Showing papers in "Journal of the Institution of Electrical Engineers - Part IIIA: Radiocommunication in 1947"

Statistical theory of d.f. fixing

Abstract: The paper deals with the location of an object of u nknown position, on which bearings are taken from two or more stations whose positions are known, and provides solutions to the two problems:-(a) Given a set of bearings, what is the most probable position of the object?(b) How far from the true position is the position indicated by the “fix” likely to be?Diagrams are given to show the results of applying the theory to typical practical cases.It is shown that, subject to certain qualifications, reliability of a fix does not depend on the size of the particular “cocked hat” from which it is derived.It is proposed that the reciprocal of the root-mean-square error expected in the position of the fix should be adopted as the conventional standard quantity for measuring the reliability of a fix.

The control and elimination of electrical (Dauphiné) twinning in quartz

Abstract: Quartz for telecommunication purposes has to be of the highest crystalline quality. A considerable proportion of raw material is rejected on account of electrical twinning. The war-time need for economy of quartz led to an investigation of means of removing this defect. The paper surveys the experimental methods used in exploring the possibilities of untwinning quartz. The basis of these methods is a heat treatment of the quartz plate while subjected to a system of stresses applied to the plate by temperature gradients, pure bending, longitudinal compression or torque. The effect of crystallographic orientation is discussed, and the new physical phenomenon of “piezocrescence” is defined.The paper also describes the practical application of the untwinning techniques to quartz-crystal plates, with particular reference to the requirements of telecommunication. It is shown that the method employing heat treatment with an applied torque may be successfully applied to many important types of quartz crystals including the BT and AT cuts. The proportion of success and the occurrence of quartz showing anomalous behaviour are discussed.

The spectrum of modulated pulses

Abstract: Some methods proposed for evaluating the spectra of modulated pulse trains are discussed. The basic pulse-frequency, -phase, -length and -amplitude modulation systems are defined and the spectrum of a train of rectangular pulses sinusoidally modulated in any one of these ways is derived. Modulation by more than one tone is also considered. It is shown that in none of these methods is there any harmonic distortion or audio crosstalk. Anharmonic distortion arises from sidebands of harmonics of the pulse repetition frequency, and graphs are given to illustrate its magnitude. The formulae are extended to cover non-rectangular pulses, and it is shown that the distortions to be expected are practically the same as for rectangular pulses. The reproduction of transients is briefly examined. It seems that they suffer little distortion in form but that their timing is modified by the pulse modulation, the maximum time-shift being half the pulse repetition period.

Radio direction-finding by the cyclical differential measurement of phase

Abstract: This paper introduces a new general type of direction-finding and beacon system in which an appreciable reduction of the usual site errors is achieved by the use of aerial structures of wide aperture, the ambiguity normally associated with such systems being resolved by the manner in which the aerials are connected.Practical forms of the new system generally consist of a circularly disposed array of vertical aerials which are cyclically connected, singly or in groups, by a process of electronic commutation to a receiving device. The basic principle can best be appreciated by considering a single vertical aerial connected to a receiver and caused to move continuously along a circular path in the horizontal plane at a uniform rate. The motion of the aerial would impose a phase modulation on any received signal, and the horizontal direction of arrival of the signal could be determined if this modulation could be related to the law of motion of the aerial.Several types of direction-finder using the same basic principle are possible; these are outlined and classified. The practical and theoretical advantages of the system are discussed, and two directionfinders, one for use in the very-high-frequency band, the other in the high-frequency band, are described.The paper is confined to an account of the more important aspects of the subject, attention being paid to the fundamental requirements of the system and the means whereby they are met in practice. The mechanism whereby site errors are suppressed is outlined, and a comparison with the orthodox Adcock types of direction-finder is made, in which it is shown that, just as a phase-modulation communication system has certain inherent superiorities over an amplitudemodulation system, so the method of phase comparison has similaradvantages over other forms of direction-finder.

