摘要:

The paper describes apparatus by means of which the British Broadcasting Corporation hopes to release for other services personnel who have been employed in keeping a listening check on the technical quality of its programmes. The apparatus performs its task not by judging from an absolute basis—as has so often been done by the staff employed for this purpose—but by comparing the content of the programme at a point where it has already been checked with that at a more distant point. Where it is impracticable to return the programme to the same point for checking purposes, it is synthesized into a band of information in which the factors to be judged are specially emphasized. The narrow band is transmitted for checking purposes over any convenient channel that can be derived from existing circuits.

DOI:10.1049/pi-3.1951.0073

出版商:IEE    

年代:1951

数据来源: IET

摘要:

The paper discusses the results of a field-strength survey on 90 Mc/s from a site at Wrotham, Kent.The effects of the height of the site and of the transmitting aerial and the profile of the transmission path on height gain are discussed in Section 1.Measurements in built-up and hilly districts are discussed in Section 2. Those in flat built-up areas indicate that buildings of normal height in London reduce the average field at a receiving-aerial height of 20 ft by about 10 to 12 db, and that an average for one street will be the average for the district to an accuracy of about ± 4 db. The variation of field strength along the street is less with horizontal than with vertical polarization. In hilly country, the minimum field strength occurs on the near-side slope of a valley and not at the lowest point. The minimum is slightly lower with horizontal than with vertical polarization, and in this case the latter is slightly to be preferred.In Section 3, graphs of measurements along ten radials using vertical polarization and three using horizontal polarization are discussed. These measurements show that, in general, signals of both polarizations are propagated equally well.A field-strength contour map of south-east England has been prepared from the radial measurements, but the values taken from these have been extrapolated to allow for the use of a high-gain (8 db) transmitting aerial, a 500-ft mast and a power of 25 kW.

DOI:10.1049/pi-3.1951.0076

出版商:IEE    

年代:1951

数据来源: IET

摘要:

The effect on radio wave propagation of departures from the normal variation with height of the refractive index of the lower atmosphere is considered with special reference to frequencies of 90 and 45 Mc/s. The relative importance of changes in refraction near the surface of the earth and of reflection from high-level inversion layers is investigated, and some illustrative examples are worked out. The number of parameters concerned in the problem prevents an all-embracing survey of the many possibilities within the scope of the present work, but a few general conclusions are reached.It is unlikely that radio ducts of sufficient magnitude to be greatly significant at the frequencies in question can be maintained over long distances in broken country. Of the two mechanisms—refraction near the surface of the earth and reflection at an elevated inversion layer—the latter is the one which is likely more often to give abnormally high field strengths at ranges of a few hundred kilometres, especially for relatively low terminal points.Consideration is also given to the effects of scattering of radio energy by turbulent eddies in the atmosphere, and an approximate calculation is made of the magnitude of this scattering for horizontally polarized waves on a frequency of 90 Mc/s, and at distances up to 500 km. It appears likely that such scattering is of less importance in the creation of unusually high field strengths up to distances of the order of 250 km than the mechanisms of abnormal refraction, including reflection at elevated inversion layers.

DOI:10.1049/pi-3.1951.0077

出版商:IEE    

年代:1951

数据来源: IET

摘要:

Measurements of radio field strength at frequencies of 90 and 45 Mc/s over distances such that direct transmission from the sender to the receiver was impossible are described. Two paths, of lengths 110 and 270 km, have been investigated at 90 Mc/s, and one, of length 160 km, at 45 Mc/s. The statistical distribution of quasi-peak field strengths as a function of time over these paths has been determined, and it is shown that signals considerably in excess of values corresponding to standard atmospheric refraction often occur. An attempt is made to interpret the observed field strengths in terms of the meteorological data obtained from the routine radio-sonde ascents made by the Meteorological Office, and some degree of correlation has been obtained.An indication is given of the bearing of the measurements on the planning of v.h.f. broadcasting and communication services.

DOI:10.1049/pi-3.1951.0078

出版商:IEE    

年代:1951

数据来源: IET

摘要:

The paper mentions first some essential points about the early development of languages, codes and symbolism, picking out those fundamental points in human communication which have recently been summarized by precise mathematical theory. A survey of telegraphy and telephony development leads to the need for “economy,” which has given rise to various systems of signal compression. Hartley's early theory of communication is summarized, and Gabor's theory of signal structure is described.Modern statistical theory of Wiener and Shannon, by which “information” may be expressed quantitatively, is shown to be a logical extension of Hartley's work. A Section on calculating machines and brains attempts to clear up popular misunderstandings and to separate definite accomplishments in mechanization of thought processes from mere myths.Finally, a generalization of the work over the whole field of scientific observation is shown, and evidence which supports the view that “information plus entropy is an important invariant of a physical system” is included.

DOI:10.1049/pi-3.1951.0082

出版商:IEE    

年代:1951

数据来源: IET

摘要:

In the calculation of the performance of modulators, frequency changers, and detectors it is found difficult to allow for any frequency dependence of the resistance characteristic of the rectifier, although when copper-oxide rectifiers are used, such dependence is very severe. An approximate method of allowing for such an effect is described in the paper, and it can be applied easily to the analysis of circuits where the terminating impedances are finite only at a finite number of modulation-product frequencies and zero at all other such frequencies. In particular, the case whereeitherthe inputorthe output frequency is low, the carrier frequency is high and only one sideband is accepted is considered in some detail, and the results are illustrated by numerical examples based on copper-oxide modulators working at frequencies up to 6 Mc/s. It is also shown how the conversion insertion-loss can be stabilized against temperature variation by a suitable choice of terminating resistance. The working and results are the same for shunt, series and ring modulators, for conditions of low input frequency with high output frequency and vice versa.

DOI:10.1049/pi-3.1951.0083

出版商:IEE    

年代:1951

数据来源: IET

DOI:10.1049/pi-3.1951.0084

出版商:IEE    

年代:1951

数据来源: IET

DOI:10.1049/pi-3.1951.0085

出版商:IEE    

年代:1951

数据来源: IET

DOI:10.1049/pi-3.1951.0086

出版商:IEE    

年代:1951

数据来源: IET

DOI:10.1049/pi-3.1951.0087

出版商:IEE    

年代:1951

数据来源: IET