Y. Pinhasi

EHF for Satellite Communications: The New Broadband Frontier

The exploitation of extremely high-frequency (EHF) bands (30-300 GHz) for broadband transmission over satellite links is currently a hot research topic. In particular, the Q-V band (30-50 GHz) and W-band (75-110 GHz) seem to offer very promising perspectives. This paper aims at presenting an overview of the current status of research and technology in EHF satellite communications and taking a look at future perspectives in terms of applications and services. Challenges and open issues are adequately considered together with some viable solutions and future developments. The proposed analysis highlighted the need for a reliable propagation model based on experimental data acquired in orbit. Other critical aspects should be faced at the PHY-layer level in order to manage the tradeoff between power efficiency, spectral efficiency, and robustness against link distortions. As far as networking aspects are concerned, the large bandwidth availability should be converted into increased throughput by means of suitable radio resource management and transport protocols, able to support very high data rates in long-range aerospace scenarios.

Efficiency enhancement of free electron Maser oscillator by mode selection with a prebunched electron beam

We present a method for enhancing the efficiency of a Free Electron Laser Maser oscillator by locking it to a preferred resonator mode. This is done by prebunching of the e beam before injection into the wiggler. In a free running oscillator, the longitudinal mode that dominates the mode competition process during the oscillation buildup period is usually the highest gain mode. However, this mode does not extract the highest energy from the e beam. Lower eigenfrequency modes would provide a higher efficiency if they could dominate the mode competition process. By prebunching the e beam at a frequency near any one of the longitudinal eigenfrequencies of the resonator (having a gain>1), we can make that mode dominant at saturation. The eigenfrequency for which the maximum efficiency is obtained is the lowest eigenfrequency of the resonator for which the net small signal gain is greater than 1. Employing an experimental setup of a prebunched beam Free Electron Maser, we demonstrated efficiency enhancement of...

Study of ultrawide-band transmission in the extremely high frequency (EHF) band

The growing demand for broad-band wireless communication links and the lack of wide frequency bands within the conventional spectrum, causes us to seek bandwidth in the higher microwave and millimeter-wave spectrum at extremely high frequencies (EHF) above 30 GHz. One of the principal challenges in realizing modern wireless communication links in the EHF band are phenomena occuring during electromagnetic wave propagation through the atmosphere. A space-frequency approach for analyzing wireless communication channels operating in the EHF band is presented. Propagation of the electromagnetic radiation is studied in the frequency domain, enabling consideration of ultrawide-band modulated signals. The theory is employed for the analysis of a communication channel operating at EHF which utilizes pulse amplitude modulated signals. The atmospheric absorptive and dispersive effects on pulse propagation delay, pulse width and distortion are discussed. The theory and model are demonstrated in a study of ultrashort-pulse transmission at 60 GHz.

Propagation of ultra wide-band signals in lossy dispersive media

Development of a channel model for continuous frequencies enables the analysis of communications in an ultra wide band wireless network in indoor environment including a single transmitting and a single receiving antenna. In this work we will describe a model taking into account multiple reflections which are a consequence of the room in which both transmitter and receiver are localized including wall, ceiling and floor reflections. Moreover, our model enables the analysis of a communication channel between adjacent and distant rooms, in those cases we take into account the wide band signal propagation through separating walls. The model developed is in the frequency domain and thus allows analyzing dispersive effects in transmission and reflection of ultra short pulses in UWB communications from building materials which the room is made of in accordance with their complex dielectric coefficients. For this purpose a library of material characteristics of various materials (concrete, reinforced concrete, plaster, wood, blocks, glass, stone and more) in the standard frequency domain for wireless networks was assembled. One of the important phenomena for UWB communications which our research has revealed is the in-wall multiple reflections resulting in echoes of the narrow pulse transmitted. Our model takes into account antenna polarization and beam shape, the effect of those traits are clearly distinguishable. Space-frequency theory of the propagation of an ultra-wide band radiation in dielectric media is presented. The transfer function of a slab of material is derived in the frequency domain, considering polarization losses via a complex permittivity. It is shown that absorptive and dispersive effects play a role in the transmission and reflection coefficients of the electromagnetic incident field. The theory is applicable in the analysis of broadband communication links operating in wireless local or personal area networks. In an indoor scenario, the construction material of the walls attenuates the propagating waves in a dispersive manner, causing amplitude and phase distortions in the transmitted signal.

