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Antennas and Propagation, 1991. ICAP 91., Seventh International Conference on (IEE)

Date 15-18 April 1991

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Displaying Results 1 - 25 of 243
  • Study on rain attenuation and fading on a terrestrial microwave link

    Page(s): 63 - 66 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (246 KB)  

    Signal strength fluctuations on the 11 GHz line-of-sight microwave link situated in Southern part of India between Puducherry and Madras with a hop length of 31.2 km have been recorded at the receiving end, Madras. The signal strengths have been recorded on a strip chart recorder on 24 hour basis from August 1989 to February 1990. The rain rate at this location is recorded with a conventional rain gauge. A preliminary examination of the records shows prominent effects of rain attenuation. The diurnal and seasonal variation of field strength and fading characteristics of this microwave link are discussed. A detailed study of median signal (dBm), fade rate/hour, fade depth (dB) and scintillation index (percent) is made. The fading effects, particularly noticeable at night during the winter months, can be classified into three distinct types. The first is characterised by slow fading of small amplitudes, the second is also slow fading with deep fades superposed and the third is characterised by fast fading.<> View full abstract»

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  • Three-dimensional approach to multivision microwave imaging

    Page(s): 436 - 439 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (288 KB)  

    In the previous paper, the authors proposed a two-dimensional approach (see Caorsi et al., IEE Trans. Microwave Theory Tech., MTT-38, p.981, 1990) to microwave imaging based on a moment-method solution and on the use of a pseudoinversion algorithm to face the ill-conditioning of the inverse scattering formulation. An extension of that approach to a multivision process was also presented (see Caorsi et al., 1991). In the present paper, starting from that formulation, they develop a multivision inverse scattering numerical solution to three-dimensional problems related to strongly scattering vector configurations. The approach is based on the solution (by means of Green's dyadic tensor) to a set of Helmoltz vector equations whose source terms are equivalent current densities. The Fredholm equations are numerically inverted, using the moment method, for different angles of illumination, and the dielectric features are related to the reconstruction equivalent current densities via the solution to a direct scattering equation for the scattering region. View full abstract»

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  • Some limitations of realistic driving sources in moment method calculations

    Page(s): 957 - 960 vol.2
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (272 KB)  

    Two means of driving mathematical models are considered which appear to offer greater realism than the simple pulse source. These are the magnetic current annular ring (magnetic frill) source and the current slope discontinuity source. It is shown that the former is conveniently considered as a toroidal transformer. This approach enables the driving electric field to be derived by inspection from the magnetic vector potential produced by the magnetizing current flowing in the transformer primary. It is indicated that, for practical computation, this source is equivalent to the pulse source. The current slope discontinuity source is portrayed as a portion of a biconical capacitor. Examination of both sources leads to the conclusion that for point matching calculations, neither is free of difficulty: implementation necessitates an integration process which tends to remove the configurational details and hence the benefit of realism. For this type of calculation, neither appears to have any clear advantage over the pulse source View full abstract»

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  • A polarimetric covariance matrix concept for random radar targets

    Page(s): 396 - 399 vol.1
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    A covariance matrix concept for reciprocal random targets is presented which describes the polarisation dependence of the mean radar observables, i.e. the signatures of powers and correlation of the backscattered wave components, by unitary similarity transformations. Thus, important polarimetric invariances are preserved and limiting conditions for the backscattered powers revealed. As an important result, the derivatives of all covariance matrix elements with respect to the transmitter polarisation can be determined, involving only the covariance matrix elements. These derivatives physically describe how rapidly the measurable radar observables vary if the transmitter polarisation is changed. Analytical and numerical methods are presented, finally, to determine the optimal polarisations of the co- and crosspolar power signatures View full abstract»

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  • Multipolarisation radar receiver for target detection

    Page(s): 400 - 403 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (180 KB)  

    The authors consider the single hit detection problem, while assuming that the target echo polarisation is constant during pulse observation time, but unknown due to its randomness. A particular target detection technique is proposed and analysed in order to mitigate the effect of target polarisation randomness. Such a technique is based on a bank of linear polarisation filters providing maximum signal response for different and preset polarisations. The performance of such a multipolarisation receiver is evaluated in terms of false alarm probability and target detection probability. The evaluation is performed for a target in a Gaussian white zero-mean background noise, using six polarisation filters having their optimum polarisations symmetrically located on the Poincare sphere View full abstract»

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  • Modelling radio propagation at millimetre-wavelengths for communications and remote sensing

    Page(s): 269 - 273 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (312 KB)  

    The author discusses the use of MM waves for communication and remote sensing of the environment. He then looks at some of the problems of atmospheric propagation modelling View full abstract»

