Klaus Solbach

Transmission Line Modeling and Asymptotic Formulas for Periodic Leaky-Wave Antennas Scanning Through Broadside

It is shown, using three specific examples-a series fed patch (SFP) array, a phase reversal (PR) array and a composite right/left-handed (CRLH) antenna-that one-dimensional periodic leaky-wave antennas scanning through broadside build a class of leaky-wave antennas sharing qualitatively similar and quantitatively distinct dispersion and radiation characteristics. Based on an equivalent transmission line (TL) model using linearized series and shunt immittances to approximate the periodic (Bloch) antenna structure, asymptotic TL formulas for the characteristic propagation constant, impedance, energy, power and quality factor are derived for two fundamentally different near- and off-broadside radiation regimes. Based on these formulas, it is established that the total powers in the series and shunt elements are always equal at broadside, which constitutes one of the central results of this contribution. This equal power splitting implies a severe degradation of broadside radiation when only one of the two elements series or shunt efficiently contributes to radiation and the other is mainly dissipative. A condition for optimum broadside radiation is subsequently established and shown to be identical to the Heaviside condition for distortionless propagation in TL theory. Closed-form expressions are derived for the constitutive (LCRG) parameters of the TL model for the specific SFP, PR and CRLH antenna circuit models, and quantitative information on the validity range of the TL model is subsequently provided. Finally, full-wave simulation and measurement LCRG parameter extraction methods are proposed and validated.

Numerical analysis of characteristic modes on the chassis of mobile phones

This paper explores the application of the concept of characteristic modes on the chassis of a mobile phone to purposeful design of its radiation characteristics. Focus is on the resonant chassis modes which dominate the overall radiation properties. Modal radiation quality factors are derived from characteristic mode eigenvalues. Antenna ¿ chassis coupling, previously only considered in terms of equivalent circuits, is discussed from a field theoretic point of view. A numerical approach for characteristic mode analysis is presented which is an eigenmode solver extension to the well known NEC2 code. Numerical results are given for wire grid models of a bar-type and a folder-type phone chassis.

Simulation Study of Harmonic Oscillators

In the last few years, the operating modes of Gunn oscillators for frequencies above 60 GHz have been discussed controversially. In this context, a general theoretical circuit model for oscillators operating in the fundamental and in the second-harmonic modes is studied. The model employs a simple cubic I - V characteristic of the active element and separate embedding circuits for the fundamental and second-harmonic frequencies. The current and voltage waveforms of both modes are contrasted. The oscillator source impedances and the external Q of the second-harmonic mode oscillator are calculated.

Cerebellar pathology in Friedreich's ataxia: Atrophied dentate nuclei with normal iron content

Background In Friedreichs ataxia (FA) the genetically decreased expression of the mitochondrial protein frataxin leads to disturbance of the mitochondrial iron metabolism. Within the cerebellum the dentate nuclei (DN) are primarily affected. Histopathological studies show atrophy and accumulation of mitochondrial iron in DN. Dentate iron content has been suggested as a biomarker to measure the effects of siderophores/antioxidant treatment of FA. We assessed the iron content and the volume of DN in FA patients and controls based on ultra-high-field MRI (7 Tesla) images. Methods Fourteen FA patients (mean age 38.1 yrs) and 14 age- and gender-matched controls participated. Multi-echo gradient echo and susceptibility weighted imaging (SWI) sequences were acquired on a 7 T whole-body scanner. For comparison SWI images were acquired on a 1.5 T MR scanner. Volumes of the DN and cerebellum were assessed at 7 and 1.5 T, respectively. Parametric maps of T2 and T2* sequences were created and proton transverse relaxation rates were estimated as a measure of iron content. Results In FA, the DN and the cerebellum were significantly smaller compared to controls. However, proton transverse relaxation rates of the DN were not significantly different between both groups. Conclusions Applying in vivo MRI methods we could demonstrate significant atrophy of the DN in the presence of normal iron content. The findings suggest that relaxation rates are not reliable biomarkers in clinical trials evaluating the potential effect of FA therapy.

