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Electrical Engineers, Proceedings of the Institution of

Issue 9 • Date September 1968

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Displaying Results 1 - 25 of 27
  • Small-scale variations of radio refractive index in the troposphere. Part 1: Relationship to meteorological conditions

    Page(s): 1227 - 1234
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1125 KB)  

    A description is given of results obtained from soundings made with a spaced-cavity balloon-borne refractometer and 10cm-band vertical-incidence radar. The experiments, carried out at Cardington, Beds., England, in 1964 and 1965, show that at heights up to 2km there is often considerable inhomogeneity in the refractive-index structure, even in nonanticyclonic conditions. Instantaneous differences in refractive index of 5 parts in 106 (5N units) were observed over distances of 11cm within discrete parcels of air in which the quotient of variance by scale size was unusually large, occasionally 10¿11m¿1. Such parcels were often located at or near the base of a layer of steep mean vertical gradient in refractive index associated with elevated inversions or isothermals. The results are discussed in relation to meteorological factors, such as wind shear and dynamic stability, and to theories of tropospheric scatter based on the concept of homogeneous turbulence. View full abstract»

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  • Small-scale variations of radio refractive index in the troposphere. Part 2: Spectral characteristics

    Page(s): 1235 - 1239
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (722 KB)  

    Spatial (wave-number) spectra of refractive-index variations have been derived from measurements of frequency spectra made with a spaced-cavity refractometer located in and near elevated inversion layers. The intensity of the 1-dimensional wave-number spectrum decreased with increasing wave number k as k¿n, with values of n generally greater than 5/3 and occasionally as large as 3 [k may be regarded as equal to 2¿/(blob size)]. The largest values of n were usually found in layers across which the value of Richardson's number was appreciably greater than unity. The variation from sample to sample in any sounding in the spectra intensity at k ¿ 100m¿1 was often appreciably less than the corresponding variation at k ¿ 1m¿1. The results illustrate the dependence of spectral characteristics on sampling time. View full abstract»

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  • Method of obtaining the impedance of an antenna in a dissipative medium from that in air

    Page(s): 1240 - 1244
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (546 KB)  

    A Taylor-series expansion is applied in the complex frequency plane in order to find the admittance of an antenna in a dissipative medium from the admittance of the same antenna in air. The admittance of a dipole antenna immersed in mediums with tan ¿ = 0.01 and 0.02 is tabulated. The values so obtained are in good agreement with measurements. The error and the range of values of the loss tangent of the medium, and the length of the antenna for which the present method is applicable, are examined from various points of view. View full abstract»

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  • Transmission performance of telephone networks containing p.c.m. links

    Page(s): 1245 - 1258
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (2136 KB)  

    Public telecommunication networks are required to transmit various types of information, and any new method of transmission must provide a quality at least as good as the present standards. This paper describes the basis on which the fundamental characteristics were chosen for the new 24-channel short-distance p.c.m. system now being introduced by the British Post Office. Representative overall telephone connections were selected and assessments made of their speech-transmission performance when certain links in them are provided by p.c.m. systems having various sets of fundamental characteristics. Special attention was given to requirements concerning load capacity, quantising distortion (including performance at very low signal levels) and circuit noise. Acceptable values for each quantity were determined and certain compression characteristics examined in the light of these requirements The final choice of fundamental characteristics was based mainly on consideration of speech, and the new systems should provide a suitable standard for any conditions of use for telephony that can be envisaged at present. It is believed that other services that the systems may be required to transmit impose less onerous conditions than those for speech, and so it is expected that they will also satisfy requirements for these other services. View full abstract»

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  • Order of complexity of linear active networks

    Page(s): 1259 - 1262
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (508 KB)  

    The upper bound on the order of complexity ¿max of general linear active networks is found. The result thus completes those already obtained for the RLC networks and for a restricted class of active networks. The result is applicable to networks consisting of independent sources, resistors, capacitors, inductors, gyrators, multiwinding ideal transformers and the four types of controlled sources. The voltage graph (NV) and the current graph (NI) associated with the active network (N) are used in the derivation. A common tree is defined as a set of branches which forms a tree in both NV and NI. The network `operator matrix¿ includes the set of equations resulting from Kirchhoff's voltage and current laws, applied to NV and NI, respectively, and the element behaviour between the corresponding branches in NV and NI. The order of complexity is obtained by an expansion of the determinant of the operator matrix and can be stated with respect to some particular common trees. In the state-space (variable) approach to network synthesis, the order of complexity represents the minimum number of reactive elements required for the realisation View full abstract»

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  • Modified cut-set matrix of an n-port network

    Page(s): 1263 - 1268
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (798 KB)  

