摘要:

The paper deals with the experiments carried out in 1943–44 to investigate the use of an earth screen or mat to reduce the polarization errors of the U-type Adcock direction-finder. These errors arise from the introduction of signal voltages into the system by the horizontally polarized component of the incident waves, when the aerial feeders are laid on, or buried in, ground of poor conductivity. The polarization error of a standard form of direction-finder, erected in the normal manner on highly conductive ground (3 × 108e.s.u., or about 3 × 103ohm-cm resistivity) at the Radio Research Station, Slough, was measured over the frequency range 3–10 Mc/s, with the aid of a local transmitter which radiated at controllable angles of incidence and polarization. An earth mat, about 31 m in diameter and of a square mesh between 0.3 and 1 m, was then installed at the direction-finder, and the measurements were repeated for various experimental arrangements of this installation.When the mat was raised above and insulated from the ground, the polarization error of the direction-finder was seriously increased; but when the mat was effectively earthed (both around its perimeter and at an intermediate radius) the error was slightly reduced. In place of the direct earth connection, a similar result was achieved by attaching radial wires to the perimeter of the mat, the length of the wires being selected to resonate at the frequency in use. By using a combination of 108 wires of 11, 15 and 25 m lengths, arranged so that there were 36 of each length, the desired reduction in the polarization error was attained more or less uniformly over the whole of the 3–10 Mc/s range.Additional experiments were carried out at Martlesham on ground of such poor conductivity (107e.s.u., or about 105ohm-cm resistivity), that direct earth connection to the mat was ineffective. The results confirmed the previous deduction that the attachment of radial wires of suitable length and disposition makes the earth mat effective. It was also shown that both mat and radial extension wires may be laid on the ground if desired. Furthermore, it was shown that, with the mat in position, it was no longer necessary to bury the aerial feeders, but was sufficient to lay the feeders on the ground just under the mat, or along the surface of the mat, if the feeder screen was well bonded to the mat. An additional advantage of the use of such an arrangement is that it smooths out the effects of any irregularities in the ground at the site, and produces a corresponding improvement in the performance of the direction-finder.Comparison of observations made at the same site with two direction-finders, one equipped with an earth mat and one without, showed that when taking bearings on signals from stations at distances ranging from 200 km to nearly 800 km, the expected appreciable improvement in overall performance due to the mat was attained.

DOI:10.1049/ji-3-2.1947.0022

出版商:IEE    

年代:1947

数据来源: IET

摘要:

This paper discusses the principles of design of spaced-loop radio direction-finders and describes a practical instrument embodying these principles. The dimensions of the model described are: size of loops, 1 m square; spacing between loops, 3 m. It has been designed for the frequency range within which the spaced-loop direction-finder can have most useful applications, namely 3–15 Mc/s (λ 20–100m). The sensitivity of the instrument is such that for an arc of silence not exceeding ±5 deg, the required field strength, for a ground wave, varies from 1.5 to 4 μV/m throughout the range. The direction-finder is provided with a simple and effective means of determining the sense of the bearing, operating on novel principles.To obtain the high accuracy of which the spaced-loop radio direction-finder is capable, care must be taken that the local surroundings of the aerials do not contain features capable of introducing errors of various types. In particular, attention must be paid to the proper disposition of power or telephone cables.In the Appendices, the effects of the essentially non-uniform character of the current distribution in loop aerials are considered, and it is shown that certain arrangements of spaced-loop aerials may be very inaccurate on account of a type of polarization error thus introduced.

DOI:10.1049/ji-3-2.1947.0023

出版商:IEE    

年代:1947

数据来源: IET

摘要:

The paper deals with the results of a series of investigations into the errors observed in short-wave direction-finding on wavelengths of 20–50 m 6–15 Mc/s). It is shown that the errors observed on a transmitter at a distance of several miles vary rapidly and erratically with azimuth, wavelength and distance. In some cases a change of bearing of 2–3 deg has accompanied a change in azimuth of 0.5 deg or a change of wavelength of only 1%. It was also found that for two different sites the average numerical error and the (statistical) probable error were roughly proportional to the wavelength. For one site, the probable error on a wavelength of 50 m (6 Mc/s) was 1.7 deg and on 30 m (10 Mc/s), 0.8 deg. A hypothesis is put forward to explain these results on the basis of an assumption that the errors are due to re-radiation from a large number of reflectors scattered at random over a considerable area around the direction-finder. The implications of such a theory are discussed, and it is deduced that for average sites the probable error will have a maximum value on a wavelength of about 100–150 m.Some tests, carried out with a portable direction-finder, show that even on apparently ideal sites, the local errors are not always negligible.

