US3432858A

US3432858A - Short dipole antenna

Info

Publication number: US3432858A
Application number: US418144A
Authority: US
Inventors: Allan R Brown
Original assignee: ALLAN R BROWN
Current assignee: ALLAN R BROWN
Priority date: 1964-12-14
Filing date: 1964-12-14
Publication date: 1969-03-11
Anticipated expiration: 1986-03-11

Description

March 11, 1959 BROWN 3,432,858

SHORT DIPOLE ANTENNA Filed Dec. 14, 1964 Sheet I of 2 1% 2 ALLA/V R. BEOWA/ Arr-aways.

March 11, 1969 A. R. BROWN SHORT DIPOLE ANTENNA Sheet Filed D60. 14, 1964 Z M, w. ME 4 7% M o A w M Arron/5Y5.

United States Patent Int. Cl. Hillq 9/00, 1/50, 9/16 ABSTRACT OF THE DISCLOSURE The antenna comprises a shortened dipole radiating element having a physical length equal to a small fraction of a wavelength at the operating frequency and a pair of spiral wound coils are connected to the outer ends of the radiating element. A transmission line is coupled to the radiating element and the antenna is resonated to the operating frequency by adjustment of the spiral wound coils. The coils are wound about the longitudinal axis of the radiating element in the same direction so that the magnetic fields produced by these coils tend to cancel and radiation is concentrated in the shortened dipole radiating element.

The present invention relates to antennas, and, more particularly, to an antenna which is of very compact dimensions in relation to the operating wavelength while at the same time being highly efiicient and economical in manufacture.

In the field of antenna design, certain arrangements heretofore proposed have attempted to provide antennas for relatively low operating frequencies which are a small fraction of the operating wavelength in the vertical dimension. For example, a so-called low profile antenna is disclosed in Boyer Patent No. 3,151,328, issued Sept. 29, 1964. While arrangements such as disclosed in this Boyer patent accomplish a reduction in vertical height of the antenna, they nevertheless leave much to be desired in the way of providing a compact antenna structure which is of small dimensions in a horizontal plane as well as having a vertical height which is a relatively small number of electrical degrees of the operating wavelength. Furthermore, arrangements such as disclosed in this Boyer patent are necessarily associated with a conductive ground plane which means that the vertical spacing of the antenna cannot be adjusted to provide for optimum low angle coverage. Also, in this Boyer patent wherein an open ring type of antenna structure is provided, it is necessary to drive the open ring near one end thereof which severely limits the physical arrangement of both the ring and the driving source therefor.

In certain other prior art arrangements a series reactor has been employed to provide a positive reactance to compensate for the shortened vertical radiating section. However, this coil has been placed in series with the trans mission line and the antenna or in the center of the shortened dipole. Certain other arrangements have provided a large area conductive element connected to the top port-ion of the shortened vertical radiator to aid in increasing the magnitude of reactance presented by the short vertical radiating element, but this is accomplished in large part by increasing the current in the upper part of the antenna and thus making it look more inductive. Furthermore, such series coil or top hat capacity arrangements have been of extremely low radiation efiiciency, and have presented very low magnitudes of input impedance to the transmitting or receiving equipment associated therewith.

"ice

-It is, therefore, an object of the present invention to provide a new and improved antenna of relatively compact dimensions which obviates one or more of the disadvantages of the above-discussed prior art arrangements.

It is a further object of the present invention to provide a new and improved antenna which is of compact dimensions in the horizontal plane and wherein the radiating elements of the antenna have a physical length which is many times shorter than the operating wavelength of the antenna.

It is a still further object of the present invention to provide a new and improved antenna wherein radiating elements which are a small fraction of the operating wavelength are employed and tuning means, in the form of. spirally-wound conductors, are connected to the top and bottom ends of the radiating elements to provide a balanced antenna structure which is tuned to a wavelength many times greater than the physical length of the radiating elements.

A further object of the present invention is to provide a new and improved antenna of compact dimensions wherein spirally-wound coils are positioned in planes per" pendicular to the ends of a pair of radiating elements. these spirally-wound elements being wound in the same direction about the longitudinal axis of said radiating elements to reduce radiation therefrom and to provide an eflicient antenna structure in Which the operating wave-- length is many times longer than the physical length of said radiating elements.

