Adcock Direction-finder Research Articles

Studies of the Adcock direction finder in terms of phase-mode excitations around circular arrays

A new analysis of the Adcock direction finder is presented in terms of phase-mode excitations associated with circular arrays. This leads to a convenient form of processing for such a d.f. system based upon a circular array/Butler matrix configuration which employs a direct phase-measuring detector for bearing measurements. The principal sources of error limiting the physical spacing of elements within the array are highlighted and methods of improving wide-bandwidth operation are presented which entail the use of directional elements. A signal processing system based around a small microcomputer is discussed for the automatic compensation of errors due to the finite number of elements.

Determination of octantal error on a 4-element adcock direction finder

It is shown that bearings taken on a signal with a 4-element Adcock direction finder can be corrected for octantal error by means of an additional measurement made on the signal. A suitable measurement would be a comparison of the sum signals (N + S) and (E + W).

Fluctuations in continuous-wave radio bearings at high frequencies

Bearing observations have been made with an Adcock direction-finder on distant transmitters in the 3–4 Mc/s range, and the auto-correlation function of their time variation has been calculated. Curves of the form ε−τ/τ0 have been fitted to the functions obtained, the resulting values of τ0 having a mean of 0.81 sec. The results differ from these in the 6–20 Mc/s band in that the standard deviation in a group of observations is not correlated significantly with the value of τ0.

H.F. bearing variations on an Adcock direction-finder

Measurements on a number of transmitters at ranges from 1000 to 5000 km show that the variance of snap bearings within a period of half an hour is of the order of 10deg2. To reduce this variance by a factor of ten, time averaging over about 5 min is required when the bearings are taken at 10 sec intervals. Bearing changes from hour to hour and from day to day have a variance of about 1 deg2 and thus set a limit to the error reduction obtainable by averaging over a halfhour period. The results are consistent with previously published estimates of bearing variances.

On the rapidity of fluctuations in continuous-wave radio bearings at high frequencies

Bearing observations were made with an Adcock direction-finder on distant high-frequency transmitters at a rate of five per second. The auto-correlation function of the observations was calculated; the analytical formula found most useful as its representation was ε−τ/τ0, where the mean value of τ0 was 0·75sec with a standard deviation of 0·51 sec. Expressions are given for the reduction in the variance of bearing error to be obtained by time-averaging.

The calculation of wave-interference errors on a direction-finder employing cyclical differential measurement of phase

This paper is concerned with the evaluation of errors on a wideaperture type of direction-finder. The errors considered are those due to the wave interference between two rays incident on the aerial system, such as may be produced in ionospheric propagation. Expressions are derived for the mean-square error taken over all phase relations of the down-coming rays, and also over a distribution in bearing of the smaller of the two rays. In all cases a comparison is made with the corresponding figures for an Adcock direction-finder.It is suggested that, for the d.f. system discussed, an aperture of four wavelengths would be sufficiently large to give a useful reduction in the component of error variance due to wave interference, over that obtained with an Adcock direction-finder.

Rotating H-type Adcock direction-finders for metre and decimetre wavelengths

Rotating H-type Adcock direction-finders for metre and decimetre wavelengths

Rotaing H-type Adcock direction-finders for metre and decimetre wavelengths

An account is given of the development at the National Physical Laboratory of rotating H-type Adcock systems suitable for use at metre and decimetre wavelengths. Instruments of high precision for the range 26–600 Mc/s (0.5–12 m wavelength) are described, and details of their performance are given. The direction-finders at the tower end of this frequency range are of high sensitivity and have seen considerable practical service. At the higher frequencies the instruments are of an experimental nature, constructed with a view to examining the problems to be encountered in exploiting this part of the band.Development has proceeded largely on empirical lines, but from this work a fair appreciation has now been obtained of the fundamental factors affecting performance and of the practical means for taking them into account. Some progress has been made, for example, in understanding the cause of polarization errors, which are of importance in some—but not all—d.f. applications at these frequencies. The first half of the paper is therefore devoted to a discussion of these factors and to a description of certain common design features which have emerged.It is probable that, at frequencies above about 300 Mc/s, the application of ground-based simple Adcock systems will be limited by their inherently low sensitivity. This disadvantage may be partly overcome by the use of passive reflecting elements, and a system is described in which a sheet reflector is used in conjunction with an H-type Adcock aerial.

