摘要:

The paper is an investigation into the possibilities of using an indirectly heated thermistor as a precise a.c./d.c. transfer device over a wide range of frequencies. The thermal drift inevitable with such a device has been examined and a procedure for its elimination developed, resulting in a very high degree of thermal stability. Experiments made on the reproducibility of the thermistor seem to indicate a complete absence of hysteresis over the working range.It is shown that the device has an extremely high sensitivity and is therefore useful for general laboratory work with simple equipment, although this advantage is somewhat outweighed by the slow response.Experimental results are given for the d.c. reversal errors due to Peltier and Thomson effects; these are found to be quite small.An analysis of the effects of frequency shows that, owing to the large thermal time-constant, the indirectly heated thermistor in its present form can be used at frequencies as low as 0.2 c/s, thus well below the working range of normal measuring devices. The response is very slow owing to this large time-constant, and methods of con struction are suggested which would improve the speed of response at power frequencies and above.

DOI:10.1049/pi-b-1.1956.0235

出版商:IEE    

年代:1956

数据来源: IET

摘要:

A description is given of a null method for the measurement of the complex permittivity of dielectrics. The direct admittance of a 3-terminal capacitor with the sample as dielectric is measured as a complex capacitance, the two components being indicated directly on two 3-terminal variable air capacitors. In addition to these the bridge network comprises transformer ratio-arms and an amplifier whose output voltage is in quadrature with that of the transformer. The bridge operates at ten fixed frequencies from 30 to 106rad/sec.

DOI:10.1049/pi-b-1.1956.0236

出版商:IEE    

年代:1956

数据来源: IET

摘要:

The lecture describes some of the scientific characteristics of the 250 ft-aperture steerable radio telescope at the Jodrell Bank Experimental Station of the University of Manchester. Some of the research problems on which the telescope will be used are then outlined against the current background in radio astronomy. These include the study of the background continuum radio emission of the Galaxy and of the galactic radio sources over a wide range of wavelengths. The main programme on the extragalactic radio emissions will involve a study of the spatial distribution of particular classes of extragalactic radio sources and the measurement of their distances, from which it is hoped that significant progress can be made with cosmological problems. The telescope will also be used as a combined transmitter and receiver to make further studies of the moon and possibly the planets. Other items in the programme include the study of very faint meteors by the radio echo technique and various problems of solar terrestrial relation ships of particular importance to the International Geophysical Year.

DOI:10.1049/pi-b-1.1956.0240

出版商:IEE    

年代:1956

数据来源: IET

摘要:

All telephone systems incorporate time-sharing techniques in order to economize in apparatus, but the extent of their application depends upon the operating speeds and adaptability of the components used. The introduction of electronic techniques enables various switching, coding and sampling techniques to be used to achieve a higher degree of time sharing than has previously been possible.In a system now under development these principles are the main basis of design. Connections are made over common leads using pulse channels each of which may be used for connections to any line. The connections are controlled by register and supervisory apparatus in which information is received, stored, manipulated and transmitted by common apparatus which operates on the connection pulse channels. Common selecting, translating and marking apparatus is used to set up all connections on a one-at-a-time basis, and all apparatus apart from the line-terminal equipment is time-shared by the connections using either sampling or switching techniques. Connection, register and supervisory apparatus is also time-shared by coding.The capital and maintenance costs of such a system are as yet indeterminate, but it is expected that they will be competitive.

DOI:10.1049/pi-b-1.1956.0241

出版商:IEE    

年代:1956

数据来源: IET

摘要:

