DOI:10.1049/pi-3.1950.0001

出版商:IEE    

年代:1950

数据来源: IET

摘要:

A critical examination is made of the somewhat loose and incomplete statement that a polar diagram is the Fourier transform of an aperture distribution. By aperture distribution it is necessary to understand, in the two-dimensional case, distribution across the aperture of the component along the aperture plane of the electromagnetic field in the plane of propagation. Furthermore, the concept of the polar diagram has to be replaced by that of an angular spectrum, except in the common case when the aperture may be considered more or less limited in width, and the field is being evaluated at a point whose distance from the aperture is large compared with the width of the aperture (and the wavelength). For example, it is convenient for some purposes to regard the problem of diffraction of a plane wave by a semi-infinite plane screen, with a straight edge, as a problem about an aperture distribution in the plane of the screen. This is a case for which the concept of a polar diagram is not in general applicable, and has to be replaced by that of an angular spectrum. The field at all points in front of a plane aperture of any distribution may be regarded as arising from an aggregate of plane waves travelling in various directions. The amplitude and phase of the waves, as a function of their direction of travel, constitutes an angular spectrum, and this angular spectrum, appropriately expressed, is, without approximation, the Fourier transform of the aperture distribution. If the aperture distribution is of such a nature that the concept of the polar diagram is applicable at sufficiently great distances, then the polar diagram is equal to the angular spectrum. But the angular spectrum is a concept that is always applicable, whereas the polar diagram is one that is liable to be invalid (for example, in the Sommerfeld theory of propagation over a plane, imperfectly reflecting earth).

DOI:10.1049/pi-3.1950.0002

出版商:IEE    

年代:1950

数据来源: IET

DOI:10.1049/pi-3.1950.0003

出版商:IEE    

年代:1950

数据来源: IET

摘要:

A new way of visualizing the Sommerfeld theory of propagation over a flat, imperfectly reflecting earth is presented. The Sommerfeld theory arises because the ray theory of propagation from a source in the presence of a flat, imperfectly reflecting earth is only an approximation. The ray theory involves the assumption that the Fresnel reflection coefficient of the earth does not vary rapidly with angle of incidence, and this assumption is not satisfied for glancing incidence of vertically polarized waves on the earth's surface at broadcasting wavelengths. The main object of the new presentation is to facilitate the solution of problems involving propagation near the surface of the earth partly over land and partly over sea, but these applications are not included in the paper.It is convenient to think of a two-dimensional problem in which the transmitter is a line source parallel to the earth's surface, having a vertical polar diagram of circular shape. Such a source may be Fourier analysed into plane waves whose directions are distributed in a vertical plane of propagation perpendicular to the line source; the amplitudes of all the plane waves are the same and they are in the same phase at the source. When these waves are reflected from the earth they produce an angular spectrum of reflected waves, the amplitudes and phases of which are determined by the Fresnel reflection coefficient. This angular spectrum could be thought of as arising, in the absence of the earth, from an aperture distribution on the vertical plane through the primary line source. The aperture distribution that produces the angular spectrum of reflected waves in this way is the exact image of the primary source in the imperfectly reflecting earth, and is given by the Fourier transform of the Fresnel reflection coefficient. For a perfectly conducting earth this aperture distribution reduces to a line source identical with the primary source and located at the optical image line. The correction required to this when the earth is not perfectly conducting is mainly the following. An aperture distribution extending indefinitely downwards from the image line must be introduced, and this consists essentially of the aperture distribution produced by diffraction of the Zenneck wave under a screen extending from the image line upwards. The field produced by the primary source in the presence of the imperfectly reflecting earth is thus the field that would be produced with an almost perfectly conducting earth, together with the field arising from diffraction of the Zenneck wave under the image line.If diffraction of the Zenneck wave under the image line is calculated by the edge-wave approximation we merely arrive at the ray theory of reflection from the earth of radiation from the primary source: the edge wave from the image line, together with the wave from an image in an almost perfectly conducting earth, makes up the wave from the Fresnel image for the imperfectly reflecting earth. But, at broadcasting wavelengths, points close to the earth are often too close to the shadow edge, formed by diffraction of the Zenneck wave under the image line, for application of the edge-wave approximation. We then have to apply the full theory of edge-diffraction based on the Cornu spiral, and this gives the Sommerfeld theory.

DOI:10.1049/pi-3.1950.0004

出版商:IEE    

年代:1950

数据来源: IET

DOI:10.1049/pi-3.1950.0005

出版商:IEE    

年代:1950

数据来源: IET

摘要:

After describing the essential parts of a concentric-line impedance-measuring apparatus, the author discusses a number of factors which give rise to discontinuities. Three methods of measuring discontinuities in a lossless system are given. It is shown how equivalent networks may be derived to represent non-uniform lengths of line, and how errors in the measurement of unknowns, caused by the presence of discontinuities, may be calculated. In particular, the effect of discontinuities on three types of measurement is described, namely the measurement of resistance and reactance, of conductance and susceptance and of the electrical characteristics of cables. A recommended technique for the measurement of unknowns is given.

DOI:10.1049/pi-3.1950.0007

出版商:IEE    

年代:1950

数据来源: IET

摘要:

The “earth-plane” measuring technique, together with a coaxial-type standing-wave indicator, was used to measure the input impedance near half-wave resonance of simple centre-fed slot aerials of various widths and of two inductively loaded (or dumb-bell type) slot aerials. The impedance characteristics of an end-fire slot aerial over an approximately 3/1 frequency range were also determined.

DOI:10.1049/pi-3.1950.0008

出版商:IEE    

年代:1950

数据来源: IET

摘要:

A theory of metallic delay dielectrics, which is more accurate than existing ones and is based on an analogy with shunt-loaded transmission lines, has been developed for the simplest case, when the delay medium consists of an array of infinitely long conducting strips. Formulae have been obtained for the refractive index, impedance and cut-off wavelength of such a medium, and the more important results are presented graphically. A procedure for designing such dielectrics to have a desired refractive index is outlined, and some limitations in the use of metallic delay lenses are pointed out.

DOI:10.1049/pi-3.1950.0009

出版商:IEE    

年代:1950

数据来源: IET

摘要:

A Calibration Centre has been set up in the Royal Air Force to maintain in calibration the test equipment used on radio and radar equipment. The main object is to provide all aircraft and R.A.F. stations with electrical standards that are known to agree with each other. Part 1 of the paper discusses the general principles governing the design of the Centre, and Part 2 gives some details of the methods and apparatus used.

DOI:10.1049/pi-3.1950.0010

出版商:IEE    

年代:1950

数据来源: IET

摘要:

The paper reviews the development of terminal insulation as used on capacitors over about three decades, and shows the gradual increase of appreciation of the need for sealing them against the ingress of moisture. Such sealing is needed in general to maintain their electrical properties unimpaired over long periods of time. Part of these improvements in sealing has evolved from the users' general need for improved quality, and part has arisen from the improvement in electrical characteristics of the capacitors themselves that has taken place over the same period, which has automatically necessitated the better sealing to retain those better characteristics. A classification of the various forms of sealing is suggested, which is partly chronological and partly based upon the form and nature of the sealing medium; while a Bibliography is included to substantiate the chronological aspect of the review.

DOI:10.1049/pi-3.1950.0011

出版商:IEE    

年代:1950

数据来源: IET