The specification and measurement of polarization errors in Adcock-type direction-finders

This critical review of the specification and measurement of polarization errors in Adcock-type direction-finders was prepared in 1944. The respective merits of the “standard-wave error” introduced by Barfield, and the “pick-up ratio” for wanted and unwanted fields suggested by several other workers, are examined. It is concluded and recommended that the former is to be used for Adcock direction-finders erected not higher than about ¼λ above ground, while the latter is appropriate for more elevated systems. Comparison of direction-finders falling into different categories can only be carried out if a complete specification of polarization error performance over all relevant angles of incidence is available.The problems involved in using a relatively nearby test transmitter are discussed. For frequencies above about 3 Mc/s it is shown that a suitable local radiator consists of a rotatable closed loop used at moderate elevations, and at distances not less than a wavelength. The effects of loop size are discussed, and it is shown that adequately large loops are practicable; for example, for frequencies in the range 3–30 Mc/s, a loop up to about 5 ft in diameter may be used, providing the distance is not less than 100 m. For frequencies below about 2–3 Mc/s it is suggested that the “local-injection” method due to Barfield may be more practicable than a radiation method.It is pointed out that the performance of a direction-finder is in general intimately bound up with the electrical properties of the site on which it is erected.

Fundamental problems in radio direction-finding at high frequencies (3–30 Mc/s)

The problems of direction-finding at high frequencies may divided for convenience into three groups, (a) those dealing with instrument itself, (b) those dealing with phenomena occurring in course of propagation of the waves, and (c) those concerned with interpretation of the bearings once the readings have been obtained.As to the instrument, the basic type of high-frequency direction-finder during the past decade has been the Adcock, using spaced vertical aerials. During the war years, attempts were made to improve performance of the simple Adcock direction-finder by the use of earth mats or arrangements of radial counterpoise wires. Some reduction of polarization error on sites of low conductivity was thereby achieved. There were advances in bearing display, particularly in regard instantaneous direct-reading types, while improvements in design components such as receivers and goniometers have resulted in all-round improvement in the performance of high-frequency ground station direction-finders.To overcome the difficulties of polarization error inherent in Adcocktype direction-finders, several types of practical spaced-loop directionfinders have been developed in recent years. These are all of the rotating coaxial-aerial type.Future improvements in instrumental design are likely to involve departure from the small-aperture spaced-aerial arrangement, and systems of wide aperture, in which the direction of the wavefront is determined by measurements made at widely separated points are forecast. Reduction of certain types of wave-interference errors, particularly those due to scattering, is likely to be achieved.Recent advances in knowledge of the propagation of waves in the high-frequency band have been concerned chiefly with ionospheric waves. A good deal of information of a statistical nature concerning the deviations suffered by these waves has been accumulated but much still remains to be done, particularly as regards a study of the characteristics and structure of the ionospheric irregularities causing deviations. During the war years various attempts to overcome some the difficulties encountered when dealing with ionospheric waves were made. Of these, the most important were the ray selection technique involving apparatus for the selection of a single ray component of complex signal) and the employment of groups or clusters of directionfinding stations in place of single stations. In the future, the use instruments of large aperture is likely to cause both of these techniques to be outmoded.Once a bearing from a single direction-finding station or a group bearings from a network of direction-finding stations has been obtained, there arises the problem of deciding what information is contained the figures as to the true direction or position of the transmitter. This is the problem of interpretation and it is essentially a statistical one. There are at least two parts to the problem, (a) to determine, a priori, the probable accuracy of the individual bearing or bearings; and (b) with this information to determine the most probable location the transmitter and establish areas within which there is a given probability of finding the transmitter. The first of these two problems is essentially the more difficult, and though there have been attempts at a solution by various schemes for “classifying” the bearings according to probable errors there is still no fully satisfactory solution. Given solution of the first, the second problem is mathematically simple, the solution being found in terms of the principle of least squares. In the future, the development of mechanical, electronic or other devices which will carry out the necessary operations automatically is likely to proceed.

A simple method of reducing the polarization error of a U-type Adcock direction-finder