A description is given of a technique for investigating the characteristics of atmospheric noise, and of the type of information obtained at very low frequencies. The results quoted are typical of those which have been obtained in southern England during a long period of recording, but a discussion of the statistics of all the data is not included.Atmospheric noise in a bandwidth of 300 c/s at frequencies in the range 10–35 kc/s is intermediate in character between fluctuation noise and discrete impulses. For example, the variation of the average voltage with bandwidth is similar to that for fluctuation noise, but the noise also contains peaks of high amplitude. The r.m.s. value of the envelope is of the order of five times the average value (compared with 1.1 for fluctuation noise), which illustrates the impulsive nature of the noise and shows that neither parameter by itself can provide a satisfactory description of the noise. The long-term trends, however, have been expressed in terms of the average field strength.A more detailed description of the noise is given in terms of the amplitude distribution of the peaks in the envelope and the amplitude probability distribution of the envelope itself. Either of these distributions can be expressed empirically in terms of two parameters, and can be interrelated at the higher voltage levels where the impulses have consistent shape determined by the characteristics of the receiver. At one location the amplitude of the noise may vary between wide limits with time, frequency and bandwidth, but the parameters which can be used to describe the noise structure are comparatively invariant.To supplement the amplitude data, information is required on the time sequence of the voltage changes, and it has been found that the larger impulses are randomly distributed in time. There is a tendency for the smaller impulses to occur in groups, in accordance with the knowledge that a single atmospheric often consists of a number of successive discharges, but it is doubtful whether the slight departure from randomness would have a significant effect on the severity of the interference with the usual types of radio service.

DOI:10.1049/pi-b-1.1956.0244

出版商:IEE    

年代:1956

数据来源: IET

摘要:

An experimental procedure is described for testing the assumption that a generator behaves as a linear source.The source admittance is obtained from measurements made when the generator is operating normally, and the problem considered is that of matching this source admittance to the characteristic admittance of the feeder connecting it to a wide-band aerial.

DOI:10.1049/pi-b-1.1956.0245

出版商:IEE    

年代:1956

数据来源: IET

摘要:

An account is given of measurements in the band 40–200 Mc/s of the discrimination likely to be achievable between common-frequency transmissions by the use of orthogonal polarizations. It is shown that the discrimination is determined primarily by the topographical nature of the receiving site, that it is substantially independent of distance from the transmitter and of frequency in the band under consideration, and that the median value is about 18dB. The perturbing effects of pick-up on the feeder and of receiving aerial misalignment are discussed.

DOI:10.1049/pi-b-1.1956.0246

出版商:IEE    

年代:1956

数据来源: IET

摘要:

Field-strength measurements at distances extending far beyond the horizon from a transmitter on a frequency of 91.3 Mc/s have been carried out in an aircraft flying at a height of 10000 ft.The analysis of the results obtained and their correlation with meteorological data suggest that many phenomena of long-distance propagation could possibly be explained by the simple hypothesis of specular reflection from temperature-inversion layers at the tropopause.

DOI:10.1049/pi-b-1.1956.0247

出版商:IEE    

年代:1956

数据来源: IET

摘要:

The general characteristics of helical aerials are summarized and the results of polar-diagram measurements in the S- and X-bands are given for single and multiple helices. Reference is made to methods of widening both the beam width and the bandwidth obtained with a helical aerial by means such as end loading and tapering.The possibility of adapting helical aerials to give wide-band linearly polarized arrays is explored experimentally with a considerable measure of success.Tests show that acceptable characteristics may be obtained with helices of ‘unorthodox’ form or with dimensions outside the limits suggested by Kraus.Encapsulation of helical aerials in foamed dielectric material in order to improve their rigidity is found to be a satisfactory and practical proposition provided that the materials are selected with some care, particularly in the X-band.

DOI:10.1049/pi-b-1.1956.0249

出版商:IEE    

年代:1956

数据来源: IET

摘要:

A theoretical and experimental study has been made of the effects of coupling films on the propagation of compressional waves from a transducer to a solid medium. In practice it has been found that ‘wringing’ the transducer to the specimen with oil as a coupling medium produces a film of non-uniform thickness. Although the variations in thickness are of the order of a wavelength of light, these variations are important at ultrasonic frequencies in the region of 50 Mc/s and above.Conditions are described under which such films can lead to the propagation of a predominantly first-order mode in the specimen, resulting in an exponential decay of the amplitudes of successive reflections, with a consequent improved accuracy of attenuation measurement.The work provides a greater understanding of the problems encountered in applications of delay lines.

DOI:10.1049/pi-b-1.1956.0252

出版商:IEE    

年代:1956

数据来源: IET