US1747262A - Loop antenna - Google Patents

Description

,Feb- 18, 1930- J. H. PRESSLEY 1,747,262

LOOP ANTENNA Filed Nov. l5. 1925 3 Sheets-Sheet 1 Feb. 18, 1930. J. H. PRssLEY- 1,747,252

' Loo? ANTENNA Filed Nov. 13. 1925 5 Sheets-Sheet 2 attori/w.,

Feb. 18, 1930.` J. H. PRl-:ssLEY 1,747,252

LOOP ANTENNA Filed Nov. 13. 1925 3 Sheets-Sheet 5 r l f f7 AIlz/). 6

Patented Feb. 18, 1930 JACKSON' I-I. PRESSLEY, OIE' FORT MNMOUTH, OCEANPORT, NEW JERSEY LOOP ANTENNA Application led November 13, 1925. Serial No. 68,886.

This invention relates to a novel improvement in coils, used as antennae in radio communication apparatus, and has for one of its objects to provide means whereby the directional characteristic of a loop antenna may more nearly approximate the ideal figure 8 diagram.

A more particular object of my invention is to provide a means whereby the displacement current effect of a solenoidal or box type of loop antenna may be eliminated.

With the foregoing and other objects in View the invention consists in the novel combination and arrangement of parts `hereinafter described and claimed, certain embodiments thereof being illustrated in the accompanying drawings, lin which:

Fig. l is a diagrammatical sketch of the conventional loop antenna.

Fig. 2 is a modification of that shown in Fig. l for the purpose of illustrating the eX- istence of a displacement current effect in this type of antenna.

Fig. 3 is a diagrammatical illustration of the directional characteristic of the displacement current effect in the conventional type of loop antenna.

Fig. 4 is a diagram of my improved device for eliminating the above referred to displacement current effect.

Fig. 5 is a diagram of a modification of that shown in Fig. 4.

F ig. 6 is a diagram of a. modiiication of that shown in Figs. 4 and 5.

Fig. 7 is a top plan view of that shown in Fig. 6.

Referring more particularly to the drawings, Fig. l shows the conventional type of loop antenna l with its plane perpendicular to the direction of travel of the electromagnetic Wave, which travel is assumed to be along the X axis. A variable condenser 2 is used to tune the loop l to resonance. With the loop in the position, shown in this figure, the approaching wave induces voltages in all of the vertical sides of the loop turns. However these induced voltages in symmetrical pairs of the vertical portions of the loop turns, such as in a and a and in Zi and ZJ', are equal in amplitude and opposite in phase, and consequently balance each other which should result in no current flowing in the loop circuit. If the loop is rotated about the Y axis by a small angle the induced voltages will not balance each other, and consequently a current will flow in the loop which, by -producing a voltage across the condenser 2, may be indicated in the output circuit.

The loop characteristic described above is for the ideal case and is a very valuable property for radio direction finding receivers, or loop directional radio transmission. In practice, however, a departure occurs from the ideal case and in many cases the loop current cannot be made zero for any position of the loop about the Y axis. The reason for this lies in what may be termed the production of parasitic voltages which destroy the pure loop effect above described. These parasitic voltages arise primarily from two sources; firstly, dissymmetry in the loop capacity to ground (this can be rectified by methods well known to the art); and secondly to what is known as displacement current effect which may be eliminated by my invention. Referring again to Figure l: Although the induced voltages in sides a and al exactly balance each other, and also in the case of b and b1, the induced voltage in a differs in phase from theinduced voltage in b, and similarly in case of a1 and 1, by a phase angle depending on the axial length d of the loop and the length of the electromagnetic waves. Because of the distributed capacity between the turns of the loop a current will be produced as a result of this phase difference. Figure 2 is a diagrammatical view of this situation. Here L represents the loop with its distributed capacity removed and the lumped capacity C0 substituted therefor. C is the tuning condenser and e is the lumped cumulated displacement voltage. From this figure it is obvious how this displacement voltage acts to produce a current through the loop and so destroy the ideal loop characteristic.

Figure 3 is a graph in polar co'ordinates representing the desired directional effect of the loop by the dotted circles and the displacement effect by the small solid line circles. These graphs represent the voltage produced across the tuning condenser by the above described effects. The resultant tuning condenser voltage is the vector sum of the individual voltages. The displacement effect is a maximum when the true loop effect is a minimum.

Figure t shows one of my methods of eliminating the displacement eiect described above. In this figure, A and B represent two vertical wires symmetrically located with respect to the plane of the loop and preferably in a plane at right angles to the plane of the loop and also mounted to the loop so as to rotate with it. Wire A is connected'to the terminalof the loop turn farthest away from it and likewise B is connected to the terminal of the loop turn .farthest away from it.

Now if the length of the wires A and B and their distance apart are proper, the resultant voltage produced in these two wires will be exactly the same as the resultant displacement voltage in the loop'and withthe connections shown it is clearly evident that a complete neutralization of voltages will oc-v cur. In practice yto obtain an exact balance for each individual loop the lengths of or the distance between the wires A and B may be made easily adjustable.

Figure l5 shows a method'of eliminating the displacement effect by the use Yof an auxiliary loop with its plane vatright angles to the main loop. In this case the auxiliary loop should be of the proper size to exactly counteract the loop displacement voltage. To obtain a liner b-alance means may be vreadily devised whereby the effectiveness of the auX- iliary loop can be made readily adjustable. In practice this method will probably not prove as convenient as the preceding one or the method shown in Figure 6 which I have found to be vmost preferred.

Referring to Figure 6, a double layered loop is shown'lwith the plane of the loop in the plane of the paper and Ye-Y is the vertical axis about which the loop can be rotated. Figure 7 is a schema-tic view looking down on the top of theloop. For convenience in following the diagram the outer turns are shown insolid lines and the inside turns in dotted lines with l asthe outer layer, 2r as the inner layer, havingthe connections 3 and 4 .between inner and outer layers, andterminal leads 5 and 6. The space s is left to provide room for the mounting shaft although the shaft may be mounted in other ways if desirable. The sections of the loop are so wound that the loop self inductance is a maximum when the sections are connected as shown. In other words the winding direction of the two sections of the outerlayer must be the same and of opposite direction tothe winding of the twolsections of the inside layer. Under'these conditions it is evident that e1 and e2 representing the displacementvoltages induced in layer 1 will be neutralized by e3 and e4 which representl the displacement voltages in layer 2. The above is on the premise that layers l and 2 have the same number of turns of the same numerical pitch. It is obvious that to those skilled in Y the art, otherarrangements and connections of the individual sections will present themselves and that the same scheme may be used with loop of more than two layers but it will in general be found preferable to use only an even number of layers.

Having thus described my invention, I claim:

l. A loop antenna'in combination with two vertical antennae, said vertical antennae being located symmetrically with respectfto the plane of said loop, said vertical antennae being adjustable as to lengt-hand distance apart from each other, said vertical antennae'being rigidly attached` to said loop and .arranged to rotate therewith, said vertical antenn'being so connected to the terminals of said loop as to neutralize any displacement voltage induced in said loop antenna.

2. In combination with a loop antenna, means for neutralizing the displacement-current eiifect thereof, said means comprising an auxiliary linear antenna type collectorixedY JACKSON H. 'PRESSLE

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