US3363254A - Broadband antenna with direction of radiation determined by frequency - Google Patents

Description

Jan. 9, 1968 I R. CARREL. ET AL 3,363,254

BROADBAND ANTENNA WITH DIRECTION OF RADIATION AFilgd Oct. 26, 1964 FIG DETERMINED BY FREQUENCY 3 Sheets-Sheet l INVENTORS ROSS LA BELL ROBERT L. CARREL PAUL B. SMITHEY ATTORNEYS Jan. 9, 1968 R. L. CARREL ET AL 3,363,254

BROADBAND ANTENNA WITH DIRECTION OF' RADIATION DETERMINED BY FREQUENCY A Filed OCT.. 26, 1964 3 SheeS-Sheet 2 ACTUAL ANTENNA 64 GROUND PLANE IMAGE ANTENNA vFIG 2 INVENTORS ROSS L` BELL ROBERT L. CARREL PAUL B. SMITHEY ATTORNEYS Jan. 9, 1968 R. L. CARREL AL 63,254

BROADBAND ANTENNA WITH DIRE O F RADIATIO DETERMINED BY FREQUE Filed oct. 2e, 1964 s sheets-sheet :5

- lNVENTORS OSS l. BELL ROBERT L. CARREL PAUL B. SMITHEY T TORNE YS United States Patent O 3,363,254 BRGADBAND ANTENNA WlTH DIRECTIGN F RADIATION DETERMINED BY FREQUENCY Robert L. Carrel, Richardson, Ross L. Bell, Dallas, and Paul B. Smithey, Richardson, Tex., assignors to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Filed Oct. 26, 1964, Ser. No. 405,351 8 Claims. (Cl. 343-7925) ABSTRACT 0F THE DISCLOSURE A log periodic type antenna positioned vertically over ground with the vertex pointed at ground and with the spacing between ground and the shortest dipole and the spacing between ground and the longest dipole being such that the angle of elevation of directivity of radiation decreases as frequency increases.

This invention relates generally to broadband antennas and, more specifically, to a broadband antenna for use in the high frequency -band for short and medium length skywave circuits ranging from 0 to approximately 1000 miles.

In the prior art there are applications in which it is desired to transmit over very short distances and also over considerably longer distances as, for example, in the range of from 0 to 1G00 miles. It has been found that the optimum transmission characteristics over different distances can be obtained at diierent frequencies.. Generally speaking, the greater the distance over which transmission occurs, the higher the frequency should be.

It also has been found that the angle of directivity of transmission with respect to the ground plane will vary for optimum results with dierent frequencies. More speciiically, the angle of directivity of radiation with respect to the ground plane should decrease as the frequency increases and the transmission path becomes longer.

There exists in the prior art a type of antenna known as a log periodic antenna which exhibits a substantially constant impedance and radiation pattern over a broad frequency band. The log periodic antenna basically is an antenna composed of a plurality of dipoles arranged in a generally triangular form with the dipoles increasing in length from the apex of the triangle. The dipoles extend outwardly from a line running from the apex of the triangle and bisecting the overall triangular shape. Further, the distance of a given point on any dipole from the apex bears a constant ratio r to the distance of a corresponding point on the dipole next farthest removed from the apex of the triangle. For a detailed description of log periodic yantenna reference is made to United States Patent 3,059,234 issued Oct. 16, 1962 to R. H. Du Hamel et al., and entitled, Logarithmically Periodic Antenna Array.

These log periodic antennas exhibit unusually desirable broadband characteristics, i.e., radiation pattern remains quite constant, and the angle of radiation directively does not vary as the frequency changes. Thus, optimum conditions for the type of operation described above, i.e., over a range of from 0 to 1000 miles, is not best niet by a conventional log periodic antenna.

It is an object of the present invention to provide a broadband antenna similar to a log periodic antenna but whose vertical pattern of radiation varies with frequency to provide near optimum radiation directivity at all frequencies in the operating band.

A second object of the invention is a broadband antenna whose vertical pattern of radiation directivity forms 3,363,254 Patented Jan. '9, 1968 ICC an angle with respect to the ground plane, which angle becomes smaller with respect to said ground plane as the frequency increases.

A third object of the invention is a multipath antenna whose elevation angle of radiation decreases as the frequency increases to provide optimum directivity of radiation over the operating bandwidth.

A fourth purpose of the invention is the improvement `of a broadband antenna, generally.

