In order to accurately compute scattered and radiated fields in the presence of arbitrary excitations, a low-frequency stable discretization of the right-hand side (RHS) of a quasi-Helmholtz preconditioned electric field integral equation (EFIE) on multiply-connected geometries is introduced, which avoids an ad-hoc extraction of the static contribution of the RHS when tested with solenoidal functions. To obtain an excitation agnostic approach, our approach generalizes a technique to multiply-connected geometries where the testing of the RHS with loop functions is replaced by a testing of the normal component of the magnetic field with a scalar function. To this end, we leverage orientable global loop functions that are formed by a chain of Rao-Wilton-Glisson (RWG) functions around the holes and handles of the geometry, for which we introduce cap surfaces that allow to uniquely define a suitable scalar function. We show that this approach works with open and closed, orientable and non-orientable geometries. The numerical results demonstrate the effectiveness of this approach.

A document by Han Na . Click on the document to view its contents.

A novel design of a broadband mm-wave patch antenna for mobile devices is introduced. The proposed antenna element operates around 40 GHz. Through utilizing a multiple resonances patch structure, 9% bandwidth has been obtained. The substrate consists of low cost material and the antenna with its microstrip feed can easily be integrated with frontend circuits. Two parasitic patches are implemented to reduce the directivity of the antenna, which leads to a wide receiving angular range and makes the antenna suitable for specified mobile applications.

The inverse equivalent source problem related to near-field antenna measurements is typically ill-posed, i.e., the forward operator suffers from non-trivial null spaces. This issue is commonly tackled by pursuing a least-squares solution of the reconstructed near fields. We propose to find a solution of the normal error system of equations which minimizes the l2-norm of the source-coefficients reconstruction deviation. In the scope of near-field to far-field transformations (NFFFTs), advantages are found in a slightly better iterative solver convergence, a reduced number of unknowns, and—most importantly—a more convenient control of the stopping criterion of the iterative solution process. Since the residual of the normal-error solution equals the reconstruction deviation, the proposed formulation includes a meaningful stopping criterion based on the measurement error. All these claims are corroborated by NFFFTs of synthetic and real-world measurement data.

A non-reciprocal self-mixing receive antenna array is presented. First, the principle of the self-mixing concept is explained and it is found that self-mixing receivers achieve a large gain over a wide angular range, which is in contrast to reciprocity. In the second part, the design of a 4×2 antenna array operating from 34 GHz to 39 GHz including the receive amplifier and mixer is introduced. Measurements in an anechoic chamber verify the simulation results, showing promising results for future applications of self-mixing arrays with a wide angular receiving range.

Some innovative near-field antenna measurement setups, e.g., UAV-based, have limited capabilites for providing a phase and magnitude reference to the receiver. Phaseless antenna measurements have always been an important topic for such scenarios, but they often suffer from the unreliability of nonconvex phase retrieval algorithms, necessity of extremely accurate magnitude measurements and strong oversampling requirements. A similar problem arises if the measured magnitudes are impaired, i.e., if the global magnitude reference is unreliable for instance due to drift. We present a linearized method to reconstruct the phases for measurements without a global phase reference as well as an inherently linear method to reconstruct the magnitudes for measurements without a global magnitude reference. The underlying assumption is that a drift in either the phase or the magnitude occurs during the field measurement. With the reconstructed, thus, fully consistent observation vector, standard electromagnetic field transformations are possible and yield accurate and reliable results.

The accurate solution of quasi-Helmholtz decomposed electric field integral equations (EFIEs) in the presence of arbitrary excitations is addressed: Depending on the specific excitation, the quasi-Helmholtz components of the induced current density do not have the same asymptotic scaling in frequency, and thus, the current components are solved for with, in general, different relative accuracies. In order to ensure the same asymptotic scaling, we propose a frequency normalization scheme of quasi-Helmholtz decomposed EFIEs which adapts itself to the excitation and which is valid irrespective of the specific excitation and irrespective of the underlying topology of the structure. Specifically, neither an ad-hoc adaption nor a-priori information about the excitation is needed as the scaling factors are derived based on the norms of the right-hand side (RHS) components and the frequency. Numerical results corroborate the presented theory and show the effectiveness of our approach.

The combined source (CS) condition is employed to augment the electric field integral equation (EFIE), whereby the CS condition is implemented as a modification of the weak form Leontovich impedance boundary condition. By doing so, both electric and magnetic surface current densities can be modeled with Rao-Wilton-Glisson (RWG) basis functions. Strict point-wise orthogonality of electric and magnetic currents is not fulfilled. However, rotated RWG functions or Buffa-Christiansen functions, which are both often utilized in method of moments formulations, are avoided. The conditioning of the CS integral equation (CSIE) is improved as compared to the standard EFIE with Love currents, whereas the very good accuracy of the EFIE is retained. Objects that show poor accuracy when solved with magentic field and combined field integral equations, e.g. objects with sharp edges, can be solved very accurately with the proposed CSIE.

A low-order discretization scheme for the magnetic field integral equation (MFIE) with considerably improved accuracy is discussed and analyzed. The formulation is based on the classical form of the MFIE discretized with Rao-Wilton-Glisson functions and introduces weak-form basis transformations, which have recently been employed with great success within the context of the so-called combined source integral equation. Only the identity operator discretization, which is known to be a major error source, is modified in order to obtain highly accurate solutions. Numerical results are given to corroborate the effectiveness of the new scheme.

The inverse source problem of near-field antenna measurements is often formulated in terms of unknown equivalent electric and magnetic surface current densities. Exploiting just the constraints for the radiated field exterior to a closed Huygens surface results in ambiguous equivalent sources due to the excitation of non-radiating currents during the solution process. Therefore, enforcing the zero-field or Love side condition for the suppression of non-radiating currents based on a left-hand side preconditioner in the form of a Calderón projector is proposed. Transformation results based on realistic measurements demonstrate the effectiveness of this approach.

The free-space radiation characteristic of an antenna under test (AUT) is determined from measurements in proximity of a scatterer by time gating reconstructed equivalent currents for the AUT and the scatterer. The presented approach effectively combines spatial filtering methods with time domain methods while mitigating their individual drawbacks. In contrast to conventional time-gating methods which usually work on the measured probe signals this approach allows to get rid of undesired echo perturbations even if measurement samples are located in the shadow region of the scatterer and the line of sight (LOS) contribution and the echo contribution are indistinguishable for the field probe. In contrast to conventional frequency domain methods, mutual coupling contributions of the AUT currents are identified and removed in the time domain representation of the reconstructed currents. Numerical examples show that, due to the sparse time domain representation of the currents, the free-space radiation of the AUT can be determined accurately even at the borders of the measured bandwidth.

Conventional phaseless near-field measurement data is not adequate for reliable transformation into the far field. We tackle this challenge by a formulation working with multifrequency phaseless measurements under the assumption of coherently measured spectra. Such data is, for instance, obtained with a transmitting antenna under test and standard receivers featuring a nonzero coherent bandwidth. The focus of this work is on an advanced algorithm to demonstrate that this ansatz is very promising for further research and real-world investigations. Empirical studies based on simulation data demonstrate that appropriately merging this multi-frequency data significantly increases the chance of successful phaseless near-field far-field transformation. As a by-product, this multi-frequency phase retrieval method supports also the retrieval of the phase of farfield antenna measurements.