摘要:

The paper surveys the television transmitting-station distribution scheme by which the B.B.C. hope to provide the greater part of the United Kingdom with a television service using the 405-line 2: 1-interlaced-scanning 25-pictures/sec British Standard transmission. The basic factors underlying the choice of scheme are discussed, together with the arrangements for dividing the available 41–68 Mc/s frequency band into five separate operating channels using the receiver-attenuation system of vestigial-sideband transmission. The proposals for operating two geographically separated transmitters, one of high power and one of low power, on each channel to complete a 10-station plan, and the precautions needed to minimize co-channel interference, are explained. Some of the problems of site selection for transmitting stations are mentioned, and a brief account is given of propagation phenomena which influence the estimation of service area. The scheme for linking the stations with the London studio centre is briefly described.The general planning and design of the complete transmitting equipment used at the B.B.C.'s new high-power television broadcasting stations and intended for use at the future low-power stations is surveyed, with particular reference to the different types of vision and sound transmitters, monitoring equipment, air and water cooling installations, vision/sound combining circuits, transmission lines and aerial systems, to indicate the progress so far made and illustrate the trend of future developments.

DOI:10.1049/pi-3a.1952.0025

出版商:IEE    

年代:1952

数据来源: IET

摘要:

A reference to the Alexandra Palace television aerial and to the circumstances preceding the expansion of high-power television services in Great Britain serves as an introduction.The various mechanical and electrical considerations which affect the design of a transmitting aerial are then reviewed, particular attention being directed to common-aerial working for vision and sound. The properties of 2-phase aerial systems are discussed, and curves enabling the horizontal radiation patterns of such aerials to be interpreted in terms of impedance measurements on the distribution-feeder system are presented.The development of the Sutton Coldfield aerial is described, details being given of the scheme of admittance correction applied to the distribution feeder. The modifications to this basic design adopted for Holme Moss and the later high-power stations are explained.The paper concludes with speculations on the probable future trend of television transmitting-aerial design.

DOI:10.1049/pi-3a.1952.0026

出版商:IEE    

年代:1952

数据来源: IET

摘要:

Performance requirements for television transmission are discussed in relation to reliability. Modulating systems are reviewed, and it is concluded that high-level modulating systems are to be preferred where the choice of system exists.The design features of a current 50-kW transmitter are discussed with special reference to shunt-regulated amplifiers, pulse stabilization methods and black-level control. Overall performance figures are quoted.Transmitter testing requirements are discussed, and a case is made for a controlled shape of step signal for transient testing and for the use of very-low-frequency testing for overall system appraisal.

DOI:10.1049/pi-3a.1952.0027

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The paper deals with coaxial-line transmitting feeder systems of the type in which the whole system is suspended from the top of the transmitting tower. The inner conductor, weighted at the lower end, consists of a locked-coil winding rope of the type common in mining practice, but modified in design so as to have low electrical loss and high uniformity in diameter along its length, and thereby to reduce to negligible proportions the reflections arising in the feeder. The paper also discusses the principles and properties of locked-coil ropes, and records the results of tests, both mechanical and electrical, on ropes developed for feeder systems; indications are given, from theoretical considerations, of the tolerances in mechanical uniformity which must be satisfied in the manufacture of the rope and of the outer conductor of the feeder. Information relative to the performance of a trial system erected on the Hayes transmitting tower in May, 1949, is included, as well as a brief description, with results of some electrical measurements, of a similar system erected at the Kirk o'Shotts Television Station in August, 1951.

DOI:10.1049/pi-3a.1952.0028

出版商:IEE    

年代:1952

数据来源: IET

摘要:

A short review of possible methods of combining vision and sound transmissions into a common aerial is given. The use of the Maxwell bridge (M.b.) principle leads to a compact design with good performance.The fundamental characteristics of a filter of this type are derived. It is shown that tapped coaxial stubs are unsuitable as frequency discriminative elements, and instead, T-connected coaxial stubs are used with the connecting arms forming part of the resonators; the mathematical analysis of these is presented.Consideration is given to problems associated with high-power transmission, such as the dissipation of heat and the presence of high voltages. An expression for the minimum insertion loss for the sound transmission is derived and generalized for any frequency-discriminative combining filter. A description is given of an M.b. combining filter, of which the performance was in close agreement with the calculations. The measured results are shown for insertion loss between vision and sound inputs, insertion loss of each transmission, and phase delay, envelope delay and input admittance for the vision transmission.Other possible uses are suggested for Maxwell-bridge-type filters.

