US3594807A

US3594807A - Vertically polarized log-periodic-like antenna with minimal tower height

Info

Publication number: US3594807A
Application number: US839305A
Authority: US
Inventors: Robert L Tanner
Original assignee: Communications Technology Corp
Current assignee: Fluke Electronics Corp; Technology for Communications International
Priority date: 1969-07-07
Filing date: 1969-07-07
Publication date: 1971-07-20
Anticipated expiration: 1988-07-20

Abstract

A vertically polarized broadband antenna including a logperiodiclike array of elements, providing the high gain at the higher frequencies, characteristic of a conventional dipole-type vertically polarized log-periodic antenna, and reduced antenna height characteristic of a conventional monopole-type vertically polarized log-periodic antenna. The log-periodiclike array includes conventional dipoles at the array front end and hybridtype elements towards the array back end. Each hybrid element has a physical length less than lambda /2 of its designed frequency of resonance. Each hybrid element is capacitively loaded to compensate for its reduced physical length. At least some hybrid elements are electrically connected to electrically conductive segments of the main support catenary, to further compensate for their reduced physical lengths.

Description

United States Patent [72] Inventor Robert L.Tanner 3,470,559 9/1969 Radford 343/7925 2] A l N gggggg Primary ExaminerEli Lieberman I Assistant Examiner-Marvin Nussbaum med July Atiorne s-Samuel Lindenber and Arthur Freilich 4s Patented July 20,19711 y g [73] Assignee Technology For Communications International Mountain View, Calif.

ABSTRACT: A vertically polarized broadband antenna in- [54] v -n POLARIZED LOGJERIODIGUKE cl uding a log-periodiclike array of elementsproviding the ANTENNA WIT" MINIMAL TOWER HEIGHT high gain at the higher tirequencies, characterist c ofa conven- 22 Claimsfl Drawing Figs. tional dipole-type vertically polarizeddog-periodic antenna,

and reduced antenna height characteristic of a conventional U.S. mono ole.type ertically polarized ]og-pe iodic antenna The 343/752'343/8l2343/886 log-periodiclike array includes conventional dipoles at the [5 I] Int. Cl ..H0 lq 11/10, array from end and hybricptype elements towards the array q 21/12 q 1/14 back end. Each hybrid element has a physical length less than of Search t of its designed frequency of resonance Each ele- 810-812816,802803,752,886 ment is capacitively loaded to compensate for its reduced h sical len th. At least some h brid elements are electricall [56] References Cited :oznected t o electrically condi ictive segments of the mail UNITED STATES PATENTS support catenary, to further compensate for their reduced 3.389.396 6/1968 Minerva et al t. 343/7925 physical lengths.

VERTICALLY POLARIZED LOG-PERIODIC-LIKE ANTENNA WITH MINIMAL TOWER HEIGHT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention generally relates to broadband antennas, and, more particularly, to an improved vertically polarized log-periodiclike antenna having all of the desirable features of a log-periodic antenna with additional structural and electrical advantages.

2. Description of the Prior Art Vertically polarized log-periodic antennas of the dipole and monopole-types are well known in the art. The advantages, as well as the disadvantages of one type over the other are, likewise, well known. Briefly, a vertically polarized logperiodic antenna (VPLPA) of the dipole-type, hereafter also referred to as a dipole VPLPA, includes a plurality of radiating dipole elements, arranged in a log-periodic array which are fed by means of a balanced feed line, consisting of a pair of feed wires. Such an antenna exhibits considerably higher gain than that realizable with a monopole-type VPLPA in which monopole elements of an array are fed by an unbalanced feed line, usually consisting of a single wire supported above ground. The feed arrangement of a monopole VPLPA as well as the fact that there are usually large currents flowing onto the ground from the lower ends of the active elements requires that the monople VPLPA be provided with an elaborate and quite expensive ground screen to render the ground highly conductive. The dipole VPLPA does not require elaborate ground screening arrangements.

The ground screening problems, complexity and cost are particularly great near the front end of the array in the monopole VPLPA, whereat the monopole elements, designed to radiate at the frequencies near the upper limit of the frequency band for which the antenna is designed, are located. The only apparent advantage of a monopole VPLPA over a dipole VPLPA designed to operate over the same frequency band is the reduced height of the support structure, primarily the main support tower, which is necessary to support a monopole VPLPA as compared with the height of the tower needed to support the dipole-type VPLPA.

It is primarily for the latter-mentioned advantage that the monopole-type VPLPA is used, whenever the antenna has to be placed where the vertical room or space is limited. A typical example of limited vertical space is a location near an airport where strict limitations are placed on tower heights so as not to pose a hazard to low-flying airplanes. Shorter antenna support towers are also desirable where the antenna may be subjected to harsh environmental conditions such as high winds. However, accompanying the use of the monopole VPLPA are the various disadvantages hereinbefore referred to. Thus, a need exists for a vertically polarized logperiodiclike antenna which exhibits the relatively short tower height characteristic of a monopole VPLPA yet does not possess the disadvantages characteristic of such an antenna. That is, a need exists for a broadband vertically polarized antenna which can be supported by a relatively short tower yet have high gain, particularly at the higher frequencies near the upper limit of the frequency band, and one which eliminates or substantially reduces the ground screening problems characteristic of a monopole-type VPLPA. Another desired characteristic of such an antenna is that it be fed from a balanced feed line arrangement so as to be compatible with feed arrangements employed in dipole-type antennas, which represent the majority of the log-periodic antennas that are actually constructed and used at present.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a new, improved, vertically polarized, broadband antenna, similar in characteristics to a log-periodic antenna.

Another object of the present invention is to provide a new, vertically polarized antenna which possesses most of the advantages, characterizing both monopole and dipole-type VPL- PAs.

A further object of the present invention is to provide a new vertically polarized log-periodiclike antenna which is supportable by a tower of height comparable with or less than the height of the tower in a conventional monopole-type VPLPA, yet provides gain at frequencies near the upper limit of the frequency band which is comparable or superior to the gain realizable at such frequencies in a conventional dipole-type VPLPA.

A further object of the present invention is to provide a new vertically polarized antenna which is supportable by a tower which is shorter than the tower of conventional monopoletype VPLPA, operable over the same frequency band, and one in which the disadvantages of such a conventional antenna are substantially eliminated or greatly minimized.

