US2687475A - Low-frequency antenna - Google Patents

Description

24, 1954 M. w. SCHELDORF 2,687,475

LOW-FREQUENCY ANTENNA Filed April 11, 1950 Patented Aug. 24, 1954 LOW-FREQUENCY ANTENNA Marvel W. Scheldorf, Palos Heights, 111., assignor to Andrew Corporation, Chicago, 111., a corporation of Illinois Application April 11, 1950, Serial No. 155,267 7 Claims. (01. 250-3355) The present invention relates to antennas and more particularly to an antenna designed to operate at very low frequencies.

Antennas for the transmission and reception of very low frequencies for vertically polarized radiation have certain electrical characteristics which makes it difficult to feed or to collect energy from them. Generally, the terminal resistance of such antennas is unusually low and the associated terminal reactance is unusually high This is brought about by the fact that the electrical length of such antennas is unusually short, and hence they may be considered to be an equivalent transmission line which is short with a very high Q. Thus, they exhibit characteristics similar to capacitor antennas having low radiation resistance These limitations occur because it is not feasible to mount in a vertical direction a conductor sufiiciently long to approach even the first resonant period. It therefore appears that it is desirable to provide a system wherein the conductors need not be vertical in their most extensive dimensions.

It is therefore an object of the present invention to provide a physically convenient antenna for very low frequency waves which has dimensions that are suitably oriented.

A further object of the present invention is to provide a very low frequency antenna having practical input impedance.

A still further object in the present invention is to provide a very low frequency antenna for producing vertically polarized waves.

A still further object of the present invention is to provide an improved very low frequency antenna whereby the radiation pattern is readily predetermined.

Still another object of the present invention is to provide an improved very low frequency antenna whereby the radiation pattern is readily modified.

A still further object of the present invention is to provide an improved very low frequency antenna having a plurality of portions excited by phase displaced currents.

Other and further objects of the present invention subsequently will become apparent by reference to the following description taken in conjunction with the accompanying drawings wherein:

Figs. 1 and 2 are plan and elevation views respectively of one antenna embodiment employing the principles of the present invention.

Fig. 3 is a representation of a radiation pattern of the antenna illustrated in Figs. 1 and 2;

Figs. 4 and 5 are plan and elevation views of another antenna embodiment constructed in accordance with the principles of the present invention;

Fig. 6 is a representation of the radiation pattern of the antenna disclosed in Figs. 4 and 5;

Figs. 7 and 8 are plan and elevation views of still another antenna embodiment constructed in accordance with the principles of the present invention; and

Figs. 9 and 10 are representations of the radiation patterns obtained from the antenna of Figs. 7 and 8, by utilizing different methods of feeding energy to the antenna components.

If an antenna radiating element comprising a narrow slot cut horizontally in a vertical conducting sheet is energized in the center on opposite sides of the slot at the proper frequency, there will be a resultant transverse electrical field and vertical polarized radiation. If the area of the conducting sheet is reduced until only edge portions of the slot remain, the metallic structure becomes substantially equivalent to a vertical rectangular loop. The radiation characteristics however, will be the same as for the slot in the conducting sheet.

In accordance with the present invention, it is proposed to utilize an antenna radiating element formed of a plurality of conductors arranged at angles to each other which have a length which is a multiple of a quarter wave length long. Thus, in Figs. 1 and 2 there is shown a radiating element comprising two parallel conductors II and I2, which are shorted at their ends by conductors I3 and I4. In this particular embodiment each half of the conductors I I and I2 is a half wave length long, and these conductors are bent at their middle so as to have the half wave length portions at angles to each other.

By so arranging the conductors as indicated in Fig. 1 where the angle is nearly the radiation pattern will be essentially circular as shown in Fig. 3. Preferably the angle included between the two halves of the conductors II and I2, is somewhat less than 90. The parallel conductors I I and I2 lie in parallel horizontal planes.

An antenna of this type has the characteristic of having a low impedance. This antenna may be considered to be a form of two paralleled horizontal balanced transmission lines of one-half wave length long each, shorted at their ends. The common points of the two lines are connected to a suitable source of oscillation.

Thus in Figs. 4 and 5, one radiating element consists of parallel conductors l5 and [6 arranged in horizontal planes and short circuited as at ll of each end. The apex of this radiation element is arranged closely adjacent to the apex of another radiation element comprising two parallel conductors I8 and i9, arranged in horizontal planes and shorted at each end as at 20. Thus each portion of the radiation elements from the apex to its outer extremity is a half wave length long, and the two radiating elements are excited at their centers by transmission lines 2| and 22. The two transmission lines 2! and 22 are connected to sources of oscillation which are in phase quadrature.

By thus exciting the two antenna elements shown in Figs. 4 and 5, by oscillations differing phase by 90 degrees there is obtained a unidirectional radiation pattern of the type shown in Fig. 6.

An antenna arrangement such as shown in Figs. 5 and 4 also has the characteristic of having a relatively low impedance.