Briefly, in accordance with the invention, a pair of colinearly arranged radiating elements are provided which have a total physical length many times less than the operating wavelength to be received or transmitted. A pair of tuning elements in the form of spirally-Wound coils are positioned in planes perpendicular to the axis of the radiating elements and are connected at their inner ends to the ends of these radiating elements. These tuning elements serve to tune the antenna structure to the operating Wavelength.

In accordance with a further feature of the invention,

the two spirally-wound coils may be wound in the same direction about the longitudinal axis of said radiating elements so as to minimize radiation therefrom and to concentrate radiation of the antenna structure in the radiating elements themselves.

In accordance with a further feature, the antenna array, which is balanced with respect to the midpoint of the radiating elements, may be positioned vertically at an optimum height, such as a distance equal to of the operating wavelength, to provide an optimum low angle propogation pattern for the antenna with resultant maximum distance for reception or transmission. A further feature of the invention consists in providing adjustable tapping points for the connection of the ends of the radiating elements to the spirally-wound tuning coils. With this arrangement, the antenna may be readily tuned to any desired operating Wavelength within a given frequency band. Furthermore, the point of connection to the spirally-wound coils can be so arranged that the antenna will be tuned to receive signals efiiciently at two relatively widely spaced operating frequencies.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

FIG. 1 is a top plan view of an antenna embodying the features of the present invention;

FIG. 2 is a side elevational view of the antenna of FIG. 1;

FIG. 3 is a sectional view through one of the coil support members of the antenna prior to insertion of the coil therein;

FIG. 4 is a sectional view illustrating an alternative embodiment of the invetnion wherein a co-axial feed element is employed to drive the antenna proper;

FIG. 5 is a plan view of an alternative embodiment of the invention, designed for operation at very low frequencies; and

FIG. 6 is a sectional view taken along the line 6-6 of FIG. 5.

Referring now to the drawings, and more particularly to FIGS. 1, 2 and 3 thereof, the antenna of the present invention is therein illustrated as comprising a pair of vertically extending radiating elements 19 and 12 which are jointed together at the adjacent ends thereof by means of a joint 14 of insulating material. Preferably, the

elements

10 and 12 are in the form of one-inch metal tubing and the joint 14 may be provided with

shoulders

15 and 16 extending into the

tubing elements

10 and 12 to provide a rigid main structure.

In accordance with an important feature of the invention, the

radiating elements

10 and 12 have a total length which is many times less than the operating wavelength to which the antenna system is tuned to transmit or receive signals. For example, the

elements

10 and 12 may have a length of four feet in an antenna arrangement suitable for receiving signals in the forty meter amateur radio band.

In order to tune the

radiating elements

10 and 12 to the desired operating wavelength, there is provided in 5 accordance with a further aspect of the invention a pair of tuning assemblies indicated generally at 18 and 20 which are positioned generally in a plane perpendicular to the axis of the

elements

10 and 12 at the upper and lower ends thereof. More particularly the tuning assembly 18 comprises a rectangular support member 22 of suitable insulating material, which is secured to the upper end of the element 10 by any suitable means and is pro vided with four radially-extending spokes 24, each of which is provided with an end portion 25 of reduced cross section. On the reduced end section 25 of each of the spokes 24, a spiral coil support member 28 is secured. Preferably, the support members 28 are of the form shown in FIG. 3 and comprise a hollow tubular construction with a vertically-extending flange or rib 30 along one edge thereof. The support members 28 may conveniently be formed with an internal diameter such that they may be press fitted onto the portions 25 of the spokes 24 to provide a rigid structure, there being provided suitable means in the form of a key way, or the like, to prevent rotation of the support members 28 with respect to the spokes 24 and to insure that the rib portion 30 is positioned vertically in the manner shown in FIG. 2 of the drawings.

Each of the support members 28 is provided with a plurality of spaced notches 32 in the rib portion 30 thereof, the notches 32 being spaced along the length of the members 28 so that a conductor 34 in the form of a spiral may have adjacent convolutions thereof snapped into place within the notches 32. In this connection, it will be noted that the spokes 24 are of unequal length so that the members 28 may all be of identical construction and when assembled on the spokes 24 they will provide the proper support for the conductor 34 in the desired spiral form.