Still a further object of the present invention is to provide a new, improved vertically polarized antenna, supportable by a tower of minimum height in which the ground screening problems, characterizing a conventional monopole-type VPL- PA, are substantially reduced.

Yet a further object of the present invention is to provide a new, improved log-periodiclike antenna, supportable by a tower of minimum height in which the elements of the array of the antenna are fed by means of a balanced feed line consisting of a pair of feed wires, as is the ease in a conventional dipole-type VPLPA.

These and other objects of the present invention are achieved by providing a vertically polarized log-periodlike antenna in which different type radiating elements, rather than elements of the same type, are incorporated in forming the antennas array, and which which are connected to a pair of feed wires and to the antenna support structure, including a main support catenary in a way so as to achieve the aforementioned objects. Briefly, the elements of the antenna array are fed by means of a balanced two-wire feed line, of the type used in conventional dipole log-periodic antennas. The array elements may be divided into at least two basic types. The shorter elements, near the front end of the array, which are designed to resonate at frequencies at and near the upper limit of the frequency band, represent one type. These elements comprise conventional dipoles, which are connected to the two feed wires as in any conventional dipole VPLPA. Thus, in the novel antenna of the present invention, the high gain at the higher frequencies, realizable in a dipole VPLPA is also achieved without the complex ground screening problems which are present in a conventional monopole-type VPLPA near the arrays front end.

The antenna array also includes a second type of elements, which hereafter may be referred to as hybrid dipole elements, or hybrid elements. Each hybrid dipole element, like a true dipole element in a conventional dipole-type VPLPA, consists of two parts or poles, each one of which is connected to a different one of the two feed wires. However, whereas in the prior art the two poles of each dipole are of equal physical length, the length of each pole being equal to M4, where A is the free space wavelength of the frequency at which the dipole is to resonate, in the present invention the two poles of each hybrid dipole element may be, and typically are, of different lengths. Also, the total physical length of each hybrid dipole element is less than M2 of the frequency at which the particular element is designed to resonate.

To compensate for such length differences, one or both poles of each hybrid dipole element are electrically loaded so that their equivalent electrical lengths are substantially equal, and the element's total equivalent electrical length is equal to )t/2. thereby enabling the element to properly resonate at the particular frequency for which it is designed. As used herein the term equivalent electrical length" refers to the impedance resonance characteristics. Thus, a dipole having an equivalent electrical length of M2 is a resonant element.

Similarly. a monopole having an equivalent electrical length of M4 represents an element at resonance Thus, the physical length of each hybrid dipole element in the novel antenna of the present invention is shorter than a conventional dipole element. designed to resonate at a corresponding frequency. Consequently, the overall height of the array in the antenna of the present invention. as compared with the height of a conventional dipole-type VPLPA for the same frequency band, is reduced, thereby enabling the array to be supported by a shorter support tower, forming part of the antennas support structure.

Unlike prior art VPLPAs, in the antenna of the present invention the feed wires, in addition to performing their electrical function by coacting, electrically with the various elements of the array, are connected to the antenna support structure in a manner which enables the feed wires to perform a load support function. Thus, the feed wires not only do not contribute to the overall structural load which the antenna support structure has to support, but rather, minimize such load, to enable a smaller and shorter support structure to be utilized.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are diagrams used to summarize the features of a conventional dipole VPLPA;

FIGS. 3 and 4 are diagrams used to summarize the features ofa conventional monopole VPLPA;

FIG. 5 is a side view of one embodiment of the novel antenna ofthe present invention;

FIG. 6 is an electrical schematic diagram of the antenna shown in FIG. 5; and

FIG. 7 is an electrical schematic diagram of three hybrid elements of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel features of the novel VPLPA of the present invention may best be explained by first summarizing the conventional structural and electrical characteristics of conventional dipole and monopole-type VPLPAs. The features of a conventional dipole-type VPLPA will be described in conjunction with FIGS. I and 2, while FIGS. 3 and 4 will be utilized to summarize the features or characteristics ofa conventional monopole-type VPLPA. In these figures, as well as in the others, to be employed in the description of the invention, similar elements will be designated by like numerals.

In FIG. 1, an antenna array 10, comprising a plurality of radiating elements 11 through 25, is shown disposed in a vertical plane above a reference surface, such as ground 30. Each element is also shown in FIG. 2, which is an electrical schematic diagram of the dipole VPLPA. Elements 11 and 25 are assumed to be at the front and back ends of the array, respectively. Each radiating element is a dipole, a pole designated by the element's reference numeral followed by the suffix letter t, and a bottom pole, which is designated by the element's reference numeral followed by the suffix letter b. The two poles which together form the dipole element, or dipole for short, are connected to a pair of feed wires, both of which are designated by the same reference numeral 35. The two wires form the feed line which hereafter will be designated by the same numeral 35. The dipoles are connected in a transposed manner whereby alternate poles on each side of the feed line are connected to the same wire for reasons well known and appreciated in the art.

As seen from FIG. I, in a conventional dipole VPLPA, the bottom ends of the bottom poles are disposed at or very near the ground surface 30, in electrical insulation therefrom, while the top ends of the top poles, the top ends being designated by the element's reference numerals followed by the letters d, are connected to a main support catenary 40, through individual respective drop rods 41. The elements are in electrical insulation from the drop rods, as represented by the small circles therebetween. Hereafter, an electrical connection in any of the figures is represented by means of a solid dot, while a circle or hollow dot represents an electrically insulating type connection. The main support catenary 40 is supported at its upper end by a main support tower 42, while the other catenary end is supported by a front pole 44. A plurality of tower guys 43 and one or more front pole guys 45 are used to secure the vertical position of the tower and front pole, respectively.

