotropic media. In this work, field analysis is performed on media that possesses a monoclinic tensor element arrangement, demonstrating the additional control over EM fields as compared to biaxial anisotropic media. A monoclinic sample is then constituted using crystallographic symmetry. Future work will yield the development and analysis of a monoclinic sample material measurement capability.
The concept of using plasma physics to create antenna array characteristics with or without multiple elements is to surround a plasma antenna or even a metal antenna by a plasma blanket in which the plasma density can be varied. In regions where the plasma frequency is much less than the antenna frequency, the antenna radiation passes through as if a window exists in the plasma blanket. In regions where the plasma frequency is high, the plasma behaves like a perfect reflector with a reactive skin depth. Hence, by opening and closing a sequence of these plasma windows, we electronically steer or direct the antenna beam into any and all directions. During transmission of the inside antenna of the smart plasma antenna design, the signal can pass through an open plasma window by turning off the plasma in the plasma window or sufficiently decreasing the density of the plasma in the plasma window to allow the signal to pass through. A smart plasma antenna prototype has already been built and demonstrated using these physical principles. A faster technique to steer the smart plasma antenna beam, is to increase the plasma density in the plasma tubes so that Fabry-Perot-Etalon effects are met and the signal will pass through. The Fabry-Perot-Etalon effects are well known in optics. The smart plasma antenna has been programmed to meet the Fabry-Perot-Etalon conditions. The characteristic decay time of the plasma after power turn-off is typically in milliseconds, so the opening time of such a barrier generally is predicted also to be in milliseconds. However such a barrier can be opened on a time scale of microseconds by using Fabry-Perot-Etalon effects. This is done by increasing the plasma density rather than waiting for it to decay. The annular cylindrical ring of plasma of the smart plasma antenna creates a Fabry-Perot-Etalon cavity under the proper conditions.
To determine the proper moisture content in the soil is critical to get maximum grow in plants and crops and its estimation it is used to regulate the amount of irrigation that it is needed. For this reason, many sensors that measure water content have been developed to give the grower some feedback of the water content. Some methods such as the ones based in gravimetric properties are accurate but labor consuming, other such as the tension meters require periodic service, the neutron probe is also accurate but expensive. The more popular sensor is based in electrical resistance measurement that gives acceptable accuracy and it is not expensive. However, this sensor has the disadvantage that needs to be buried in the soil. Here, we are exploring the characteristics of electromagnetic propagation and its scattering properties as a tool to identify the physical soil composition. The presence of water changes drastically the dielectric properties of the soil affecting the reflected signal. In this research, we are assessing the viability of a sensor based in FMCW radar technology for water detection with the advantage of being portable and low cost. The research involves the fabrication of a directive antenna operating in a broadband regimen, transmitter/ receiver circuit and the signal processing of the return signal adjusted to the detection of moisture in soil. We present the calibration methods and graphic results of the intensity of the reflected signal of dry bare soil, wet soil, and soil covered by plants.
Recently, the 5 GHz bands have become increasingly attractive for the wireless industry due to the large amount of unlicensed spectrum available and less congested than the 2.4 GHz band. For this reason, WiFi standards 802.11a/h/j/n/ac operate in the 5 GHz band and its use is also being proposed for LTE-U (LTE-Unlicensed), which will use the unlicensed spectrum in this band for increasing the capacity of LTE (Long-Term Evolution) networks. However, this band is allocated on a primary basis to aeronautical and satellite services, meteorological and military radars and other federal and non-federal uses. In this context, it is necessary to perform additional propagation and interference studies in order to characterize the 5 GHz mobile radio channel considering its current uses and future applications. This work contains measurements and characterization of 5 GHz mobile radio channel along indoor and indoor to outdoor paths using a novel narrowband propagation measurement system. This system, developed at the Institute for Telecommunications Sciences (NTIA/ITS), consists of a vector signal analyzer and highly stable oscillators. The stability, precision and flexibility of the system permit post-processing the baseband complex received signals in order to analyze the channel characteristics. We have performed narrowband measurements on the 5.41 GHz mobile radio channel in four different scenarios: indoor with line of sight, indoor through walls, indoor through different floors and indoor to outdoor. The data collected has been processed for estimating the path loss (including the attenuation factors and path loss exponents), fading statistics and Doppler power spectrum for each scenario. We also show that the processing permits the extraction of accurate information about the scattering environment. The results permit characterizing the 5 GHz mobile radio channel for future spectrum sharing designs, as well as for network planning and capacity planning for WiFi and cellular applications in this band.
