Martin Leibfritz

Fully Probe-Corrected Near-Field Far-Field Transformation Employing Plane Wave Expansion and Diagonal Translation Operators

Near-field antenna measurements combined with a near-field far-field transformation are an established antenna characterization technique. The approach avoids far-field measurements and offers a wide area of post-processing possibilities including radiation pattern determination and diagnostic methods. In this paper, a near-field far-field transformation algorithm employing plane wave expansion is presented and applied to the case of spherical near-field measurements. Compared to existing algorithms, this approach exploits the benefits of diagonalized translation operators, known from fast multipole methods. Due to the plane wave based field representation, a probe correction, using directly the probes far-field pattern can easily be integrated into the transformation. Hence, it is possible to perform a full probe correction for arbitrary field probes with almost no additional effort. In contrast to other plane wave techniques, like holographic projections, which are suitable for highly directive antennas, the presented approach is applicable for arbitrary radiating structures. Major advantages are low computational effort with respect to the coupling matrix elements owing to the use of diagonalized translation operators and the efficient correction of arbitrary field probes. Also, irregular measurement grids can be handled with little additional effort.

Full probe-correction for near-field antenna measurements

Nowadays antenna measurements are often carried out exploiting the benefits of near-field transformation techniques. The transformation algorithms usually applied require near-fields being measured in two polarisations in order to calculate an equivalent spherical multipole representation of the antenna under test (AUT). Although it is desirable to use custom probes for various applications, especially in research. Custom probes generally do not satisfy the first-order requirement. On the other hand it is well known that a treatment of custom probes as first-order probes will lead to significant errors when calculating the far field from these insufficiently corrected near-field measurements. In this paper it is shown that this error is even worse when dealing with near-field to near-field transformations as required for antenna diagnostics. However the impact on the quality of the transformed near-field data is significantly reduced when applying our proposed probe correction algorithm. The results of the near-field transformations therefore provide significantly improved far-fields as well as an adequate basis for calculating the electromagnetic fields in the proximity of the AUT for diagnostic purposes

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.

A Dual-Polarised Dielectric Near-Field Probe

This paper presents a novel dielectric near-field probe as an alternative for the conventional open-ended waveguide probes. The latter tend to be bulky in the mobile communication frequency bands (GSM 900 and GSM 1800). Furthermore they show significant interaction with the AUT as compared to the proposed dielectric probe.

Diagnosis of antenna-arrays using near-field antenna measurements and A {itpriori} information

Nowadays antenna measurements are often carried out by exploiting the benefits of near-field transformation techniques. However, as spherical near-field measurements are very time consuming it is desirable to use the near-field data being measured in order to determine an antennas far-field characteristic also for diagnostic purposes, especially if the far-field characteristic differs from the expected one. This paper explains how to gain diagnostic information from measured near-field data. For this purpose, the standard microwave holography as well as the limits of its spatial resolution are discussed. Following this a new equivalent-source method is introduced allowing the reconstruction of electromagnetic near-fields with enhanced resolution as compared to the holographic approach. The quality of the reconstructed near-field data will be significantly enhanced when applying the proposed diagnostic algorithm.

Ultra-Wideband dielectric rod antenna with biconical dipole and reflector

A directive ultra-wideband (UWB) antenna exhibiting a gain of more than 7 dBi in the whole FCC UWB frequency band is presented. The excitation of the so called dipole mode on the rod is achieved by a biconical dipole. Placing this configuration in some distance to a reflector results in a flat gain curve. Besides an input reflection of -10 dB from 3 up to 20 GHz, this novel antenna has an excellent transient behavior over a wide azimuth angle, whereas the dimensions can be kept very compact.

Near-field far-field transformation exploiting the benefits of plane wave expansion and diagonal translation operators

Nowadays, near-field antenna measurements combined with a near-field far-field transformation are a common antenna measurement approach. Among the well known methods employing spherical multipole-expansions, various other techniques are available. In this paper, a new near- field far-field transformation algorithm based on plane wave expansion is presented. In contrast to existing algorithms, the new approach exploits the benefits of diagonalised translation operators. Compared to other plane wave techniques, like holographic projections which are suitable for highly directive antennas, the new approach is applicable for almost arbitrary radiating structures. The major advantage of the presented method is a low computational effort with respect to the coupling matrix elements owing to the use of diagonalised translation operators.