Free electron maser experiment with a prebunched beam

An experimental project aimed at demonstrating Free Electron Maser (FEM) operation with prebunching is under way at Tel-Aviv university. The FEM utilizes a 1.0 A prebunched electron beam obtained from a microwave tube. The electron beam is bunched at 4.87 GHz and is subsequently accelerated to 70 keV. The bunched beam is injected into a planar wiggler (B, = 300 G, A, = 4.4 cm) constructed in a Halbach configuration with 17 periods. The wiggler utilizes a new scheme for horizontal focusing based on the use of two long permanent magnets at the sides of the wiggler. We plan to study FEM gain enhancement and radiation features due to the prebunched (superradiant) mode of operation. In an oscillator configuration the experiment should be enable study of seed injection by prebunching. Simulation of FEL operation shows an expected gain of approximately 100% and an rf output power of 5 kW. In this paper we review the design of the main parts of the experimental set-up, and present recent analytical, numerical, and experimental results.

Propagation analysis of ultrashort pulses in resonant dielectric media

The space-frequency theory of the propagation of an ultrawideband radiation in dielectric media is presented. Characterization of the material via its susceptibility leads to a transfer function, which describes the response of the medium in the frequency domain. This description enables the consideration of broadband signals, taking into account inhomogeneous absorptive and dispersive effects of the medium. Analytical expressions are derived when a pulse-modulated signal is propagating in a general dielectric material. Conditions for apparent “superluminal” and pulse compression effects are identified. The theory is applied for a special case of transmission inside a resonant medium, revealing analytical approximations for the parameters of a Gaussian propagating pulse in terms of initial pulse width, carrier frequency, and medium parameters. Constraints of the derived analytical expressions are discussed, pointing out conditions of approximation validity. We demonstrate the approach by studying the propagation of ultrawideband signals, while transmitted in the vicinity of the 60 GHz absorption peak of the atmospheric medium at millimeter wavelengths.

Resonator design and characterization for the Israeli tandem electrostatic FEL project

Abstract The design and measurements of a resonator operating near 100 GHz and intended for use in a tandem FEL are presented. The designed resonator employs two parallel curved plates as a waveguide. In FEL operation the TE 01 mode is excited. The resonator employs two wave splitters as reflectors. The wave splitters are segments of an overmoded rectangular waveguide which is connected at one end to the waveguide as described above, and is shorted at the other end by a metal plate with an aperture in the center for e-beam passage. Gain calculations were made in the low gain regime. At the operating frequency the curvature of the plates and the gap size were chosen so as to maximize the gain. A multimode analysis of the wave splitter was made. Calculations show, that the optimal splitter width and length allow achievement of very low diffraction losses at the aperture (∼ 2%). This means that the aperture can be made sufficiently large to allow efficient beam entrance into the resonator without degrading its Q -factor. A resonator prototype was constructed and its performance was evaluated experimentally.

Selective amplification of the lower-frequency branch via stimulated super-radiance in a waveguided free electron laser oscillator driven by short electron bunches

In this letter, we propose a mechanism to extend the spectral range of a waveguided free electron laser (FEL) oscillator driven by a rf LINAC toward significantly longer wavelengths without changing the undulator or accelerator design parameters. This mechanism involves selective amplification of a lower-frequency branch supported in a FEL due to the waveguide dispersion. Based on simulations performed for the terahertz FEL under construction at the Radboud University Nijmegen, we conclude that these long wavelengths can be efficiently amplified via stimulated super-radiance as the electron bunches are shorter than the radiation wavelength.

Efficient electrostatic-accelerator free-electron masers for atmospheric power beaming

The electrostatic-accelerator free-electron laser (EA-FEL) operating at mm wavelength is considered as a source for energy transfer through the atmosphere to a high altitude platform. The high average power and high efficiency attainable from appropriately designed EA-FEL make it a suitable candidate as an efficient source of mm-waves for power beaming from a ground station. Various aspects of the FEL as a high power oscillator (operating voltage, e-beam current, gain and efficiency) are reviewed; design tradeoffs are described. The study includes consideration of typical requirements of power beaming to a high altitude platform such as atmospheric absorption versus frequency and transmitting and receiving antenna requirements. A conceptual design of a compact, moderate voltage (0.5-3 MeV), high current (1-10 Amp) EA-FEM operating in the mm-wavelength band is presented as an efficient power source for space beaming. The FEM design parameters are presented based on analytical and numerical models. Expected performance parameters of an FEL (gain, energy conversion efficiency, average power) are discussed as related to the proposed application.

Non-Imaging MM-Wave FMCW Sensor for Pedestrian Detection

This paper describes a millimeter-wave sensor that is able to detect pedestrians, thereby reducing the likelihood of human road injuries or fatalities. The sensor consists of a transmit/receive channel module, operating in the millimeter-wave range (W-band) using frequency-modulated-continuous-wave mode. The laboratory prototypes of the sensor have been designed and tested in real-life environment. An analysis of system performance and experiments conducted has indicated a high-resolution, detection ability of both adults and children at a distance of up to 100-150 m.