摘要:

The paper describes the development of picture-telegraphy equipment since Alexander Bain's first proposals in 1842, and particularly the applications to radio working since the first transatlantic tests in 1924. The technique depended on the type of radio circuit available, commencing with low-frequency high-power transmitters, then high-frequency beam-telegraph transmitters, frequency-modulated sub-carriers on double-sideband telephone transmitters and finally frequency-modulated single-sideband transmitters.The use of conventional amplitude-modulated transmitters has never been possible, first because such methods were not admissible on low-frequency circuits and later because the final fading ratios were too high to permit satisfactory reproduction of tone values.Methods of visual recording which preceded photographic reception included the use of hot wax, ink and hot-air jets and electrolytic paper. Spurious patterns caused by mechanical imperfections and means of avoiding them are described. Synchronizing methods have developed from elementary tuning-forks to quartz crystals.The comparatively low limit to speed of operation passed from equipment difficulties to that set by the propagation of high-frequency signals through the ionosphere. The effect of multi-path transmission is discussed.Consideration of bandwidth requirements discloses the inefficiency of facsimile methods for the transmission of telegrams.

DOI:10.1049/pi-3.1952.0077

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The equipment described is designed for the transmission and reception of pictures by sub-carrier frequency modulation, the same mechanical and optical system being used for transmission and reception. All mechanical, optical and electrical parts and all testing equipment required for normal operation are contained within a single console. The mechanical system is of the normal drum type rotating at the standard speed of 1 r.p.s. The drum diameter is 3.52 in and is scanned at 100 lines per inch, giving the standard index of co-operation of 352. Speed control is from a crystal oscillator. Provision is made for alternative positive or negative photographic reception. The f.m. swing is adjustable both in magnitude and sign, the final standards adopted being 2300 c/s for black and 1500 c/s for white.

DOI:10.1049/pi-3.1952.0078

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The paper deals with development work on which the authors have been engaged since 1945 on machines for the transmission of pictures and facsimiles over land-lines and radio.Faithful tone reproduction of photographic material has been attained, and machines have been produced to fulfil the needs of the Press. Machines with more precise scanning, up to 500 lines per inch, have been developed. A method of controlled retoning of a picture is also described.Further developments have been made using an electrochemical action instead of photographic paper as the recording medium. Machines have been produced to receive tone pictures and to act either independently or as monitors to the photographic machines.Recent applications of the electrochemical recorder to facsimile and telegraph purposes are described.

DOI:10.1049/pi-3.1952.0079

出版商:IEE    

年代:1952

数据来源: IET

摘要:

An instrument is described with which the relative permittivity and loss of disc samples of dielectric materials may be measured over the frequency range 100–300 Mc/s.The measuring cavity is essentially a short-circuited coaxial transmission line, terminated by the sample capacitance. The line length is adjustable, so that each sample may be measured at a number of different frequencies.All measurements are based upon audio-frequency calibrations of the measuring capacitors, and the design allows these to be used with only small corrections at 300 Mc/s. The corrections are analysed, and a comparison is made of different methods used for the excitation of measuring cavities.The accuracy is estimated to be ±1% in relative permittivity, and ±1½% ±0.00002 in tan δ.

DOI:10.1049/pi-3.1952.0083

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The method of moments, as used in statistics, is applied to the problem of synthesizing a network to have a prescribed impulse response; in particular, to have a sine-squared response. One of the advantages of the method is that the basic delay associated with the output of the network does not have to be specified beforehand but is allowed to vary so as to get the best approximation to the desired waveform from a network of given complexity.A numerical example leads to a design for a network consisting of three inductors and six capacitors in a ladder structure working between resistive terminations. The departure of the impulse response from the sine-squared curve does not exceed 2% of the peak amplitude of the pulse. The approximation could be improved by the use of more components.

DOI:10.1049/pi-3.1952.0084

出版商:IEE    

年代:1952

数据来源: IET

摘要:

An analysis is given of the behaviour of some crystal-oscillator circuits with the aim of finding the most favourable working conditions and the limiting conditions for oscillation. These questions are of special interest at very high frequencies, where overtone oscillators are becoming more and more important. Some overtone circuits are described, and a few problems affecting frequency stability are discussed. A simple and accurate method of measuring the parameters of the crystal equivalent circuit is also described.

DOI:10.1049/pi-3.1952.0085

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The theory and operation of an equipment developed for laboratory use in the frequency range 50–500 Mc/s is described. The principle of the equipment is to compare the modulus of any impedance with that of a standard impedance, namely a 100-ohm 0.1-watt resistor mounted so as to keep the lead length constant and at a minimum.From four values of impedance moduli determined experimentally the resistive and reactive components of the unknown can be calculated.This generalized method of determining an unknown impedance can be simplified for particular, and important, cases, such as the measurement of high- and medium-value resistors, inter-electrode impedances of valves, inductances, capacitances. Impedances in the range 1–100000 ohms can be determined. The error in the measurement of the modulus of an unknown impedance is less than ±5%, whilst that of the phase angle varies with the value of impedance being measured. Examples of measurements made on resistors, crystal valves, and short-circuited coaxial transmission lines are given.

DOI:10.1049/pi-3.1952.0086

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The input impedance of a rectifier modulator is different in its circuit relationships from that of a passive linear network; for a given input frequency it depends on the value of the load impedance at all the frequencies which can exist in the circuit and often also on the impedance of the signal source from which it is measured.The calculation of the input impedance appears to be impracticable in the general case; but in special cases where the terminating impedances can be restricted to zero or infinity or to a pure resistance at the various frequencies which can exist in the circuit, and where the resistance/time function of the rectifier can be expanded as a Fourier series independent of the signal frequencies—i.e. controlled solely by the carrier—calculation is possible, and a selection of results is tabulated.

DOI:10.1049/pi-3.1952.0087

出版商:IEE    

年代:1952

数据来源: IET

摘要:

The transmission of waves along a straight cylindrical conductor is discussed and an analysis made of the method of launching by means of a conical horn. It is shown that the efficiency of launching depends on the horn angle and aperture and that the launching loss is probably due to the generation of complementary waves which dissipate their energy as heat in the conductor. Details are given of experiments on a surface-wave transmission line at a wavelength of 6.5 cm, and suggestions are made for reducing the launching loss.

DOI:10.1049/pi-3.1952.0091

出版商:IEE    

年代:1952

数据来源: IET

DOI:10.1049/pi-3.1952.0092

出版商:IEE    

年代:1952

数据来源: IET