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  • Evolution toward an expert system for design of feeds and reflector antennas

    Page(s): 274 - 275 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (92 KB)  

    The authors briefly review the requirements and progress being made towards an expert system for antenna design. This includes simulation software and a database View full abstract»

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  • Statistical studies of propagation and related meteorology over a 500 m path

    Page(s): 673 - 676 vol.2
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (164 KB)  

    In order to improve the availability of propagation data at frequencies above 30 GHz, the Rutherford Appleton Laboratory has built and operated the 500 m Millimetre-Wave Experimental Range at Chilbolton (MWERAC) in Hampshire (latitude 57° 8'N, longitude 1° 26' W, elevation 84 m), to carry out propagation studies in the millimetric, infra-red and optical wavebands, in conjunction with a comprehensive set of meteorological observations (precipitation). Measurements from the range have been compiled into a three-year database of propagation and related meteorology, which is being used for two distinct kinds of study; case studies into individual events, in order to investigate in detail the interaction between radiowaves and the prevailing meteorology and statistical studies of propagation and related meteorology, to develop and test prediction procedures and for direct application to systems design. The authors present the main results of a statistical analysis of the complete three-year database, and compares these with some predictions based on the procedures recommended by CCIR View full abstract»

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  • The effects of dry snow on reflector antennas

    Page(s): 17 - 20 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (192 KB)  

    Fading due to snow accretion on the reflector antenna is a major problem in satellite communication in Finland and other northern countries. Dry snow may stay on antennas for many days or even weeks, if the temperature is below 0°C, making fading events longer. Dielectric losses of dry snow are very small and can be normally discounted, but asymmetrically gathered dry snow tilts and distorts the antenna beam due to the different phase delays of the different wave paths. Theoretically, the worst case would be a dry snow layer on the lower half of the reflector causing a phase delay of 180°, when the signal entirely disappears from line of sight. The author expands research in this neglected area. The measured radiation patterns of a 1.8 m reflector antenna are presented for three cases of a parabolic reflector partially covered by dry snow. The measurements were made at 20 GHz in one case and simultaneously at 20 and 30 GHz in two cases View full abstract»

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  • Finite element formulations using reactive power flow

    Page(s): 276 - 277 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (108 KB)  

    Advances in microwave engineering technology, such as the use of the millimetre wave spectrum, necessitate the use of accurate modelling tools to analyse microwave circuits and propagation problems. The author describes an alternative approach to the derivation of suitable energy functionals to be used with the universal finite element method View full abstract»

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  • A propagation model for slow fading in urban mobile radio systems

    Page(s): 160 - 163 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (200 KB)  

    Applies ray methods to produce a model of the propagation effects of buildings in a macrocellular (wide area coverage) mobile radio environment at VHF/UHF. The aims of the study are twofold: firstly to examine computationally efficient methods of evaluating multiple building diffraction effects close to grazing incidence and secondly, to determine simple but quantitative parameters describing the propagation environment, avoiding the ambiguities of the mostly empirical methods in common use View full abstract»

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  • Superconducting electrically-small dipole antenna

    Page(s): 446 - 447 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (152 KB)  

    A superconducting electrically-small dipole antenna was constructed and compared with a copper antenna of similar dimensions, both operating at low VHF frequencies. The antennas had lumped inductive loading to ensure that the feeding network was also electrically small. The superconducting antenna was formed from high-Tc ceramic superconductor and was parasitically driven in order to avoid problems of electrical (YBa2Cu3Ox) contact to this material. Both antennas were tested by immersion in a bath of liquid nitrogen in an electromagnetically-transparent container. The superconducting antenna achieved a higher Q-factor than the copper equivalent, the Q-factors being about half the values expected from simple theory. The efficiency of the superconducting antenna was also deduced to be about double that of the copper antenna View full abstract»

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  • The teaching of propagation over flat and spherical surfaces

    Page(s): 282 - 285 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (132 KB)  

    Propagation over flat and spherical surfaces is the practical basis of radiowave propagation. In each case the theoretical solution for the received field strength, where it is known, will depend on the transmitter power, the antenna power gain, the geometry of the propagation path, the ground constants, the refractive index variation over the air path and the frequency. A measure of uncertainty is associated with calculations alone, and they must be supplemented by a number of measurements. The purpose of this paper is to emphasise what can be calculated, so as to minimise the cost of experimental measurements. These calculations will relate to propagation over planar and spherical surfaces since it will be assumed that propagation in free space is understood View full abstract»

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  • A scattering matrix formulation for Gaussian beam-mode analysis

    Page(s): 201 - 204 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (196 KB)  

