Cylindrical Near-field Measurements Research Articles

Examination of Far-Field Mathematical Absorber Reflection Suppression through Computational Electromagnetic Simulation

The mathematical absorber reflection suppression (MARS) technique has been used to identify and then suppress the effects of spurious scattering within spherical, cylindrical, and planar near-field antenna measurement systems, compact antenna test ranges (CATRs), and far-field measurement facilities for some time now. The recent development of a general-purpose three-dimensional computational electromagnetic model of a spherical antenna test system has enabled the MARS measurement and postprocessing technique to be further investigated. This paper provides an overview of the far-field MARS technique and presents an introduction to the computational electromagnetic range model. Preliminary results of computational electromagnetic range simulations that replicate typical MARS measurement configurations are presented and discussed which, for the first time, confirm through simulation many of the observations that have previously been noted using purely empirical techniques.

Near-Field Antenna Measurements Using Photonic Sensor of Mach-Zehnder Interferometer

We have been developing a photonic sensor system to measure the electric near-field distribution at a distance shorter than one wavelength from the aperture of an antenna. The photonic sensor is a type of Mach-Zehnder interferometer and consists of an array antenna of 2.4 mm height and 2 mm width on a LiNbO3 substrate (0.5 mm thickness, 8 mm length, and 3 mm width) supported by a glass pipe. The photonic sensor can be considered to be a receiving infinitesimal dipole antenna that is a tiny metallic part printed on a small dielectric plate at microwave frequency. Those physical and electrical features make the photonic sensor attractive when used as a probe for near-field antenna measurements. We have demonstrated that the system can be applied to planar, spherical, and cylindrical near-field antenna measurements without any probe compensation approximately below 10 GHz. We show the theories and the measurements using the photonic sensor in the three near-field antenna measurement methods.

Hybrid Fast Fourier Transform-plane wave based near-field far-field transformation for “body of revolution” antenna measurement grids

[1] Near-field measurement and transformation techniques are widely applied to characterize radiation patterns of antennas. Spherical and cylindrical near-field measurements have been researched extensively and various techniques with different probe compensation capabilities and complexities exist. Among those techniques applicable for (almost) arbitrary probes, the crucial computational efficiency has been achieved through the use of Fast Fourier Transform based preprocessing of the measurement data. It is shown in this paper that the Fast Fourier Transform based preprocessing can also be utilized in conjunction with the plane wave based fully probe-corrected near-field far-field transformation with low numerical complexity. The collection of probe signals is split into smaller subsets for individual orthogonal azimuthal Fourier modes by an Inverse Fast Fourier Transform. These smaller subsets can be transformed to the far field very efficiently with full probe correction. The technique presented in this paper is applicable for arbitrary “body of revolution” antenna measurement grids, including the important cases of cylindrical and spherical measurement grids. The “body of revolution” grids are rotationally symmetric around the z-axis and the probe signals must be available equidistantly in ϕ.

Optimized Design of a Compact Probe for Accurate Near Field Measurements

A rectangular corrugated horn antenna is designed for use as a probe for planar, cylindrical or spherical near-field measurements. The performance of the new design is compared with that of standard rectangular probes. The proposed probe has a nearly omnidirectional radiation pattern with very low cross polarization over a wide bandwidth. Moreover, the small size of the antenna is very suitable for use as a probe. Simulations and optimizations were carried out with a software tool based on the moment method.

Reconfigurable Slot-Array Antenna With RF-MEMS

Cylindrical near-field measurement systems are being widely demanded, especially for imaging applications at millimeter-wave frequencies. One of their most important drawbacks is the long time required to perform a full measurement of an antenna. To avoid multiple receivers, the probe is mechanically moved to the points where the field must be measured. The mechanical movement increases the time required to take a full measurement. The process may be sped up in several ways. This letter proposes the use of a pattern reconfigurable slot-array antenna as a probe. The switching among the different patterns is carried out electronically by RF-MEMS, which are placed over the different slots. Simulations and measurements of a model in the X-band are shown in this letter.

Cross-Polarization Uncertainty in Near-Field Probe Correction [AMTA Corner

The probe correction of near-field measured data can be considered to be composed of two parts. The first part is a pattern correction that corrects for the effects of the aperture size and shape of the probe, and can be analyzed in terms of the far-field main component pattern of the probe. The second part is due to the non-ideal polarization properties of the probe. If the probe responded to only one vector component of the incident field in all directions, this correction would be unnecessary. However, since all probes have some response to each of two orthogonal components, the polarization correction must be included. The polarization correction will be the focus of the following discussion. Previous studies have derived and tested general equations to analyze polarization uncertainty. This paper simplifies these equations for easier application. The results of analysis and measurements for planar, cylindrical, and spherical near-field measurements will be summarized in a form that is general, easily applied, and useful. Equations and graphs will be presented that can be used to estimate the uncertainty in the polarization correction for different AUT/probe polarization combinations and measurement geometries. The planar case will be considered first, where the concepts are derived from the probe-correction theory and computer simulation, and are then extended to the other measurement geometries.

Conical Near-Field Antenna Measurements [AMTA Corner

A near-field measurement technique for the prediction of asymptotic far-field antenna patterns from data obtained from a modified cylindrical or plane-polar near-field measurement system is presented. This technique utilizes a simple change in facility alignment to enable near-field data to be taken over the surface of a conceptual right cone [1, 2], or right conic frustum [3, 4], thereby allowing existing facilities to characterize wide-angle antenna performance in situations where hitherto they would perhaps have been limited by truncation. This paper aims to introduce the measurement technique, and to describe the novel probe-corrected near-field-to-far-field transform algorithm. The algorithm is based upon a cylindrical-mode expansion of the measured fields. Preliminary results of both computational electromagnetic simulations and actual range measurements are presented. As this paper recounts the progress of ongoing research, it concludes with a discussion of the remaining outstanding issues, and presents an overview of the planned future work.

