November io, 1923] 



NA TURE 



691 



a single rotating coil directly receiving the energy of 

 the waves. Thus the Bellini-Tosi system is in theory 

 exactly equivalent to the ideal single turn rotating 

 loop. The system, as in case of the single frame coil, 

 is liable to a certain amount of " antenna " efifect. It 

 is also necessary in erection for care to be taken that 

 the similarity of the two loops and their circuits is 

 ensured, and that the planes of the loops are accurately 

 at right angles. 



In the Robinson system two coils, which differ as 

 regards their area -turns, are fixed rigidly at right 

 angles and pivoted about a vertical axis. The coils 

 are connected in series and so arranged that the 

 direction of winding of one of the coils can be reversed 

 with regard to the other by means of a switch. In 

 this way the electromotive force induced in the former 

 coil can be added to or subtracted from that induced 

 in the latter. When the coil with the larger area-turns 

 is placed in the minimum position for signals induced 

 by the arriving waves, the smaller coil is in the maxi- 

 mum position. In this position, on throwing over the 

 switch from one position to another, no change of 

 signal strength will be heard in the telephones attached 

 to the receiving circuit, and the larger coil will be per- 

 pendicular to the direction of travel of the waves. 

 Consideration of the theory of this system shows that 

 the operation of the reversing switch is really equivalent 

 to swinging a single frame, or the search coil in a 

 Bellini-Tosi installation, through an angle on either 

 side of the minimum position. The amount of this 

 equivalent angle of swing depends on the ratio of the 

 area-turns of the two coils of the Robinson system. 

 This ratio is usually arranged so that for good sensi- 

 tivity this equivalent angle is 20° to 30°. 



Since, in using the system, the equivalent coil is 

 swung to positions 20° to 30° on either side of the 

 minimum by the action of the switch, the received 

 signals are not reduced to zero intensity. The system 

 is therefore suitable for use where the finding of the 

 zero position is difficult through extraneous noises or 

 interference, and it is, accordingly specially adaptable 

 for use in aircraft. 



All the three systems of direction finding are liable 

 under certain conditions to errors which may be classed 

 under three heads : (a) variable errors arising from 

 causes influencing the direction of travel of the waves 

 during their propagation through space, (p) errors due 

 to the effect of the local surroundings of the receiving 

 station, (c) instrumental errors. 



A discussion of the three systems of direction finding 

 as regards their basic principles and as regards their 

 liability to the above three classes of errors has recently 

 been published as Special Report ^ No. i of the Radio 

 Research Board under the Department of Scientific 

 and Industrial Research. The conclusion reached in 

 this report based on a large number of careful observa- 

 tions is that each system is liable to errors of the same 

 order of magnitude. The errors due to the location 

 of the directional finder can be avoided, however, by 

 careful selection of the site of the station, while instru- 

 mental errors can be reduced to practically negligible 

 amounts by suitable design and arrangement of the 



' A (li«;u<^i-)n of the practlMl nyitem* of Dirccli.in Fin lin.< bv Rorcpiion, 

 Dr. R. L. Smi;h Riw, an I R. H. BirAold (RiJi) Ko^jrch Iliard Sppcial 

 Kepjrt No. I), pjbti'ihcd by H.M. Slationsry O.fiaj. Pii.-^ 9/. net. 



Xf). 2819, VOL I 12] 



apparatus. Until recently it appears to have been 

 generally held that observations with undamped waves 

 were more liable to error than those with damped 

 waves. Experiment has shown, however, that the 

 errors' observed were occasioned by the heterodyne 

 method of recef)tion of undamped waves, and that 

 they may be eliminated almost entirely by careful 

 screening and arrangement of the heterodyne with 

 regard to the receiving apparatus and aerial system. 



The variable errors, falling under class (a), however, 

 present much greater difficulty and so far no means of 

 eliminating them have been found. Since they are 

 introduced by changes in the direction of travel of the 

 wave front during propagation the explanation of the 

 factors which give rise to them is to be sought in the 

 study of the propagation of waves. The fact that all 

 the systems of direction finding are in their essentials 

 equivalent to rotating the single turn rotating loop, is 

 of great importance in this respect because in con- 

 sidering the effect of different wave fronts it is only 

 necessary to consider the behaviour of the simplest 

 type of aerial (i.e. the single coil type). The results of 

 experiments carried out on one system then can safely 

 be considered as applicable to the other two. 



By any of the three methods discussed the direction 

 of the horizontal component of the resultant magnetic 

 field in the wave front can be determined. In practice, 

 however, it is the direction of the horizontal component 

 of the line of motion of the waves which is actually 

 required. Should neither the direction of travel of 

 the waves nor their resultant magnetic field be hori- 

 zontal, then the setting of a coil in the minimum 

 position for signal strength will have no necessary 

 relation to the direction of arrival of the waves, and 

 errors will be recorded by all three systems. On the 

 other hand, provided the resultant magnetic field 

 remains horizontal, the wave front may be inclined at 

 any angle ; or again, provided the wave front remains 

 vertical, the resultant magnetic field may have any 

 angle therein, without causing errors to occur in the 

 observation of the direction. The variable errors are 

 far greater by night than by day. The variation 

 produced may arise very suddenly and the observed 

 bearing may change at the rate of several degrees a 

 minute, or the deviation in the bearing may remain 

 steady for a considerable period. The magnitude of 

 night variations, which are far greater than those due 

 to location or to instrumental errors, may be judged 

 from the following observations recorded in the Special 

 Report of the Radio Research Board already referred 

 to. In one series of experiments where ol«ervations 

 were carried out with a Standard Robinson set and a 

 portable type ik>llini-Tosi set, erected in the same field 

 at Slough, on various fixed transmitting stations 

 employing waves between 2000 and 5000 metres, the 

 maximum variation for Karlsburg observed with the 

 Robinson set was 27*1° and with the Bellini-Tosi .set 

 28°, for Moscow 9'9°, 9*2°, for Coltano io"8'^ and 7*2° 

 respectively. In another series of experiments, on 

 waves of 2000 to 9000 metres, carried out at Orfordness 

 with a permanent Bellini-Tosi apparatus and a standard 

 Robinson set, the total variations at night ranged on 

 various occasions from 5*0° to 54° for the Bellini-Tosi 

 set and 5*2° to 51 '3° for the Robinson set. 



It should be remarked, however, that these large 