Dielectric-rod aerials

Abstract: This paper describes theoretical and experimental work carried out in the Admiralty Signal Establishment, and at the same time includes all the relevant material that the authors have been able to extract from German reports. It was decided to carry out this investigation when it became known that considerable importance had been attached to dielectric-rod aerials in Germany during recent years. The material of German origin is taken mainly from a translation by the authors of a paper by Mallach of the Vierjahresplan Institut fur Schwingungsforschung, Berlin.The fundamental principles governing the radiation characteristics of dielectric rods are considered, and brief reference is made to other dielectric aerials.

Some theoretical and practical considerations of pulse modulation

Abstract: This paper gives a summary of theoretical and practical studies on the properties of pulse-phase modulation, developed mainly in 1943.The properties of pulse-phase modulation are studied by means of Fourier transformations. Although some approximations are introduced, the calculations lead to the following definite conclusions:(1) Pulse-phase modulation introduces no amplitude distortion except at sub-multiples of the recurrent frequency.(2) The harmonic distortion, if any, is negligible and this method of modulation can be used for high-quality broadcasting.(3) Pulse-phase modulation is subject to a special type of distortion called “cross-distortion,” produced by side bands of the recurrent frequency appearing in the signal bandwidth. Curves of the approximate amount of this type of distortion are given, and it is shown that, in practical multi-channel systems, this distortion is negligible, provided that the recurrent pulse frequency is at least double the highest signal frequency to be transmitted, and preferably equal to, or greater than, three times this frequency.This study is followed by considerations on the signal/noise ratio in pulse-phase modulation. Pulse-phase modulation is compared with amplitude modulation and a formula, giving the improvement in the signal/noise ratio due to pulse-phase modulation, is established by very simple considerations. It is shown that this ratio improves as the frequency bandwidth used in pulse-phase modulation.It is shown how an improvement of 3 db in signal/noise ratio can be obtained by suppressing the noise on the synchronizing pulse, and a practical circuit developed and applied in 1943 by the author is described.Finally, a typical example of pulse technique is given. In practical circuits the modulator and demodulator pulses are not perfectly shaped, because of the departure from linearity due to finite time-constants. This introduces harmonic distortion. It is shown how this distortion can be practically eliminated by designing circuits so that the time constant is equal at modulation and demodulation.

Noise problems in pulse communication

Abstract: Formulae for random noise in the audio-frequency output in pulse-communication receivers are given for pulse-length-, pulse-phase- and pulse-frequency-modulation systems in which the received pulses are sliced; they are also given for pulse-amplitude modulation. Noise resulting from random distribution of the i.f. phase at the onset of pulses is also considered. Optimum slicing level giving minimum output noise is evaluated.Signal/noise ratio and threshold formulae are obtained from these data for the pulse-modulation systems and are compared with those for conventional amplitude- and frequency-modulation systems having the same mean or peak power. Final conclusions as to the merits of all the above systems are drawn from graphs of output signal/noise ratio, in which the performance below threshold is also taken into account.

The development of a high-frequency cathode-ray direction-finder for naval use

Abstract: The operation of the direction-finder described in this paper is based on the familiar principle by which two balanced amplifiers, connected to Bellini-Tosi crossed loops, operate the appropriate deflecting plates of a cathode-ray tube. The bearing is displayed as a trace on the c.r.t. fluorescent screen, providing a direct instantaneous reading. The sense of the bearing is obtained by combining the signals from an omni-directional sense aerial with the signal from either of the loops. The sense signal modulates the intensity of the electron beam, which is being swept, either vertically or horizontally, by the signal provided by the respective loop. The relative phase of the two signals is such that one or other end of the fluorescent trace is blacked out, depending upon which of the two lobes of the figureof- ight loop characteristic is effective for a given wave-direction.The first experimental equipment, designed for surface vessels, was already in operational use at the end of 1941, but the development proceeded until 1944, when the equipment reached its final form. With small modifications the same equipment was adopted for use at naval shore-stations, operating with an Adcock aerial system.The paper describes the latest design of the shipborne equipment and also discusses, more generally, some of the problems of the twinchannel receiving technique encountered during the development.