As is appreciated by those familiar with the art, in a true logperiodic array, the physical length of each dipole corresponds approximately to one-half the free space wavelength ()\/2) of the signals at the frequency at which the dipole element is designed to resonate or be excited. The relative lengths of adjacent adjacent dipoles, as well as their relative distances in a straight line, such as along the feed line 35, from an array apex 50 (see FIG. 2), are fixed in a log-periodic relationship. The array apex 50 represents a point at which the locus of all the top ends of the top poles in the form of a straight line converges onto the locus of the bottom ends of the bottom poles, which in a conventional dipole VPLPA is represented by ground. Defining the length and distance of a dipole element from the apex as L,, and X,,, respectively, and the length and distance of the adjacent next largest dipole element, disposed towards the back end of the array, as L,, and X,,,,, the rela' tive lengths and distances of the dipole elements may be expressed as As is appreciated by those familiar with the art, when the bottom ends of the dipoles are insulatably connected to ground, as is the case in every conventional dipole-type VPL- PA, the feed wires 35 extend across the array 10 in a straight line. Therefore, from a structural point of view, they cannot contribute to supporting the array. Rather, the feed wires represent an added load, which has to be supported by the antenna support structure, comprising the main catenary 40, the tower 42 and front pole 44. As is further appreciated, the height of the tower 42 is always greater than the total physical length of the longest element, which is equal to approximately M2 of the lowest frequency in the band for which the antenna is designed. In a conventional dipole-type VPLPA, designed to operate at a lowest frequency of 3 megacycles (mc.), the tower height is generally not less than 220 feet.

As previously pointed out, such a dipole-type VPLPA is nearly always preferred over a monopole-type VPLPA, except where the total tower height of the dipole antenna exceeds established height limitations. In such a case, herebefore, resort was made to a monopole-type antenna, one example of which is diagrammed in FIG. 3. Basically, in such an antenna, the antenna support structure is the same as the one shown in FIG. 1, comprising the tower 42, the front pole 44, the main catenary 40 and the various drop rods 41, from which the top ends of the radiating elements are suspended. However, instead of employing radiating elements which are dipoles, only single pole or monopole, quarter wavelength (ll/4) elements, designated in FIG. 3 as 11! through 25! are employed. The monopole elements or monopoles, are connected to a singlewire feed line 35.

As shown in FIG. 3, the line 35 is fed from a source 50 with respect to ground in unbalanced fashion, and is coupled to the elements at a location a short distance above their lower ends, which may be grounded, as shown in FIG. 3, by a coupling device 36, frequently a capacitor of appropriate size or a transmission line coupling section, illustrated schematically in FIG. 3. Another monopole construction frequently employed is illustrated in FIG. 4, where the bases of the monopole elements are connected directly to the feedline 35 and additional networks 37 are connected in series with the line to produce added phase delay between radiators.

As previously pointed out, the only advantage of such a monopole VPLPA over the dipole-type is the reduced tower height. However, this advantage is achieved at the price of several major disadvantages, including reduced gain which is particularly undesirable at higher frequencies. Another major disadvantage is the need for an elaborate ground screening arrangement, designated in FIG. 4 by numeral 55. The cost and complexity of the screening arrangement is particularly great near the front end of the array, whereat are located the short monopole elements, designed to be excited or radiate at the higher frequencies near the upper limit of the frequency band. This is true, since a screen with a much finer mesh must be at the front end of the array to insure that the screen wires are sufficiently close to one another with respect to the wavelengths of the frequencies which they are to screen. In Figure 4, the finer mesh screen, near the front end of the array, is designated by numeral 57.

As previously briefly stated, the novel antenna of the present invention is an improved vertically polarized antenna, which possesses substantially all the advantages realizable by a conventional dipole-type VPLPA, yet is supportable by a tower of a height which is generally not greater than the height of a comparable prior art monopole-type VPLPA, without exhibiting the disadvantages which characterize a monopoletype VPLPA. Alternately stated, the antenna of the present invention is a vertically polarized antenna which possesses all the advantages of both dipoleand monopole-types VPLPA's with substantially none of the disadvantages of either antennatype. These advantages are realized by providing an antenna with elements which form an array which is not a true logperiodic array as accepted in the art. Rather the elements form an array which is like a log-periodic array. The differences between a true log-periodic array and the one incorporated in the present invention will be discussed after first describing a basic embodiment of the invention. Since the novel antenna incorporates a log-periodiclike array, the antenna will be referred to as a vertically polarized logperiodiclike antenna or VPLPLA. For convenience, however, it may also be referred to simply as the novel antenna.

The novel antenna incorporates an array of radiating elements which can be thought of as being of two basic types. A plurality of the elements in the array which are designed to resonate at the higher frequencies in the design bandwidth, and which are disposed at and near the front end of the array, consists of conventional dipole elements, such as those shown in FIGS. 1 and 2. The dipole elements represent one type of element. Following these dipole elements in the array, towards the back end thereof, is included a plurality of hybrid-type dipole elements, which represent the second type of elements. Each hybrid element, like a conventional dipole element, comprises two individual separate poles. However, whereas in a true dipole, the two poles are of equal physical length, in the present invention, the two poles or parts of each hybrid dipole element, or simply hybrid element, are not of equal length.

These and other features of the novel antenna may best be explained by referring to FIG. 5, which is a side view of one embodiment of the novel VPLPLA of the present invention, and to FIG. 6, which is a schematic electrical diagram of the novel antenna, shown in FIG. 5. The novel antenna 60 includes a plurality of radiating elements 61 through 86, which are disposed in a vertical plane above ground 36 to form a log periodiclike array 90 which is supported above ground by means of a support structure. The structure includes a main support tower 42, vertically supported by means of guys 43, a front pole 44, vertically disposed above ground by means of front pole guys 49 and a main catenary 92. The catenary 92 differs from a conventional catenary, as will be described hereafter in detail.

Each of the radiating elements all through as, which as shown in FIG. 6 comprises two poles, designated by the elements reference numeral followed by the suffix letters t and b, is electrically connected to a pair of transposed feed wires 95 in a manner identical with that employed in a conventional dipole antenna (see FIG. 2). The two wires 95 comprise a feed line 95. However, whereas in a conventional dipole VPLPA, the feed line is disposed in a straight line and performs only an electrical function, by being electrically connected to the various dipoles to electrically coact therewith, in the present invention, the feed line 95, in addition to performing a similar electrical function, also performs a significant antenna support function. This is achieved by connecting the feed wires to the various elements so that the feed wires are capable of acting as a support catenary, whose ends are mechanically and structurally coupled to support tower 42 and the front pole 44, by means of

coupling elements

96 and 97, respectively. Consequently, the feed wires acting as a support catenary, hereafter also referred to as the feed line catenary, not only do not contribute to the overall load which the main catenary 92 has to support, but rather they reduce said load so that the sag of the main catenary 92 is significantly reduced, thereby enabling the utilization ofa shorter main tower 42.

