Aerial Array Research Articles

A new approach to the design of super-directive aerial arrays

A new approach to the design of super-directive aerial arrays

A new approach to the design of super-directive aerial arrays

The current distribution required for maximum directivity of an array with a finite number of elements and any specified geometrical configuration is shown to be completely defined by the self- and mutual resistances of the elements and by a certain component of the voltage (the “resistance voltage”) across the terminals of each element. This voltage component is required to vary from element to element in the same way as the instantaneous local values of a sinusoidal disturbance travelling across the array, in the direction under consideration, with the velocity of an electromagnetic wave.As a consequence, the maximum gain of the array is expressible either as a double sum containing only the mutual conductances between the individual elements multiplied by trigonometrical factors depending on their spacing, or as an expression identical (except for a numerical factor) with that for the distant field of the array.These theorems hold, slightly modified, for arrays of non-identical elements.The theory has been applied to the numerical calculation of certain simple arrays. It appears that, for arrays of a given size, directivities greater than those obtained by conventional design methods can be achieved without excessive losses.This has been substantially confirmed by an experimental array of four elements operating at 75 Mc/s. The theoretical gain was 10.1 db, while 8.7 db was measured. Of the discrepancy, 0.6 db was calculated to be due to losses in the feeder system and a further 0.2 db to losses in the dipoles. The bandwidth was about ±½ Mc/s for a drop in gain off ½ db. The degree of super-directivity is indicated by the fact that a physically identical array fed with equal-amplitude currents phased for maximum field strength in the end-fire direction would have a gain of 4.6 db.

Radio reflections from aurora

This paper outlines observations and experiences associated with a continuing investigation of radio echoes from aurora at Saskatoon, Canada. For the three frequencies investigated—3000, 106, and 56 Mc/s—echoes have been observed with the last two. Echoes occur when the auroral forms exhibit some ray structure and, then, only from parts of the aurora at elevations less than 15° above the horizon. Using range-amplitude and range-azimuth displays of echoes in combination with the corresponding photographs of the aurora, it is shown that the echoes originate from levels close to the lower edge of the aurora. Height distributions of the centers from which the echoes come agree closely with height distributions found from parallactic photographs of aurora in other regions. Using aerial arrays for the 106- and 56-Mc/s frequencies that are mounted on a rotatable tower so that each beam scans simultaneously the same part of the sky, the 106-Mc/s echoes are found to occur most frequently within the auroral zone and the 56-Mc/s echoes some distance to the south. Relative frequencies and diurnal variations of the two groups of echoes, as well as their heights and geographical distributions, indicate that echoes originate from a threshold process. It is suggested that the echoes arise by critical reflection from centers of high electron density (1.4× 08 and 4×107 electrons per cm3, respectively) that are formed close to but above the levels where the electron collision frequency exceeds the gyro-frequency of the electrons.

New equipment for impedance matching and measurement at very high frequencies

The slotted transmission line has long been used as a standard device for determining impedance. Over the v.h.f. range, however, the constructional difficulties due to the physical length required for the slotted line make it primarily a laboratory instrument. The need for an alternative device of comparable or improved accuracy, capable of giving much more rapid measurements, and sufficiently robust to be used on field tests of aerial arrays, has led to a modified approach.A test section of low-loss coaxial line, which may be as short as one-third of the equivalent slotted line, is fitted with fixed probes to measure the relative voltage amplitudes at three known points. From these ratios the unknown impedance can be found, either analytically or graphically, each ratio defining a circular locus on the Smith chart. The frequency range covered by a line of this type is approximately 3:1; near mid-frequency the present equipment can measure standing-wave ratios up to 5:1, corresponding to resistive loads ranging from 15 to 375 ohms for a line of 75 ohms.An important advantage of the new equipment is that the adjustment of a load to a prescribed value is shown by the simultaneous zero reading of two meters.

Galactic Radiation at Radio Frequencies. I. 100 MC/S. Survey

This paper contains an account of a survey of galactic radiation at a frequency of 100 Mc/s. An aerial array with a 17� beamwidth, on an equatorial mounting, was used to plot the distribution of intensity over the section of the celestial sphere between Declination +30� and -90�. The method of eliminating the effect of the aerial polar diagram from the observations is described and the final distribution, expressed in terms of equivalent black-body temperature, is presented in galactic coordinates on a series of equal-area charts. I.