The fading machine, and its use for the investigation of the effects of frequency-selective fading

Abstract: The function of the fading machine is to simulate the frequency-selective fading which is characteristic of long-distance short-wave radio channels. Fading of this type is usually due to interference between waves which have traversed multiple paths between sender and receiver, and its effect is often to produce distortion of the modulation of radio waves. Many radio transmission systems have been devised with the object of minimizing the effects of selective fading; the fading machine enables different systems to be compared in the laboratory under precisely controlled fading conditions. The equipment described in the paper incorporates three transmission paths, the group time-delay differences between which may be varied in steps from 0 to 2 milliseconds. The phase differences between the paths may be varied manually (“static” fading) or continuously (“dynamic” fading), with fading rates ranging from 0.1 to 10 fades per second. Random noise, either fading or non-fading, may be included so as to synthesize a complete short-wave radio channel. The fading machine may also be used to simulate diversity reception obtained by spaced aerials. Examples are given of the use of the equipment to assess the merits of double-sideband, single-sideband and frequency-modulated transmission systems with telephony or telegraphy modulation, under conditions of severe selective fading and high noise level.

Radio-wave propagation research in the Department of Scientific and Industrial Research during the years 1937-1946

Abstract: The fundamental study of the propagation of radio waves has formed an important part of the programme of the Radio Research Board, which was established under the Department of Scientific and Industrial Research in 1920. The object of the present paper is to survey the progress made in this field during the decade ending in 1946, and to indicate the manner in which the investigations concerned have been developed to meet the requirements of those responsible for applying radio waves to various practical uses. The researches described cover the radio-frequency spectrum from about 10 kc/s up to 50000 Mc/s or more. They deal with the transmission of radio waves along the surface of the earth, and illustrate the manner in which this transmission is influenced by the electrical properties of the ground and the curvature of the earth and by the corresponding properties and slopes of natural and man-made protuberances on the ground. The characteristics of the ionosphere have been studied intensively for over twenty years, and during the past decade considerable advance has been made in our knowledge of this region of the atmosphere and of the manner in which radio waves are propagated through it. With the advance of radio technique and developments to shorter and shorter wavelengths, it has become clear that wave propagation in the lower atmosphere is greatly affected by the vertical gradient of the temperature and humidity of the air; the associated phenomena have been studied directly, and, in addition, the electrical properties of the constituents of the atmosphere, solid, liquid and gaseous, have been determined at the very high radio frequencies relevant to this portion of the investigation. Finally, the important part played by radio noise of either atmospheric or extraterrestrial origin has been studied with a view to understanding more clearly the manner and circumstances in which such noise can either influence the successful transmission of intelligence by radio waves, or provide a means of increasing our knowledge of such phenomena as those associated with thunderstorms and activity on the surface of the sun. This survey attempts to show what an important part the study of wave propagation plays in numerous applications of radio technique, and demonstrates that there is still much further work to be done before the full requirements of those using the technique can be met in a satisfactory manner.

An analysis of the performance of multi-aerial Adcock direction-finding systems

Abstract: This paper presents a theoretical discussion of the performance of crossed fixed-aerial Adcock direction-finder systems employing more than two aerial pairs. Comparison is made with the conventional four-aerial Adcock system, with regard to both spacing error and sensitivity.Improvements are obtained by increasing the total number of aerials over the four of the conventional Adcock system, so that, for a given accuracy, a gain may be achieved either in frequency coverage or sensitivity, or in both together. It is also shown that an increase in the number of aerials above eight is of little practical advantage, since the spacing for small errors approaches the absolute limit of 1.22λ.A modified system, in which two aerials are connected in parallel to replace each of the single vertical aerials, is also discussed. This system has a performance nearly as good as the fully symmetrical eight-aerial system, and when used in conjunction with a twin-channel receiver it is expected to give the better performance.Finally it is shown that a star connection of the aerials provides a correct sense signal, only when the aerial spacing is less than 0.76λ.

Low-reading absolute altimeters

Abstract: Efforts have been made over the past twenty years to design a lowreading absolute altimeter for use in aircraft. At the present time the most successful instrument of this type is the frequency-modulation radio altimeter.The paper deals in detail with the problems involved in the design, installation and use of this type of equipment.The discussion is illustrated by reference to various instruments that have been designed.