In this paper an advanced analysis regarding the interaction between antennas installed on a spacecraft is presented. In particular, data coming from a GNSS satellite near field measurement campaign have been considered and the MV-INSIGHT software has been used to perform the analysis. Such a software, starting from the measured field of a DUT, computes equivalent currents on a surface conformal to the test object. The availability of the equivalent currents is a key point for an in depth analysis of DUT such as a spacecrafts since it allows to obtain exclusive diagnostic information like coupling between antennas and satellite structure. The near-field data have been collected in the Hybrid ESA RF and antenna Test Zone (HERTZ) at ESTEC. Such a hybrid NF/FF system has recently been installed in the existing dual reflector CPTR. The installed system has been designed to perform spherical, cylindrical and planar NF measurements in the broad frequencies range 0.4-50 GHz.
For satellite applications payload measurements are a crucial part of the radio frequency validation campaign before the launch. Parameters like Equivalent Isotropic Radiated Power (EIRP), Input Power Flux Density (IPFD), Gain over Noise Temperature (G/T), Gain over Frequency (G/F), Group Delay, and Passive Intermodulation (PIM) are to be measured in suitable facilities on satellite level. State-of-the-art payload measurements are conducted in compensated compact range facilities which offer a real-time test capability which is easy to setup and use. Closed link tests are straightforward to realize with two compact range feeds employing feed scanning. The measurement techniques as well as the error budgets are well known. Near-field facilities are widely used for antenna pattern measurements. However, there is not much literature available discussing in particular measurements of G/T, G/F, and Group Delay in the near field. Measurements of the above parameters in the near field seem to be feasible, however, the processing of the measured data has to be adapted and further calibration measurements are required. In this paper methodologies for payload parameter measurements in compact range and near field facilities will be described. A comparison of payload measurement campaigns in near field and compact range facilities will be drawn. The techniques will be compared in terms of measurement timing and effort, practicability for satellite applications, and achievable accuracies.
The spherical multipole based near-field far-field transformation is one of the most widespread algorithms for field transformation due to its very low computation time achieved by employing the fast Fourier transform (FFT) and imposing the utilization of first order probe antennas which obtain regularly distributed near-field samples on a spherical surface. Thus, huge efforts in highly accurate scanner system and antenna design are invested to fulfill the transformation algorithm requirements. In comparison, the recently developed inverse source reconstruction methods are very undemanding as they allow to use arbitrary probe antennas and arbitrarily shaped measurement surfaces as long as the probe’s relative position and orientation with respect to the device under test (DUT) is accurately known. Furthermore, the diagnostics capabilities of the algorithms give insight into the radiation mechanisms of the antenna. Although multilevel fast multipole boosted inverse source reconstruction algorithms such as the fast irregular antenna field transformation algorithm (FIAFTA) provide an excellent linearithmic complexity, their computation time is still higher than the one of the spherical transformation. The flexibility to process near-field samples on an irregular grid is yet only of interest for some challenging measurement scenarios where it is easier to determine the exact position and orientation of the probe than to accurately position it at certain grid points. Moreover, most antenna measurement facilities are already equipped with positioner systems for spherical scans. Therefore, a spherical multipole based transformation with higher order probe correction capability is proposed to perform a fast near-field far-field transformation. Once the far-fields and thus, the plane wave representation of the antenna has been obtained, a hierarchical plane wave representation is utilized to efficiently determine the equivalent sources of the antenna. For best sources localization and diagnostic features, equivalent surface currents on a Huygens’ surface enclosing the antenna are used. Their organization in a hierarchical octree is the key to a fast transformation from the antenna far-field to its equivalent sources. In this way, the blend of the spherical multipole based transformation and the hierarchical plane wave based field representation allows to profit from the benefits of both transformation approaches.