Still another embodiment of the present invention is illustrated in Figs. 7 and 8. One radiating element comprises two parallel conductors 22 and 23, arranged in a horizontal plane and short circuited at each end by the conductors 24. This element has an over-all length of one half wave length. Arranged at right angles to this element is another similar element of a half wave length long comprising two parallel conductors 25 and 26 arranged in a horizontal plane and short circuited at their ends by the conductor 27. The two radiating elements are connected at their centers to transmission lines 28 and 29, which are connected to sources of oscillation which are substantially in phase quadrature.

This embodies an antenna arrangement of a substantially circular radiation pattern as shown in Fig. 9 and the impedance characteristic of the antenna is relatively high.

If the arrangement illustrated in Figs. '7 and 8 is modified to the extent of utilizing transmission line members which are only one quarter wave length long, and the feed lines 28 and 29 are connected to the ends of said radiating elements, it is possible to produce a cardioid radiation pattern such as that illustrated in Fig. 10.

On each of the antennas heretofore described, it is possible to pass a ground plane midway between the parallel conductors Without disturbing the performance thereof. In actual practice, only half of the structure shown could be built, utilizing the earths surface for a return path. The cost of a good grounding system might well be more than the cost of a higher structure with both circuits in metallic conductors, so the two-conductor path is preferable.

Whereas, there has been shown only a single conductor for each current path, other constructions might be utilized such as cage and fan arrangements to reduce the conductor losses and to improve the reactive component of the terminal impedance.

While full wave, quarter wave lengths and half wave length elements have been shown in the drawings, these may be reduced considerably by end loading. Such end loading is not a structural problem with the antenna element relatively close to the earth.

From the foregoing it is believed to be evident to those skilled in the art that by means of the present invention there is provided an arrangement where very low frequency vertically polarized transmissions can be practically produced, it is possible to control to a certain extent the impedance characteristics of the antenna, and the radiation pattern.

While for the purpose of illustrating and describing the present invention, certain embodiments have been illustrated in the drawing, it is to be understood that the invention is not to be limited thereby when such other variations and. embodiments are contemplated as may be connected with the spirit and scope of the invention, as defined in the accompanying claims.

I claim:

1. An antenna for very low frequency vertically polarized radiations comprising two pairs of parallel conductors each connected together at their ends, each pair of said conductors being bent at their centers to form an obtuse angle, the apices thereof being spaced apart in parallel horizontal planes, and connections from the apices of each pair of conductors to a source of oscillations.

2. An antenna for very low frequency vertically polarized radiations comprising two pairs of parallel conductors each a wave length long and short circuited at their ends, each pair of conductors being bent at their centers to form an obtuse angle, the apices thereof being spaced apart in parallel horizontal planes, and connections from the apices of each pair of conductors to different sources of oscillation displaced in phase with respect to each other.

3. An antenna for very low frequency vertically polarized radiations comprising two pairs of parallel conductors each a multiple of a half wave length long, and short circuited at their ends, each pair of conductors being bent at their. centers to form an obtuse angle of approximately l20, the apices thereof being spaced apart in parallel horizontal planes, and connections from the apices of each pair of conductors to quadrature phase displaced sources of oscillation.

4. An antenna for very low frequency vertically polarized radiations comprising two pairs of parallel horizontal conductors each a multiple of a quarter wave length long and short circuited at both their ends, the conductors of one pair being at an angle to the conductors of the other pair, said angle being bisected by a vertical plane passing through the midpoint of each of said parallel conductors, and connections to the midpoints of each conductor, each pair of conductors being connected to different sources of oscillation.

5. An antenna for very low frequency vertically polarized radiations comprising the structure set forth in claim 4 wherein the sources of oscillation are displaced in phase by 6. An antenna system for very low frequency vertically polarized radiations comprising two symmetrically positioned horizontal conductors each arranged parallel to a return path, connections from the ends of each conductor to its return path, each conductor being bent at its middle to form an obtuse angle, the apices thereof being spaced apart in a common plane, each conductor having an effective length equal to one wavelength of the radiated frequency, and connections from the return paths and said apices of said conductors to different oscillation sources displaced in phase relative to each other.

7. An antenna system for very low frequency vertically polarized radiations comprising two symmetrically positioned horizontal conductors each arranged parallel to a return path, each conductor having its ends connected to its return path, each conductor being bent at its middle to form an obtuse angle, the apices thereof being spaced apart in a common plane 1 and arranged symmetrically, each conductor having an effective length equal to one wave length of the radiant frequency and connections from the apices of said conductors and said return paths to quadrature phase displaced oscil- 20 lations.

References Cited in the file of this patent Number 5 D. 156,078 Re. 23,346 1,952,326 2,258,406 2,147,168 10 2,183,562 2,254,697 2,268,664 2,507,528

5 Number UNITED STATES PATENTS Name Date Disch Nov. 22, 1949 Ercolino Mar. 20, 1951 Ludenia Mar. 27, 1934 Carter Apr. 16, 1938 Landon Feb. 14, 1939 Hansell Dec. 19, 1939 Godet Sept. 2, 1941 Landon Jan. 6, 1942 Kandoian May 16, 1950 FOREIGN PATENTS Country Date Great Britain Dec. 13, 1917 OTHER REFERENCES Radio, June 1939, pages 21-22.

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