An even further and more significant reduction in the tower height is realized, however, by incorporating in the array hybrid dipoles, including a longest hybrid dipole 86, whose physical length is in the order of one-fourth wavelength (it/4) of the lowest frequency in the antenna bandwidth, at which hybrid dipole 86 is designed to resonate. Consequently, the height of the tower 42 is in the range of one-fourth of the wavelength of the lowest frequency of the designed band width, rather than more than one-half the wavelength of such a frequency, as is the case in a conventional dipole-type VPL- PA.

In accordance with the teachings ofthe present invention, the array 90 includes elements 61 through 73 located at and near its front end, adjacent front pole 44. These elements, which are true dipoles, represent :1 first type of elements. Their top ends are supported by catenary 92 and their bottom ends are shown secured to ground through cable 97 (FIG. 5).

The array also includes hybrid elements 74 through 86, disposed towards the back end of the array following the true dipoles. Whereas, the physical length of each of elements 61 through 73 is substantially equal to its electrical length, i.e., one-half wavelength of the frequency at which it is to resonate, with each of its poles being equal to a fourth wavelength, in each of hybrid elements 74 through 86, the actual physical length of the element differs and is smaller than its equivalent electrical length.

The difference between the physical and equivalent electrical length of each hybrid element increases for elements successively disposed towards the back end of the array. Also, the two poles or parts of each hybrid element need not be, and usually are not, of equal physical length. However, in order to insure that each hybrid element resonates at its design frequency, each is loaded to compensate for its reduced physical length and in such a way that the equivalent electrical length of its two poles are equal, i.e., resonate at the same frequency.

The hybrid elements 74 through 86, shown in FIGS. 5 and 6, may be divided into three subgroups designated G1, G2 and G3, comprising hybrid elements 74-77, 73-81 and $2-86, respectively, In group Gil, in each hybrid element (such as 74) its top pole (such as 74!) is disposed in a vertical direction between the feed line 95 to which it is electrically connected and the catenary 92. Herein, the catenary may be assumed to comprise of an electrically conductive cable, although broken byinsulators into electrically inactive segments. The onefourth of each element is insulatably connected to the catenary, as represented by the small circles in FIG. 5. The bottom pole of each hybrid element in group G1 is connected between the feed line 95 and ground in electrical insulation from the latter. In this group, the physical length of the bottom pole is assumed to be less than the physical length of the top pole. Consequently, the bottom pole of each hybrid element in the group is loaded, such as by means of a capacitor, designated by the elements numeral followed by the letter c," to ground. The capacitance value of each capacitor is selected so that from an electrical point of view the bottom pole has an equivalent electrical length which is equal to the equnalent electrical length of the top pole at the frequency at which the hybrid element is to resonate That is. in the particular example, the capacitance is selected to insure that the equivalent electrical length of each bottom pole is equal to one quarter wavelength of the frequency at which the element is to resonate In the embodiment shown in FIGS. and 6, the top pole of each hybrid elements 78 through 86. which is disposed in a vertical direction between the feed line 95 and the catenary 92, has a physical length which is less than a fourth wavelength of the frequency at which the element is designed to resonate. The top pole of each of these elements is electrically connected to a selected segment of the electrically conductive cable which forms the top catenary 92. The electrically conductive segment of the catenary, which is connected to each hybrid element, is designated by the element's reference numerals followed by the suffix letter The catenarys electrically conductive segment, which is electrically connected to each of these elements, is selected to be of a length and disposed in a direction so that it electrically compensates for the reduced physical length of the top pole with respect to a fourth wavelength at the frequency, at which the element is to resonate. Consequently. the top element has an equivalent electrical length substantially equal to a fourth wavelength. The only difference between the hybrid elements in group G2 and those in group G3 is that in the former the catenary segment which is electrically connected to each hybrid element is only a portion of the catenary segment or part between adjacent hybrid elements, while in the latter the full catenary segment between adjacent hybrid elements is utilized to augment the physical length of the top pole of each hybrid element.

From the foregoing it is thus seen that unlike.prior art arrangements in which the catenary performs merely a support function, in the present invention selected segments thereof are utilized to perform, together with the hybrid elements to which they are connected, a significant electrical function. In FIG. 5, numeral 101 represents a single section ofthe catenary 92 to which the top ends of elements 61-77 are connected in electrical insulation therefrom, while numerals 102-104 represent catenary sections which are in electrical insulation from electrically conductive parts ofdifferent adjacent hybrid elements. Since one or more segments of the catenary are used for electrical as well as mechanical purposes the term cable" is employed, to distinguish the catenary, from other electrically conductive elements which do not perform any mechanical function. The term cable" should be interpreted in a broad sense to include one or more electrically conductive wires or elements, capable of performing the required load support function.

From the foregoing it should be appreciated that the longest element in the array 90, which is hybrid element 86, has a physical length which is significantly less than a N2 of the frequency at which it is designed to resonate. This is true since either of its two

poles

86b and 86c is less than M4. Consequently, the overall array height is reduced as compared with the height of an array in a conventional dipole VPLPA, designed to include the same frequency. As a result, the novel array 90 can be supported by a shorter tower than is needed in a conventional prior art dipole VPLPA. Furthermore, by utilizing the feed line 95 to act as a feed line catenary, which supports part of the antenna's structural load, the structural load which the main catenary 95 has to support is reduced. Consequently, the required catenary sag is reduced, enabling a further reduction in tower height.

lt should be appreciated, that in the novel VPLPLA of the present invention, since true dipole elements, such as elements 61 through 73, are incorporated in the front end of the array, high gain is realizable at the higher frequencies, at which these elements resonate, as is the case in a conventional dipole antenna. Also, since true dipole elements are employed at the higher frequencies. the difficult ground screening problems near the array front end, which are present in a monopole type VPLPA (see FIG. 4) are eliminated by the present invention. Unlike a true dipole VPLPA in which ground screening is not required at all, in the present invention some ground screening is needed for the hybrid elements which are capacitively coupled to the ground. However, since such couplings are only present for hybrid elements, which are excitable at lower frequencies, the ground screening problem is greatly minimized and, therefore, does not represent a significant disadvantage. Although capacitive coupling is diagrammed in FIG. 5 as being accomplished by means of capacitors, it is appreciated that any known technique, such as use of line sections, may be employed to provide the necessary capacitance between the bottom ends of the various hybrid elements and ground.