DOI:10.1049/ji-3-2.1947.0024

出版商:IEE    

年代:1947

数据来源: IET

摘要:

A description is given of a simple U-type rotatable radio direction-finder, which was used to investigate the improvement in performance of short-wave direction-finders obtained by the use of large earth screens, and to determine the smallest ratio of screen-size/wavelength for which the polarization errors were reduced to a satisfactorily small value (≃1 deg).The screens were constructed of sheet metal, wire netting and radial wires, and the results showed that provided a metal sheet was not less than 4λ in diameter, polarization errors as small as 1 deg could be attained. For wire-netting screens of the same diameter, a mesh size not greater than ½ was required to attain this performance.

DOI:10.1049/ji-3-2.1947.0025

出版商:IEE    

年代:1947

数据来源: IET

摘要:

The nature of the radiation from a lightning flash is discussed, and it is shown that the main source of inaccuracy in the crossed-loop cathode-ray types of direction-finder at present in use by the Meteorological Office is due to polarization errors. Various methods are then considered for improving the accuracy of thunderstorm location, and the conclusion is drawn that a cathode-ray direction-finder, using a spaced-aerial system and with brilliance modulation applied to the display tube, is likely to be more successful than other systems. A description is then given of a direction-finder, having greatly improved instrumental accuracy, which has been designed as an experimental tool for the development of the improved method of location.

DOI:10.1049/ji-3-2.1947.0026

出版商:IEE    

年代:1947

数据来源: IET

摘要:

A description is given of an experimental spaced-loop direction-finder for use with vertically polarized waves in the frequency range 30–100 Mc/s (10-3 m). The screened loops are 28 cm square and are mounted coaxially with a spacing of 1.57 m. The sensitivity of the system in conjunction with a good commercial receiver is such that a field strength of 20 μV/m is required for a silent swing of ± 5°. Means are provided for selecting the required minima of the polar diagram. Instrumental errors are liable to become excessive at the resonant frequencies of the aerial system or its screen, but may be minimized by careful construction and adjustment of the system. Reciprocal error can be reduced to less than ½ deg at all frequencies in the above range.No satisfactory method exists for measuring any small residual polarization errors of a spaced-loop system, because the curvature of the wavefront from a local transmitter introduces polarization errors which would not be present with a distant transmitter. Errors measured with a local test transmitter at an angle of elevation of 25° with a dipole aerial inclined at 60° to the vertical have been compared with calculated errors due to wavefront curvature alone. It is estimated that there were no instrumental polarization errors in excess of 2 deg in the frequency range 30–100 Mc/s under the above test conditions.A brief account is given of a more sensitive system with loops 92 cm square, designed for a small frequency range around 30 Mc/s (10 m). In the Appendix an analysis is made of the polarization errors due to the proximity of a local test transmitter.

DOI:10.1049/ji-3-2.1947.0027

出版商:IEE    

年代:1947

数据来源: IET

摘要:

Each line circuit is marked with signals directly indicative of its number. These are applied through two non-linear resistors which function as static switches and offer a much higher resistance to the direct biasing voltage than to the superimposed 750-c/s signals.The two resistors, which are connected to each line, are interconnected in the form of a co-ordinate system, which provides an almost static marking distribution.The signals pass along the connection to the point requiring the identification, where they are decoded by a standard 2-v.f. receiver and an all-relay distributor. The equipment functions satisfactorily at 50 impulses/sec, making it possible to identify any subscriber's line on a 10 000-line exchange in less than half a second.

DOI:10.1049/ji-3-2.1947.0032

出版商:IEE    

年代:1947

数据来源: IET