The design of transmitter drives and receivers for single-sideband systems

Abstract: Many of the long-distance short-wave radio links between Great Britain and overseas countries are being converted from double-sideband (d.s.b.) to single-sideband (s.s.b.) operation because of the improved performance so obtained; this has necessitated new designs of s.s.b. generating equipment and radio receivers suitable for commercial operation. The type of s.s.b. transmission used incorporates two channels, each 6 kc/s wide, on either side of a reduced level pilot carrier which is nominally 26 db below the peak level of the sidebands. The function of the pilot carrier is to provide automatic gain-control (a.g.c.) and automatic frequency-control (a.f.c.) in the receiver.The equipment described includes:—(a) A drive equipment for generating a low-power two-channel reduced-carrier s.s.b. signal for subsequent amplification in the highpower stages of a radio transmitter.(b) A monitor receiver which is associated with the drive equipment and transmitter, and which enables either channel of the radio frequency (r.f.) signal at various stages in the transmitter to be demodulated to audio frequency (a.f.) for aural or electrical tests of quality and distortion.(c) Single-sideband receivers suitable for the reception of twochannel reduced-carrier transmissions, for use at the receiving station of a radio link. Receivers for single-aerial operation and triple-diversity spaced aerial operation are described.The performance requirements are stated, and the design, layout and performance of typical equipments are discussed.

The estimation of the probable accuracy of high-frequency radio direction-finding bearings

Abstract: The paper describes a method of estimating a priori the probable accuracy of high-frequency direction-finding bearings based on the estimation of the probable value of the standard deviation or variance to be associated with the observation. To apply the method the bearing observations must be made according to a prescribed form, which consists in taking about ten “snap” readings of the bearing spread over about five minutes. The arithmetic mean of this group of bearings is taken as the observation, and the scatter or spread of the readings in the group provides one of the quantities necessary for the estimation of the probable variance. Other components entering into the estimation of the variance are (a) site error component based on past history of the direction-finder, (b) ionospheric lateral deviation component based on known data, and (c) observational error component based on the quality (i.e. flatness) of the minimum. In a practical trial of the method, good agreement was obtained between estimated and actual bearing errors. For example, a check applied at the 90% probability level showed that 85–90% of the bearings did have their variance correctly estimated.

The errors in bearings of a high-frequency direction-finder caused by reradiation from a nearby vertical mast

Abstract: In most ships there is one metallic structure (e.g. a mast) which is considerably taller than any other object on board. It is found that the largest errors in a high-frequency direction-finder installed in a ship are generally encountered when this conductor is in resonance. In this paper all other sources of error are neglected and an investigation is made of the error and blurring caused by a single reradiating vertical mast. The results are applied to assess the suitability of different sites for the d.f. aerial.

Recent developments of aircraft communication aerials

Abstract: The paper describes the progress made in recent years in aircraft aerial design. The impedance characteristics of several typical kinds of aerial are stated.

A common-wave duplex pulse-communication system

Abstract: The paper details the principles of a duplex pulse-communication system using only one carrier frequency and suited for air-to-air and air-to-ground radiotelephony. The duplex facility is achieved by suppression of the receivers at each end of a communication link. Pulsephase modulation is employed and the pulse-recurrence frequencies of the transmissions in the two directions are unsynchronized. By using a memory pulse-phase discriminator, the introduction of noise of any considerable level is avoided.

A time-multiplex radio-frequency phase-comparison method for navigational systems

Abstract: A method of using time multiplex to provide a continuous wave navigational aid is discussed theoretically. The practicability of the system has been demonstrated by the construction and experimental use of low-power equipment. The use of the scheme for various navigational problems is discussed and the present programme of development is outlined.

Variable-frequency crystal oscillators

Abstract: The frequency of a crystal oscillator can be varied by means of a variable capacitor, but the range obtainable is extremely small. Methods of increasing the range by associating inductance with the crystal are described. It is estimated that the range of a 3-Mc/s oscillator, using an AT-cut plate, could be increased from 0.8kc/s (267 parts in 106) with variable capacitance alone, to 4kc/s (1 330 parts in 106) or more with added inductance, while preserving far better frequency stability than that of a tuned-circuit oscillator. Experiments have confirmed these results, a range of 7.7 kc/s (2560 parts in 106) being obtained with one 3-Mc/s plate. Spurious responses limit the range with some plates, but changes of rather more than 1000 parts in 106, with good frequency stability, are not difficult to achieve.Many arrangements of inductance and capacitance, capable of giving the same range, are possible. Circuits are described whereby the frequency can be made a linear function of a controlling reactance or capacitance, and the possibility of linear variation by means of a reactance valve is briefly considered. Various fields of application for an oscillator of the kind described are indicated.

