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.

Attenuation and group delay effects on millimeter wave (MMW) propagation in clouds and fog are studied theoretically and verified experimentally using high resolution radar in an indoor space filled with artificial fog. In the theoretical analysis, the frequency-dependent attenuation and group delay were derived via the permittivity of the medium. The results are applied to modify the millimeter-wave propagation model (MPM) and employed to study the effect of fog and cloud on the accuracy of the Frequency-Modulated Continuous-Wave (FMCW) radar operating in millimeter wavelengths. Artificial fog was generated in the experimental study to demonstrate ultra-low visibility in a confined space. The resulted attenuation and group delay were measured using FMCW radar operating at 320–330 GHz. It was found that apart from the attenuation, the incremental group delay caused by the fog also played a role in the accuracy of the radar. The results were compared to the analytical model. It was shown that although the artificial fog has slight different characteristics compare to the natural fog and clouds, in particle composition, size, and density, the model predictions were good, pointing out that the dispersive effects should be considered in the design of remote sensing radars operating in millimeter and sub-millimeter wavelengths.

Further enhancement of the intense coherent superradiant and stimulated-superradiant emission from prebunched electron beams is possible, in schemes of prebunched beam radiation devices, and particularly free electron laser (FEL). The enhancement of coherent power and spectral power by use of a waveguide, particularly at the zero-slippage condition, is evaluated. A special scheme of a stimulated-superradiance FEL oscillator is analyzed and is shown to feature ultimate radiative energy conversion efficiency (near 100%).

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.

The global spread of bacterial resistance to antibiotics promotes a search for alternative approaches to eradication of pathogenic bacteria. One alternative is using photosensitizers for inhibition of Gram-positive and Gram-negative bacteria under illumination. Due to low penetration of visible light into tissues, applications of photosensitizers are currently limited to treatment of superficial local infections. Excitation of photosensitizers in the dark can be applied to overcome this problem. In the present work, dark antibacterial activity of the photosensitizer Rose Bengal alone and in combination with antibiotics was studied. The minimum inhibitory concentrations (MIC) value of Rose Bengal against S. aureus dropped in the presence of sub-MIC concentrations of ciprofloxacin, levofloxacin, methicillin, and gentamicin. Free Rose Bengal at sub-MIC concentrations can be excited in the dark by ultrasound at 38 kHz. Rose Bengal immobilized onto silicon showed good antibacterial activity in the dark under ultrasonic activation, probably because of Rose Bengal leaching from the polymer during the treatment. Exposure of bacteria to Rose Bengal in the dark under irradiation by electromagnetic radio frequency waves in the 9 to 12 GHz range caused a decrease in the bacterial concentration, presumably due to resonant absorption of electromagnetic energy, its transformation into heat and subsequent excitation of Rose Bengal.

An electron beam, prebunched at the synchronous free-electron laser frequency and passing through a magnetic undulator, emits coherent (superradiant) synchrotron undulator radiation at the bunching frequency. If an external electromagnetic wave is introduced into the interaction region, at the same frequency and at a proper phase, the radiation process will be stimulated (stimulated prebunched beam radiation). We report first experimental measurements of stimulated superradiant emission in a prebunched free-electron maser. Measurements are in good agreement with theory.