For today's sophisticated antenna applications, the accurate knowledge of 3D radiation patterns is increasingly important. To measure the antennas under far-field conditions over a broad frequency band is hereby hardly impossible. By near-field to far-field transformation, one can overcome the difficulties of limited measurement distances. In common spherical near-field antenna measurement software, the transformation based on spherical mode expansion is typically implemented. These software tools only provide to correct the influence of first order azimuthal probe modes. The influence of the probe’s higher order modes though increases with shorter measurement distances. To measure a broad frequency range in one measurement set-up and to save time, dual ridged horns are popular candidates since they operate over a wide frequency range. The drawback is that they are probes of higher order. In this contribution, we will present an investigation on near-field measurements which are transformed into the far-field deploying the transformation technique based on spherical modes which is extended by a higher order probe correction capability. The resulting diagrams comparing first and higher order probe correction show that a correction is important in particular for the cross polarization In addition, the near-field data is transformed with an algorithm which employs a representation by equivalent currents. In this method, a higher order probe correction based just on the probe’s far-field pattern is integrated. The equivalent currents supported by an arbitrary Huygens surface allows to reconstruct the current densities close to the actual shape of the AUT which is mandatory for precise antenna diagnostics. Another issue needs to be accounted for regarding limited measurement distances and spherical modal expansion. While representing the AUT and the probe in spherical modes the radii of the spheres grow the more modes are included which depends on the sizes of the TX and the RX antennas. It has to be ensured that both spheres do not interfere. All measurements were carried out in the anechoic chamber of our laboratory in which measurements starting at 1 GHz are practicable according to the dimension of the chamber and of the absorbers. Due to our restricted measurement distance of 0.57 m, all the above mentioned rules need to be considered. In conclusion, small anechoic chambers are also capable of delivering precise antenna measurements over a broad frequency range due to algorithms capable of higher order probe correction.
We have successfully designed and developed an innovative “CLose-range Antenna Scanner System” (or CLASS) suitable for measuring the far-field radiation pattern of installed antennae at short distances. The system consists of three key components: (1) a uniquely designed lens horn antenna that generates plane waves in close proximity, (2) a mechanical x-y scanner to scan the antenna-under-test, and (3) a customized stitching software to compute the far-field antenna pattern from the measured field information. The developed system has a scan area of 4.6 x 4.6 m, with resolutions of ±0.1mm in both the x and y traverse directions. The scanner structure is designed in a scalable fashion to cater for measurement of antenna installed at various locations (e.g. front and sides) on a platform. The system is capable of measurement from 1 to 18 GHz and generates far-field radiation pattern with a gain accuracy of ±1 dB.
A multi-probe array (MPA) spherical near-field antenna measurement system, comprised of COTS equipment, has been developed for testing UHF antennas mounted in an aircraft rotodome. The spherical probe radius is 5 meters, which accommodates a 24 ft. diameter rotodome. The probe array, arranged in a circular arc about the test zone center, provides rapid time multiplexed samples of dual polarized spherical theta angle measurements. These measurements are collected at incremental steps of spherical phi angles, provided by a floor azimuth turntable. The rotodome is mounted on the azimuth turntable, and is rotated 360 degrees during a data collection. During one azimuth rotation, completed in a few minutes, a full set of 3D, dual polarized, multi-frequency near-field pattern data is collected. The data is transformed to full 3D far-field patterns in another few minutes, providing a complete rotodome test time within 15 minutes. The entire system is contained within a room 42’ x 42’ x 25’. This paper will describe the test requirements, physical requirements of the DUT, size constraints of the facility, and measurement speed goals. Alternate solutions and range geometries will be discussed, along with why the MPA solution is best given the requirements and size constraints. The system will be described in detail, including discussion of the room design, RF instrumentation, multi-probe array, positioning equipment, and controllers. Measurement results will be presented for test antennas of known pattern characteristics, along with other performance metrics, such as test times.
This paper presents a method for determining the phase center of an antenna based on pattern measurements made over multiple frequencies with a two axis spherical positioning system. Mathematical calculations are used to determine the best-fit sphere for the measured phase data. The origin of the sphere is the phase center of the antenna at the frequency of interest. This method provides fast, flexible multi-frequency measurement of an antenna’s phase center. Results validating the proposed method are presented using both simulated data and measured data. In order to determine the accuracy of this method, a dipole is translated a precise distance within the measurement coordinate system. The difference between phase center measurements made before and after the translation gives an indication of the potential accuracy of the measurements. Also, major contributors to phase center measurement uncertainty are discussed with consideration to reducing the overall phase center measurement uncertainty.
