US1947021A - Directional aerial - Google Patents

Description

Feb. 13, 1934. w. VAN B; ROBERTS DIRECTIONAL AERIAL Filed March l, 1929 @we/woz -ER VAN B. ROBERTS Patented Feb. 13, 1934 LM'LZI DmEc'noNAL AERIAL Walter van B. Roberts, Princeton, N. J., assigrior to Radio Corporation of America, a corporation of Delaware Application March 1, 1929.- Serial No. 343,710

2 Claims.

In radio direction finders known heretofore of the type which use a rotatable loop aerial much diiculty has been encountered in obtaining a system in which at zero response in the receiver energized thereby, the loop aerial will be pointing directly toward the source of energy.

This inaccuracy in true pointing position is due in part to distorted wave front and in part to characteristics inherent in the apparatus used such as, for instance, l-out of phase currents caused by the vertical aerial or electrostatic effeet of the signal voltage on the tuned loop when the loop is unsymmetrical with respect to ground. This eiect is present whether the wave front is distorted or not; 2-currents due to signal voltage across a loop of some axial length caused by a distorted wave front having horizontal component of the electric field. This voltage is in phase with the electrical eld and the amount thereof changes as the loop is rotated, and the current set up thereby is either in phase or 180 out of phase with respect to the normal loop eiect thereby distorting the normal loop characteristic curve. This eiect is present whether the loop is symmetrical or not, and results in shifting the point of zero response so that the loop does not point directly toward the source of energy when signal response is minimum. Aerial systems of the above type are subject to various other inherent defects caused either by unsymmetrical apparatus, or thereby when eected by unsymmetrical signal waves, however, those pointed out above will serve for purposes of illustration.

Heretofore various means have been provided to compensate the various errors above referred to by adding current in the proper phase to more or less compensate the error causing currents introduced in each of the above mentioned ways. Such means known heretofore has necessitated the use of considerable additional apparatus.

The primary object of this invention is to provide a new and simple method of correcting the null points of a loop shifted from true pointing position by the effect of a component of the electric eld along the axial length of the loop.

- Another object of this invention is to provide a loop in combination with simple means for compensating the error therein which once adjusted will remain permanently adjusted for all wave lengths received on the loop.

A better understanding of the invention and the operation thereof will Vbe had from the following detailed description of the inventionand therefrom when read in connection with the annexed drawing throughout which like references indicate like parts and in which Figure 1 illustrates how the horizontal component of the electric field acts on a tuned coil having appreciable length;

(Cl. Z-33) Figure 2 shows a method of winding an inductance coil to overcome the difficulties encountered with apparatus as illustrated in Figure 1;

Figure 3 shows novel correcting apparatus arranged to correct the error in the coil of Figure 1 in accordance with the present invention; while Figure 4 shows a receiving circuit connected with an aerial system which includes my novel method of reducingerror applied to a balanced frame aerial ,and a vertical aerial coupled to said frame aerial to obtain in the receiver a cardioid characteristic curve.

Referring to the drawing and in particular to Figure 1 which shows a solenoidal coil or loop 1 tuned by means of a variable capacity 2. When the axis X of the coil coincides with the direction of propagation of the signals we would normally have no current produced in the coil. But, if the wave front is distorted as indicated by the heavy line W F of Figure 1 in addition to the expected vertical electric field which is perpendicular to the axis of the coil there would exist an electric component along the flrectionof signal propagation, and then when the axis of the coil coincides with the direction of signals a considerable current will be produced by this horizontal electric component. This is, of course, due to the fact that the incoming wave which tends to vary the potential of all objects it meets, hits one end of the coil rst and thus causes a diierence of potential between the ends of the coil. This diiference in potential in turn sets up currents. As a result such a coil generally does not give .a true indication of direction at its position of minimum signal response. The object of the present invention is to provide a simple means for compensating and eliminating the error introduced in the above manner.

The difficulty may be obviated by using two coils wound in opposite directions with their axis parallel as shown in Figure 2. The above mentioned trouble does not exist in this case because when an incoming electric field is in the direction of the axis of the coil itself it strikes both ends of the tuned circuit at the same instant, thus maintaining the potentials at the two ends equal and hence producing no current even though the eld of the approaching wave is distorted. Figure 2 is only one of a number of symmetrical arrangements which may be used to avoid the trouble.

It is possible however, to cure the trouble without going to the extra complication of making the structure symmetrical. A modication showing an arrangement for eliminating such errors in accordance with the present invention is illustrated in Figure 3. In this modification two conductors 3 and 4 from the right and left sides respectively of coil 1 are run out to receive compensating differences of potential. The length of the conductors are determined-by trial and error and'once determined for a certain Vcoil need not be again altered. It will beV seen that theY incoming wave strikes the conductor 4 extending thus offsets the fact that the wave strikes `the right hand end of the coil first. Likewise a wave approaching from the left 'of Figure 3V will strike the conductor 3 first and thereby compensate the effect which would otherwise be present in the coil due to the fact that the wave strikes the left hand end thereof first. v

My invention is preferably used in connection withY other means known in the art for convenientoperation of direction finders including auxiliary means fordetermining the sense as well as direction of the source of radiant energy.