Airborne automatic direction-finders

Abstract: The paper is concerned with the use, in aircraft, of automatic direction finders of the switched-cardioid type. Operating principles are described, and the underlying design considerations are dealt with. Representative examples of German and American origin, used widely during the war, are described, and reference is made to a new equipment being developed. A section is devoted to new developments in connection with iron cored loops, and cathode followers for coupling the loop to the receiver. The more general aspects of continuous-wave (C.W.) direction-finding are treated only to the extent necessary to ensure completeness.

The development of single-receiver automatic adcock direction-finders for use in the frequency band 100–150 Mc/s

Abstract: The paper describes the development of automatic direction-finders primarily intended for measuring the bearings of aircraft, on signals radiated in the frequency band 100–150 Mc/s. The basic system employs fixed elevated-H Adcock antennas and a single receiver, and, with the exception of the earliest model, all instruments give cathoderay oscillograph indication.An examination of the advantages and limitations of fixed*antenna automatic direction-finders employing two or more receivers leads to an account of the reasons for the adoption of a single-receiver system in the present development. The principles and evolution of this system are explained with reference to the original experimental models, whose performance is discussed.A naval direction-finder based on one of the experimental models is described in detail, and statistics are presented to show that the probable instrumental error varies from about 0.6–1.25 deg over the frequency band, after compensation for octantal error. Bearings can be measured on signals of field strength down to 7 μV/m or less.The paper concludes with a description of direction-finders for use on land. One of these, under current development, will be capable of unattended operation on two alternative frequency-channels, with bearing indication and full remote-control at points up to twenty miles distant.

External cross-modulation in the 100-Mc/s band

Abstract: The introduction of very-high-frequency equipment to the Fleet Air Arm involved the fitting of numerous v.h.f. transmitters and receivers in ships, and the number was further increased by the discovery that these frequencies were also convenient for ship-to-ship working. It was found that when a number of transmitters and receivers were fitted in the same ship, the number of workable channels available in the band was much smaller than was the case ashore, where grouped transmitters and receivers are set up on well separated sites.Experiments were carried out in 1944 by members of the Communications Department, Admiralty Signal Establishment, which proved that the effect was due to external cross-modulation arising in parts of the ship's structure, which act as non-linear conductors.A description is given of an apparatus which has been recently developed to assist in locating accurately the parts of the ship's structure responsible for the interference, as part of an investigation into the possibility of reducing the effect.

Carrier-frequency-shift telegraphy

Abstract: After the history of the development of carrier-frequency-shift telegraphy has been briefly outlined, methods of frequency-shift transmission are described and discussed. Methods of detection, by band-pass filter selection, or by a discriminator, are described. Practical advantages of frequency-shift operation are discussed and some factors influencing the design of carrier-frequency-shift transmitters and receivers are considered.A theoretical analysis of the threshold value of signal and the time shift produced by random noise is given in an appendix, and formulae are derived for comparing keyed-carrier and carrier-frequency-shift systems.It is shown that the discriminator method of detection gives no theoretical advantage over normal keyed-carrier operation, but that in a typical example, post-discriminator filtration gives an improvement of 14 db and band-pass filter selection an improvement of 10 db in threshold value.

Crystal oscillators and their application to radio transmitter control

Abstract: The paper discusses the mechanism of operation of crystal oscillators, with particular reference to the frequency stability obtainable. The quartz crystal unit is described with special regard to the temperature performance obtainable from the various cuts used to cover a frequency range on fundamental modes of 4–20 000 kc/s, and some drivecircuit analysis necessary for the establishment of stability criteria is given. Although it is not desirable to lay down hard and fast rules for design, a chart is included which gives an indication of the type of crystal and drive circuit required for given conditions of stability, frequency, and temperature range. Finally, examples of practical designs are quoted giving long-and short-period stabilities ranging from 30 × 10−6 to 1 × 10−8.

