Spherical Near-field Antenna Measurements Research Articles

Superdirective Magnetic Dipole Array as a First-Order Probe for Spherical Near-Field Antenna Measurements

The theory as well as numerical and experimental results are presented for a superdirective array composed of closely spaced electrically small resonant magnetic dipole elements. The array operates on a metal ground plane and can exhibit a maximum directivity of 11.5 dBi, 15.2 dBi, and 17.8 dBi (including 3 dB due to the ground plane), for 2, 3, and 4 magnetic dipoles, respectively. The array is self-resonant and is directly excited by a 50-ohm coaxial cable through the ground plane. The array radiates essentially the |μ| = 1 spherical modes, which, despite a narrow bandwidth, makes it an excellent first-order probe for spherical near-field antenna measurements at low frequencies.

Numerical Investigation of the System-Matrix Method for Higher-Order Probe Correction in Spherical Near-Field Antenna Measurements

The system-matrix method for higher-order probe correction in spherical near-field scanning is based on a renormalized least-squares approach in which the normal matrix closely resembles the identity matrix when most of the energy of the probe pattern resides in the first-order modes. This method will be “stressed-tested” in the present paper by employing probes for which up to 49% of the pattern energy resides in the higher-order modes. The condition number of the resulting normal matrix will be computed, and its “distance” from the identity matrix displayed. It is also shown how the condition number of the normal matrix can be further reduced.

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.

Reduction of Truncation Errors in Planar, Cylindrical, and Partial Spherical Near-Field Antenna Measurements

A method to reduce truncation errors in near-field antenna measurements is presented. The method is based on the Gerchberg-Papoulis iterative algorithm used to extrapolate band-limited functions and it is able to extend the valid region of the calculated far-field pattern up to the whole forward hemisphere. The extension of the valid region is achieved by the iterative application of a transformation between two different domains. After each transformation, a filtering process that is based on known information at each domain is applied. The first domain is the spectral domain in which the plane wave spectrum (PWS) is reliable only within a known region. The second domain is the field distribution over the antenna under test (AUT) plane in which the desired field is assumed to be concentrated on the antenna aperture. The method can be applied to any scanning geometry, but in this paper, only the planar, cylindrical, and partial spherical near-field measurements are considered. Several simulation and measurement examples are presented to verify the effectiveness of the method.

A time domain spherical near-field measurement facility for UWB antennas employing a hardware gating technique

Abstract. A spherical near-field antenna measurement facility employing a time domain hardware gating technique is presented. On-off keyed sinusoidal impulses are used as stimuli requiring wideband antennas with a bandwidth in excess of 400 MHz. The received signal is evaluated in the time interval after reaching the steady state and before multipath components arising in the non-ideal anechoic chamber distort the signal. An application specific pulse generator synthesizing sinusoidal impulses with a sub-nanosecond settling time and a low-cost equivalent time (ET) sampling receiver developed and optimized for this particular purpose are described. Measurement results of typical ultra-wideband (UWB) antennas show a significant improvement of the measured antenna pattern compared to conventional techniques.

Phaseless spherical near-field antenna measurements for low and medium gain antennas

Abstract. The characterisation of antenna radiation patterns by measurements in the near-field and a following near-field far-field transformation require accurate amplitude and phase data. Especially at higher frequencies phase measurements are demanding in terms of instrumentation and measurement accuracy. Phaseless techniques which require amplitude only data on one or more measurement surfaces are therefore of special interest. In this paper a phaseless spherical algorithm based on spherical modal expansion is presented. The algorithm works with amplitude only near-field data measured on two spheres of different radii and is therefore not restricted to certain types of antennas as for planar measurements for example. Simulated as well as measured results of low and medium gain antennas are shown.

Iterative Algorithm for Probe Calibration in Spherical Near-Field Antenna Measurement

Probe correction in spherical near-field measurements is a widely studied technique used to compensate for probe polarization and pattern effects. While the polarization compensation can be carried out in a straightforward way, the pattern compensation requires the knowledge of receiving coefficients which are not easily obtained. With this aim, several techniques, which are known as probe calibration techniques, have been introduced. However, none of them are accurate enough. This communication introduces an algorithm for the accurate computation of first-order probe receiving coefficients. It also shows how accurate gain may be determined by using these new coefficients.

Theory and Practice of the FFT/Matrix Inversion Technique for Probe-Corrected Spherical Near-Field Antenna Measurements With High-Order Probes

A complete antenna pattern characterization procedure for spherical near-field antenna measurements employing a high-order probe and a full probe correction is described. The procedure allows an (almost) arbitrary antenna to be used as a probe. Different measurement steps of the procedure and the associated data processing are described in detail, and comparison to the existing procedure employing a first-order probe is made. The procedure is validated through measurements.

Experimental Demonstration of the Extended Probe Instrument Calibration (EPIC) Technique

A chamber calibration technique for spherical near-field antenna measurements proposed by Pogorzelski is experimentally demonstrated. The chamber was purposely degraded by introducing a metal plate situated so as to produce a strong specular reflection from the antenna under test to the chamber probe. The effects of this artifact were easily observed in the raw data. The chamber with the artifact was calibrated using an open ended waveguide calibration antenna and the resulting calibration coefficients were used to correct the raw measurement producing a result very similar to the measurement carried out in the undegraded chamber over about 30 to 35 dB of dynamic range.