The foregoing described teachings have been incorporated in the construction of a novel VPLPLA, designed to operate over a frequency range from 2 to 30 megacycles. A logperiodiclike array of 28 dipoles, 13 of which were of the hybrid-type, was constructed. The hybrid elements varied in length from a shortest hybrid dipole of 44 feet in length to a longest one of approximately 1 l 1 feet, the latter being designed to be excitable at 2 megacycles. Capacitors, varying in capacitance values, from 16 picofarads (pf) to 400 picofarads were employed to capacitively couple the bottom ends of the hybrid dipoles to ground. Electrically conductive catenary segments, varying in length from approximately 2 feet to over 30 feet, were electrically coupled to the top ends of various ones of the hybrid elements to significantly reduce the maximum array height. The feed wires were employed as a feed line catenary, so that the total antenna array was supportable by a main support tower 42 of a height of one not more than feet. By comparing such height with a tower height of feet, which represents the minimal tower height necessary to support a conventional monopole-type VPLPA with all its attendant disadvantages hereinbefore described, it becomes apparent that the novel VPLPLA of the present invention represents a major significant advance in the state of the art. From an electrical performance point ofview, the performance of the novel antenna of the present invention can only be duplicated by a dipole-type VPLPA. However, such an antenna would require a tower which is more than twice the 125 feet height ofthe tower ofthe present invention.

As previously stated, the array 90, incorporated in the novel antenna, is a log-periodiclike array rather than a true logperiodic array. The latter type array is generally defined as consisting of identical type elements, such as true M2 dipoles, whose lengths and distances from the array apex are related by expression From the foregoing it EiBfiidBciEr that array 90 is riot a true log-periodic array in at least one respect. The elements of array 90 are not of the same type. Only some of its elements, such as elements, such as elements 61 through 73, are true M2 dipoles, while the rest of the elements are hybrid elements. Thus, if for no other reason array 90 should be regarded as a log-periodiclike array.

in practicing the teachings of the present invention it has been discovered that in order to achieve the advantageous properties of the novel antenna, the relative lengths of all the elements in the log-periodielike array 90, need not be defined by a single value of o", as is the case in a true log-periodic array. That is, array 90 may be formed of groups of elements, with the relative lengths of the elements in each group being defined by a different value of 0'. For example, for the elements in array 90, shown in FIG. 5, their lengths relationships may be expressed as,

where I: may be any one ol elements 61 through 72. and in may be any one of elements 74 through 85. and wherein 0,. It should however be pointed out that in order to retain the electrical performance. typical of a log-periodic array. the difference between a, and 0- should not be excessive In an embodiment of the invention actually reduced to practice, an array was formed with a or,=l-l25 and a a =l-l83 with a difference of about 5 percent While the lengths of all the elements in the array 90 need not be. and often are not. related by a single value of o their distances from the array apex 100 are related by a single 0' value. Thus. the relative distances may be expressed as.

line, representing an X axis.

It should further be pointed out that in the novel antenna the array apex 100 is above ground, unlike the apex of the array in a conventional vertical polarized log-periodic array, which is at ground level. Thus, the location of the array apex represents another feature of the log-periodiclike array which deviates from a true log-periodic array.

There has accordingly been shown and described herein a novel VPLPLA which includes substantially all the electrical performance characteristics of a dipole-type VPLPA, yet is supportable by a main support tower whose height is substan' tially less than the height of a support tower, needed to support a conventional dipole type vertically polarized logperiodic antenna. The reduced tower height of the antenna of the present invention as well as its advantageous electrical properties are realizable by incorporating therein a logperiodiclike array, comprising a plurality of conventional true dipole elements, as well as plurality of hybrid dipole elements. The true dipole elements are incorporated at and near the front end of the array to be excitable at the higher frequencies in the frequency band for which the antenna is designed, in order to provide the high gain realizable by true dipoles in a typical dipole-type VPLPA.

The hybrid dipole elements in the antenna of the present invention, each one of which has a physical length which is less than its equivalent electrical length in terms of the wavelength of the frequency at which it is to resonate, are incorporated in order to reduce the overall antenna height toward the back end of the array, and thereby enable the antenna to be supported by a short tower. The tower height is further reduced by incorporating in the array a feedline which in addition to performing electrical functions also performs a catenary support function, thereby enabling a main catenary having a smaller sag to be used. Each hybrid dipole element comprises element comprises two poles, one or both of which are electrically loaded at ends thereof, farther from the points at which the poles are at electrical contact with the feedline. Loading is used to compensate and control the equivalent electrical length of each pole to be equal to a fourth wavelength (M4) of the frequency at which the element is to resonate.

In the particular embodiment hereinbefore described, each hybrid dipole element includes two poles which are disposed in a vertical direction, with the bottom pole being capacitively loaded by means of a capacitor which is coupled to ground. The top ends of the top poles of some of the hybrid elements are electrically coupled to electrically conductive segments of the catenary to effectively increase their lengths to match the equivalent electrical lengths of the corresponding opposite bottom poles of the elements. Although such an arrangement is deemed to be preferable, it is apparent that if desired the top ends of the top poles of the various hybrid elements may be loaded by other than the electrically conductive segments of the catenary to increase their equivalent lengths. For example,

Ill

the top poles may be top loaded by bifurcating or branching each wire at a point near its top and attaching the two branching wires both to the catenary by means of insulators as shown in FIG. 7. To simplify FIG. 7, only three hybrid dipole elements are shown therein with two branching wires designated by each elements numeral followed by the letters x and y. The additional capacitance of the added wire provides a top loading effect similar in function to that provided by the segment of catenary to which the radiators are attached as shown in FIG. 6. It should be apparent that more than two branching wires may be employed for leading the top pole of each hybrid element. For example, in FIG. 5, all the elements of the array are shown to be coupled to the catenary directly without any drop rods therebetween. Clearly, the array may be modified so that some element, particularly the shorter elements near the front end, are supported by the catenary 92 through drop rods. Among other possible modifications which may be made within the spirit of the invention, the array of the novel antenna may be designed to include elements whose relative lengths are definable by more than two values ofa. Also, if desired the tower 42 (FIG. 5) may be constructed of an electrically conductive material and made to resonate at a frequency below the frequency of resonance of the longest element 86. In order, however, not to extend the tower height beyond that necessary to support the top of catenary 92, the equivalent electrical length of the tower may be made to be M4 of its design frequency by connecting its top end to an electrically conductive segment 43x of one of the guy wires 43. Segment 43x is electrically insulated from the rest ofthe guy by an electrical insulator represented by 43y.

