US2252447A - Impulse direction-finding method - Google Patents

Description

Aug. 12, 1941. G. ULBRICHT IMPULSE DIRECTION-FINDING METHOD 5/200 TUBE Filed Feb. 11, 1938 mrpaLst F050.

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r m U r e h t n u G Gttorneg Patented Aug. 12, 1941 IMPULSE DIRECTION-FINDING METHOD Gunther Ulbricht, Blankenfclde Mahlow, Germany, assignor to Telefunken Gesellschaft fiir Drah-tlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application February 11, 1938, Serial No. 190,105 In Germany February 3, 1937 of this tube appears across the plate load im- 4 Claims.

This invention relates to impulse direction finders and has for its object the provision of an apparatus for and a method of increasing the effective separation between two successive impulses, such as the ground and space wave impulses in a directionfinder.

This invention will be better understood from the following description when considered in connection with the accompanying drawing, in which Figure 1 illustrates the result of the overlapping of two successive impulses,

Figure 2 illustrates an embodiment of my invention which utilizes capacitive coupling between the radio frequency rectifier and the audio frequency rectifier,

Figures 3a and 3b represent, respectively, an impulse as received and the impulse which is derived therefrom. Fig. 3c is a special case of the impulse shown in Fig. 3b,

Figures 4a and 4b represent, respectively, received and derived ground and space waveimpulses of substantial spacing and received.and derived ground and space wave impulses of small spacing such as is shown in Fig. l',

Figure 5 is a schematic drawing of an alternative embodiment of my invention which utilizes transformer coupling.

The working ranges obtainable with the impulse direction-finding methods are limited for the reason that when the major or original signals and the echo or space signals are received at a great distance from the transmitter it is no longer possible to separate the two as they occur in such rapid succession. In fact, they merge into one another and overlap partly as shown in Fig. 1 where the amplitudes of the main signal H and of the echo E are plotted against the time t.

According to the invention an increase in the working range or distance can be attained by reducing the time duration or width of the incoming space and ground wave impulses at the receiver, with a view to separating them. more clearly as to their time of arrival. To accomplish this the radio frequency impulses are rectified and the resultant unipotential impulse signals are coupled through energy storing means to an audio-frequency rectifier.

Referring to Fig. 2, one embodiment of my invention is shown which utilizes a resistancecapacitance coupling system. A loop antenna L is used to receive the impulse signals. The signal is then impressed on the grid of a radio frequency rectifier tube HG. The rectified output pedance n, and a voltage is impressed on the grid of the audio frequency rectifier NG by the coupling capacitor C. The rectified voltage which appears across the output impedance T2 'is impressed across the vertical deflecting plates of a Braun tube B. The horizontal deflecting plates of the Braun tube are connected to a sweep oscillator which is synchronized to the frequency at which the impulses occur. This provides a horizontal scanning line on which a mark appears whenever an impulse is received. Impulses arriving at difierent times appear in different positions on the line and may thus be identified. The use of a loop antenna and a Braun tube indicator for impulse direction finding are in themselves old.

The operation of my invention will now be explained.

'Afteri radio frequency rectification in rectifier HG the voltage across the resistance '11 corresponding to one impulse has the form shown in Fig. 3a. The voltage appearing across the resistance R is the derived curve as shown in Fig.

. 3b. This will be understood when it is remem-' bered that the voltage which is induced through a capacitor, or a transformer, i zero when the inducing voltage is constant, and a maximum when the rate of change of the inducing voltage Mathematically, the

relation between the inducing and the induced voltage is expressed by the process of diiferentiation. Thus a sine wave voltage, or current, induces a'voltage which follows a cosine curve. An impulse such as that shown in Fig. 3a is not the equivalent of one half of a sine wave cycle, for in the sine wave the rate of change is a maximum at the point of intersection with the abscissae while in the impulse the rate of change becomes zero at the point of intersection with the abscissae.

The width of the impulse is decreased because the derived voltage starts from zero, goes through a maximum and returns to zero in the time during which the original voltage is going from zero to maximum. The return of the original voltage to zero induces another impulse of opposite polarity. The two derived impulses are shown at FI and F2 of Fig. 3b. The areas Fl and F2 above and below the neutral or zero line must be the same since no direct current will flow through the condenser C. The form of the areas is a function of the size of condenser C and of the resistance R.

In the impulse direction-finding method the signals follow one another periodically, say, at a frequency of f1=300 cycles per second. If the coupling means C and R are chosen so small that only the high harmonics of frequency h are transmitted, then the signals which appear across R assume the shape shown in Fig. 30.

Now, these signals are fed to the grid of the audio frequency rectifier NG. Owing to rectification, the negative portion below the neutral line drops out, and there is obtained in the output of the tube across the resistance r2 signals of the original shape, though narrowed or constricted. In this manner both the ground wave impulse and the echo signal will be reduced in width. If the ground wave and the echo impulses are received with as great a separation as that illustrated in Fig. 4d at a, their identification would be possible according to the usual methods, but by utilizing the method herein explained their separation would be improved as shownin Fig. 4a at a. However, if the ground wave and the echo impulses are received in very close sequence as shown at b in Fig. 4b, or in Fig. 1, then, as noted above, direction finding would normally be quite inaccurate. By my invention, the two impulses are completely separated as shown at b in Fig. 4b,

and direction finding thusmade" accurate where" it is otherwise inaccurate.

In Figs. 4a and 4b the vertical dotted lines are drawn in to show the relative time relationbe tween the received impulses'at a'and' b and the derived impulses at a" and b; The dotted lines below the abscissae show that portion of the derived voltage which has been removed by the audio rectifier NG.

Referring now to Fig. 5, an alternative arrangement is illustrated which substitutes. a transformer coupled stage for the impedance-capacitance coupling shown in Fig. 2. The theory'of operation is the same as before, as it is well known that the voltage induced in the secondary 'ofa' 2. The combination which includes means re-' ceptive to short radio frequency ground wave impulses, said means being receptive to undesired reflected impulses which arrive at a normally indistinguishable intervals after said ground wave impulses, means for rectifying said ground wave and said reflected impulses, a capacitor coupled to said rectifying means for obtaining an impulse transformer is the derivative of the primarycur rent.

ing the received impulses, obtaining a derived The basic idea of this invention is. not confined to its use as here suggested and the energy storl which is the derivative of each of said rectified impulses, the time duration of said derivative impulse being small with respect to the duration of each of said received impulses, means for rectifying said derived impulses, and means for indicating the successive occurrence of said derived and rectified impulses to distinguish said ground wave impulse from said reflected impulse.

3. The combination which includes means receptive in short radio frequency ground wave impulses, said means being receptive to undesired reflected impulses which arrive at a normally indistinguishable interval after said ground wave impulses; means for rectifying said ground wave and said reflected impulses, a transformer coupled to said rectifying means for obtaining impulseswhich are the derivatives of said rectified impulses, the time duration of said derivative impulses being small with respect to the duration of corresponding received impulses, means for rectifying said derived impulses, and means for indicating the successive occurrence of said derived and rectified impulses to distinguish said grx'gund wave impulses from said reflected imp se.

'4. The combination which includes means receptive to short radio frequency impulses, means for rectifying said impulses to obtain unipotential impulses containing high order harmonics, means for obtaining derivative impulses from said high order harmonics of said rectified impulses, means for rectifying said derivative impulses, and means for visually indicating the successive occurrence of said rectified derived impulses.

GUNTHEB, U'LBRICH'I.

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