    The modified cut-set matrix of an n-port network is defined and its properties are enumerated. The importance of this concept in considering the parallel interconnection of n-port networks containing R, L and C elements is demonstrated. A procedure is obtained for the generation of a class of continuously equivalent networks of a given RLC n-port network. The class is characterised by the property that all networks belonging to this class have the same modified cut-set matrix. Using this procedure, it can be shown that, for a resistive n-port network in which every pair of vertexes is connected by a finite positive conductance, a large number of continuously equivalent networks containing only nonnegative conductances can always be obtained. The usefulness of this result in the synthesis of RLC n-port networks is indicated View full abstract»

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  • Synthesis applications of the modified cut-set matrix

    Page(s): 1269 - 1274
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (713 KB)  

    A procedure for the realisation of a general K network, which is a generalisation of the conventional 2n-node network realisation of a dominant matrix, is given. It is shown that a set of dominant matrices can be realised by 2n-node n-port networks having the same modified cut-set matrix. This technique is used in evolving a procedure for the realisation of 2-element-kind n-port networks. View full abstract»

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  • Some aspects of electrical machines with open magnetic circuits

    Page(s): 1275 - 1283
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1324 KB)  

    Every electrical machine consists of electric and magnetic circuits interlinked. Good machines have circuits of low resistance and reluctance, the latter generally achieved by the use of a steel structure. If magnetic flux is made to thread a relatively long air path, compensation for the higher reluctance otherwise introduced can be sought by making the area of the flux path large. The paper shows how useful shapes of machine can be developed in which a large part or all of the magnetic circuit consists of air. Machines included are open-bar transformers, single-sided linear induction motors, and homopolar d.c. machines with superconducting field windings. Vertical forces in open-sided linear motors are calculated. The paper concludes with a brief discussion of the possibilities of electromagnetic current collection for traction systems employing a moving transformer View full abstract»

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  • Discussion on reluctance motors

    Page(s): 1283 - 1285
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  • Alternative convertor for h.v. d.c. transmission. Three single-phase bridges connected in series

    Page(s): 1289 - 1296
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1138 KB)  

    The Graetz (3-phase) bridge circuit is the arrangement usually chosen for a 3-phase d.c. convertor, because it has the best valve and transformer utilisation, and hence lowest valve costs. This bridge has the disadvantage that the alternating current is fixed relative to the direct current, which means that the reactive-power requirement is dependent upon the active power. If three single-phase bridge circuits are used, however, it is possible to vary the relationship between the alternating and the direct currents, and hence it is possible to change reactive power without varying active power. This facility is particularly useful when a number of convertors are connected in series on the d.c. side, when the direct current is common to all convertors and voltage control is used to set the active power level. The operation of a convertor using three single-phase bridge circuits is analysed, and it is shown that, although the bridge is more expensive on valves, it has operational advantages over the conventional Graetz bridge. The theoretical and practical operating limitations are discussed, and it is shown that the convertor can be operated continuously over its complete power range with a reactive-power requirement of only 70% of the rated convertor apparent power, compared with 100% for a Graetz bridge. The cost of terminal plant other than valves is shown to be the same for both convertor circuits. In a brief economic assessment, it is shown that the overall cost of the new convertor may be only 14% higher than a similarly rated Graetz bridge View full abstract»

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  • Currents and potentials along extensive underground conductors

    Page(s): 1299 - 1304
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (674 KB)  

    The current and potential along an extensive underground conductor are determined for voltage, current and conductive energisations. The general expressions for current and potential are derived, and it is shown that the approximate relations may be obtained from differential equations based on the equivalent circuit for the underground conductor. In the case of conductive energisation, the point, parallel and perpendicular earth electrodes are considered. it is shown that parallel or perpendicular earth electrodes, being sufficiently short, may be replaced by an equivalent point earth electrode View full abstract»

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  • Frequency-dependent power-system static-load characteristics

    Page(s): 1307 - 1314
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (960 KB)  

    The importance of power-system load characteristics under transient conditions has been stressed in the literature, but little information is available, particularly regarding frequency dependence. General conditions regarding the behaviour of active- and reactive-load components with frequency are investigated in relation to their effects in producing positive and negative damping in a system. The importance of reactive-power variation with series and shunt compensation by capacitors and reactors is discussed. Frequency-dependent response of various passive loads usually met in practice is presented along with an illustrative 4-machine example involving such loads. The information can serve as a guide in determining the behaviour of loads under transient conditions, and subsequently can assist in giving more accurate assessment of power-system behaviour. View full abstract»

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  • Improved Lyapunov function for transient power-system stability

    Page(s): 1315 - 1317
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (374 KB)  

    A Lyapunov function is determined for the problem of transient power-system stability; only the stability of a synchronous machine connected to an infinite bus is considered. It is shown that the obtained Lyapunov function includes governor action, pole saliency and damping torques, and is hence an extension of earlier results. View full abstract»

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  • Calibration and comparison of partial-discharge and radio-interference measuring circuits

    Page(s): 1332 - 1340
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1159 KB)  

