摘要:

Contemporary differences of opinion among leading authorities about dimensions are numerous and often extreme. It is sometimes overlooked that the use made of the theory of dimensions is purely algebraic, and mistakes in elementary algebra are the cause of several of the more extreme opinions. When these have been excluded, it is shown that dimensional quantity appears as a synonym for independent variable, although considerations of convenience demand the arbitrary exclusion from dimensional equations of many independent variables.It is shown that the difference between rationalized and unrationalized units depends on the value of the unit given to a quantity that appears sometimes as an independent variable. This quantity has not always been recognized as such and has not received a name, but it might be called a ‘configuration measure’.

DOI:10.1049/pi-b-1.1957.0140

出版商:IEE    

年代:1957

数据来源: IET

摘要:

A series of v.h.f. measurements over long sea paths was made over a period of 15 months. The results suggest that, in general, higher field strengths are reached for a given percentage of the overall time than in the case of over-land paths.

DOI:10.1049/pi-b-1.1957.0142

出版商:IEE    

年代:1957

数据来源: IET

摘要:

The paper describes the developments leading to the inauguration of the B.B.C. very-high-frequency service of sound broadcasting, which works in the band 87.5–100Mc/s. The need for a very-high-frequency service is explained, and the considerations leading to the choice of frequency modulation rather than amplitude modulation, and of the national planning standards, are summarized.In July, 1954, the Postmaster General authorized the first stage of the development plan, comprising nine stations. Later it was agreed that another station should be included in this stage, which is now virtually complete. In July, 1956, the B.B.C. was given authority to build a further six stations, which will be in service early in 1958. These sixteen stations will cover 96% of the population of Great Britain and Northern Ireland with three programmes. At a later date the B.B.C. plans to build additional stations to extend the population coverage to about 98%. At the majority of the stations three programmes are radiated from a common aerial; special precautions are taken to minimize the generation of intermodulation terms.The experience gained during the first year's service is described, with special reference to the performance of commercial receivers. The present stage of development of very-high-frequency sound-broadcasting services abroad is briefly summarized.

DOI:10.1049/pi-b-1.1957.0143

出版商:IEE    

年代:1957

数据来源: IET

摘要:

Equipment and performance requirements for frequency-modulated broadcasting transmission are given, with particular reference to the British Broadcasting Corporation's recently introduced network of v.h.f. transmitters.The general planning and design of the complete transmitting equipment used at the new high-power v.h.f. sites is surveyed, including problems of unattended operation, automatic phasing of paralleled amplifiers and 3-programme common-aerial working, to indicate the progress made in this new field of broadcasting.The design features of a current 10kW f.m. transmitter operating in Band II (87.5–100Mc/s are described, with special mention of the method of generating the frequency-modulated carrier, its automatic centre-frequency control and means for automatic phasing of the high-power amplifiers operated in parallel. Details are given of a newly introduced system of transmitter control designed to increase the service reliability, thus reducing the demands on the time of visiting maintenance personnel.Reference is made to the B.B.C.'s requirement of combining three independent 10kW f.m. carriers spaced 2.2Mc/s apart to feed one half of a wideband horizontally polarized aerial, the other half being fed with an identical set of transmitters kept in phase within a few degrees. The necessary filter circuits, giving sensibly zero coupling between transmitters and negligible crosstalk, are described.Overall performance figures are quoted.

DOI:10.1049/pi-b-1.1957.0144

出版商:IEE    

年代:1957

数据来源: IET

摘要:

An f.m. system is described which employs a directly-frequency-modulated quartz-crystal oscillator, the design of the circuit and of the crystal plate being treated in some detail. Easy methods of setting-up derive from the use of an oscillator having a high Q-factor. The system has the advantages of simplicity, reliability and ease of maintenance.

