US 3732572 A
A log periodic antenna comprising foreshortened dipoles, each having a profile corresponding to the interior cross section of a hollow ridged waveguide. One or more of the elements of the array, usually those which are resonant at relatively low frequencies (HF and low VHF bands) are so foreshortened and each is formed with conductive wire for an open-type element or conductive sheet or film for a solid type dipole. The dipole has the profile of the interior cross section of either a double ridged or a single ridged waveguide and is oriented with the plane of the dipole either parallel to or transversely of the plane containing the axis of the array. A coplanar array of log periodic dipole arrays consisting of such foreshortened dipoles exhibits substantially improved gain and radiation pattern characteristics as compared to a coplanar array of conventional log periodic dipole arrays.
Description (OCR text may contain errors)
United States Patent [191 Kuo 1 May 8, 1973 s41 LOG PERIODIC ANTENNA WITH 3,193,831 7/1965 Yang ..343/792.5
FORESHORTENED DIPOLES Primary Examiner-Eli Lieberman  Inventor. gilllliltllel Chung-Shu Kuo, Cupertmo, Atmmey Norman J. UMalley et aL  Assignee: GTE Sylvania Incorporated, Moun-  ABSTRACT  Continuation-impart of Ser. No. 50,782, June l9,
 U.S. Cl ..343/792.5, 343/802 [5l] Int. Cl. ..H0lq 11/10  Field of Search ..343/792.5, 802
 References Cited UNITED STATES PATENTS 3,543,277 ll/l970 Pullara ..343/792.5 3,371,348 2/1968 Simons ..343/792.5 3,573,839 4/1971 Parker ..343/792.5
l -24 24 I2 W tain View, Calif.
Filed: Nov. 22, 1971 Appl. No.: 200,681
Related U.S. Application Data A log periodic antenna comprising foreshortened dipoles, each having a profile corresponding to the interior cross section of a hollow ridged waveguide. One or more of the elements of the array, usually those which are resonant at relatively low frequencies (HF and low VHF bands) are so foreshortened and each is formed with conductive wire for an open-type element or conductive sheet or film for a solid type dipole. The dipole has the profile of the interior cross section of either a double ridged or a single ridged waveguide and is oriented with the plane of the dipole either parallel to or transversely of the plane containing the axis of the array. A coplanar array of log periodic dipole arrays consisting of such foreshortened dipoles exhibits substantially improved gain and radiation pattern characteristics as compared to a coplanar array of conventional log periodic dipole arrays.
7 Claims, 13 Drawing Figures PATENTED 81975 3,732,572
SHEET 1 UF 3 23 23c 23 22 x f INVENTOR.
-24 SAMUEL CHUNG-SHU KUO l BY 24. 3 lE-E AW R w Q ATTORNEY LOG PERIODIC ANTENNA WITH FORESHORTENED DIPOLES BACKGROUND OF THE INVENTION This is a continuation-in-part of application Ser. No. 50,782 filed June 19, 1970, now abandoned.
This invention relates to log periodic antennas, and more particularly to a log periodic array with foreshortened dipoles.
The broadband medium gain performance of log periodic dipole arrays has long been recognized for use in the high frequency or HF (3-30 MHz) and the very high frequency or VI-[F (30-300 MI-Iz) bands. The linear relationship of the standard dipole to operating wavelengths, however, results in difficulty capacitive th in using such arrays in space limited applications. For example, at 3 and 30 MHz the dipole span is 50 and meters, respectively. The need for more compact antennas has led to consideration of foreshortening or size-reducing dipoles by various techniques such as series inductance loading, transmission line loading and various types of capacitive loading of the elements as described, for example, in the article entitled Reduced-Size Log Periodic Antennas by D. F. DiFonzo, Proccedings of the 9th National Communication Symposium October 1963, pages 121 to 130, inclusive (IEEE). These techniques, however, have not provided the required transforming action, the wideband active region and/or the high attenuation of the active region required for antenna gain, pattern uniformity and the front to back ratio necessary for high performance broadband operation.
An object of this invention is the provision of a log periodic dipole array antenna having dipoles foreshortened by 35 percent or more and electrical performance substantially equivalent to that of a corresponding conventional log periodic dipole array.
Another object is the provision of a simple technique for designing a log periodic antenna with foreshortened dipoles when the available space is limited.