Both the inner end 340 of the conductor 34 and the outer end 34b thereof are preferably terminated adjacent one of the support members 28, and electrical connection to the conductor 34 near the innermost turn thereof is provided by means of a flexible conductor 36, one end of which is connected to the upper end of the radiating element 10, by any suitable means, such as a clamp, screw, or the like, and the other end of which is arranged to be adjustably connected to the innermost turn of the conductor 34 by means of a pinchclip connector 38.

The lower tuning assembly 20 is constructed in a manner identical to the assembly 18 described in detail above. Thus, the support 40 is secured to the bottom end of the radiating element 12 and is provided with the spokes 42 to which are secured the support members 28 which in turn support in the appropriate spaced relation the convolutions of a spiral wound conductor 46. Connection is made from the bottom end of the radiating element 12 to the innermost turn of the conductor 46 by means of the flexible conductor 48.

The adjacent ends of the

radiating elements

10 and 12 are connected to the conductors 5t and 52 of a suitable transmission line 54 and a matching impedance, which may comprise either a capacitive element 56 or an inductive element 58 connected to the adjacent ends of the

radiating elements

10 and 12. For example, the

impedance element

56 or 58 may be physically supported on the screws 60 which secure the

elements

10 and 12 to the insulating member 14 and also provide a means of connection of the leads and 52 to the radiating elements.

Considering now the operation of the above-described antenna structure of FIGS. 1 to 3, inclusive, it will be understood that this antenna structure may be mounted by any suitable support means. For example, the bottom end of the radiating element 12, or in the alternative, the bottom support member 46- may be secured to the upper end of a supporting mast or pole. In the alternative, a suitable supporting arrangement can be provided to connect to the

radiating elements

10, 12 in the vicinity of the midpoint thereof, it being understood, of course, that any support means will be provided with suitable dielectric characteristics such as not to interfere with the operation of the antenna proper.

The length of the spirally-wound coils 34 and as is so chosen that they provide sufficient distributed capacity and inductance that the antenna may be tuned to a wavelength which is many times greater than the physical length of the

radiating elements

10, 12. This desired resonant condition can be achieved either with or without the

center loading devices

56, or 58. However, in actual practice, the

tuning assemblies

18 and 20 are so constructed that the entire antenna is electrically equivalent to a small capacitance or a small inductance, and the appropriate type of

loading device

56 or 58 is then provided to resonate the whole antenna so that it appears as a resistive load for the transmission line 54.

In a particular embodiment, each of the

coils

34 and 46 comprised a length of 34 feet of /8 inch diameter aluminum wire, the inner end 34a of each coil being spaced from the radiating element by a distance of 5 /2 inches and adjacent convolutions of each coil being spaced apart by a distance of 3 inches so that the total diameter of each of the spirally-wound coils was 3 /3 feet. With such an arrangement, it was found that the antenna could be tuned over the entire forty meter band by moving the connector 38 over an arc of approximately 120 degrees, i.e., the are shown in FIG. 1 of the drawings, the extended length of each of the coils 34, 46- thus being in the order of one quarter wavelength at the operating frequency of forty meters. In this connection, it will be understood that slight variations may exist between the points of connections of the upper andlower spiral coils to their respective radiating elements. However, these points of connections are preferably in the same general area of each spirally-wound coil to provide a completely balanced structure.

It will be noted that the spirally-wound coils 34 and 46 both extend about the axis of the radiating elements 10' and 12 in the same direction. Accordingly, when viewed from an intermediate point on the antenna, such as the connection of the transmission line 54 to the radiating elements 1-0 and 12, the spirally-wound coils 34 and 46 are wound so that they are bucking and hence the magnetic fields produced by these coils are in opposite senses and tend to cancel one another. Such an arrangement tends to reduce undesired radiation from the spiral coils themselves and to concentrate radiation in the radiating

elements

10, 12. Radiation from the

elements

10, 12 is further facilitated since these elements are at the high current portion of an equivalent dipole structure.