Design of Yagi aerials

An account is given of work carried out at the Telecommunications Research Establishment on the design and use of Yagi aerial arrays during the period 1938-42. An attempt is made to give sufficient information in tabular form for an aerial of given characteristics to be designed approximately. Final adjustment by experiment will be required unless a previous design is copied exactly. The advantages and limitations of multiple Yagi arrays are considered and a simple theoretical explanation of the mode of operation of a Yagi aerial is given. The types of polar diagram obtained when the design parameters are changed are deduced from vector diagrams. Only aerials of length greater than one wavelength are considered. These normally consist of three or more directors in addition to an exciter and a reflector.

Mr. C. S. Franklin

THE twenty-seventh award of the Faraday Medal has been made to Mr. Charles Samuel Franklin for his distinguished work in radio engineering, and in particular for his original studies of short-wave wireless transmitting and receiving circuits and his invention of the beam aerial, by means of which the practical use of short-wave transmission for communication purposes was established. Mr. Franklin was born in 1879 and received his engineering and scientific training at Finsbury Technical College under Prof. Silvanus Thompson. In 1899 he joined Marconi‘s Wireless Telegraph Co., then known as the Wireless Telegraph and Signal Co., and was continuously associated with research and development in radio engineering with that Company until 1935, when he retired from active work. During his period with the Company, many outstanding developments stood to his credit, including reaction patents for wireless receiver circuits, the early British broadcasting stations, directional receiving systems for long-wave communication circuits, rotating beam aerials, shortwave wireless transmitting and receiving circuits, and in particular the beam aerial array associated with his name, which was the fundamental invention comprised in the beam system of wireless telegraphy. He also developed the concentric cable system, which is essential in television transmission practice. Mr. Franklin was the first recipient of the James Alfred Ewing Medal, instituted in 1936 in memory of Sir Alfred Ewing and awarded by the Institution of Civil Engineers for specially meritorious contributions to the science of engineering in the field of research.

Radio direction-finding by the cyclical differential measurement of phase

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.

Radar Echoes from the Moon

ACCORDING to recent announcements in the daily Press, scientists of the U. S. Army Signal Corps have succeeded in detecting, with the aid of specially designed radar equipment, echoes of radio waves reflected from the moon. The possibility of such an achievement was foreshadowed by Sir Edward Appleton in his Kelvin Lecture to the Institution of Electrical Engineers delivered, in April 1945. A similar prediction was made independently by Pietro Lombardini in a paper entitled “The Possibilities of Astronomical Radio Soundings with Metre Waves” (Commentationes-Pont. Acad. Sci., 8, No. 2, 13 ; 1944). In the latter paper, the author estimates that by using a wave-length of 1.5 m. and a large aerial array for both transmission and reception, it should be just practicable to detect a radio echo from the moon, although it would appear to be problematical whether this technique could be extended to the planets. In Sir Edward Appleton's lecture (Inst. Elect. Eng., 92, Pt. 1, 340 ; Sept. 1945), the relevant formula was given for the energy we might expect to get back from the moon if a beam of radio waves was directed on to it. It was calculated that, with a powerful sender, and with sending and receiving aerials of high gain, it should be possible to get back detectable radio echoes after their 2½ sec. journey to the moon and back. The success of the experiments just announced from America would appear to afford a confirmation of the accuracy of this prediction. These experiments are to be distinguished from those made a few years ago in various countries, during which a radio wave component in the solar radiation spectrum was detected on a very sensitive receiver of the type used for radiolocation (see, for example: “Radio Waves from the Sun”, Nature, Sept. 1, 1945, p. 273).

Wide-band aerials and transmission lines for 20 to 85 Mc/s

Wide-band aerial and transmission-line systems are described covering the frequency bands 20 to 30 Mc/s, 40 to 50 Mc/s and 50 to 85 Mc/s. These systems are capable of dealing with pulse transmissions of 600 kW peak power. The aerials have vertical polar diagrams such that the major part of the energy is radiated at low angles of elevation, and horizontal polar diagrams either circular or with reduced backward radiation. While the theoretical background of such aerials is well established, the problems of electrical and mechanical design have hitherto received little attention, and some interesting solutions to the special problems encountered are described. The aerial arrays are built up of full-wave centre-fed dipoles of wire-cage construction, with an input impedance of 600 ohms. Openwire transmission lines are used throughout. When more than one dipole is used, branch feeding is employed and 300-ohm 4-wire transmission lines are introduced to eliminate frequency-sensitive matching devices. Wire-mesh reflectors are used to obtain the horizontal polar diagram required, and their effect on dipole impedance is discussed. The problem of the mismatch introduced by the reactance of supporting insulators on transmission lines has been investigated, and a solution is described which is attractive both from the electrical and mechanical viewpoints. Other problems arising from the use of 4-wire transmission lines are reviewed. An exponential transformer is described which simplifies the construction of arrays consisting of four full-wave dipoles, its function being to transform an impedance of 300 ohms to 600 ohms. Reference is made to the use of compensating stubs for increasing the bandwidth of the aerials.

