US 3209362 A
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Description (OCR text may contain errors)
Sept. 28, 1965 S. D. SPIEGELMAN LOG-PERIODIC BOW-TI E ANTENNA Filed Jan. 8, 1964 INVENTOR, STANLEY D. SPIEGELMAN.
United States Patent 3,209,362 LOG-PERIODIC BOW-TIE ANTENNA Stanley D. Spiegelman, Plainview, N.Y., assignor to the United States of America as represented by the Secretary of the Army Filed Jan. 8, 1964, Ser. No. 336,609 4 Claims. (Cl. 343792.5)
The present invention relates to a bow-tie log-periodic antenna and more particularly to a broadband, linearlypolarized, unidirectional antenna which has the property that the location of its phase canter (effective center of radiation) is essentially independent of frequency.
This antenna was developed to fill the requirement for a broadband primary feed for a paraboloidal reflector. Proper operation of such primary feeds require that the phase center be located at the focus of the paraboloid. If the phase center is displaced from the focus along the focal axis, the result is a loss in gain. If the displacement is transverse to the focal axis, the secondary beam from the reflector will be tilted away from the focal axis.
The general purpose of this invention is to provide a unidirectional, broadband antenna the phase center of which remains substantially fixed. To obtain this, the present invention contemplates a unique arrangement of two log-periodic antenna elements, in the shape of a bow-tie, and fed in parallel against a ground plane. It will also be seen that this arrangement does not require a balanced feed and may therefore be fed directly by a coaxial line.
It is, therefore, the object of this invention to provide a broadband, linearly-polarized, unidirectional antenna having a fixed phase center.
It is also an object of this invention to provide a broadband, linearly-polarized, unidirectional antenna having a fixed phase center which can be fed directly by an unbalanced feed.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof, and wherein:
FIG. 1 shows a side elevation view, partly in section, of a preferred embodiment of the invention; and
FIG. 2 shows a view of one of the antenna elements looking in the direction of the arrows 22 of FIG. 1.
Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 an antenna 11 having a highly conductive ground plane and two antenna elements 12 and 13. The outer conductor 14 of a coaxial line is connected to the ground plane 10 while the inner conductor 15 is connected to the antenna elements 12 and 13 at their apex or common feed point 20. The ground plane 10 should extend a substantial distance beyond all sides of the elements 12 and 13.
The antenna elements 12 and 13, which are identical, are conventional log-periodic antenna elements being generally triangular in shape and having staggered conducting teeth 16 and 17 extending laterally from a center conductor 18. As mentioned in FIG. 1, element 12 is a mirror image of element 13. Elements 1'2 and 13 and ground plane 10 are all mounted perpendicular to the plane of the paper as viewed in FIG. 1. The radiation characteristics of such antenna elements are well-known and are assumed to be common knowledge in the antenna art. Briefly, at a given frequency, the radiation from a log-periodic antenna element originates from a comparatively narrow radiating region centered around the tooth whose length is equal to one-quarter wavelength.
In the present device the radiating tooth will be fed against the ground plane 10. It is now readily seen that the effective phase center of element 13 together with its image, formed by the ground plane 10, will be located at the ground plane or at the axis of symmetry of element 13 and its image. The location of the phase center can be calculated approximately from the following formula:
(i=2 001; 3 cos where d is the distance from apex 20 to the phase center along plane 10 as viewed in FIG. 1, A is the wavelength at the operating frequency, a is the angle formed between the outer edges of teeth 16 and 17, and 11/ is the angle formed between the element 13 and its image or in other words, p is twice the angle formed between element 13 and the ground plane 10. From this equation it is seen that the location of the phase center is dependent upon the wavelength at the operating frequency.
Since antenna element 12 is identical to element 13, the phase center for element 12 and its image will be located on the ground plane 10 and spaced from point 20 the same distance as the phase center for element 13 but in the opposite direction from apex 20. The resulting phase center for both elements 12 and 13 when combined will be located at the mid-point or at the apex or feed point 20.
With antenna element 12 oriented as a mirror image of element 13 a single radiation lobe is excited up and along the plane of symmetry of the antenna as viewed in FIG. 1. This condition is referred to as the sum mode. If one of the elements 12 or 13 were rotated such that teeth 16 were coming out of the paper in FIG. 1 to produce two-fold symmetry a split beam radiation lobe would be produced. This condition is referred to as a difference mode and is undesirable for a paraboloid feed.
It has been determined experimentally that the single lobe is approximately circular in cross-section over a fairly wide range of values for \l/ centered about the value 11:11. It has also been determined experimentally that the optimum impedance performance is obtained when 1,0 is equal to a.
A minimum value for a which will permit successful operation has been determined. As stated above, the eifective phase center of each antenna element (together with its image) is located at the ground plane at a distance d from the feed point 20 as given by the equation:
2 cos The spacing between the phase centers of the two elements 12 and 13 is then twice this distance or 2d. The effective currents at the two radiating regions on each element are in phase. Therefore, the radiation pattern of the antenna 11 will be a single-lobed, unidirectional pattern directed up and along the plane of symmetry, provided the distance 2d is less than approximately one wavelength. Introduction of this assumption into the equation for d yields:
(1 d cot circular cross-section and optimum impedance. With this assumption, the above equation yields:
sin 5+2 sin 3- 1 0 This restriction is satisfied for values of at greater than approximately 45. The validity of this restriction has been satisfied experimentally. A log-periodic antenna model with on equal to 30 gave unsatisfactory multi-lobed performance. On the other hand, models constructed with or equal to 45, 53, and 60 gave satisfactory performance.
When this antenna is used as a primary feed for a paraboloid many desirable features exist. The phase center remains fixed at the feed point of the antenna; the bandwidth of the antenna is restricted at the low frequency end only by the length of the longest tooth, and at the high frequency end only by the care exercised in maintaining fine details in the vicinity of the feed point; with on greater than 45 and on equal to the resulting singlelobed, unidirectional pattern with a circular cross-section provides optimum illumination of the paraboloidal reflector; the antenna may be fed directly from a coaxial transmission line supplying a desirable broad band feed.
Of course, the above advantages are only some of the more important while many modifications and variations of the present invention are possible in the light of the aobve teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.
What is claimed is:
1. An antenna comprising a pair of log-periodic antenna elements, a ground plane, and a coaxial transmission line; each said antenna elements being generally triangular in shape and having a central conductor extending from the apex to the base; a plurality of conducting teeth extending laterally from said central conductor and being alternately arranged on opposite sides thereof; the
outer edges of said teeth being tapered to form the sides of the triangular shape; the plane of each said antenna element and said ground plane all being perpendicular to a common plane; said common plane bisecting said triangular shape and passing through each said central conductor, each said apex, and said ground plane; each said antenna being mounted at equal angles with respect to said ground plane with each said apex being closest to said ground plane; and the inner conductor of said coaxial line connected to each said central conductor at said apex while the outer conductor of said coaxial line is connected to said ground plane.
2. An antenna according to claim 1 and wherein said apex angle is approximately equal to twice the angle between each said antenna element and said ground plane.
3. An antenna according to claim 2 and wherein said apex angle is greater than 45.
4. An antenna according to claim 1 and wherein the distance between the corresponding teeth of each said antenna element is less than approximately one wavelength at the operating frequency of said corresponding teeth.
References Cited by the Examiner UNITED STATES PATENTS 3,079,602 2/63 Du Hamel et al 343-807 3,101,474 8/63 Wickersham et al. 343792.5 3,134,979 5/64 Bell 343--792.5
ELI LIEBERMAN. Primary Examiner.