The Calculation and the Measurement of the Coupling Properties of Dielectric Image Lines of Rectangular Cross Section

In this paper the coupling properties of uniformly coupled dielectric image lines of rectangular cross section are calculated employing an exact method for the determination of the phase constants of the even and the odd mode. The theoretical results are verified experimentally. The results are also applied to the calculation of the coupling characteristics of practical dielectric image line directional couplers.

Electric Probe Measurements on Dielectric Image Lines in the Frequency Range of 26-90 GHz

Electric-field probes, using miniature semirigid coaxial cable in connection with standard metal waveguide detectors, are described and shown to be well-suited for direct standing wave measurements on dielectric image lines. Experiments in the frequency range of 26-90 GHz for the detemination of phase and attenuation constants of image lines as well as experiments for the determination of very small reflections from matched dielectric image-line terminations and the scattering coefficients of dielectric image-line discontinuities are described.

Circular Polarization of Periodic Leaky-Wave Antennas With Axial Asymmetry: Theoretical Proof and Experimental Demonstration

This paper includes two contributions. First, it proves that the series and shunt radiation components, corresponding to longitudinal and transversal electric fields, respectively, are always in phase quadrature in axially asymmetric periodic leaky-wave antennas (LWAs), so that these antennas are inherently elliptically polarized. This fact is theoretically proven and experimentally illustrated by two case-study examples, a composite right/left-handed (CRLH) LWA and a series-fed patch (SFP) LWA. Second, it shows (for the case of the SFP LWA) that the axial ratio is controlled and minimized by the degree of axial asymmetry.

Radiation Efficiency of Longitudinally Symmetric and Asymmetric Periodic Leaky-Wave Antennas

This letter derives efficiency formulas and establishes fundamental limitations for longitudinally symmetric periodic leaky-wave antennas (LWAs). The proposed approach is based on an equivalent transmission-line model for periodic structures, which is derived from a lattice topology providing a perfect decoupling between the series and shunt immittances and their respective radiation contributions. A central result of this letter is that a 2-D array composed of uniformly excited 1-D LWAs cannot exceed a radiation efficiency of 50%. The presented theory is validated by comparing measured and finite-difference time-domain (FDTD) simulated radiation efficiencies with the ones predicted by the lattice model for several symmetrical and asymmetrical series-fed patch (SFP) array antenna and composite right/left-handed (CRLH) antenna configurations.

A Low-Cost Microwave Radiometer for the Detection of Fire in Forest Environments

This paper deals with the development of a microwave noise-adding radiometer, which is purposely designed for the fire detection in forest environments. The sensor operates at 12.65 GHz and exploits commercial Satellite Television (SAT-TV) components such as a parabolic dish and a low-noise block. First, a simple system model is presented to estimate the radiometric contrast due to the presence of fire (increase in the antenna noise temperature with respect to the background) at a certain distance from the receiving antenna. Then, the design of the sensor is addressed, underlining the key technologies that allow the required performance to be attained at low industrial costs. An experimental characterization of the developed radiometer is reported focusing on both accuracy and sensitivity issues. Due to a continuous gain calibration based on the noise-adding procedure, the antenna noise temperature can be retrieved with an absolute error of 4 K without any thermal stabilization of the instrument electronics. Last, real fire detection experiments have been carried out both in laboratory and open-space environments. A radiometric contrast of 8.8 K has been recorded for a wooden fire of 0.2 placed at a distance of about 30 m from the antenna.

Optimal antenna location on mobile phones chassis based on the numerical analysis of characteristic modes

This paper contributes to the solution of the antenna-chassis coupling problem for handheld devices in mobile communication. That problem, up to now addressed in terms of equivalent circuits, is discussed here from a field theoretical point of view. A numerical analysis of characteristic modes on the chassis of the device provides a resonant mode which dominates the overall radiation properties. The modal excitation coefficient which relates the above mentioned chassis dominant wavemode to that of a given antenna element (exciter) should be maximized to achieve an optimal modal coupling. This paper establishes conditions to achieve maximum modal excitation coefficient which lead to an optimal placement of the antenna element on the chassis. A numerical approach for the evaluation of the optimal location of the exciter (electric and magnetic) on the chassis is performed. In this paper a theoretical approach based on some assumptions of the evaluation of the modal excitation coefficient is presented for the cases of an electric and a magnetic exiter. Numerical results are given for a bar-type and a folder-type phone chassis.