Aerial Systems for Short Radio Waves

A RECENT meeting of the Radio Section of the Institution of Electrical Engineers was devoted to the presentation of two papers dealing with the theory and experimental performance of special aerial array systems for short and ultra-short radio waves.

On estimating the total output of a curtain array of aerials and the current distribution among its members

This paper starts by calculating the electric field of an infinite sheet of cophased current and also the field of a flat grating of N equally spaced wires of infinite length. It shows that the field of the grating approximates closely to that of an infinite sheet at near-by points and approximates to that of N concentrated currents at distant points: thus the wave front starts nearly plane and merges gradually into a cylinder. Section 4 shows that the field close to a single Franklin aerial approaches rapidly to that of a filament of infinite length and Section 5 shows that the field close to a “broad-side curtain array” approaches rapidly to that of a grating of filaments of infinite length.Having established that a broad-side array, having the dimensions commonly used in practice, can be validly represented by an idealized grating the author shows that the total output of such an array can be measured by means of a single monitor aerial. The monitor aerial may be placed at any distance less than a wavelength from the plane of the array, but it should be near the ground and near the middle of the panel and in a plane mid-way between two consecutive wires.Then the problem is considered of using a monitor aerial to test whether successive members of the curtain are carrying similar currents: it is shown that the response of a monitor aerial is unduly sensitive to phase and insufficiently sensitive to the magnitude of the current in the member to which it is closest. It is shown that equality of loading should be tested by a monitor loop and not by a monitor aerial, though it is recognized that the use of a loop involves serious instrumental difficulties at wavelengths less than, say, 5 metres. An example is given to show how maladjustments of phase can be located by systematic analysis of the inequalities in response of a monitor aerial which is moved parallel to the plane of a curtain array. In short, a monitor loop is a suitable device with which to test equality of loading, and a monitor aerial to test equality of phase.

Empire Broadcasting

SINCE the opening of the Empire Broadcasting Station at Daventry on December 19 (see NATURE, 131, 16, Jan. 7, 1933) the British Broadcasting Corporation has received a large number of cablegrams reporting reception of the transmissions in all parts of the world. A very large number of letters has also been received from listeners, and extracts from some of these are published in recent issues of World Radio, which is now the official organ of Empire broadcasting. Good quality reception is reported from such places as Bagdad, the Federated Malay States, Zululand, Tanganyika, various parts of India, New Zealand, and North and South America. In some places, particularly South Africa, local atmospheric conditions have marred the reception to some extent, but on the whole it would appear from the first few weeks' tests that the inauguration of the new Empire broadcasting service has been a conspicuous success. The aerial arrays and the transmitters for the Empire Broadcasting Station at Daventry were designed, constructed and installed by Messrs. Standard Telephones and Cables Ltd.

Marconi Beam Stations for Shanghai

NEW Marconi transmitting and receiving stations are to be erected near Shanghai for the operation of the proposed short-wave beam services between China and Europe and the United States. A unique feature of the installation will be the inclusion of auxiliary Marconi apparatus enabling the transmitters to be utilised for broadcasting services when they are not in use for telegraphic communications. The auxiliary apparatus for this purpose consists of a modulating equipment which can be connected to either of the telegraph transmitters to provide telephone signals of broadcasting quality. A complete set of Marconi studio equipment is also to be supplied to enable the service to be conducted in accordance with the most modern practice. The aerial system of the transmitting station is of particular interest. For the commercial telegraph services to Europe, of which the principal is the service to Great Britain, two bays of beam aerials, accurately oriented to concentrate their signals on the receiving stations, are to be erected, one being tuned to the wave-length of 17 metres and the other to 26 metres, the wave-lengths allotted to these circuits. A third beam aerial array will be directed on San Francisco for the American service. In addition, there will be four omni-directional aerials, one of which will be used for broadcasting. The others are provided to afford the station the maximum flexibility in the range and extent of its telegraphic services. At the receiving station four high-speed commercial service receivers of the Marconi beam type are to be installed, with two beam aerial systems directed on Europe and two on San Francisco. Four omni-directional receiving aerials are also to be supplied for the reception of short-wave signals from other countries with which beam services are not required. On the completion of these stations near Shanghai a direct commercial wireless circuit will be provided for the first time between Great Britain and China.