While connection of the spiral coils 34 and 46 in the above-described bucking relation is desirable where the area of these coils is substantial, as shown in the embodiment of FIGS. 1 to 3, inclusive, it will be understood that this bucking relationship is not required if the diameter of the spirally-wound coils is somewhat smaller. However, when the maximum diameter of the spirally-wound coils is made considerably smaller, the adjacent turns of each coil are closer together and tend to increase the losses in the

tuning assemblies

18 and 20 due to interaction between adjacent turns of each coil on the other closely spaced turns of the spiral. In this connection, it is noted that whereas the antenna in FIGS. 1, 2 and 3 is shown with the radiating elements and 12 extending vertically, the antenna assembly may, if desired, be positioned with the

elements

10, 12 horizontally, and the spiral coils 34 and 46 positioned in vertical planes. When so arranged, the antenna assembly has the customary double lobe pattern of a conventional dipole and will still have the advantages of extremely compact dimensions for the antenna. It is pointed out, however, that the important advantage of reduction in vertical height is achieved when the

elements

10, 12 are mounted vertically and when so arranged the antenna is particularly suitable for those applications where a quarter wavelength vertical antenna cannot be erected.

A further important feature of the invention is that the antenna structure is balanced and hence requires no separate ground plane or counterpoise. As a result, the antenna may be mounted at the optimum height to obtain maximum low angle coverage. For example, the antenna may be mounted on a pole at a height of approximately of the operating wavelength and will provide optimized low angle coverage whereas such positioning of a conventional quarter wavelength vertical antenna would in many applications be impossible. It will of course be understood that the antenna of the present invention does not need to be mounted at a height of of the operating wavelength to achieve efiicient operation since any reasonable spacing above the ground will provide for adequate reception or transmission.

In the event that two widely spaced frequency bands are to be received, the antenna of the present invention may be adapted to receive both bands in a highly eflicient manner. Thus, if both the 10 meter and 40 meter amateur bands are to be received, the connection of the spiral coils 34 and 46 to the ends of the radiating

elements

10 and 12 can be modified so that the overall antenna is tuned to resonance in both of these bands. To accomplish this, the connectors 38 are connected to a point on an intermediate one of the turns of each of the spiral coils 34 and 46', the exact point being determined by appropriate resonance tests so that the antenna is tuned to resonance in both of these bands. A similar adjustment in the connection of the bottom end of the radiating element 12 to the coil 46 would, of course, be made. In this connection it will be understood that any suitable arrangement can be employed to vary the length of the spiral coils 34 and 46 to achieve tuning of the antenna to the desired operating frequency, the illustrated clip connector 38 being shown merely by way of example.

In FIG. 4 of the drawings there is shown an alternative embodiment of the invention wherein a co-axial transmission line is employed for connection to the antenna proper and may also be used as a physical support for the antenna structure. According to this figure, the upper radiating element of the antenna is formed by the portion 64 of the center conductor of a co-axial, transmission line 66, the portion 64 extending above the upper end 65 of the co-axial line 66. The bottom radiating element 12 is formed by an outer sleeve 68 which is electrically connected to the sheath 67 of the cable 66 at the upper end thereof by means of a metal end ring 70. The tuning assembly 18 is connected to the upper end of the radiating element 64 and the tuning assembly 20 is connected to the bottom end of the sleeve 68.

In order to match the transmission line 66 to the antenna structure, there is provided a tapered dielectric member 72 which is of a thickness sufiicient to [fill the space between the center conductor 64 and the sleeve 67 at the upper end of the cable 66 and tapers down to zero thickness along the length of the co-axial cable '66. The dielectric member 72 thus provides an impedance transforming section which matches the characteristic impedance of the co-axial line 66- to that of the antenna structure. In other respects the arrangement of FIG. 4 operates in exactly the same manner as described in detail heretofore, it being understood that the physical length of the section 64 and the sleeve 68 are again many times smaller than the operating wavelength.