Time-Domain Spherical Near-Field Antenna Measurement System Employing a Switched Continuous-Wave Hardware Gating Technique

A time-domain spherical near-field antenna measurement system capable of gating out erroneous signal components, which arise due to multipath propagation in nonideal anechoic chambers, is presented. The developed hardware (HW) gating technique evaluates a switched sinusoidal signal, which is synthesized by an application-specific pulse generator and acquired by either a commercial real-time digitizing oscilloscope or an application-specific equivalent-time sampling receiver developed for this particular purpose. The low-cost measurement system has been optimized for acquisition speed, dynamic range, and resolution. Its operating frequency range covers 1.5-8 GHz, and it is applicable to antennas exhibiting a typical 3-dB bandwidth in excess of 400 MHz. Test measurements of an omnidirectional and a directional antenna, respectively, have been carried out to demonstrate the performance of the novel HW gating technique. It is shown that the HW gating technique can significantly improve the absolute average deviation of the erroneous 3-D far-field pattern.

Extended Probe Instrument Calibration (EPIC) for Accurate Spherical Near-Field Antenna Measurements

A probe calibration technique is described that removes artifacts from spherical near-field antenna measurements. The technique is based on preliminary calibration measurements of a known antenna, a process that characterizes the probe and any stationary objects in the measurement chamber including the walls in terms of a set of correction coefficients. These coefficients are then used in a correction algorithm that recovers the correct antenna pattern from the corrupted measurement data.

Modified $\theta $-Scanning Technique for First/Third-Order Probes for Spherical Near-Field Antenna Measurements

This paper presents a modified thetas-scanning technique for first/third-order probes for spherical near-field antenna measurements. Unlike the traditional thetas-scanning technique, this new scanning technique gives a possibility, due to the cutoff property of the azimuthal spherical modes, to filter out the influence of the undesired third-order azimuthal modes of the probe near the poles of the measurement sphere. Without increasing the measurement time, the technique allows reducing the errors caused by the application of the computationally efficient first-order probe correction technique (incorrectly) to first/third-order probes. The technique is optimal for spherical near-field systems, that employ, for example, an open-ended rectangular (or square) waveguide probe, that closely approximates a first/third-order probe, and apply the well-known first-order probe correction technique. In such systems, compared to the traditional thetas-scanning technique, the modified thetas-scanning technique can be used to significantly reduce the errors caused by the incomplete probe correction.

Double $\phi$-Step $\theta$-Scanning Technique for Spherical Near-Field Antenna Measurements

Probe-corrected spherical near-field antenna measurements with an arbitrary probe set certain requirements on an applicable scanning technique. The computational complexity of the general high-order probe correction technique for an arbitrary probe, that is based on the Phi scanning, is O(N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> ), where N is proportional to the radius of the antenna under test (AUT) minimum sphere in wavelengths. With the present knowledge, the computational complexity of the probe correction for arbitrary probes in the case of the thetas scanning is O(N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> ), which is typically not acceptable. This paper documents a specific double Phi-step thetas scanning technique for spherical near-field antenna measurements. This technique not only constitutes an alternative spherical scanning technique, but it also enables formulating an associated probe correction technique for arbitrary probes with the computational complexity of 0(N4) while the possibility for the exploitation of the advantages of the thetas scanning are maintained.

A First/Third-Order Probe Correction Technique for Spherical Near-Field Antenna Measurements Using Three Probe Orientations

A probe correction technique is described for spherical near-field antenna measurements based on sampling the near field for three probe orientations in each measurement direction. The technique applies to odd-order probes whose radiated field contains (significant) power only in the first (mu = plusmn1) and third- order (mu = plusmn3) azimuthal spherical modes. The technique is ideally suited as an optional probe correction technique for high-accuracy measurements in existing measurement systems that employ a rectangular or square waveguide probe and the traditional first-order probe correction technique.

Probe Correction Technique for Symmetric Odd-Order Probes for Spherical Near-Field Antenna Measurements

A probe correction technique for symmetric odd-order probes for spherical near-field antenna measurements is presented. The technique provides computational relaxations in the probe correction compared to the known general high-order and odd-order probe correction techniques.

Application of the iterative probe correction technique for a high-order probe in spherical near-field antenna measurements

An iterative probe-correction technique for spherical near-field antenna measurements is examined. This technique has previously been shown to be well-suited for non-ideal first-order probes. In this paper, its performance in the case of a high-order probe (a dual-ridged horn) is examined

Iterative probe correction technique for spherical near-field antenna measurements

An iterative probe correction technique is presented to correct for the errors caused by the application of the traditional first-order probe correction for nonideal first-order probes in spherical near-field antenna measurements. The technique may be applied, with certain restrictions, for more general probes as well.

Real-time spherical near-field handset antenna measurements

A new spherical near-field measurement system, dedicated to personal-communications applications, has been installed at e-tenna Corporation. The system, sold by Satimo, uses an electronically scanned probe array, and offers a dramatic reduction of the measurement time (by a factor ranging from 60 to 1,000) with respect to conventional mechanical scanners. This test range has undergone extensive acceptance testing, which has verified the specified measurement accuracy. The speed of the system, combined with the availability of full-sphere measurement data, enables rapid measurement of antenna efficiency. The system also enables the measurement of other parameters, such as the percentage of radiated power dissipated into the body of a human test subject operating a cellular telephone.

Book Review: Spherical Near-Field Antenna Measurements

Book Review: Spherical Near-Field Antenna Measurements