Antenna measurements at increasing working frequencies carry the difficulty of reliably measuring the signal phase, due to effects of cable bending, thermal drift, etc, and the resulting impedance mismatch which introduces uncertainty in the measurement results. In this paper we investigate the problem of phaseless measurements and phase retrieval for planar near-field measurements, together with the application of probe correction of the retrieved results, to the best of our knowledge the first experimental case of probe correction in phaseless near-field antenna measurements. A phase retrieval method based on an iterative Fourier technique (IFT) is proposed and tested with measurements of a Standard Gain Horn at 60GHz acquired at the planar near-field (PNF) scanner facility at the Technical University of Denmark. The obtained results indicate good agreement with a measured reference pattern within the region of validity when the probe correction is applied after performing the phase retrieval from a pair of uncorrected probe signals. Application of the probe correction before the phase retrieval, on the other hand, shows not satisfactory results. Additional improvements are obtained by introducing spatial filtering at the AUT aperture, thus enhancing performance of the algorithm by reducing phase noise of retrieved fields. Also, a “double-iterated” approach is explored, with additional phase-retrieval iterations after probe correction, with the aim of introducing true electric fields into the IFT.
Highly accurate antenna measurements can require precise alignment and positioning of the probe antenna to the antenna under test. The positioning of the antenna during acquisition can involve the movement of several simultaneous axes of motion. This places a global positioning accuracy requirement on the positioning system. To achieve precision in global positioning and alignment, an understanding of dominant error factors such as load induced deflection/resonance, thermal deflection, positioning error sources and mechanical alignment tolerances is essential. This paper focuses on how global accuracy and stability were achieved, addressing these factors, on a recently delivered large far field antenna measurement system. The system involved eight axes of positioning with the ability to position 950 lbs antenna under test 19.5 ft above the chamber floor achieving 0.007 inch and 0.005 degrees positioning accuracy relative to the global range coordinate system. Stability of the probe antenna after motion was within 0.001 inch. Key Words: Global Position Accuracy, Far Field, Position Stability, Simultaneous Motion, Position Error Correction, High Accuracy, Precise Motion
Compensated Compact Range Facilities are the state-of-the-art RF test facilities for spacecraft payload modules and/or antennas. The outstanding features of the compact range technique are the (a) real-time testing capability, (b) easy to use far-field measurement technique, (c) extremely high frequency capability, (d) end-to-end payload testing at multiple test zones due to scanning features, and last but not least the (e) considerable low cross-polar contribution over the full frequency band between 1 - 200 GHz which is one of the important parameters for telecommunication antenna testing. Upcoming spacecraft antennas with single feed per beam configuration and broadband transponder requirements (up to 500 MHz) need rapid test environments for antenna and payload (end-to-end) measurement campaigns. For the desired wide frequency spectrum the Ka-Band and even higher bands (U, and V) are of interest for the next generation of telecommunication spacecraft antennas. Compensated Compact Ranges provide an excellent test environment for such scenarios. Recent developments for the range feeds up to 200 GHz, a new heavy load and highly accurate specimen positioner design, and the easy enlargeable reflector system within the existing chamber complete the picture of a state-of-the-art test facility for present and future spacecraft testing. The paper will explain the advantages of the selected system design and preferred technology with its resulting features to optimally cover the future requests focusing to new developments in the high frequency range. For typical spacecraft antenna scenarios a comparison between Compact Range and Near-Field facilities will demonstrate the applicability in the frequency range from 1 to 200 GHz. Beside the developed test set-up for the required measurement parameters, typical measurement times and achievable performance with its related error budget will be depicted.
Unlike most antenna performance parameters (directivity, beamwidth, and efficiency, e.g.), phase center is not strictly defined and warrants further clarification when used. Put simply, the phase center is the point at which antenna radiation seems to emanate and is determined as the center of a spherical surface of constant phase in the far field. For practical antennas, however, such a point is fictional and can only be established by minimizing the phase variation on a portion of the spherical surface over a smaller angle of interest, generally where the radiation intensity is greatest (e.g. the 3dB beamwidth). Most commonly, the phase center is defined for a two dimensional planar cut parallel to the direction of propagation, for example the E or H plane of a horn. Knowledge of the phase center is particularly critical in the feeds of reflectors or lenses, where it is required to be located at the focal point of the reflecting or refracting structure to maximize aperture efficiency. Due to its electro-mechanical properties the horn antenna has often been used as the feed for the above mentioned configurations. For wideband applications, the stabilization of the phase center over the entire frequency band poses a significant challenge since this point generally tends from the mouth to the throat of a horn as frequency is increased. The design discussed in this paper involves a feed horn operated in conjunction with a Lunenburg Lens for increased directivity and gain over 18-45 GHz bandwidth. A design overview is discussed with the primary focus on phase stabilization considerations. Methods for determining the phase center of the design are also discussed and compared. These include analytical solutions using the aperture current approximation, simulations using method of moments and finite element method from FEKO and HFSS, respectively, as well as measurements taken in the anechoic chamber at the University of Colorado Boulder.