    The authors have show that scattering matrix theory enables Gaussian beam mode analysis to be extended in a straightforward way to take account of multiple reflections and scattering between modes. The use of propagating modes as basis functions, unlike those of fourier optics, allows the treatment of a vastly expanded range of problems. As well as the example of the Fabry-Perot interferometer used to illustrate this technique, other applications include, for example, calculation of the reflection coefficient of a thick dielectric lens, and of the Q of a confocal resonator with finite-sized reflectors, and has an obvious application to the analysis and design of beam-waveguide feed systems for large antennas. Although they have illustrated the theory for the simplest case of cylindrical symmetry, it is equally applicable to more complicated field distributions, or indeed to problems involving polarization, just by increasing the number of basis functions used. Despite this increase in N, substantial economies of computer time may be achieved in many cases through consideration of the symmetries of the problem View full abstract»

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  • A new integrated optic technique for time delays in wideband phased arrays

    Page(s): 918 - 921 vol.2
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (244 KB)  

    Phased-arrays antennas are being employed in new generations of radar and communication systems that require the antenna to provide wide field-of-view (FOV), very narrow beamwidths, and wide instantaneous frequency bandwidths. Phased arrays designed to meet these requirements must have their aperture broken into subarrays with time delay units (TDUs) to minimize the dispersive effects of transmitting and receiving wideband waveforms through large arrays that use modulo 360 degree phase shifters to perform electronic beam scanning. The authors describe an integrated optic TDU. The TDU relies on a folded path optical waveguide to achieve time delays ranging from a few picoseconds to several nanoseconds. The folded path configuration utilizes thick phase gratings as reflective elements. These elements provide large angle deflection, high diffraction efficiency or reflectivity, wavelength multiplexing capabilities and form the basis for an electronically addressable switch. As a switching element they also offer high isolation due to their high diffraction efficiency View full abstract»

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  • Development of a dual-reflector feed for the radiotelescope in Arecibo. An overview

    Page(s): 21 - 26 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (480 KB)  

    The spherical reflector antenna of the Arecibo Observatory will be equipped with a dual-reflector feed. This is often referred to as the Gregorian due to the Gregorian shapes of the two reflectors. The Gregorian has larger bandwidth, lower losses and provides better aperture distribution than the old line feeds. The authors describe the main ideas and review the methods and algorithms that have been developed to synthesize the shapes of the two reflectors, and to analyze them. Results are given of analysis by forward ray tracing using geometrical optics with included edge diffraction corrections. At low frequencies, analysis by physical optics integration is performed. Finally, the authors describe a `Mini-Gregorian' that has been constructed, built and tested to verify the dual-reflector feed concept View full abstract»

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  • Extension of the DBP field strength prediction programme to cellular mobile radio

    Page(s): 164 - 168 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (284 KB)  

    Results from field strength measurements at Mannheim in the 900 MHz range were used to extend the validity range of an existing field strength prediction method to urban small cell mobile radio situations. Some aspects of the measurements concerning base station antenna height, street orientation, and the base station and mobile antenna radiation pattern are presented in more detail; the performance of the new prediction programme version is shown by comparison to measured values View full abstract»

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  • RCS of tuned small superconducting loop antennas

    Page(s): 448 - 451 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (164 KB)  

    The RCS of electrically small loops made from copper and a high temperature superconducting compound YBa2Cu3Ox are measured, and compared with computer predictions, to confirm the gain enhancement due to increasing the efficiency of the radiating element alone. The small loop antennas are modelled using a method of moments technique employing piecewise sinusoidal subdomain basis and testing functions spanning two segments. Loss is introduced by modifying the diagonal and off-diagonal elements of the impedance matrix according to the wire conductivity, and tuning is achieved by adding a capacitive reactance to the centre element of the matrix View full abstract»

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  • Performance analysis of an LMS adaptive array with inverted-F elements on a rectangular conducting body

    Page(s): 286 - 289 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (248 KB)  

    The authors present a theoretical performance analysis of a two-element LMS adaptive array antenna in the mobile communication environment when the inverted-F antenna (IFA) is used as the array elements. The array SINR (signal to interference-plus-noise ratio) performance is analyzed three-dimensionally when the array elements are placed on a finite rectangular conducting body which may be a model of a vehicle or a tower. The inter-element coupling and diffraction effect of the conducting body are taken into account in the calculation. The IFA array performance is compared with a similar array with quarter-wave monopole elements. A factor called SINR pattern angle ratio (PAR) is used to discuss the quantitative evaluation of the system performance View full abstract»

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  • Mapping electron content near the main trough

    Page(s): 906 - 909 vol.2
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (232 KB)  