In accordance with the invention there is provided a broadband antenna comprising a plurality of dipoles having a common feed and arranged substantially parallel with each other to form an overall conliguration having the general shape of an isosceles triangle, but with the equal sides of the triangle being concave in nature rather than straight. Such dipoles are spaced apart in the usual log periodic arrangement as discussed in United States Patent 3,059,234 and are positioned in a single vertical plane parallel to each other and substantially perpendicular to the center line of the overall isosceles triangular shape. The antenna is mounted above ground with the vertex of the overall triangular configuration pointing toward the ground plane and with the center line of the isosceles triangle substantially perpendicular to said ground plane.

In accordance with a feature of the invention, `appropriate supporting structure for the antenna comprises a pair of towers each having a cable running from near the top of the tower to a point under the vertex of the antenna array. The dipoles are Istrung horizontally across the two supporting cables, which cables under the tension and weight of the dipoles, become catenaries, thus forming the generally concave shape of the overall antenna configuration.

In accordance with another feature -of the invention, the dipoles are fed from a common pair of leads which `are zigzagged across each other to connect alternately to oppositely positioned halves of each dipole structure.

The above and other objects and features of the invention will be more fully described in connection with the drawings, in which:

FIG. 1 shows a sketch of the invention as mounted above ground between two towers;

FIG. 2 shows the antenna array without the supporting structure, and the image of the antenna array created below the ground plane; and

FIGS. 3, 4, and 5 show radiation patterns in the H plane for low, medium and high frequencies in the operating bandwidth of the antenna structure of FIG. 1.

Referring now to FIG. l, there is shown a view of the antenna which consist-s of an array of dipoles 2li-20u through 33-33a connected between towers 35 and 36. The dipole array is held in position by two catenary cables 39 and 40 which are strung from a point near the tops of the towers 35 and 36 to ground positions adjacent either side of the apex of the antenna array. A

In discussing the antenna array, the individual dipoles will be designated by reference characters identifying the two half sections of each dipole. For example, the longest dipole in FIG. 1 is designated by the reference characters 20 and 20a with the reference character 20 representing the left half section of the dipolerand the reference character 20a representing the right half section. When the entire dipole is to be designated the phraseology dipole 20 for example, will be used. If, however, only the left half section is meant, then the phraseology dipole section 2li will be employed. Thus the reference characters 20 through 33 will carry two meanings depending on the specific term employed.

The dipoles 20 through 33 can be fed by a coaxial cable 66, for example, which supplies the signal to balun t) which, in turn, feeds the output signal to the two zigzag feed wires 45 and 46. The use of a zigzag two-wire feeding arrangement has been found to be more desirable than using two parallel lines to feed the half sections of the dipoles. By using the zigzag configuration, lower standing wave ratios are obtained than could be obtained with the parallel wire arrangement.

As mentioned above, the dipoles are secured to catenaries 39 and 40. More specifically, each of the dipoles, with the exception of the top dipole 20, is secured to the catenaries 39 and 40 through insulating connectors, such as insulating connectors 52 and S3 associated with the dipole sections 21 and 22. The connectors 52 and 53 are, in turn, secured to the catenary by suitable tying means designated generally by reference characters 54 and 55. The catenaries 39 and 40 may be of any suitable material such as, for example, Daeron.

Consider now the mathematical relationship in the present antenna, particularly as they compare to a conventional log periodic antenna. In a conventional log periodic antenna as discussed in the above-mentioned U.S. Patent 3,059,234, the overall shape of the antenna is triangular in nature with each dipole having a radial distance from the apex which bears a constant ratio 1- to the radial distance of the dipole next farthest removed from the apex. Also, in a conventional log periodic antenna, the length of each dipole bears the constant ratio 1- to the length of the dipole next farthest removed from the apex. The present invention varies from such conventional relationship.

More specifically, as the distance from the vertex 41 0f the antenna increases, the lengths of the dipoles increase in an ever increasing ratio. The ratio of the radial distance of each dipole from the vertex to the radial distance of the dipole next farthest removed from the vertex still remains constant, however, as in the case of a conventional log periodic antenna structure.