A brief description of the nature of polarization error, and the historical background of earlier work and measuring techniques, introduces the instrumental problems associated with improving the performances of U-type Adcock direction-finders, particularly on sites of poor conductivity.An Army transportable U-type Adcock direction-finder, having surface feeders and an earthing system designed for speedy installation, was tested on a site of good conductivity, and compared well with similar but more elaborate systems, indicating a standard-wave error of only a few degrees. A series of tests on ground of abnormally low conductivity, over the frequency range 2–10 Mc/s, showed that a simple counterpoise of eight radial elements could have an effectiveness comparable with that of elaborate earthing arrangements of a more permanent nature.Quantitative comparisons of the effects of various earthing arrangements (buried feeders and extended earthing plates associated with Marconi direction-finders Types DFG24/2 and DFG25) are necessarily restricted to the small frequency range 2–5 Mc/s, on account of limitations which prevent the local source from simulating plane (or natural) wave conditions and also to enable a simple workable analysis, applicable only to electrically small systems, to be used. However, this lower region of the high-frequency band is a part where reduction of polarization error is particularly required.The computed performance of a simple unscreened U-Adcock direction-finder reveals that adding a feeder screen without adequate earth return at its ends, can have a deleterious effect, but with care a fair improvement over a closed loop (reduction in standard-wave error from 35 deg to 20 deg) can be achieved on an exceptionally poor site. On a good site the performance of a U-Adcock direction-finder, even without feeder screens, is far superior to a loop. Tests on an average type of unfavourable site to be found in this country, show that the use of an eight-radial counterpoise, which, on average reduces polarization error by 4 to 1, results in a performance comparable with that for a very good site.Data available from polarization-error measurements on a directionfinder with a large circular earth mat refer only to limited measuring conditions and do not allow a strict comparison, but from general considerations a greater efficacy in reducing polarization error as compared with an 8-wire radial system can be expected. The present tests, though limited, reveal fundamental points which would suggest that a layer of crushed coke might confer similar benefits to an earth mat.The simplicity of the counterpoise of eight long radials is such, however, that its use is suggested for all but the best sites, so as to make performance less dependent on weather, and because it facilitates the approach of an external cable with a minimum of disturbance. Even with the suggested coke layer its use should still be beneficial.

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

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.

Fundamental problems in radio direction-finding at high frequencies (3–30 Mc/s)

The problems of direction-finding at high frequencies may divided for convenience into three groups, (a) those dealing with instrument itself, (b) those dealing with phenomena occurring in course of propagation of the waves, and (c) those concerned with interpretation of the bearings once the readings have been obtained.As to the instrument, the basic type of high-frequency direction-finder during the past decade has been the Adcock, using spaced vertical aerials. During the war years, attempts were made to improve performance of the simple Adcock direction-finder by the use of earth mats or arrangements of radial counterpoise wires. Some reduction of polarization error on sites of low conductivity was thereby achieved. There were advances in bearing display, particularly in regard instantaneous direct-reading types, while improvements in design components such as receivers and goniometers have resulted in all-round improvement in the performance of high-frequency ground station direction-finders.To overcome the difficulties of polarization error inherent in Adcocktype direction-finders, several types of practical spaced-loop directionfinders have been developed in recent years. These are all of the rotating coaxial-aerial type.Future improvements in instrumental design are likely to involve departure from the small-aperture spaced-aerial arrangement, and systems of wide aperture, in which the direction of the wavefront is determined by measurements made at widely separated points are forecast. Reduction of certain types of wave-interference errors, particularly those due to scattering, is likely to be achieved.Recent advances in knowledge of the propagation of waves in the high-frequency band have been concerned chiefly with ionospheric waves. A good deal of information of a statistical nature concerning the deviations suffered by these waves has been accumulated but much still remains to be done, particularly as regards a study of the characteristics and structure of the ionospheric irregularities causing deviations. During the war years various attempts to overcome some the difficulties encountered when dealing with ionospheric waves were made. Of these, the most important were the ray selection technique involving apparatus for the selection of a single ray component of complex signal) and the employment of groups or clusters of directionfinding stations in place of single stations. In the future, the use instruments of large aperture is likely to cause both of these techniques to be outmoded.Once a bearing from a single direction-finding station or a group bearings from a network of direction-finding stations has been obtained, there arises the problem of deciding what information is contained the figures as to the true direction or position of the transmitter. This is the problem of interpretation and it is essentially a statistical one. There are at least two parts to the problem, (a) to determine, a priori, the probable accuracy of the individual bearing or bearings; and (b) with this information to determine the most probable location the transmitter and establish areas within which there is a given probability of finding the transmitter. The first of these two problems is essentially the more difficult, and though there have been attempts at a solution by various schemes for “classifying” the bearings according to probable errors there is still no fully satisfactory solution. Given solution of the first, the second problem is mathematically simple, the solution being found in terms of the principle of least squares. In the future, the development of mechanical, electronic or other devices which will carry out the necessary operations automatically is likely to proceed.

The use of earth mats to reduce the polarization error of U-type Adcock direction-finders

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 × 108 e.s.u., or about 3 × 103 ohm-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 (107 e.s.u., or about 105 ohm-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.