    Methods of calibrating circuits for measuring partial discharges and radio-interference (r.i.) voltages are discussed. It is shown that discharge-measuring circuits, whether using a peak-measuring discharge detector or an r.i. meter, are best calibrated by applying a charge-quantity pulse calibrator of known repetition frequency in parallel with the test sample. The calibration of circuits for measuring r.i. voltages is more complex, and certain anomalies arise when a sine-wave signal generator is used to calibrate the circuit in terms of voltage input. It is established that, for a pulse-repetition rate of 100 pulse/s, it is possible to relate the results obtained from a conventional peak-reading discharge detector to those from a quasipeak r.i. meter; e.g. for a meter having 60¿ input impedance and a bandwidth of 9 kHz, the relationship is: 1 ¿V is equivalent to 2.6pC. At repetition rates other than 100 pulse/s, it is shown that theoretically the maximum error involved by using the above relationship is 6 dB between 25¿2000 pulse/s. Practical measurements confirm that the error does not exceed ±6dB over a wide range of pulse sizes and repetition frequencies. Comparative measurements have shown that the conventional r.i. meter is approximately an order of magnitude less sensitive than a tuned-circuit discharge detector. Methods of improving the sensitivity of the former are described. It is concluded that for many applications either a discharge detector or an r.i. meter will enable both discharge magnitude and r.i. voltage to be determined with sufficient accuracy, considering the erratic behaviour of pulses in practice. View full abstract»

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  • Solution of 2-dimensional field problems by boundary relaxation

    Page(s): 1341 - 1348
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1024 KB)  

    A numerical method is described for solution of 2-dimensional electric- and magnetic-field problems of the exterior type. Such problems are temporarily converted into interior problems by defining an arbitrary closed boundary, and then improving field values on the boundary iteratively, until a solution valid both within and without the artificial boundary is obtained. Within the boundary, the solution is found at a finite number of points, by any of the well known finite-difference methods. The final result is independent of the choice of artificial boundary, and corresponds exactly to the solution that would be obtained by applying finite-difference techniques to an infinite array of points. An empirical study of the convergence properties of this process is described, and typical computing speeds are indicated. Use of this method is illustrated by a variety of simple problems. View full abstract»

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  • Further correlation of experimental data for electric arcs in transverse magnetic fields

    Page(s): 1349 - 1354
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (688 KB)  

    Data obtained for short arcs moving in a transverse magnetic field in argon and helium, together with some results for an arc held stationary in an argon flow, have been correlated by the use of similarity parameters presented in a recent paper. The results obtained in this form are compared with previous results for arcs moving in air, and a very close similarity between the magnetic characteristic for all three gases is found. The range of conditions over which arc velocity may be predicted is thereby considerably extended. It is also shown that, although the arc-column electric field can be successfully correlated with other main variables for a single gas, it is not yet possible to extend this correlation from one gas to another. View full abstract»

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  • Digital-computer analysis of relay feedback systems

    Page(s): 1355 - 1362
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (911 KB)  

    The paper describes several ways in which an accurate analysis may be made of relay feedback systems. Methods are described which enable the signals at any part of the loop to be determined when the loop input is a d.c. signal. If the loop transfer function is simple, a solution may be possible using a desk calculator, but when the loop becomes more complex a digital computer becomes necessary. In cases where the loop input is a sinusoidal signal of frequency well removed from the oscillation frequency, it is shown how the analysis used for the d.c.-input case may be extended to enable the components at any part of the system to be determined. A method is also described which shows how the frequency spectrum, of the loop output may be determined. When the input of the loop is a sine wave, the output spectrum will in general consist of a series of sidebands displaced about the oscillating frequency of the loop and its harmonics. It is shown how the amplitude of all these components may be determined for any size and frequency of input sinusoid if a digital computer is available. Examples of solutions obtained for various feedback systems are given, and comment is made on the way in which system design is influenced by the results of the analysis. View full abstract»

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  • Computational procedure for the minimal realisation of transfer-function matrices

    Page(s): 1363 - 1368
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (478 KB)  

    The paper presents a relatively simple method for determining the minimal (lowest-dimension) state-vector realisation of a matrix transfer function. The procedure is closely related to the original method given by Kalman, but has the advantage of producing a minimal realisation which also possesses the minimum number of arbitrary parameters. View full abstract»

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  • Prediction of jump resonance in systems containing certain multidimensional nonlinearities

    Page(s): 1369 - 1372
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (463 KB)  

    An expression is derived for the incremental describing function of a nonlinearity whose first-harmonic output has a phase shift with respect to the input. Its application in predicting jump resonance in feedback systems containing certain multidimensional nonlinearities is demonstrated by three typical examples. View full abstract»

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Aims & Scope

The Proceedings of the Institution of Electrical Engineers was published by the IET between 1963 and 1979.

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