Still another object of the invention is the provision of a dipole which is substantially the electrical equivalent of the conventional linear dipole but which is less than two-thirds the length of the latter.
A further object of the invention is the provision of a coplanar array of log periodic antennas with foreshortened dipoles in which the pattern break up and gain drop phenomenon has been eliminated or substantially reduced.
SUMMARY OF THE INVENTION A dipole having an outline in the shape of the interior of a hollow ridged waveguide resonates at a substantially lowe frequency than a linear or conventional dipole of the same physical length. Substitution of such ridged-waveguide configured dipoles for conventional linear dipoles in a log periodic array effects reduction in the width of the array without materially changing its electrical performance. A coplanar array of log periodic dipole arrays with the required halfwavelength spacing between array axes has greater interelement spacing when constructed with such foreshortened dipoles which is believed to acount for the improved gain over the band of interest.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic plan view of a conventional log periodic dipole array;
FIG. 2 is an enlarged side view of the array of FIG. 1 taken on line 2-2 of FIG; 1 and showing the feed line connections to the dipole elements;
FIG. 3 is a schematic plan view of a log periodic dipole array electrically equivalent to the array of FIG. 1 and embodying the invention, the antenna comprising a mixed array of conventional dipoles and dipoles foreshortened in accordance with the invention;
FIG. 4 is a transverse section taken on line 4-4 of FIG. 3;
FIG. 5 is a view similar to FIG. 4 showing a modified form 'of the invention in which the plane of the dipole element extends transversely of the axis of the array;
FIG. 6 is a plan view of a wire or open-type dipole having the outline of a double ridged waveguide on which certain dimensional parameters are indicated;
FIG. 7 is a plan view similar to FIG. 6 in which the dipole is constructed with solid or sheet-type conduc- I01;
FIG. 8 is a plan view of a solid dipole having the outline of a single-ridged waveguide interior;
FIGS. 9, 10 and 11 are curves representing percent reduction of conventional dipole length achievable with the indicated ranges of design parameters of a dipole embodying the invention;
FIG. 12 is a plan vie of a coplanar dual array system in which the log periodic antennas are formed with dipoles foreshortened in accordance with this invention; and
FIG. 13 is an enlarged section of one of the arrays of FIG. 12 taken on line 13-13 of FIG. 12.
DESCRIPTION OF PREFERRED EMBODIMENTS This invention concerns foreshortening a conventional dipole without significantly changing its electrical characteristics by making the profile of the dipole the same as the interior cross-sectional profile of hol' low rectangular ridged waveguide as defined in The International Dictionary of Physics and Electronics (Von Nostrand, 1956) at page 981. The principle upon which this invention is based rests on the analogy between rectangular waveguide and the slot antenna, the latter being an efi'ective microwave antenna whose resonant wavelength is equal to twice the length of the slot if the width of the slot is small compared to its length. The cutofi' wavelength of the fundamental mode of a rectangular waveguide is twice the width of the guide; equating that cutofi frequency with the resonant frequency of the slot antenna, it is noted that there is a correspondence between the waveguide cutoff and slot resonant frequencies. Ridged waveguide has a eutofif frequency lower than that of rectangular waveguide of the same width and height and without ridges. By analogy a lower resonant frequency for a slot antenna is achievable by shaping it with the interior profile of a ridged waveguide. Experimental measurements verify this conclusion. The efiect of lowering the resonant frequency in this manner is to increase the effective electrical length of the slot antenna without changing its physical length. Thus the physical size of a slot antenna may be foreshortened without changing its electrical length. By Babinets principle, slot and dipole antennas are analogs, and therefore the physical length of a dipole is likewise capable of being foreshortened when it is formed with the shape of a ridged waveguide. Test results indicate that, in accordance with this invention, the physical size of the conventional dipole can be foreshortened as much as 45% without significant change in it electrical characteristics by configuring the dipole with the interior cross-sectional profile of a ridged rectangular waveguide.
The empirical relationship for the resonant length l of a rectangular slot having a width w and centered in the transverse plane of a rectangular waveguide was originally suggested by J. C. Slater in Microwave Transmission, McGraw-Hill Book Company (1942), pages l85l87, and is as follows (accurate to within a few percent):
l= )t,/2 1 (2aw/b)t,,,,) (1) where A, is the resonant wavelength of the rectangular slot, )t is the guide wavelength of )t a is the height of the rectangular waveguide and b is its width.