Referring now to FIGS. 5 and 6, there is disclosed therein a further embodiment of the invention which is particularly suitable for the transmission or reception of extremely low frequencies. In this arrangement a plurality of supporting poles 76 are arranged in a spiral fashion (when viewed from above) about a center post 78. The antenna comprises a central vertically extending conductive element 80, which is mounted on the center post 78 and a pair of

conductors

82 and 84 which are supported by the posts 76 and extend in spiral fashion outwardly from the center post 78. The conductor 82 is connected to the upper end of the radiating element at the center post 78 and the conductor 84 is connected to the bottom end of the radiating element 80' at the inner end of the bottom spiral. With this arrangement, an antenna capable of receiving extremely low frequencies may be constructed in a simple and economical manner while providing a relatively efiicient structure for transmitting or receiving at these frequencies. In the embodiment of FIGS. 5 and 6, connection of a transmission line comprising the conductors 86 and 88 is made to an appropriate point near the outer end of each spiral by means of the intermediate connecting

portions

90 and 92, respectively. With this arrangement the

conductors

90, 92 provide a delta matching arrangement whereby the impedance of the antenna may be readily matched to the characteristic impedance of the transmission line 86, 88. In this connection it will be understood that the connection point of the transmission line 86, 88 to the outer ends of the spirally-

wound conductors

82 and 84 will be chosen so that the overall antenna structure is resonated at the desired operating frequencies. It is also pointed out that the antenna arrangement of FIGS. 1 to 3, inclusive, may also be fed from a point near the extremities of the spiral coils 34, 46, if desired, in which case the radiating element and a suitable delta matching network would be employed to match the transmission line to the chosen point on the

coils

34, 46, as will be readily understood by those skilled in the art.

While the tuning assemblies, such as the

assemblies

18 and 20, described in detail heretofore in connection with FIGS. 1 to 33, inclusive, have been illustrated as comprising uniformly wound spirals, it will be understood that various modifications from an exact spiral form may be employed insofar as the principles of the present invention are concerned. Thus, the

conductors

34 and 46 may be wound in the form of a polygon, rectangle, or may be of irregularly shaped configuration, it being only necessary that these elements provide the necessary distributed capacity and inductance to tune the radiating

elements

10 and 12 to the desired operating frequency. Furthermore, while the radiating

elements

34, 46 have been illustrated as having all of the turns thereof in a common plane, it will be understood that the spiral may lie along a conical, parabolic or other curved surface insofar as the principles of the present invention are concerned.

While there have been illustrated and described various embodiments of the present invention, it will be apparent that various changes and modifications thereof will occur to those skilled in the art.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent of the United States is:

1. An antenna comprising a pair of co-linear radiating elements, means for coupling to said radiating elements at the adjacent ends thereof, and tuning means connected to each of said aligned radiating elements at the outer ends thereof and comprising a conductor Wound in a generally spiral manner in a plane perpendicular to said radiating elements, said tuning means being effective to tune the antenna to a wavelength may times greater than the length of said radiating elements and the extended length of said spiral wound conductor being approximately equal to one quarter of said Wavelength.

2. A self-resonant dipole antenna system comprising a pair of co-linear radiating elements, means for coupling to said radiating elements at the adjacent ends thereof, and tuning means connected to each of said aligned radiating elements at the outer ends thereof and comprising a conductor wound in a generally spiral manner in a plane perpendicular to said radiating elements, said tuning means being eifective to tune the antenna to a wavelength many times greater than the length of said radiating elements, both said tuning means being Wound in the same direction about said radiating elements and being approximately one quarter wavelength in extended length, thereby to minimize direct radiation from said tuning means.

3. An antenna comprising a pair of co-linear radiating elements, means for coupling to said radiating elements at the adjacent ends thereof, and tuning means connected to each of said aligned radiating elements at the outer ends thereof, said tuning means each comprising conductor means positioned in a plane perpendicular to said radiating elements and arranged to provide sutficient distributed capacity and inductance that the antenna may be tuned to a wavelength many times greater than the length of said radiating elements, said tuning means comprising multi-turn spirals having at least three turns and being arranged about the ends of said radiating elements so that the magnetic fields produced by said tuning means tend to cancel, thereby to minimize direct radiation from said tuning means.