It is appreciated that those familiar with the art may make other modifications and/or substitute equivalents in the arrangements as shown, without departing from the true spirit of the invention.

Iclaim:

1. An antenna, designed for operation over a selected frequency band including a first frequency and a second frequency, lower than said first frequency, comprising:

a plurality of radiating elements, the equivalent electrical length of each element being a function of the wavelength of signals of a different frequency in said frequency band;

support means for supporting said elements in a selected orientation with respect to a reference surface to form an array of elements wherein the elements are arranged in an order corresponding to their equivalent electrical lengths, with the element whose equivalent electrical length is a function of the wavelength of signals of said first frequency being at a front end of said array, and the element whose electrical equivalent length is a function of the wavelength of signals of said second frequency being at a back end of said array, said array including at least a first element whose physical length is equal to its equivalent electrical length and at least a second element whose physical length is less than its equivalent electrical length, said second element including impedance loading means for compensating for the difference between its physical length and its equivalent electrical length, the elements being supported in spaced relationship from one another to inhibit the coupling of energy therebetween; and

feed means electrically coupled to said elements, said feed means being coupled to said first element at the physical center thereof and to said second element at other than the physical center thereof.

2. The arrangement as recited in claim 1 wherein said feed means further include means coupled to said support means to mechanically coact therewith in supporting said array of elements in said selected orientation with respect to said reference surface, with said feed means acting as a load-bearing member.

3. The arrangement as recited in claim 1 wherein said reference surface is the ground and said selected orientation is a vertical plane with respect to ground, and said impedance loading means comprise capacitive means for capacitively coupling said second element to ground.

4. The arrangement as recited in claim 2 wherein said support means include a main support catenary extending from said array back end to the front end thereof. said catenary having at least one electrically conductive segment, electrical insulating means for connecting selected ones of said elements, at and near the array front end. to said catenary in electrical insulation therefrom, and means for connecting at least one element, located toward the array back end, to said catenary to be in electrical contact with said at least one electrically conductive segment thereof.

5. The arrangement as recited in claim 1 wherein said selected orientation is a vertical plane above said reference surface. with said elements forming a vertically polarized logperiodiclike array of elements, defining an apex above said reference surface about the array front end, each element having at least a portion thereof parallel to corresponding portions of the other elements, and said support means include a main support catenary for directly supporting the top ends of substantially all the elements at and about the array back end in said vertical plane with said feed means further serving as a secondary support catenary to thereby reduce the maximum height of said main support catenary,

6. The arrangement as recited in claim 5 wherein said main support catenary includes at least one electrically conductive segment and said array includes at least one element electrically connected at its top end to said electrically conductive segment ofsaid main support catenary.

7. A vertically polarized log-periodiclike antenna, designed to operate over a frequency band including a first frequency and a second frequency, lower than said first frequency, the antenna comprising:

a plurality of dipoles, each having an equivalent electrical/ length which is a function of one-half the wavelength of the signals of a different frequency in said band at which the dipole is designed to resonate, said dipoles including a first group of dipoles each dipole in said first grouphaving a physical length which is equal to its equivalent electrical length and a second group of dipoles, each dipole in said second group having a physical length which is less than the dipole's equivalent electrical length, said dipoles being arranged to form a log-periodiclike array with the dipole having the shortest equivalent electrical length being disposed at the array front end and the dipole having the longest equivalent electrical length being disposed at the array back end, with the dipoles in said first group being disposed toward the array front end and the dipoles in said second group being disposed toward the array back end;

feed line means;

means for electrically coupling said dipoles to said feed line means at points on said feed line means which are related distances from an apex of said array, each dipole in said first group being coupled to said feed line means at the physical center of said dipole and each dipole in said second group being coupled to said feed line means at other than the dipoles physical center; and

antenna support means for supporting said dipoles in a vertical plane above a reference surface, said support means including a main support tower of a height which is less than one-halfthe wavelength of the signals ofa frequency at which the longest dipole is designed to resonate.

8. A vertically polarized log-periodiclike antenna as described in claim 7 wherein each dipole in said second group includes loading means for loading the dipole to compensate for the difference between its physical and equivalent electrical lengths.

9. A vertically polarized log-periodiclike antenna as described in claim 8 wherein each dipole comprises first and second poles, said support means including means for supporting each first pole above said feed line means and each second pole below said feed line means, each dipole in said second group having at least one pole with a physical length less than one-half the dipoles equivalent electrical length, said loading means including means coupled to said at least one pole of each dipole in said second group.

10. A VCl'tlCall) polarized log-periodiclike antenna as described in claim 9 wherein said at least one pole of each dipole in said second group is the second pole below said feedline means.

11. A vertically polarized log-periodiclike antenna as described in claim 10 wherein each of the poles of the dipole, having the longest equivalent electrical length, has a physical length which is less than one-half of the dipole's equivalent electrical length, said loading means including capacitive means for loading the second pole of the longest dipole and cable-loading means extending from the first pole of said longest dipole for loading said first pole to compensate for the difference between its actual physical length and one-half the equivalent electrical length of the longest dipole.

12. A vertically polarized log-periodiclike antenna described in claim 11 wherein said antenna support means includes a main support catenary having at least one electrically conductive segment comprising said cable loading means.