Very-long-wave phase differences between spaced aerial systems

Abstract: The paper describes some investigations into the propagation of very-low-frequency radio waves, in which the variations of phase difference between signals received at two points about ten wavelengths apart were measured. Observations were made using two telegraph transmitters on frequencies of 17.2 kc/s and 18.4 kc/s at distances of 1030 km and 5650 km respectively; propagation with ground wave predominant, sky wave only, and both sky and ground wave, was observed. Two sets of measurements were made at each distance, to give an indication of seasonal variations.The results indicated that a navigational aid using these frequencies and a base line of about ten wavelengths would give a probable error of equivalent bearing of less than 0.2 deg near the median line of the system at these distances, and probably at most intermediate distances.Larger variations which were observed at certain times of the night appeared to be due to the sunrise or sunset line crossing the propagation path obliquely.An analysis of the periodicity of variations showed that observations would have to be averaged for a period of the order of half an hour to gain any appreciable increase in accuracy.

Note on a short-range radio position-finding system using modulated continuous waves

Abstract: The paper describes a position-finding system by which the polar co-ordinates of any suitably equipped aircraft can be determined at a single base station using phase-comparison range-finding apparatus and a v.h.f. automatic direction-finder in conjunction with conventional communication equipment.‡An experimental model is described in which azimuth and range are displayed on a single cathode-ray oscillograph, which may be installed in an airport control tower. Range is indicated without ambiguity up to 100 nautical miles, and there is no azimuthal ambiguity. So far, the model has been used primarily to demonstrate the principles of the system, in conjunction with an aircraft fitted with an experimentally modified Service type of v.h.f. communication equipment. The results of a limited number of tests suggest that, with airborne equipment specifically designed for use with the system, the probable error in range measurement should be less than 1 mile. Azimuthal accuracy is not here discussed, but in a companion paper it is shown that the probable instrumental error of the direction-finder is less than 1.5 deg, implying a probable lateral error less than 2.5 miles at the maximum non-ambiguous range of 100 nautical miles.

Some developments in commercial point-to-point radiotelegraphy

Abstract: Developments in point-to-point radiotelegraphy have been concerned chiefly with the improvement of reliability to facilitate the introduction of time-division multiplex systems using direct-printing equipment, and to ensure continuous communication over difficult circuits such as those to the antipodes.Methods of reception using frequency-shift keying are discussed in relation to distortion, interference and multi-path effects; figures are given for the results of tests with this system compared with those of the on/off keying system. In one case, the lost capacity due to poor radio conditions was reduced by 39%.Radiotelegraph relays are described which cover interruption of direction communication, and charts are included to show the recovery of lost time. Owing to the introduction of radio relay stations at Barbados and Colombo, the periods when no communication is possible between this country and Australia have been reduced to negligible proportions. The effects of ionospheric disturbances on the London–Montreal route have also been greatly reduced by the use of radio relay stations at Ascension and Barbados; the station at Ascension has been in use since 1939.Equivalent phase modulation of short-wave transmitters is discussed or use with long-distance facsimile circuits.Concerning aerials, the development of horizontal-dipole receiving arrays to augment the vertical broadside-beam array, is dealt with; also a review of wideband coupling devices for coupling balanced aerials to coaxial feeders. Mention is made of the advantages of remotely controlled and unattended radio transmitters in reducing congestion on radio-station sites and in minimizing personnel.The paper also deals with telegraph printing systems; the doublecurrent cable-code system is described, and compared with 5-and 7-unit systems for undetectable errors and flexibility of operation.A frequency-modulated v.f. system for the control of radio stations is described.Finally, under the general heading of miscellaneous telegraph equipment, the paper contains a description of a high-speed photo-electric morse transmitter and an electronic keying device.

The Consol navigation system

Abstract: The Consol navigation system embodies a chain of independently operated medium-frequency radio stations with respect to each of which bearings may be taken with the aid of a standard communication receiver. The principles of operation of the constituent stations are given and the equipment required is briefly described. Factors influencing the accuracy of the system and ranges to be expected in various parts of the world are discussed.