In Figure which shows the at present preferredembodiment of my invention the loop antenna 1 is provided with crossed electric pickup conductors 3 and l provided for the purpose of eliminating error normally caused by the current due to horizontal components of an electric field. The length of these pickup conductors is ascertained by trial and error for a particular loop and operate in the same manner in which the conductors 3 and 4 of Figure 3 operate. vSense of direction is obtained by means of a'vertical aerial 5 tuned by a variable capacity 6, coupled through a series inductance '7 to an inductance 8 in series with the loop circuit 1, 2. By tuning the aerial 5 and adjusting the coupling rbetween coils 'l and 8 current ldue to the vertical aerial eiect in Y1 can be introducedin the loop circuit in proper Yphase and amplitude to combine with the current therein to obtain a cardioid characteristic curve in the receivers. Energy from the aerial system is fed to the input elements'of a receiver R by means o1 a tuned inductance 9 variably coupled to an inductance 10 in series with the loop 1. To prevent the vertical component of the electric field of the wave from setting up circulating currents in the tuned circuit the entire loop circuit including the coupling inductances is symmetrically balanced. This is attained by providing an inductance 11 electrically equivalent to the inductance 8 but in opposition thereto in series with the leads 12 and 13 connecting the inductances 10 and 8 in series with the loop l. To further insure the complete balance of the apparatus the electrical midpoint of the inductance 10 is connected to ground through a connector 14.

For the sake of a more quantitative description of the various currents which may be produced in theV tuned circuit 1, 2 of Figure 4 before the compensating electric pickup adjustments have been made and the arrangement `has been properly balanced with respect to ground, I will tabulate them as follows:

1. The normal loop eect This is a voltage proportional to the rate ofV 2. Ejectrof horizontal component of electric field This is proportional to the electriceld andY to the cosine of the angle between the loop axis and the signals and produces a current 90 out of phase with the voltage because the voltage is applied across the tuned circuit rather than in seriesrin it'. Thus, the current is in time phase from the left end of the coil to the right rst and with the normal loop effect current and as a result makes the zero position of the loop fail A to coincide with .the direction of signals.

` 3, Effect of vertical electric field If the loop is not grounded at its electric cen- -ter the vertical eld will produce current in exactlythe same fashion as discussed under item 2. However, since the vertical field will act the same *.way, whatever direction the signals are travelling the current produced will be constant and its phase'will be the same as the phase of the currents in items 1 andv2.

4. The eect of the tufted 'vertical aerial on the loop If the auxiliary open antenna is tuned to the signal the current in its coil will be in phase with the electric iield and hence the magnetic eld of its coupling coil 5 will Vbe inthe same phase as the magnetic eld of the incoming signals. Thus, the current'V in the loop circuit produced by the coupling inductance in the open antenna Y 5 will be in phase with the normal loop current and by adjusting the coupling to such a value that the two currents aremequal in amplitude a cardioid diagram is obtained. Y

If this cardioid diagram is to be a perfect one, that isLif the system is to have a true pointing position it is, of course, necessary that the two adjustments for wiping out currents produced by the electricxeld,` items 2 and 3 above, should have been made, namely, the grounding of the loopvcircuit at` such a point that no circulating currentis produced by the vertical electric field, and Vapplying `the compensating electric pickups as disclosedin this invention to insure that any `horizontal component lof the electric field will produce no circulatingcurrent in the tuned circuit. 'if Y After the Vaerial has been installed' andthe length of the conductors ascertained by trial using signals from a known source, the operation of 4this vsystem is the same as the operation of direction finders of this type known heretofore. Having fully described my'invention and the operation thereof, I claim: 'f

ll. In a directional aerial system the'combination of a loop `aerial of appreciable axial length,

and apair of crossed openended linear conductors parallel with the axis of said loop, opposite ends of. said conductors beingfconnected to -the ends of said loop aerial; Y f `2. A Ydirectional aerial system comprising: a frame aerial, a'vertical aerial coupled thereto, means for insuring therelectrical symmetry oi said loop including a connection between the electrical Ymidpoint of said loop and ground, a receiver coupled to said aerials, and means including a pair vof crossed linear conductors parallel to' the axis of Vsaid loop and connected to opposite ends of said frame aerial for preventing the horizontal` component of a distorted wave front from setting up out of phase currents in said frame aerial.

WALTER VAN B.- ROBERTS.

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