13. A vertically polarized log-periodiclike antenna as described in claim 7 wherein the equivalent electrical lengths of the dipoles in said first group are related by 0-, where L, being the equivalent electrical length of any dipole in said first group excluding the longest dipole and L,,,, is the next longer dipole, and the equivalent electrical length of at least some of the dipoles in said second group are related by 0' where L,,, being the equivalent electrical length of any dipole in said second group excluding the longest dipole and L,,, is the next longer dipole.

14. A broadband dipole-type vertically polarized logperiodiclike antenna for providing the high electrical gain at the upper frequencies in said band, characteristic of a dipole type vertically polarized log-periodic antenna, the antenna comprising:

a log-periodiclike array having a front end and a back end;

and

array support means for supporting said array in a vertically polarized plane above an electrically conductive surface, said array including a feedline and a plurality of dipoles coupled thereto, each dipole comprising a top pole above said feed line and a bottom pole below said feed line, the dipoles including a first group of dipoles near the front end of said array, each dipole in said first group having a physical length substantially equal to its equivalent electrical length which is substantially one-half of the wavelength of signals at a frequency at which said dipole is to radiate most strongly, said dipoles including a second group of dipoles each having a physical length which is less than its equivalent electrical length which equals onehalf the wavelength of the signals of a frequency at which the dipole is to radiate most strongly, with the bottom pole of each dipole in said second group having a physical length which is less than one-half of the dipoles equivalent electrical length, and bottom loading means coupled to the bottom pole of each dipole in said second group to the compensate electrically for the difference between the physical length of the bottom pole and onehalf the equivalent electrical length of the dipole.

15. A broadband dipole-type vertically polarized logperiodiclike antenna as described in claim 14 wherein said array support means including a tower of height less than the equivalent electrical length of the longest dipole in said array.

16. A broadband dipole-type vertically polarized logperiodiclike antenna as described in claim 15 wherein said second group of dipoles includes first and second subgroups, the physical length of the top pole of each dipole in said first subgroup being substantially equal to one-half the dipoles equivalent electrical length. and the physical length of the top pole of each dipole in said second subgroup being less than one-half of the dipole's equivalent electrical length. and cable loading means electrically coupled to the top poles of the dipoles in said second subgroup for compensating for the differences between their physical lengths and one-half of their dipole's equivalent electrical lengths 17. A broadband dipole-type vertically polarized logperiodiclike antenna as described in claim 14 wherein the equivalent electrical lengths of the dipoles in said first group are related by on, where L, being the equivalent electrical length of any dipole in said second group, excluding the longest dipole in the group, and L,,,;, is the equivalent electrical length of the adjacent longer dipole.

18. A broadband dipole-type vertically polarized logperiodiclike antenna as described in claim 17 wherein said cable loading means comprise a plurality of electrically conductive cables extending from the top end of substantially every pole in said second subgroup to said main support catenary 19. A broadband dipole-type vertically polarized logperiodiclike antenna as described in claim 17 wherein said bottom loading means comprise a plurality of capacitors each coupled between the bottom end of a different bottom pole of a dipole in said second group and said conductive surface, said array support means include a main support catenary having electrically conductive segments insulated from one another, said electrically conductive segments comprising said cable loading means.

20. A broadband dipole-type vertically polarized logperiodiclike antenna as described in claim 19 wherein the top end of each top pole of each dipole in said second group is directly mechanically or electrically coupled to said catenary.

21. A broadband dipole-type vertically polarized logperiodiclike antenna as described in claim 16 wherein said bottom loading means comprise a plurality of capacitors each coupled between the bottom end of a different bottom pole of a dipole in said second group and said conductive surface, said array support means include a main support catenary having electrically conductive segments insulated from one another, said electrically conductive segments comprising said cable loading means.

22. A broadband dipole-type vertically polarized logperiodiclike antenna as described in claim 21 wherein the top end of each top pole of each dipole in said second group is directly mechanically or electrically coupled to said catenary.

Claims

1. An antenna, designed for operation over a selected frequency band including a first frequency and a second frequency, lower than said first frequency, comprising: a plurality of radiating elements, the equivalent electrical length of each element being a function of the wavelength of signals of a different frequency in said frequency band; support means for supporting said elements in a selected orientation with respect to a reference surface to form an array of elements wherein the elements are arranged in an order corresponding to their equivalent electrical lengths, with the element whose equivalent electrical length is a function of the wavelength of signals of said first frequency being at a front end of said array, and the element whose electrical equivalent length is a function of the wavelength of signals of said second frequency being at a back end of said array, said array including at least a first element whose physical length is equal to its equivalent electrical length and at least a second element whose physical length is less than its equivalent electrical length, said second element including impedance loading means for compensating for the difference between its physical length and its equivalent electrical length, the elements being supported in spaced relationship from one another to inhibit the coupling of energy therebetween; and feed means electrically coupled to said elements, said feed means being coupled to said first element at the physical center thereof and to said second element at other than the physical center thereof.

4. The arrangement as recited in claim 2 wherein said support means include a main support catenary extending from said array back end to the front end thereof, said catenary having at least one electrically conductive segment, electrical insulating means for connecting selected ones of said elements, at and near the array front end, to said catenary in electrical insulation therefrom, and means for connecting at least one element, located toward the array back end, to said catenary to be in electrical contact with said at least one electrically conductive segment thereof.

5. The arrangement as recited in claim 1 wherein said selected orientation is a vertical plane above said reference surface, with said elements forming a vertically polarizEd log-periodiclike array of elements, defining an apex above said reference surface about the array front end, each element having at least a portion thereof parallel to corresponding portions of the other elements, and said support means include a main support catenary for directly supporting the top ends of substantially all the elements at and about the array back end in said vertical plane with said feed means further serving as a secondary support catenary to thereby reduce the maximum height of said main support catenary.

6. The arrangement as recited in claim 5 wherein said main support catenary includes at least one electrically conductive segment and said array includes at least one element electrically connected at its top end to said electrically conductive segment of said main support catenary.