Bistatic RCS Calculations From Cylindrical Near-Field Measurements—Part I: Theory

A theory is presented for computing scattered far fields of targets from cylindrical near-field measurements. The targets are illuminated by plane waves and measured in a radio anechoic chamber on a cylindrical scan surface. The scattered field on the scan cylinder is obtained by background subtraction. The near-field data is truncated at the top, bottom, and angular edges of the scan cylinder. These truncation edges can cause inaccuracies in the computed far fields. Correction techniques are developed for the top and bottom truncation edges. The cylindrical wave expansions automatically apply angular tapers to the near-field data that reduce the effect of the angular truncation edges. The taper functions depend on the angular sample spacing and are related to the currents induced on perfectly electrically conducting PEC cylinders in related scattering problems. The method of stationary phase is employed with asymptotic expressions for the taper functions to determine the area on the scan cylinder that is most important for computing the far field in a given direction. The theory is validated through numerical examples involving electrically large scatterers. The edge-correction techniques significantly increase the accuracy of the computed far field. A companion paper presents experimental results

Bistatic RCS Calculations From Cylindrical Near-Field Measurements—Part II: Experiments

Bistatic radar cross section (RCS) is computed from cylindrical near-field measurements obtained in a radio anechoic chamber with the target illuminated by a compact-range reflector. Near-field measurement is convenient where the RCS of complex targets is not amenable to computation or where computational results require experimental confirmation. A companion paper addresses the theory of cylindrical near-field scanning with reference to our experimental system. RCS of canonical targets derived from near-field measurement are in good agreement with theory. This paper compares the far fields computed from the near-field measurements with numerical solutions. Separating the target-scattered fields from incident and background fields presents a major challenge in an indoor bistatic radar configuration. We discuss the errors introduced by a residue of the incident field that is not canceled by the background subtraction method currently in use. A slow drift in system parameters and probe column oscillations are among the contributing factors

Strategy to avoid truncation error in planar and cylindrical near-field antenna measurement setups

A simple and effective method to avoid the truncation error in antenna near-field measurements is presented. The method can be applied to planar or cylindrical near-field setups, whenever it is possible to vary the distance between the antenna under test and the probe during the scanning procedure.

Application of cylindrical near-field measurement technique to the calibration of spaceborne radar antennas: NASA scatterometer and SeaWinds

Modern spaceborne radar scatterometers, such as the NASA scatterometer (NSCAT) and SeaWinds radar instruments, require precise determination of the normalized backscattered radar cross section within a few tenth of a decibel. This is needed to achieve the desired wind velocity and direction measurement accuracy of 2 m/s and 20/spl deg/, respectively. This high level of precision demands a priori prelaunch accurate knowledge and determination of the radar antenna's absolute gain and relative radiation patterns characteristics over wide angular range. Such characterizations may be performed on a far-field range, compact range, or in an indoor near-field measurement facility. Among the unique advantage of the near-field measurement is that most of the information of the radar antenna radiation properties can be obtained anywhere outside the near-field measurement surface. Two recently designed radar scatterometers are considered in this paper, NASA scatterometer (NSCAT) and SeaWinds, to demonstrate the utility of a newly completed cylindrical near-field measurement range. As an example of an advanced calibration methodology, the data based on a recently measured JPL/NASA scatterometer (NSCAT) radar antenna are used to experimentally demonstrate the role of the probe pattern compensation, probe multiple reflection effects, probe mispositioning effects, scan area truncation effects, etc. A measurement test on a standard gain horn (SGH) has been performed to achieve and verify the absolute gain calibration accuracy. A comparison between direct far-field measured data and those obtained from cylindrical near-field measurements for the SeaWinds radar antenna was found in excellent agreement. It is demonstrated that the near-field measurement technique is a viable approach in accurately characterizing the performance of spaceborne radar antennas.

An approximate expression to estimate signal-to-noise ratio improvement in cylindrical near-field measurements

A very simple approximate expression for the process gain (PG) for the cylindrical case is derived. The different approximations and assumptions required to obtain this expression are shown. This expression might be useful for most practical cylindrical near-field measurements, providing a very simple mean to assess the near-field dynamic range requirements to obtain a desired far-field signal-to-noise ratio (SNR).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Far-field errors due to random noise in cylindrical near-field measurements

A full characterization of the far-field noise obtained from cylindrical near-field to far-field transformation, for a white Gaussian, space stationary, near-field noise is derived. A possible source for such noise is the receiver additive noise. The noise characterization is done by obtaining the autocorrelation of the far-field noise, which is shown to be easily computed during the transformation process. Even for this simple case, the far-field noise has complex behavior dependent on the measurement probe. Once the statistical properties of the far-field noise are determined, it is possible to compute upper and lower bounds for the antenna radiation pattern for a given probability. These bounds define a strip within the radiation pattern with the desired probability. This may be used as part of a complete near-field error analysis of a particular cylindrical near-field facility.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Antenna far-field predictions from two phaseless cylindrical near-field measurements

A phaseless cylindrical near-field/far-field transformation algorithm is described. The technique uses the measurement of two near-field intensity distributions and knowledge of the antenna geometry. Verification measurements are presented for a waveguide array.

Effect of random errors in antenna cylindrical near-field measurements

A closed expression for the autocorrelation of the far field noise due to random near field noise in a cylindrical near to far field transformation is derived. This expression is employed to obtain bounds for the computed radiation pattern for a given near field signal to noise ratio.