When the ratio of w/l is small, equation (1) reduces to l a M (2) Accordingly the resonant frequency of a narrow slot is approximately equal to the cutoff frequency of a rectangular waveguide having the same cross-sectional shape of the slot. Since the cutoff frequency of a ridged waveguide is lower than that of a rectangular waveguide of the same width and height, it follows that the resonant frequency of a similar ridged slot is correspondingly lowered. The same is true of the analog of the slot antenna, the dipole.
Referring now to the drawings, a conventional log periodic dipole array having an axis Z is shown in FIGS. 1 and 2 and comprises a plurality of axially spaced dipole elements 11 connected to a feed line J12 which extends the length of the array along its axis. The spacing between adjacent dipoles and the dipole lengths increase in one axial direction, from right to left as viewed in FIG. I, in progressive increments of a predetermined constant ratio 1 over the frequency band of interest. Energy is fed to the dipoles from a feed point F and in the embodiment shown, feed line 12 comprises a coaxial line connectable to external circuits at connector 13 and having an outer conductor 14 and an inner conductor 15. A conductive rod 16 parallel to and coextensive with coaxial line 12 is connected to the inner conductor 15 at feed point F. In order to insure the 180 degree phase reversal of energy fed to successive dipoles as required for radiation along the axis Z, axially successive dipole elements on the same side of axis Z are alternately connected to outer conductor l4 and rod 16 as shown in FIG. 2. The dipoles are fed by the two-conductor feed line which supports a slow wave from feed point P to the opposite end of the array and energizes the dipole or dipoles which are resonant at or near the operating frequency. The construction and operation of conventional log periodic antennas are well known in the art and therefore the foregoing description is sufficient to lay a foundation for an understanding of the invention described below.
The utility of a conventional log periodic antenna is often dependent upon space available for the longer dipoles in the array. As shown in FIG. I, the requirements for a space-limited application having a maximum width W available for the array cannot be satisfied with a conventional array because the three low frequency dipoles are longer than W. This problem is solved in accordance with this invention by substitution in array 17 of foreshortened dipoles l8, l9 and 20 having outlines corresponding to the shape of the interior cross-sectional profile of a double-ridged waveguide, see FIG. 3, for those dipoles of the conventional array whose lengths exceed the limit W; the lengths of dipoles l8, l9 and 2d are selected as described in detail below so as to be equal to or less than the limiting dimension W. The remainder of the dipoles 11 at the high frequency end of array 17 are identical to those in the conventional array 10 and therefore the size of the array is modified only to the extent required by the space limits of the particular application. Antenna 17 is a mixed array of linear and foreshortened dipoles.
Each of the elements of the foreshortened dipoles, such as dipole 18, consists of a wire-like stem 22 directly connected at one end to one of the feed lines and a body 23 connected to the other end of the stem. Body 23 consists of a wire-like conductor 23a con.- figured to define the outline of a totally enclosed plane geometric pattern which, in the embodiment illustrated, is a rectangle. The dimensions of the stern and body are selected as described below so that the foreshortened dipole becomes essentially the electrical equivalent of the corresponding linear dipole. The plane of each foreshortened dipole element, i.e., the plane of body 23, may be parallel to the axis Z of the array as shown in FIGS. 3 and d or, alternatively, may be oriented with the planes of the elements extending .transversely of the array axis, for example, perpendicular to the array as shown in FIG. 5. Antennas constructed with foreshortened dipoles 18, 19 and 20 in planes that are either parallel to or transversely of the array axis have been found by actual test to have substantially the same electrical performance. In short, the electrical performance of the foreshortened dipole is independent of the planar orientation of body 23. The foreshortened dipoles shown in FIGS. 3, 4i and 5 are formed with a wire or a wire-like conductor which defines the outline of each body and provides an opening 24 in the central part of the body.