    Applications involving radio waves propagating through the ionosphere are subject to errors due to the effects of the medium, the electron content along the propagation path often being a key parameter. At high latitudes, steep gradients in electron content in the vicinity of the ionospheric trough are of particular importance. The sub-auroral region containing the trough is also characterised by the boundary between the relatively homogeneous mid-latitude ionosphere and the irregular auroral ionosphere. The small-scale irregularities are responsible for the scintillation of radio signals and so are also of considerable interest to propagation applications. The authors discuss the mapping of electron content and scintillation in the sub-auroral region over northern Europe. The authors describe the experiment which made use of signals transmitted from the constellation of seven NNSS satellites in near-circular polar orbits at heights of 1100 km. They describe how the measurements have been combined to give detailed information about electron content and irregularity behaviour in the vicinity of the trough as a function of both latitude and time View full abstract»

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  • A hybrid method of moments-improved UTD formulation for the radiation of a linear antenna in the presence of a wedge

    Page(s): 721 - 724 vol.2
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (140 KB)  

    The authors apply the improved UTD (uniform geometrical theory of diffraction) by O.M. Buyukdura and R.G. Kouyoumjian (1985) to the problem of the radiation from a monopole antenna mounted closed to the edge of a perfectly conducting edge. The primary aim is to test the accuracy of the improved UTD as compared to that of the ordinary UTD. A by-product of the formulation is that it can be used to assess the effect of the vicinity of an edge to the radiation pattern of an antenna. This finds application in antenna ranges where measurements are made with antennas placed on finite ground planes in the hope of approximating an infinite ground plane. The present formulation can be used to determine the size of a ground plane in order to keep the error arising from the finiteness thereof within acceptable limits View full abstract»

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  • Development of EMC fed arrays for dual linear polarisation

    Page(s): 205 - 208 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (176 KB)  

    Presents the experimental development of a printed structure based on EMC (electromagnetic coupled) microstrip patches designed for an SAR remote sensing instrument. The single element validation has been the basis to manufacture an array of eight elements at C-band, the results of which are shown. Using the same philosophy, an element at L-band has been manufactured and tested, and the results are presented View full abstract»

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  • Far-field prediction of a MM-wave reflector antenna from one planar near-field intensity scan

    Page(s): 766 - 769 vol.2
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (260 KB)  

    The performance of a new near-field planar-scan single-intensity phase retrieval minimisation algorithm for predicting the far-field has been presented and compared with holographic measurements. The algorithm requires an aperture estimate with diffraction effects for optimum performance. The approach of combining diffraction modellings, minimisation and error-reduction has been successful and efficient in dealing with substantial astigmatic deformation and diffraction effects in a front-fed reflector. More accurate determination of axial and astigmatic feed misalignments have been obtained from a weighted modulus-error function. At present, the method is clearly capable of useful far-field prediction and the results are therefore very encouraging for the phase retrieval metrology of millimeter and sub-millimeter reflector antennas View full abstract»

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  • Estimating the selectivity of a radio channel from routine measurements on operational links

    Page(s): 661 - 664 vol.2
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (248 KB)  

    Multipath propagation is one of the main causes of outage on high-bit-rate digital line-of-sight links. These impairments are due mainly to the selectivity of the propagation channel. Predictive methods have been devised to estimate this effect. In most of them, the behaviour of the atmospheric propagation channel is represented by a statistical propagation model and that of the transmission equipment by a set of signature curves. The propagation model consists of both a mathematical model representing the transfer function (TF) of the channel over the frequency bandwidth of interest (generally a few tens of MHz) and the associated probability distribution of the model parameters. To get a general predictive method applicable to any link, a critical point is to know how this probability distribution changes with the relevant characteristics of the link (frequency, hop length, antenna aperture etc.). The authors show how this knowledge can be obtained from routine measurements on operational links, and present some experimental results obtained this way View full abstract»

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  • Near-field measurement of active phased array antennas under low-PRF pulsed conditions within milliseconds with a standard HP 8510B

    Page(s): 416 - 419 vol.1
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (228 KB)  

    Vector network measurement has been, and is, used in characterizing microwave devices. Until now, vector network analyzers (NWA) were designed to operate with a continuous wave (CW) driving signal. However, an increasingly important class of microwave devices have a requirement not normally associated with standard CW vector network measurement. These devices must be characterized under pulsed-RF conditions. Now vector network analyzers designed for measuring pulsed-RF signals are beginning to become available (Hewlett Packard HP85108A and Wiltron 360-PS20). But since these NWAs are mainly designed for pulse profile measurements and are very costly (also due to the use of two frequency synthesizers), a method has been developed to use a standard CW HP 8510B NWA with one frequency synthesizer for measuring pulsed signals with a low pulse repetition frequency (PRF) View full abstract»

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