The functional result of the arrangement shown in FIG. 1 is as follows. It has been found in an array, such as FIG. 1, that as the height of a given dipole above ground exceeds one-quarter wavelength and approaches one-half wavelength, the vertical radiation pattern assumes a maximum directivity at an angle which becomes smaller with respect to the ground plane until, at a certain frequency, the angle of radiation reaches a minimum of about 30. Worded in another way, the dipole elements, such as dipole 20, which resonate at the lowest frequency are positioned a distance a little greater than )./4 above the ground plane. Such positioning allows field reinforcement at the zenith and the resultant pattern is cosinusoidal, with the maximum located at the zenith. On the other hand, a dipole element which resonates at the highest frequency of the operating range is positioned about M2 above ground, thus allowing field cancellation at the zenith while reinforcement occurs at an elevation angle of approximately 30, depending upon local soil conductivity. In order to achieve the desired radiation characteristics, it is necessary to sacrifice the constant impedance characteristic that log periodic antenna exhibit. However, the advantages obtained by the variation in the elevation angle outweight the loss in the constant impedance characteristics for particular applications of the antenna. Thus, in summary, if a frequency whose half wavelength is equal to the length of dipole is being radiated, the direction of radiation will be concentrated at` the zenith with respect to the ground plane. On the other hand, if the frequency of the signal being radiated has a half wavelength substantially equal to the length of dipole 29, then the direction of radiation will have a much larger horizontal component.

Reference is made to FIGS. 3, 4, and 5 which show the H plane radiation pattern for the antenna of FIG. l at three different frequencies. In FIG. 3 there is shown the H plane radiation for a low frequency such as might be radiated from dipoles 20 or 21. FIG. 4 shows the H plane radiation pattern for intermediatefrequencies, i.e., signals that might have a half wavelength equal to the length of dipoles 25 or 26, for example. FIG. 5 shows the H plane radiation pattern for a signal of a frequency having a half wavelength equal to the length of dipoles 32 or 33, for example.

FIGS. 3, 4, and 5 show graphically that as the frequency increases the direction of radiation changes and forms an increasingly smaller angle with the ground plane, thus providing different paths for different frequencies. The higher frequency signals transmitted will radiate at a smaller angle with the ground plane than lower frequencies and, consequently, will have longer radiating paths. The lower frequencies are employed'for transmission paths of shorter length.

While the antenna will operate effectively with different physical configurations, it is necessary that the over-all configuration and the distance of the dipoles from ground follow the general principles outlined above, i.e., that the longer dipoles be hom about one-quarter to about three-eighths of a wavelength above the ground plane and that the shortest dipoles be about a half wavelength from the ground plane. The actual curve formed by the ends of the dipoles can vary. One preferred embodiment, however, is that they form a cubic parabola as expressed by the following equation:

Y3=HX2 where X is the distance from the vertical center line (point 41 of the antenna array), where Y is the vertical distance from the ground plane of a dipole, and where H is the height of the antenna array.

A more general expression for the curve defined by the ends of the dipoles is as follows, however:

where pis any exponent between 1 and 2.

It has been found that to extend the lower frequency range of the antenna, inductors can bey added. More specifically, in FIG. 1 inductor 44 can be added to the dipole 20 to increase the effective length thereof. To provide the required phasing for feeding the effectively lengthened dipole element there have also been added inductors 42 and 43 in each of the conductors of the transmission lines 45 and 46.

The angle which defines the extremities of the zigzag transmission line configuration is such that the distance between the inner ends of the two sectionsV of any given dipole is approximately one-quarter to one-fth the length of the entire dipole. The ratio of one-quarter to one-fifth is not critical and could extend some on either side thereof without seriously aEecting the operation of the structure. 'Ihe two towers 35 and 36 are supported by suitable guy wires, such as guy wires 37 and 38.

Referring now to FIG. 2, there is shown a sketch of the antenna over a ground plane. The actual antenna is designated by the reference character 63 and is mounted vertically above ground in the same manner as shown in FIG. 1. However, the actual supporting structures are not shown. The image antenna 65 provides the image against which the actual antenna feeds.

It is to be noted that the form of the invention shown and described above is but a preferred embodiment thereof and that various changes may be made in the configuration without departing from the spirit or the scope of Vthe invention. More specifically, the various general configurations of the log periodic antenna, shown in U.S. Patent 3,127,611 issued to R. H. Du Hamel et al. on Mar. 3l, 1964, and entitled, Side Loaded Logarithmically Periodic Antenna, can be modified in accordance with the principles set forth herein to accomplish the purposes hereof.