The design parameters of dipole w, for example, are shown in FIG. 6. The ratio of body dimension B to the overall dipole length A is directly proportional to the length-reduction factor, i.e., that factor increases as the value of BIA increases. The limit of the ratio of IBM is dictated by (l) the dipole resistance, which decreases as B/A' increases, and (2 the spacing between adjacent dipoles in a log periodic dipole array. Other dimensions affecting the dipole length-reduction factor are the dimension D of stem 22 and the inter-body spacing S of the dipole elements, the length-reduction factor increasing as the ratio of D/B increases. The optimum value of S/A for the maximum reduction of dipole lengths is approximately 0.5, the length-reduction factor generally decreasing as the ratio S/A becomes substantially greater or less than 0.5.
The effect of design parameters S, A, B and D on the dipole foreshortening sought to be achieved may be correlated in such a manner as to enable the antenna designer readily to select dipole parameters to meet a design requirement. Examples of such correlation of these data are shown in FIGS. 9, l0 and 11 wherein curves plotted from such data represent variations in the percentage of size reduction in accordance with variations in design parameter ratios. These curves are plotted from data derived from a series of foreshortened dipoles with dimensions changed by trail and error. The antenna designer, having calculated the dimensions of a conventional log periodic array and faced with a space limiting requirement, determines the amount of reduction required in the length of each oversize standard dipole in order to meet the requirement. Using the curves of the type illustrated in FIGS. 9, and 11, he selects the design parameters S, A, B and D which effect the desired foreshortening and proceeds to construct the particular dipoles in accordance with the derived data.
By way of example, assume that a linear dipole having a length of 10 inches exceeds the space limits of the particular application by 3 inches. The antenna designer therefore needs a 30 percent reduction of dipole length. Referring to the curves of FIGS. 9, 10 and 11 he selects curve 27 in FIG. 10 and determines therefore that the foreshortened dipole should have the following parameter ratios:
S/A 0.5 BIA 0.20 D!!! 0.1
With these ratios, the designer may then select the actual dimensions of the foreshortened dipole which may, for example, be as follows:
7 inches 1.4 inches 3.5 inches 0. 14 inches A B S D It will be seen from the foregoing description that the overall length A of dipoles 18, 19 and 20 of array 17 is a constant equal, to or less than W and that the dimension B of each dipole is different since the amount of reduction required for each of the dipoles is different. It should also be noted that the overall axial length of the array 17 has not changed with respect to the length of the conventional array 10 and thus the practice of the invention achieves a reduction in he transverse dimension of the array without necessitating enlargement of the antenna in another direction as a tradeoff. The aperture of the array 10 defined by the area within the trapezoid G,'I-I, K and L, see FIG. 1, is unavoidably reduced in the size-reduce array 17 of FIG. 3 and to this extent there is a diminution in gain in the latter array. In other respects, the electrical performance of array 17 is substantially the same as that of a conventional array. If the available space in the axial direction of the array permits, more foreshortened dipoles may be added to the low frequency end of the array to offset the aperture reduction and thus compensate the loss of gain.
The construction of the foreshortened dipole with opening 24 in the body portion may be modified without changing the electrical properties by the addition of a brace-like extension 28 of stem 22 across the opening as indicated in broken lines in FIG. 6. Arrays actually constructed with such dipoles not only exhibited increased mechanical rigidity but also had equivalent or even slightly improved electrical performance compared to those with unbraced or fully open element bodies.
Another form of a dipole embodying the invention is shown at 30 in FIG. 7 wherein the dipole outline or plan profile is identical to that of dipole 18 and each element 31 is formed from a continuous conductive planar sheet or film. This construction is useful in applications permitting formation of the dipoles and the feed lines 12' by printed circuit techniques on a dielectric base. The electrical performance of this dipole is substantially identical to that of the dipoles shown in FIG. 6.
The foregoing embodiments of the invention feature dipoles having the outline of the interior cross section of a double-ridged waveguide. It has been found that dipoles may also be formed with the outline of the interior cross section of a single ridged waveguide and such a dipole 32 in sheet form is shown in FIG. 11. The electrical performance of dipole 32 is essentially identical to that of dipole 30.