DOI:10.1049/pi-b-1.1957.0145

出版商:IEE    

年代:1957

数据来源: IET

摘要:

Tubular metal waveguides of circular cross-section supporting the H01-mode and operated at a frequency well above the cut-off value for the guide are known to be capable, in ideal circumstances, of providing a screened microwave channel having an attenuation of only a few decibels per mile. The paper is concerned with the practicability of this arrangement at a frequency of 35 Gc/s employing straight lengths of copper and aluminium tubes, 0.9–2.74 in inside diameter, manufactured and installed to commercial tolerances. Using pulses of 0.1 microsec duration at this frequency, it is demonstrated that with solid-drawn tubes of standard production attenuations about 30% above the theoretical value are readily obtainable, and the conditions of propagation are entirely stable. Arrangements for launching the wave both from the usual H01rectangular-guide source and directly from a magnetron oscillator are discussed. Attention is drawn to the possible application of such a waveguide to trunk communication and to high-power transmission. Although progress has been made on the problem of propagation round bends in the guide, a really satisfactory solution has not yet been found, and further work on that aspect is in progress.

DOI:10.1049/pi-b-1.1957.0148

出版商:IEE    

年代:1957

数据来源: IET

摘要:

The principle of the branch-waveguide directional coupler has been applied to the design of a new form of hybrid junction for microwave frequencies. An equal division of power between the main and subsidiary waveguides is achieved by arranging the voltage coupling coefficients of the branch waveguides in accordance with the coefficients in a binomial expansion.Details of design and performance of such a hybrid junction are given for the 3 cm waveband. Its properties are superior to those of the hybrid-T and rat-race and compare favourably with those of the short-slot hybrid. In addition, the geometrical configuration of this form of hybrid junction is convenient for the construction of compact waveguide circuits.

DOI:10.1049/pi-b-1.1957.0149

出版商:IEE    

年代:1957

数据来源: IET

摘要:

Two sets of data are fundamentally required to describe a television picture, one giving the significant changes of brightness, and the other the positions of such changes. The total information content is calculated according to Shannon, and the means are discussed for reducing bandwidth by redistributing the data in time so as to achieve a constant rate of transmission. Maximum compression is achieved by treating the two sets of data as independent quantities, and using two channels for their transmission. A compromise may be adopted, however, in which almost the maximum bandwidth compression is achieved using a single channel at the cost of an increase of signal power.The method is flexible in that it permits of an exchange to be made between the size of picture element and the continuity of grey scale, for a minimum stated bandwidth.

DOI:10.1049/pi-b-1.1957.0151

出版商:IEE    

年代:1957

数据来源: IET

摘要:

The general properties of linear accelerator waveguides loaded with spaced discs of ceramic dielectric are investigated theoretically and these waveguides are shown to have a higher shunt impedance than all-metal waveguides. Theoretical and experimental methods of waveguide design follow, a combination of which is used to design an experimental linear electron accelerator. Early experience with the accelerator shows considerable excess attenuation, which is probably associated with a high secondary-emission coefficient of the carbonaceous deposits formed; this is later eliminated by using mercury instead of oil-diffusion pumps. Provided that suitable precautions, such as the use of a mercury pump, are taken it is concluded that the way lies open for further development of dielectric-loaded accelerators, with considerable potential advantages.

DOI:10.1049/pi-b-1.1957.0152

出版商:IEE    

年代:1957

数据来源: IET

摘要:

Junction-transistor linear-sweep circuits of the ‘bootstrap’ type are described for which deviations from perfect sweep linearity of much less than 1% are estimated to be possible. The operation of the circuits depends on the properties of the emitter-follower amplifier stage; these properties are shown to be very well suited for such applications. The basic bootstrap circuit is closely related to its thermionic-valve counter part and is very versatile, being readily arranged in externally gated, monostable or astable forms; a variety of useful waveforms such as rectangles or delayed pulses may be generated, in addition to the linear voltage sweep. One form of monostable circuit described is particularly precise in operation, the timing duration being defined by the comparison of an accurate linear voltage sweep waveform with a reference potential by means of a transistor blocking oscillator comparator. The limitations imposed on the circuits by the frequency-dependent properties of the transistors are investigated, and it is shown that these result in initial transients at the beginning of the sweep involving time-constants of the order 1/ωαfurthermore, the transients concerned are not of a serious nature. Finally, quite fast, yet accurate, linear sweeps are shown to be possible, examples being given of circuits generating sweeps of less than 1 microsec duration.

DOI:10.1049/pi-b-1.1957.0155

出版商:IEE    

年代:1957

数据来源: IET