DOI:10.1049/pi-3a.1952.0029

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The planning and successful operation of a national television service involve an understanding of the mode of propagation of the radio waves concerned. At the present time the frequency bands provisionally allocated for television broadcasting are located between limits of 41 and 960 Mc/s, and the service at present under development in the United Kingdom is confined to the use of five channels in the band 41–68 Mc/s. For the service to be successful, each transmitting station must establish, over the normal reception area, a satisfactory ratio of the wanted signal to the prevailing noise or other interfering signals. This involves, first, understanding in detail the manner in which the waves carrying the programme signals are propagated within the service area of the station to a radius of the order of 50 to 100 km; and secondly, determining, on a statistical basis, the manner in which the waves from any other station operating in the same frequency channel are propagated to considerable distances in sufficient strength to impair, by interference, reception of the wanted programme.The paper reviews the results of investigations conducted in this country on the propagation of radio waves of the frequencies assigned for the existing and future television services. Within the service area, this propagation is largely determined by the nature of the terrain over which the waves travel, including the effect of hills and valleys, trees, buildings and similar departures from the ideal smooth-earth condition. The effect of the diffraction of waves round the curved surface of the earth, or over hills, has been studied, as well as the bending of the waves due to the vertical gradient of refractive index in the atmosphere. Within the service area, the field strength from the transmitter remains reasonably constant, but variations may occur as a result of interference between the direct wave and the waves reflected or scattered by moving objects such as aircraft. Apart from such interference, the field strengths attained in practice are in reasonable agreement with those estimated from theory.In recent years, considerable investigation has been carried out on the propagation of radio waves in the metre waveband (frequencies 30 to 300 Mc/s) at distances of a few hundred kilometres. At such ranges, well beyond the normal optical horizon, the propagation is affected to a major degree by meteorological conditions. The general level of field strength and the variations superimposed on this are determined first by the changes in mean gradient of the refractive index of the atmosphere, and secondly by the presence of temperature inversions which, at heights of one or two kilometres, may act as reflecting layers. Furthermore, at distances approaching the limits of practical reception, turbulence in the atmosphere may be a sourceof scattering of radio waves and may result in an extension of the normal range of transmission.Under suitable conditions, transmission of waves in the 30–300- Mc/s band may also take place to distances of several thousand kilometres by way of the ionosphere, but experience suggests that occurrences of reception due to indirect transmission of this kind are comparatively rare with normal transmitter powers and receiver sensitivities.Apart from the use of radio links at frequencies in the region of 900 Mc/s, comparatively little opportunity has so far arisen for the study of radio-wave propagation in the band 470 to 960 Mc/s, in which much of the future development of television is likely to take place. At higher frequencies, between 3,000 and 10,000 Mc/s, several investigations of wave propagation have been made in the last decade in the course of work connected with the development of centimetre wave radar. Much of this work was carried out over sea paths, but some of it, conducted over land, has already provided certain information on propagation characteristics, and this will prove of interest in connection with more detailed investigations which are now in progress. It seems likely, however, that the use of such frequencies in a television service will be confined to radio links for point-to-point distributipn.

DOI:10.1049/pi-3a.1952.0032

出版商:IEE    

年代:1952

数据来源: IET

摘要:

A simple technique for the measurement of the path attenuation, i.e. the attenuation between the input to the transmitting aerial and the output of the receiving aerial, of paths for radio-relay systems operating at frequencies of the order of 4000 Mc/s is described. The transmitter consists of a coaxial-line oscillator, amplitude-modulated at 1000 c/s; the receiver comprises a silicon-crystal detector followed by a 1000-c/s high-gain narrow-band a.f. amplifier and diode rectifier operating a meter. With a transmitter output power of 1.0 watt, path attenuations up to 95 dB can be measured with an accuracy of better than 0.5 dB. A method of measuring the path attenuation directly in terms of the setting of a piston-attenuator, i.e. without a precise knowledge of the transmitter output power or the receiver sensitivity, is given. The equipment can be used for the recording of fading on microwave links; a fading analyser that measures the proportions of the total time that the received signal level falls below six assigned levels is also described. Examples are given of tests carried out on typical overland and oversea paths at 4000 Mc/s.

DOI:10.1049/pi-3a.1952.0033

出版商:IEE    

年代:1952

数据来源: IET

摘要:

A review is given of the circumstances and characteristics of ionospheric reflections of v.h.f. waves, with reference to their effects on television reception. It is shown that sustained v.h.f. propagation via the F2 region may occur for limited periods during a defined phase of the cycle of solar activity, whilst irregular and intermittent ropagation via forms of sporadic ionization in the E region may be experienced during certain months each year. The range of frequencies affected embrace those currently in use for television services in the United Kingdom and elsewhere. A summary is presented of the circumstances under which interference effects to the reception of these services may be expected, owing to the use of common frequencies for remotely-situated short-distance v.h.f. services.

DOI:10.1049/pi-3a.1952.0034

出版商:IEE    

年代:1952

数据来源: IET

摘要:

A review is given of the effects of irregularities in the terrain and of non-standard refraction in the troposphere on radio field strength characteristics in the v.h.f. band. The results of an analysis of existing data on long-range transmission in this band in the United Kingdom are then applied to the problem of determining the spacing required between two transmitters working on a common frequency when the degree of interference in each local service area caused by signals from the more distant transmitter must not exceed various specified limits. The effects of non-standard tropospheric refraction, in particular, are such as to make this spacing much greater than it would be if such departures from standard did not exist.

DOI:10.1049/pi-3a.1952.0035

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The paper discusses the problem of covering the United Kingdom with television service in the frequency band between 41 and 68 Mc/s. It is proposed to use five high-power transmitters, with a further extension of five low-power transmitters to cover other populated areas outside the range of the main transmitters; this will necessitate common-channel working.An accurate forecast of the service of a transmitter using very high frequencies can be obtained only by erecting a test transmitter on the proposed site and measuring the field strength in the required area. The aerial used for these tests is attached to a balloon which replaces the mast required at the final site. Such tests were carried out in the North of England, Scotland, Wales and the West of England; the results are shown, and the reasons for the final choice of sites are given. Estimated figures for the population covered are also given.The effect of raising the transmitter aerial on the received signal is very dependent on the ground profile between transmitter and receiver.Height/gain curves are shown for various transmitter sites.Experiments were made at two sites, to observe the effect of the reradiation from structures near the transmitter sites which might have given rise to ghost images on a television service. Short-duration pulses were radiated from the test transmitter, and the delay time and amplitude of the echoes were measured. The echoes observed were caused by obstacles, such as buildings, close to the receiving point; those due to structures near the transmitter site were negligible.

DOI:10.1049/pi-3a.1952.0036

出版商:IEE    

年代:1952

数据来源: IET