The purpose of a radio direction finder (df) is to measure the direction of arrival of a radio signal. Traditionally the azimuth angle of arrival, i.e., the angle of arrival in the horizontal plane, has been considered sufficient to define the direction

Direction-Finding Antennas

The paper sets out to show how the more fundamental problems encountered in the development of the commutated-aerial direction-finding system have been solved during a long-term development plan. Reasons are given for the choice of a circular array of aerials, which are commutated sequentially to the receiving equipment. It is shown how such an arrangement permits great economy in the number of aerials required for a given system performance, maximum adjacent aerial spacing being much larger than might at first appear permissible, and the method for computation of the signal bearing is described in some detail. Significant features of the development are described, particularly where problems are peculiar to the system, such as realization of adequate selectivity despite aerial commutation and the avoidance of the effect of aerial interaction. Final practical circuits are described for signal processing and demodulation, and for the display of bearing, and it is shown that the solutions adopted should permit realization of the performance predicted by theoretical study some 10 years ago. Although practical experience in the field is not yet large, adequate practical measurements have been made to substantiate most of the theoretical predictions with regard to performance.

The practical evolution of the commutated-aerial direction-finding system

An account is given of the progress in naval radio direction-finding both on land and in H.M. Ships during the last ten years.On land, the Admiralty has been specially concerned in highfrequency (3–30 Mc/s) direction-finding. Some parts of the very-high-frequency range (30–300 Mc/s) are also of interest, notably in the band used for communication with fighter aircraft, 100 to 156 Mc/s. In the field of h.f. direction-finding, the factors limiting the accuracy at present obtainable with the standard Adcock-type apparatus are critically reviewed. Brief descriptions are given of several new systems in h.f. direction-finding on land, in which it is suggested further research would be profitable.The paper describes further developments in ship-board m.f. direction-finding, and the research and development work carried out in the h.f., v.h.f., u.h.f. (300–3000 Mc/s) and s.h.f. (3000–30000 Mc/s) bands.It is shown that accurate direction-finding on high frequencies in ships offers considerably greater difficulties as compared with those encountered on m.f. This is due to the greater disturbances caused by the secondary fields radiated by various parts of the ship's structure and rigging. It is also shown how the essential conditions have been derived for siting the direction-finder aerial in a position where the effects of secondary radiation will not excessively degrade the performance. The paper considers the principles of design employed in the ship-board h.f. direction-finder and explains how the various requirements of accuracy, sensitivity, coverage of the whole h.f. band with a minimum of design and operating complexity, reliability, sensefinding and quickness in operation, involve a compromise in which the highest performance in any one particular is unattainable. The essential performance data are given for the apparatus widely fitted during the recent war.The various aspects of research and development in v.h.f. directionfinders for ships are considered and it is shown that the ship conditions are less adverse in the v.h.f. case than they are in h.f. direction-finding.In keeping the paper to a reasonable length, only a brief introduction is given to the work done in the wide field of u.h.f. and s.h.f. directionfinding. Many of the techniques in these frequency bands merge with those of radar, with the important difference that direction-finding must always attempt to cover the widest possible frequency range with the simplest possible apparatus.The applications of radio direction-finding to naval purposes are briefly stated.

Naval radio direction-finding

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.

Fundamental problems in radio direction-finding at high frequencies (3ߝ30 Mc/s)

We present a method to enhance the accuracy of platform-based, high-frequency (HF) direction-finding (DF) systems using dynamic mode selection. To improve bandwidth and efficiency, the metallic platform supporting the DF array is used as the primary radiator by employing electrically small antennas to excite linearly independent combinations of its characteristic modes (CMs). However, the radiation characteristics of these modes vary dramatically over the HF band, making it difficult to achieve consistently good DF accuracy with a fixed system. To alleviate this, we propose a dynamic mode selection strategy to achieve enhanced DF accuracy. This strategy is based on the assumption that the number of available CMs of the platform is greater than the number of available coherent receive channels. Thus, dynamic mode selection allows for choosing the optimal antenna combination to obtain the best DF accuracy at each frequency. This strategy is demonstrated for an airborne DF system employing five electrically small antennas and up to four coherent receive channels. We demonstrate the efficacy of the proposed approach using computer simulations and scaled-model experiments. Simulation and measurement results show that dynamic mode selection can significantly enhance the DF accuracy of platform-based HF DF systems using a limited number of coherent receive channels.

/pdf/performance-enhancement-of-platform-based-hf-direction-3xs7yhnwx1.pdf

Performance-Enhancement of Platform-Based, HF Direction-Finding Systems Using Dynamic Mode Selection

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.

R.M. Godfrey

Papers

Radio direction-finding by the cyclical differential measurement of phase