7. A vertically polarized log-periodiclike antenna, designed to operate over a frequency band including a first frequency and a second frequency, lower than said first frequency, the antenna comprising: a plurality of dipoles, each having an equivalent electrical length which is a function of one-half the wavelength of the signals of a different frequency in said band at which the dipole is designed to resonate, said dipoles including a first group of dipoles each dipole in said first group having a physical length which is equal to its equivalent electrical length and a second group of dipoles, each dipole in said second group having a physical length which is less than the dipole''s equivalent electrical length, said dipoles being arranged to form a log-periodiclike array with the dipole having the shortest equivalent electrical length being disposed at the array front end and the dipole having the longest equivalent electrical length being disposed at the array back end, with the dipoles in said first group being disposed toward the array front end and the dipoles in said second group being disposed toward the array back end; feed line means; means for electrically coupling said dipoles to said feed line means at points on said feed line means which are related distances from an apex of said array, each dipole in said first group being coupled to said feed line means at the physical center of said dipole and each dipole in said second group being coupled to said feed line means at other than the dipole''s physical center; and antenna support means for supporting said dipoles in a vertical plane above a reference surface, said support means including a main support tower of a height which is less than one-half the wavelength of the signals of a frequency at which the longest dipole is designed to resonate.

9. A vertically polarized log-periodiclike antenna as described in claim 8 wherein each dipole comprises first and second poles, said support means including means for supporting each first pole above said feed line means and each second pole below said feed line means, each dipole in said second group having at least one pole with a physical length less than one-half the dipole''s equivalent electrical length, said loading means including means coupled to said at least one pole of each dipole in said second group.

10. A vertically polarized log-periodiclike antenna as described in claim 9 wherein said at least one pole of each dipole in said second group is the second pole below said feedline means.

11. A vertically polarized log-periodiclike antenna as described in claim 10 wherein each of the poles of the dipole, having the longest equivalent electrical length, has a physical length which is less than one-half of the dipole''s equivalent electrical length, said loading means including capacitive means for loading the second pole of the longest dipole and cable-loading meAns extending from the first pole of said longest dipole for loading said first pole to compensate for the difference between its actual physical length and one-half the equivalent electrical length of the longest dipole.

12. A vertically polarized log-periodiclike antenna as described in claim 11 wherein said antenna support means includes a main support catenary having at least one electrically conductive segment comprising said cable loading means.

13. A vertically polarized log-periodiclike antenna as described in claim 7 wherein the equivalent electrical lengths of the dipoles in said first group are related by sigma 1 where Ln being the equivalent electrical length of any dipole in said first group excluding the longest dipole and Ln 1 is the next longer dipole, and the equivalent electrical length of at least some of the dipoles in said second group are related by sigma 2 where Lm being the equivalent electrical length of any dipole in said second group excluding the longest dipole and Lm is the next longer dipole.

14. A broadband dipole-type vertically polarized log-periodiclike antenna for providing the high electrical gain at the upper frequencies in said band, characteristic of a dipole type vertically polarized log-periodic antenna, the antenna comprising: a log-periodiclike array having a front end and a back end; and array support means for supporting said array in a vertically polarized plane above an electrically conductive surface, said array including a feedline and a plurality of dipoles coupled thereto, each dipole comprising a top pole above said feed line and a bottom pole below said feed line, the dipoles including a first group of dipoles near the front end of said array, each dipole in said first group having a physical length substantially equal to its equivalent electrical length which is substantially one-half of the wavelength of signals at a frequency at which said dipole is to radiate most strongly, said dipoles including a second group of dipoles each having a physical length which is less than its equivalent electrical length which equals one-half the wavelength of the signals of a frequency at which the dipole is to radiate most strongly, with the bottom pole of each dipole in said second group having a physical length which is less than one-half of the dipole''s equivalent electrical length, and bottom loading means coupled to the bottom pole of each dipole in said second group to the compensate electrically for the difference between the physical length of the bottom pole and one-half the equivalent electrical length of the dipole.

15. A broadband dipole-type vertically polarized log-periodiclike antenna as described in claim 14 wherein said array support means including a tower of height less than the equivalent electrical length of the longest dipole in said array.

16. A broadband dipole-type vertically polarized log-periodiclike antenna as described in claim 15 wherein said second group of dipoles includes first and second subgroups, the physical length of the top pole of each dipole in said first subgroup being substantially equal to one-half the dipole''s equivalent electrical length, and the physical length of the top pole of each dipole in said second subgroup being less than one-half of the dipole''s equivalent electrical length, and cable loading means electrically coupled to the top poles of the dipoles in said second subgroup for compensating for the differences between their physical lengths and one-half of their dipole''s equivalent electrical lengths.

17. A broadband dipole-type vertically polarized log-periodiclike antenna as described in claim 14 wherein the equivalent electrical lengths of the dipoles in said first group are related by sigma 1, where Ln being the equivalent electrical length of any dipole in said first group Excluding the longest dipole in the group and Ln 1 is the equivalent electrical length of the adjacent longer dipole, and at least some of the dipoles in said second group have equivalent electrical lengths which are related by sigma 2 Lm being the equivalent electrical length of any dipole in said second group, excluding the longest dipole in the group, and Lm 1 is the equivalent electrical length of the adjacent longer dipole.

18. A broadband dipole-type vertically polarized log-periodiclike antenna as described in claim 17 wherein said cable loading means comprise a plurality of electrically conductive cables extending from the top end of substantially every pole in said second subgroup to said main support catenary.

19. A broadband dipole-type vertically polarized log-periodiclike antenna as described in claim 17 wherein said bottom loading means comprise a plurality of capacitors each coupled between the bottom end of a different bottom pole of a dipole in said second group and said conductive surface, said array support means include a main support catenary having electrically conductive segments insulated from one another, said electrically conductive segments comprising said cable loading means.

20. A broadband dipole-type vertically polarized log-periodiclike antenna as described in claim 19 wherein the top end of each top pole of each dipole in said second group is directly mechanically or electrically coupled to said catenary.

21. A broadband dipole-type vertically polarized log-periodiclike antenna as described in claim 16 wherein said bottom loading means comprise a plurality of capacitors each coupled between the bottom end of a different bottom pole of a dipole in said second group and said conductive surface, said array support means include a main support catenary having electrically conductive segments insulated from one another, said electrically conductive segments comprising said cable loading means.

22. A broadband dipole-type vertically polarized log-periodiclike antenna as described in claim 21 wherein the top end of each top pole of each dipole in said second group is directly mechanically or electrically coupled to said catenary.