The photo-Dember effect is known as source of impulsive THz radiation after excitation with femtosecond optical pulses. The origin of the emission is the ultrafast separation of electron and holes in strong carrier gradients due to different diffusion coefficients. For a simple semiconductor surface the time dependent polarization is oriented perpendicular to the excited surface which complicates efficient out coupling of THz radiation. We investigate a new scheme for generating strong carrier gradients parallel to the surface. In that case the photo-Dember currents are oriented parallel to the surface and the generated THz radiation can be easily out coupled. This concept can be scaled up so that multiple phase coherent photo-Dember currents contribute to the THz emission. These passive THz emitters reach electric field amplitudes comparable to high-efficiency externally biased photoconductive emitters.
We demonstrate the phase-locking of Quantum Cascade Lasers emitting at 2.4 and 2.7Terahertz, to the repetition rate of a commercial Er-doped fiber-fs-laser. We observe a linewidth of the beat-note signal with a signal-to-noise of 80dB in 1Hz bandwidth.
This paper describes the development of an array processing system and its integration into the local position measurement system (LPM). The design focus is laid on achieving high speed array processing performance in order to meet existing timing requirements. A field programmable gate array (FPGA) controls the radio frequency receivers and executes the array signal processing algorithms for local positioning. By physically separating the signal processing unit's memory, the FPGA's capacities can be fully exploited, and high signal processing throughput rates can be utilized. To verify the array receiver's performance, measurements in a typical multipath environment are processed by the system.
In this contribution we review some of our recent result in achieving power transmission enhancement through sub-wavelength apertures at both microwaves and optical frequencies. We aim at exciting highly localized magnetic resonances in order to design devices whose size is comparable to that of the aperture and suitable, thus, for practical applications. The proposed approaches may be applied indeed to the design of ultra-diffractive imaging systems, high-resolution spatial filters, high-precision lithography systems.
In this paper, accuracy of the Inverse Fast Fourier Transform (IFFT) algorithm is analysed, when used to compute the potential distribution in grounding grid transient analysis. The accuracy of the classical IFFT algorithm is questionable when the computations are carried out at relatively large distances between field points and source points. Accuracy deficiencies of the classical IFFT algorithm and the modified IFFT algorithm used in the well-known program package CDEGS are analyzed. Accuracy of these algorithms is tested using a highly accurate algorithm for computing the inverse Fourier transform based on numerical integration. Computation of the scalar potential distribution due to a point current source in unbounded and uniform earth is used as an illustrative test example.
A prediction model of GPS multipath requires a constant trade-off between computation time and realistic modeling. In previous papers we have presented a GPS multipath simulator based on physical optics. Regarding the design of this deterministic model, we have carefully justified the choices we have made to obtain reasonable computation times. They concern both the electromagnetic method and the precision of the data used to describe the 3D scene. To account for the limits of these choices, we define in this paper a new statistical model and we propose an innovative hybrid deterministic-statistical GPS multipath simulator adapted to airport navigation. This hybrid simulator is based on the deterministic simulator via Monte-Carlo simulations. Its statistical origin is the variability of the environment. For a particular scene its outputs are the statistical moments of the GPS range error. We show the necessity of this statistical component for realistic prediction purposes in comparison to pure deterministic predictions by means of simulations on a test-case.
Recently introduced concept of active non-Foster metamaterial is reviewed with an emphasis on the counter-intuitive background physics. A basic building block of this active metamaterial is an active ‘tank circuit’ that contains both conventional reactance and non-Foster negative reactance. It is shown that active metamaterial is stable if equivalent permittivity or permeability is arbitrarily small but positive number (ENZ or MNZ metamaterial). It is also shown that this metamaterial may be extremely broadband (several octaves) with almost no dispersion. The propagation within the operating band may support counter-intuitive superluminal phase and group velocities. However, causality is always preserved due to finite bandwidth of any realistic non-Foster element. Several illustrative examples of practical 1D and 2D active ENZ metamaterials developed at University of Zagreb are presented.
In overall expansion of power distribution and sub-transmission (10 kV - 110 kV) networks, conditioned with growing consumption of electrical power, there is an increasing of possibility for crossing underground power cables with home electrical installation cables, telecommunication equipment cables and other equipment. Furthermore, surge arresters on middle-voltage levels usually are not applied along overhead lines. Above mentioned can affect on equipment or even human life safety. It is known that interference of the different electrical installations and equipment is the main theme of researching in Electromagnetic Compatibility (EMC). In this article, a possibility of using the Finite Element Method (FEM) for EMC calculations in practice, is explored and presented. For that purpose, a real problem from distribution power network will be used for an illustrative example. It is worth of mentioning that FEM technique is quite a new tool in power networks calculations area.
Practical realization of an electromagnetic cloak based on the transformation electromagnetics requires the existence of highly anisotropic metamaterial, permittivity and/or permeability of which may have values very close to zero (ENZ and/or MNZ metamaterials). Published experiments on the SRR-based cloaks proved the validity of the basic idea but also showed two significant drawbacks: very complicated design and an extremely narrow bandwidth. Another possible design might use the transmission-line-based metamaterial, which is usually stated as being inherently broadband. Here, it is shown that the bandwidth is actually constrained by the basic physics and not by applied technology. There is always some minimal value of capacitance or inductance that can be achieved within some differential volume of space. Thus, one must use some kind of resonant circuit in order to achieve the ENZ or MNZ behavior. The circuit simulations showed that is indeed possible to construct a 2D TM z fully anisotropic cloak based on the transmission lines loaded with lumped elements. Such a cloak might have a bandwidth broader than the SRR-based cloak but it is again fundamentally constrained by the bandwidths of used resonant LC circuits.