Radio direction-finding on wavelengths between 2 and 3 metres (100 to 150 Mc/s)

This paper describes a preliminary investigation of radio direction-finding on wavelengths between 2 m. and 3 m. (100 to 150 Mc./s.) with the aid of instruments fitted with rotating aerial systems similar to those used in earlier work in the band 6 m. to 10 m. (30 to 50 Mc./s.). The sensitivities of the loop and Adcock direction-finders used are such that bearings with a swing of ±5° can be obtained with field strengths of 20 and 40 μV per metre respectively. The reciprocal error of the instruments has been reduced to 1°, and the polarization error can be kept below 5° for angles of incidence and polarization to the vertical of 60°, although readjustment of the compensation arrangement is required to maintain this over an appreciable wave-band. The two direction-finders have been operated at various distances up to 30 miles from an experimental transmitter developing 100 watts on a wavelength of 3 m., and the average errors varied from about 10° at 1 mile to 3.5° at 20 miles with a vertical transmitting aerial. These errors are somewhat larger than the corresponding values found for the longer-wave band of m. to 10 m. (30 to 50 Mc./s.). When an attempt was made to use a direction-finder inside a wooden hut, the observations were found to be subject to an octantal error 2° to 5° amplitude, the value increasing as the wavelength was reduced. This error was found to be due to reflection effects from the walls of the hut, and experiments made with various wooden boards showed that it depended upon the nature of the wood and direction of the grain. A synthetic material with no grain direction was found to have only a small effect.

Radio direction-finding on wavelengths between 2 and 3 metres (100 to 150 Mc/s)

This paper describes a preliminary investigation of radio direction-finding on wavelengths between 2 m and 3 m (100 to 150 Mc/s) with the aid of instruments fitted with rotating aerial systems similar to those used in earlier work in the band 6 m to 10 m (30 to 50 Mc/s). The sensitivities of the loop and Adcock direction-finders used are such that bearings with a swing of ± 5° can be obtained with field strengths of 20 and 40 μV per metre respectively. The reciprocal error of the instruments has been reduced to 1° and the polarization error can be kept below 5° for angles of incidence and polarization to the vertical of 60°, although readjustment of the compensation arrangement is required to maintain this over an appreciable wave-band. The two direction-finders have been operated at various distances up to 30 miles from an experimental transmitter developing 100 watts on a wavelength of 3 m., and the average errors varied from about 10° at 1 mile to 3.5° at 20 miles with a vertical transmitting aerial. These errors are somewhat larger than the corresponding values found for the longer-wave band of 6 m to 10 m (30 to 50 Mc/s). When an attempt was made to use a direction-finder inside a wooden hut, the observations were found to be subject to an octantal error of 2° to 5° amplitude, the value increasing as the wavelength was reduced. This error was found to be due to reflection effects from the walls of the hut, and experiments made with various wooden boards showed that it depended upon the nature of the wood and direction of the grain. A synthetic material with no grain direction was found to have only a small effect.

The performance and limitations of the compensated loop direction-finder

The principle of both the single-dipole and double-dipole compensated loop direction-finder is described and reference is made to several patents which embody applications of these principles. A brief account is given of the theoretical and practical work carried out at Slough on the single-dipole system. It is shown that this system can in certain circumstances be made to have a considerably higher pick-up factor for a given overall area than the Adcock direction-finder but that it is limited to use on ground of good conductivity. The double-dipole system, on the other hand, is independent of ground conductivity but has no superiority over the “H” type of Adcock system as regards pick-up factor.

Analysis of the effect of scattering in radio transmission

It has been known for a long time that signals from strong commercial stations can be received in the skip zone. Early investigations with a short-wave Adcock direction-finder showed that the signals within the skip zone are in general characterized by absence of bearing. The present paper is a complete investigation into the causes of such effects. It was found that they are due to momentary irregularities and small clouds in the E region of the ionosphere. These irregularities produce scattered signals. The investigation, first carried out with the Adcock direction-finder, was later made more definite and accurate by using short impulses from highpower stations. The later investigations entirely confirmed the original deductions. The effects of such scattering on long-distance transmission, direction-finding, etc., are discussed. The phenomenon is considered to be a major factor in practically all transmissions.

The performance and limitations of the compensated loop direction-finder

The principle of both the single-dipole and double-dipole compensated loop direction-finder is described and reference is made to several patents which embody applications of these principles. A brief account is given of the theoretical and practical work carried out at Slough on the single-dipole system. It is shown that this system can in certain circumstances be made to have a considerably higher pick-up factor for a given overall area than the Adcock direction-finder but that it is limited to use on ground of good conductivity. The double-dipole system, on the other hand, is independent of ground conductivity but has no superiority over the “H” type of Adcock system as regards pick-up factor.

The calibration of four-aerial Adcock direction-finders

The paper describes how it is possible to determine the minimum radius at which a four-aerial Adcock direction-finder may be calibrated for instrumental error without introducing spurious effects due to the proximity of the calibrating transmitter to the direction-finder. It is shown that this minimum radius depends upon the spacing of the aerials, the wavelength, and the inherent accuracy of the direction-finder to be calibrated. In most cases of practical interest the minimum radius is found to be between 0.6 and 3.5 Such distances, while usually easily attainable on the short-wave band, may not be possible in the case of medium or long wavelengths.