We claim:

1. An antenna structure having the general shape of an isosceles triangle and comprising:

a plurality of dipole-like elements whose centers lie on the bisector of the unequal angle of said isosceles triangle; each dipole-like element consisting of two half elements extending generally transversely out from either side of said bisector with the outer ends of said dipole-like elements defining the equal sides of said isosceles triangular shape; the half elements of the dipole-like elements on either side of said bisector being substantially parallel to each other; the radial distance from the vertex of the triangle to any given point on any given dipole-like element bearing a constant ratio 1- to the radial distance to the corresponding point on the dipole-like element next farthest removed from said vertex; the said dipole-like elements having an increasing length as their distance from said vertex increases to define the equal sides of the general isosceles shape of the antenna structure as being concave in nature; means for mounting said antenna over the ground plane with the bisector of said unequal angle being substantially perpendicular to said ground plane and with the vertex of said triangular antenna configuration pointed towards said ground plane; the shortest of said dipole like elements being positioned above the ground plane a distance slightly greater than -I/Z where )t1 equals the wavelength of the frequency at which said shortest dipole resonates; and the longest of said dipole-like elements being positioned above the ground plane a distance slightly greater than 2/4 where A2 equals the wavelength of the frequency at which said longest dipole-like element resonates. 2. An antenna structure in accordance with claim 1 comprising:

feeding means for feeding said dipole-like elements; said feeding means comprising a pair of conductors,

each of which is connected to alternate half sections of successive ones of said dipole-like elements; each of the conductors of said pair of conductors being connected to opposite half sections of any given dipole-like element. 3. An antenna structure in accordance with claim 1 in which the concave sides of said triangular conguration follow the expression:

YP=HX where Y is the distance of a dipole Ifrom the ground plane;

H is the total height of the antenna from the ground plane; and

X is one-half the length of a dipole and where:

4. An antenna structure in accordance with claim 3 comprising:

feeding means for feeding said dipole-like elements;

said feeding means comprising a pair of conductors,

each of which is connected to alternate half sections of successive ones of said dipole-like elements,

each of the conductors of said pair of conductors being connected to opposite half sections of any given dipole-like element.

5. An antenna comprising:

a plurality of radiating elements positioned substantially parallel to each other and forming a generally isosceles triangle configuration;

the midpoints of said `radiating elements lying substantially along the bisector of the unequal angle of said triangular configuration; the ends of said radiating elements defining the equal sides of said triangular configuration;

the radial distance of any point on a given radiating element measured from the vertex of said triangular configuration bearing a constant ratio -r to the radial distance of the corresponding point on the radiating element next farthest removed from said vertex;

and the lengths of said radiating elements increasing as their distance from said vertex increases to define the equal sides of said triangular conguration;

means for mounting said antenna over the ground plane with the bisector of said unequal being substantially perpendicular to said ground plane and with the vertex of said triangular antenna configuration pointed towards said ground plane;

the shortest of said radiating elements being positioned above the ground plane a distance approximately equal to )t1/2 where A1 equals the wavelength oi the frequency at which said shortest radiating element resonates; and

the longest of said radiating elements being positioned above the ground plane a distance approximately equal to A2/4 where k2 equals the wavelength of the frequency at which said longest radiating element resonates;

and where each of said radiating elements is divided into two half sections extending generally outward from said bisector of said unequal angle. 6. An antenna in accordance with the claim 5 com prising:

feeding means for feeding said radiating elements; said feeding means comprising a pair of conductors, each of which is connected to alternate half set:J tions of successive ones of said radiating elements;

each of the conductors of said pair of conductors being connected to opposite half sections of any given radiating element.

7. An antenna in accordance with claim 5 in which the concave sides of said triangular configuration follow the expression:

YP=HX where:

Y is the distance of a radiating element from the ground plane; H is the total height of the antenna from the ground plane; and X is one-half the length of a radiating element; and

where:

8. An antenna in accordance with claim 7 comprising:

feeding means for feeding said radiating elements;

said feeding means comprising a pair of conductors, each of which is connected to alternate half sections of successive ones of said radiating elements;

each of the conductors of said pair of conductors being connected to opposite half sections of any given radiating element.

References Cited UNITED STATES PATENTS 3,120,767 10/1965 Isbell 343-792.5 3,219,457 10/1965 Carr 343-7925 ELI LIEBERMAN, Primaly Examiner.

HERMAN K. SAALBACH, Examiner.

P. L. GENSLER, Asssstant Examiner.

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