During the last years, new algorithms, based on time filtering, spatial or modal filtering, have been designed for echo reduction techniques applied to antenna measurements. These algorithms have been used for different applications where the effect of the echoes is important, as far field system, VHF or UHF applications, automotive systems, small antennas, etc. The authors, in previous papers, have analysed the effect of different algorithms: time filtering (fft, non uniform dft or matrix pencial), modal filtering based on Spherical modes (MV-Echo) and spatial filtering based on Integral Equations (Insight) and holographic techniques (fft and dft) to cancel the effect of the reflections. This comparison has been applied to the measurements of a dipole antenna (SD1900) using a StarLab system. It is observed that each of the algorithms is better for different situations, depending on the source of the echo. For instance, time filtering techniques are good for reflections coming from different distances with respect the direct ray, but not so good for close reflections. In addition hey need a large frequency band to work properly. Spatial algorithms can correct the effect of positioners or other structures close to the antenna under test, but they are better for planar near field acquisitions and worse for classical single probe spherical near field where the antenna is rotated and probe is fixed (e.g. roll-over-azimuths systems). Moreover, they require extra information of the AUT geometry. This paper presents first a comparison of each algorithm and then, a combination of time and spatial techniques based on uniform or non-uniform DFT to take advantage of the benefits of each algorithm for different origins of the reflections.
An uncertainty budget for Indoor Radar Cross Section (RCS) measurements contains many contributors. Typically, one of the largest contributors comes from the Quiet Zone quality. To quantify the ripple and the tapper in the Compact Range Quiet Zone of the CEA’s indoor facility CAMELIA, a diagnosis method has been implemented, exploiting the radar response of a moving sphere located on a polystyrene mast. This polystyrene mast is fixed on the top of a linear-translating table over an azimuth positioner. The combination of the two axis capabilities allows to locate the PEC sphere in a horizontal plane cut of the quiet test zone volume. The other cuts at different altitudes are performed by changing the height of the polystyrene mast. This method samples the magnitude of the illuminating field at fixed spatial points (controlled by a laser tracking) in the Test Zone to determine the magnitude of the ripple and thus the Quiet Zone. These experimental data are then statistically processed to determine the measurement uncertainty at a given frequency. This paper introduces and analyses the results of a measurement campaign dedicated to the characterization of the Quiet Zone of the CEA’s indoor facility CAMELIA.
Rapid advances in material fabrication capability, such as 3D printing, have made the realization of engineered complex media (i.e., anisotropic and bianisotropic materials) possible. One of the primary aspects prompting the interest in complex media is the added control over scattered electromagnetic fields due to the increase in the number of constitutive parameters. Isotropic media are characterized by the 2 well-known scalar parameters of permittivity and permeability. However, in general, it requires 18 and 36 parameters to describe anisotropic and bianisotropic media, respectively. Although the increase in parameter space provides more control over electromagnetic response, the penalty to pay is the added complexity in theoretical analysis when compared to isotropic media. One method that has been developed for the analysis of complex media is the six-vector field formalism which casts Maxwell’s equations into matrix form for ease of manipulation. Although this formalism handles fully populated permittivity and permeability tensors, inversion of a block 3x3 (i.e., 6x6) matrix is required which is mathematically intensive and physical insight can be obscured since a cofactor-based inversion is often employed in the solution process. The goal of this work is to develop a scalar potential formulation capable of handling gyrotropic media. Advantages and limitations of the formulation will be discussed and relevant examples will be provided to demonstrate the simplicity and physically-intuitive nature of the technique. Future work involving the use of the scalar potential formulation in the analysis of antenna, guided wave structures and material characterization of complex media will also be discussed to demonstrate the promising aspects of the technique.