Joaquim Azevedo

An Empirical Propagation Model for Forest Environments at Tree Trunk Level

The estimation of the received signal strength inside forest environments is very dependent on the vegetation density. However, most published empirical models only provide calculation of the path loss as a function of distance and frequency. The importance of these models is their simplicity. This paper presents a model that was developed from an extensive measurement campaign carried out for different vegetation densities and types of trees. Noting the typical decaying behaviour of signal propagation, the objective was to estimate the main parameters of the log-normal model that fits the measured data. Since in many applications both the transmitter and the receiver are inside the forest, the propagation path is mainly characterized by tree trunks. In this case, it was found that the signal attenuation is dependent on the vegetation density, defined by the product of the tree density and the average diameter of the trunks. In addition to the measurements used to develop the model, other experiments were conducted to test it. The study also included a comparison with other methods in order to evaluate the performance of the model.

Synthesis of Planar Arrays With Elements in Concentric Rings

The circular symmetry of concentric ring arrays becomes an advantage for situations where the radiation patterns are symmetric in the azimuthal direction. For planar arrays with elements in an equispaced grid, the fast Fourier transform can be applied in the calculations involved between the array factor and the source distribution. However, for circular arrays the elements are in a non-equispaced grid, making the application of fast computation algorithms difficult. Therefore, two techniques are proposed to synthesize circular arrays. The first one provides the control of the pattern by sampling the array factor. In this case, the fast Fourier transform can be applied. The windows technique is also utilized to control the sidelobe levels and the ripple structure of the shaped beam. The second technique considers the symmetric nature of concentric circular arrays and the control of the array factor is performed imposing some specific points of the pattern.

SHAPED BEAM PATTERN SYNTHESIS WITH NON-UNIFORM SAMPLE PHASES

For a long time power pattern synthesis has been considered to produce shaped beam patterns due to the number of degrees of freedom when compared with field pattern synthesis. However, severaladvantages exist if synthesis techniques are based on field patterns, since the relationship between the array factor and the source distribution is a Fourier transform. In this case, quicker and efficient algorithms are obtained with the Fast Fourier Transform (FFT) to perform the calculations. Therefore, this work presents a new technique that synthesizes shaped beam patterns through the control of non-uniformly samples of the array factor, both in amplitude and phase. The sample phases increase the number of degrees of freedom. To produce complex patterns, the sample phases of the shaped beam region are used to create more oscillations in the ripple structure than the ones produced with the realpatterns. Not onl y this procedure yields a narrower transition region between the beam zone and the sidelobe zone but also smaller dynamic range ratios are obtained. Furthermore, it is described how to impose nulls in prescribed directions of the complex patterns. Some examples are presented to demonstrate the application of the synthesis technique.

Sleeping ZigBee networks at the application layer

ZigBee/IEEE 802.15.4 is one of the most used standards for low-power applications. However, full function devices must be always active to route data in a mesh network. The objective of this work is to implement a sleeping technique at the application layer that enables sleep mode for all nodes of a ZigBee network. A time synchronisation mechanism to deal with the clock drift of the sensor nodes was developed. The technique also enables the recovery of lost messages. A large network is organised into smaller groups to reduce latency and packet collisions. The active interval of each node is dynamically adapted to the network operation to optimise the energy consumption. The proposed technique was applied to a real testbed and the increase in energy efficiency was evaluated. The results demonstrated energy savings of about 95% for networks containing up to 20 nodes per group and wake up periods longer than 2 min.

On the use of Fourier transform with acoustical fields

The authors present a theoretical introduction to the possibility of using the Fourier transform to study the fields produced by distributions, either continuous or discrete, of acoustic source distributions. With this approach one can develop and simplify methods of analysis and synthesis of fields produced by acoustic sources. The procedure is based on the theorem of the small complex translation. In this case, and with a convenient choice of variables, the far field pattern is related to a window on the Fourier transform of the source distribution. The analysis and synthesis are made using the Fourier transform properties and the tools provided by signal processing theory using the Fourier transform.<<ETX>>

Design and power production of small-scale wind turbines

Wind is one of the most used renewable energy sources for general applications due to its power availability. There are several studies that use wind energy in small-scale systems, however few analyse the lower then 10000 Reynolds number range and the usual approach consists in apply a commercial turbine, or adapt one from other system, to a commercial generator, providing low efficiency system. This paper uses Schmitz theory to design wind turbines in the centimetre scale based on three airfoils, BW3, E62 and S1223. A wind tunnel was used to determine the power coefficient and maximum power transfer, with a small generator. However, the problem with the use of wind energy in urban areas is the variability of the available power since most of the time the wind speed is very low. Therefore, this work includes a study on remote monitoring using a wind turbine to feed a wireless sensor node and a vertical axis wind turbine was used to compare the systems behaviour. The results demonstrate these systems are capable to feed the load and charge a battery even with very low average wind speed.

Dealing with the useful window related with the far field pattern

With a convenient representation, the radiation pattern of a spatial distribution of radiating sources is always related with that distribution by a window in the inverse spatial Fourier transform of the distribution. This fact is used to change the distribution of sources without disturbing the radiation pattern. On the other hand, the error, in the mean square sense, made by source elimination is also related with the Fourier transform of the distribution. So it spreads all over the domain and not only inside the useful window. This must be taken in account and, with some signal processing, it is possible to find the error only in the far field pattern using the spatial distributions of sources.