By way of example and comparison, an antenna comprising a mixed array of the type shown in FIG. 3 and a conventional array of FIG. 1 both with the following parameters and performance characteristics, were constructed and successfully tested:
Conventional Log Size-Reduced Periodic Dipole Log Periodic Array Mixed Dipole Array Taper angle a 10 10 Scale factor 1 0.91 0.91 Number of elements 13 13 Operating frequency range 0.7-3.0 Ghz 0.7-3.0 Ghz Antenna boom length 12 inches 12 inches Size of largest dipole element 7 inches 4.2 inches Averaged E-plane radiation pattern beamwidth 59 62 Gain 9 db 8 db VSWR less than 1.511 less than 1.5:1
These are performances of the size-reduced portion of the sizereduced mixed log periodic dipole array. In all other respects, electrical performance of both arrays were the same.
The invention also has practical application in substantially improving performance of coplanar arrays of log periodic antennas by providing a unique solution to a gain dropout problem experienced with such arrays. Specifically, the radiation pattern of a coplanar system of conventional log periodic dipole arrays deteriorates and the gain drops drastically at several frequencies periodically throughout the operating band. This is believed to be cause by mutual coupling between adjacent elements of the arrays. This problem is solved with a coplanar system shown in FIGS. 12 and 13 comprising identical arrays 34 and 35 with axes Z and 2 each array comprising a trapezoidally shaped base layer 36 of dielectric material such as fiberglass, an axially extending coaxial feed line 37, and a plurality of axially spaced dipole elements 38 formed as conductive film on layer 36 and on opposite sides of array axis. Elements 38 comprising each dipole are located on opposite sides of the dielectric sheet and are alternately connected to the feed lines comprising the outer conductor 37a and the extension 37b of the inner conductor of a coaxial line, which feed lines are on opposite sides of the sheet. Both arrays 34 and 35 lie in the same plane and diverge from each other at an angle selected to provide half wavelength spacing between the axes Z and Z of the arrays at the particular frequency of operation. All of the dipoles in each array are foreshortened as described above. While coplanar arrays of the type described have been built and successfully tested without any periodic gain dropout or pattern discontinuities, such arrays formed with wiretype dipoles of the type shown in FIG. 6 may also be used with the same advantage.
1. A log periodic dipole antenna having an axis and comprising a pair of axially extending feed lines,
a plurality of axially spaced dipoles connected to said feed lines and extending in directions transversely of said axis,
at least three of said dipoles having lengths substantially less than M2 where )t is the wavelength at the resonant frequency of the respective dipole,
each of said three dipoles having two identical elements on opposite sides of said axis, each of said elements having a stem connected to one of said feed lines and a rectangular body longitudinally connected to the end of said stem opposite from said one of said feed lines.
2. The antenna according to claim 1 comprising a mixed array of said last named dipoles at the low frequency end and a plurality of linear dipoles at the high frequency end of the antenna.
3. The antenna according to claim 1 in which the plane of said body is parallel to the plane containing said axis of the antenna,
4. The antenna according to claim 1 in which the plane of said body extends transversely of the plane containing said axis of the antenna.
5. The antenna according to claim 1 in which the dimension of the body of said one element transverse to said axis is approximately equal to the length of said stem between connections to the feed line and the body.
6. A dual array antenna comprising first and second arrays having axes diverging at a predetermined angle and defining the plane of the antenna,
each of said arrays comprising a pair of feed lines extending along the axis of the array, a plurality of axially spaced dipoles connected to said feed lines and extending transversely of said axis, the lengths and axial spacings of said dipoles increasing in one direction along said axis in progressive increments of a predetermined ratio definitive of a log periodic structure,
at least three of said dipoles having lengths substantially less than M2 where A is the wavelength at the resonant frequency of the respective dipole, each of said three dipoles having two identical elements on opposite sides of said axis, each of said elements having a stem connected to one of said feed lines and a rectangular body longitudinally connected to the end of said stern opposite from said one of said feed lines, the dipoles of said first and second arrays lying in planes parallel to said plane of the antenna with the dipoles of one array spaced from adjacent dipoles of the other array by an increasing distance in the direction of divergence of said axes.
7. A multielement dipole array having an axis and comprising a pair of axially extending feed lines,
a plurality of axially spaced dipoles connected to said feed lines and extending in directions transversely of said axis, at least three of said dipoles having lengths substantially less than M2 where A is the wavelength at the resonant frequency of the respective dipole,
each of said three dipoles having two identical elements on opposite sides of said axis, each of said elements having a stem connected to one of said feed lines and a rectangular body longitudinally and colinearly connected to the end of said stem opposite from said one of said feed lines.