This paper presents a two-step design methodology with a 2-D modelling approach based on FDTD and Genetic Algorithms to synthesise flat lenses for beam shaping applications. This methodology has been applied to design flat lenses radiating a stable flat-top beam over a 10% bandwidth in H-plane and a nearly omni-directional radiation in E-plane. The results are validated numerically and experimentally in V-band using single-shell and double-shell lenses.
Spectrum sensing is an essential part of the cognitive radio systems which can be the solution for radio spectrum shortage. There are various methods of the signal detection; many of them are based on the energy detector. Another group of methods is based on known detected signal parameters as the method for DVB-T signal detection described in this paper. Because DVB-T uses OFDM with cyclic prefix, the computation of cyclic prefix correlation and the consequent correlation function level monitoring are proposed. The simulations of this method and their results are described in the paper. Test DVB-T signal with various C/N ratios is obtained by sampling of the output of the vector signal generator and complete signal processing is implemented in Matlab with Parallel Computing Toolbox support. The performance of investigated method for DVB-T signal detection is discussed in the paper.
We investigated the use of low-temperature-grown (LTG) GaAs photoconductive antenna (PCA) excited at 1560 nm for the detection of terahertz (THz) pulses. The decrease in the excitation spot size with a PCA of properly reduced gap length was found to be efficient to enhance the signal intensity. With a spot size of 2.7 µm exciting a 1.5 µm gap PCA, the amplitude of the detected THz wave is an order of magnitude larger than that obtained in conventional 5 µm gap PCAs. The enhancement of the signal intensity by the reduction of the spot size stems mostly from the super-linear response of the LTG-GaAs excited at 1560 nm. The results suggest that the present scheme is very useful in fiber laser based time domain THz systems.
One of the main drawbacks when designing microwave circuits or antennas based on metamaterial particles is its inherent low bandwidth and not very good efficiency. An attempt to overcome this problem is based on using negative impedance converters (NICs). Although the use of NICs have been proposed as a solution to increase the bandwidth of electrically small antennas, they suffer from many problems such as stability performance, bias and the maximum frequency that can be achieved. In addition, the application of NICs has been restricted to low frequency applications in order to avoid the previous problems. This paper makes a study on the performance of NICs for active metamaterial applications. The main contributions of the present paper is that it takes into account the non-linear equivalent circuit of the NICs to find out its performance for active metamaterial applications. From that study it can be concluded that NICs can work up to a few GHz.
Enhanced Leaky lens antennas provide new opportunities for the development of wireless links over extremely large bandwidths without dispersions. In this contribution the potentials in this respect are presented. We provide the experimental characterization of a planarly fed ultra-wideband leaky lens antenna including a brief description of two antenna prototypes with radiating elements realized in printed circuit board technology. The impedance parameters and the radiation patterns of the antennas have been measured, showing excellent pattern quality and efficiency. The link between two antennas has then been characterized in the frequency and time domains in terms of mutual coupling impulse response, respectively.
Small printed dipole-like antennas which provide circular polarization with wide axial-ratio bandwidths are presented. The first antenna is an asymmetrical dipole with a slitted ground plane which is fed by an L-shaped microstrip feedline using a via. The other antenna employs a pair of asymmetrical dipole arms which are located on opposite sides of the substrate and fed by a stepped microstrip line which connects to the shorter dipole arm. It also employs groundplane slits. By utilizing the combined orthogonal surface currents along the asymmetric arms and the microstrip line structure, circular polarization is realized. Measurements show the printed dipole antenna with a via can achieve a 23% axial-ratio bandwidth whereas the antenna without via can achieve 33.5% axial-ratio bandwidth.
Terahertz imaging and sensing have attracted much attention in recent years, because they can be applied to many application fields. A terahertz real-time imaging technologies considered increasingly important in the future (Terahertz Camera) was developed. The terahertz camera consists of a light source (Terahertz quantum cascade laser) and a un-cooled micro-bolometer array, which can easily get real-time image. As an application of the terahertz camera, a label-free bio-materials detection technology has developed and demonstrated.
Interference between two wireless body area networks at 2.45GHz and 5.8GHz has been investigated. A series of data sets have been gathered in an indoor environment - standard office - for random realistic activities and body orientations, and have been subject to statistical analysis. It is clear that significant variations of the path gain can occur due to the changing positions and orientations of the wireless network users, and the changing postures of their bodies. Chi-square goodness-of-fit test was used to assess whether the signal fading belongs to one of a number of known of probability distributions (namely, gamma, lognormal, loglogistic, Nakagami, Weibull and Rician). Our results show that the gamma distribution is an excellent candidate for describing the fading between two wireless body area networks. This paper presents results of characterization of interfering signal strength variations with the distance, and a number of significant statistical parameters, including level crossing rate and average fade duration.