4. An antenna, comprising a pair of elongated radiating elements, means positioning said elements in end-toend relation along a given axis, means for coupling to the adjacent ends of said radiating elements, a pair of tuning elements connected to the outer ends of said radiating elements, said tuning elements each comprising a conductor having one end thereof connected to the outer end of said radiating element and extending outwardly from said radiating element in the general form of a spiral having at least three turns, the distributed capacity and inductance of said tuning elements being sufficient to tune the antenna to a Wavelength many times greater than the length of said pair of radiating elements.

5. An antenna, comprising a pair of elongated radiating elements, means positioning said elements in end-toend relation along a given axis, means for coupling to the adjacent ends of said radiating elements, a pair of tuning elements connected to the outer ends of said radiating elements, said tuning elements each comprising a conductor having one end thereof connected to the outer end of said radiating element and extending outwardly from said radiating element in the form of a spiral, the distributed capacity and inductance of said tuning elements being sufficient to tune the antenna to a wavelength many times greater than the length of said pair of radiating elements, each of said conductors having an extended length of approximately one quarter of said wavelength, and means for varying the effective length of said tuning means, thereby to vary the wavelength to which the antenna is tuned.

6. An antenna, comprising a pair of elongated radiating elements, means positioning said elements in end-to-end relation along a given axis, means for coupling to the adjacent ends of said radiating elements, a pair of tuning elements connected to the outer ends of said radiating elements, said tuning elements each comprising a conductor having one end thereof connected to the outer end of said radiating element and extending outwardly from said radiating element in the form of a spiral having at least three turns, the distributed capacity and inductance of said tuning elements being suflicient to tune the antenna to a wavelength many times greater than the length of said pair of radiating elements, and means for varying the point of connection of one of said radiating elements to the adjacent tuning element, thereby to vary the Wavclength to which the antenna is tuned.

7. An antenna system, comprising a vertically extending radiating element, a pair of tuning elements connected to the top and bottom ends of said radiating element, said tuning elements each comprising a multi-turn spiral conductor having at least three turns positioned substantially in a horizontal plane and having the inner end thereof connected to the adjacent end of said radiating element, both of said spiral conductors being wound in the same direction about the longitudinal axis of said radiating element, a transmission line, and means for coupling said transmission line to said spiral conductors at points on said conductors which match said radiating element and said tuning elements to said transmission line, said tuning elements having sufficient distributed capacity and inducance to tune the antenna system to an operating wavelength which is many times greater than the length of said radiating element.

8. An antenna, comprising a pair of conductive vertically extending radiating elements arranged end-to-end and insulated from one another, a pair of tuning elements positioned in horizontal planes adjacent the top and bottom ends of said pair of radiating elements, each of said tuning elements comprising a multi-loop conductor having one end thereof connected to the adjacent radiating element, and having a plurality of loops successive ones of which enclose successively larger horizontal areas, said tuning elements having sufiicient distributed capacity and inductance to tune said antenna to an operating wavelength which is many times greater than the length of said pair of radiating element and having an extended length in the order of one quarter wavelength of the operating frequency, and means connected to said adjacent ends of said radiating elements for matching said antenna to a desired load circuit.

9. An antenna comprising a straight conductive radiating element having a physical length which is a small fraction of a wavelength at the operating frequency, and tuning means effective to tune the antenna to said operating frequency, said tuning means comprising a first tuning coil connected to one end of said radiating element and extending in a generally spiral manner with respect to the longitudinal axis of said radiating element, and a second tuning coil connected to the other end of said radiating element and extending in a generally spiral manner with respect to the longitudinal axis of said radiating element,

the extended length of each of said coils being in the order of one quarter wavelength at said operating frequency and both said first and second tuning coils being wound in the same direction around the longitudinal axis of said radiating element so that the magnetic fields due to said coils tend to cancel and undesired radiation from said coils is minimized.

10. An antenna arrangement as set forth in claim 9, wherein said radiating element comprises a pair of colinear radiating members, a transmission line, and means coupling said transmission line to the adjacent ends of said co-linear members, said tuning coils being connected to the outer ends of said co-linear radiating members.

11. An antenna arrangement as set forth in claim 9, wherein said tuning coils are in the form of flat spirals each positioned in a plane perpendicular to the longitudinal axis of said radiating element.

References Cited HERMAN KARL SAALBACH, Primary Examiner.

15 W. H. PUNTER, Assistant Examiner.

US. Cl. X.R.