Yagi-Type Antennas and Method

The present invention relates to antennas and methods of designing the same, being more particularly concerned with antennas embodying parasitic elements which, if used in a series of elements, have generically become termed "Yagi" antennas. That term is used herein in such a generic sense, it being understood that the invention is applicable wherever the same kind of operation involving the use of parasitic directors and reflectors may be employed.

Since Yagi announced his discovery that the directivity and gain of a driven dipole could be shaped and controlled through the use of parallel longitudinally spaced in-line parasitic reflector and director elements, respectively placed behind and in front of the driven element, there has been copious research into and utilization of this technique. The literature abounds with various types of modifications and utilizations of these principles as, for example, in my earlier U.S. Pat. Nos. 3,259,904; in 2,981,951; 3,114,913; 3,155,976; and 3,623,109; and in many articles, two more recent and comprehensive examples of which are D. K. Cheng and A. C. Chen "Optimum Element Spacings for Yagi-Uda Arrays", IEEE Transactions on Antennas and Propagation, Vol. AP-21, No. 5, September, 1973, commencing page 615; and "Yagi Antenna Design", National Bureau of Standards, U.S. Department of Commerce, NTIS PB-262885, December, 1976. In order to understand the operation of such parasitic arrays, copious theoretical analyses have also been made, but the number of variables involved in the spacing, number or elements, length-to-thickness ratios of the elements, and bands of frequencies over which the elements may be required to operate, have resulted in many different types of arrangements, arrived at more as a result of experimentation and art, than precise theory.

When this type of antenna became a standard for VHF radio and television, including UHF home reception, again many different variations of element design were adopted to enable such antennas to provide usually substantially maximum gain and uniform characteristics over multiple bands of frequencies, with some designs specially employed for particular selected problems, as described, for example, in the above-mentioned Letters Patent and the references therein. Depending, however, upon the variation of longitudinal spacings of the elements and the particular ranges of length-to-diameter or width of the parasitic elements themselves and the position of parasitic elements intended to operate at different frequencies or to perform different functions over different frequencies bands, the designs have been relatively critical and highly specialized in nature.

Additionally, the type of excited dipole or antenna system used has been found to require somewhat different approaches in connection with the use of parasitic elements. A balanced feed dipole, for example, generally requires a highly symmetrical type of parasitic array consideration; whereas an unbalanced feed, as by connecting a coaxial line to a folded dipole, introduces entirely different problems including asymmetrical patterns and impedance unbalances and the like which have given rise to specialized types of consideration in the configuration, spacings and nature of the parasitic elements, generally experimentally derived.

Underlying the present invention is a discovery that appears to be particularly adapted to such arrays employing unsymmetrical or unbalanced feed systems used with the driven dipole or similar elements. Summarily stated, it has been found that a vastly improved performance can surprisingly be obtained, consistently and in a stable manner, if certain parasitic elements, particularly the first director, spaced roughly a quarter of a wavelength in front of the driven element (generally of the order of two-tenths of the wavelength), is made unsymmetrical with respect to the center line of the array instead of having equal dimensional extensions on each side of the center line of the array, as is universally customary in arrays of this character. This lack of symmetry in the director may be extended to subsequent director elements, but the degree of improvement is less noticeable; so that it is the first element that is the most critical in requiring this asymmetrical treatment. In some instances, the second and/or third director of a large number of directors in the array may be thus rendered unsymmetrical, and in some systems a parasitic reflector may similarly be made unsymmetrical as later explained.

As reported, for example, in the National Bureau of Standards publication, supra, it has been observed that one cannot just continue to add parasitic directive elements and continue to improve the gain and other antenna performance characteristics. To the contrary, in some instances, the gain will proceed to drop as additional elements are added. In addition, particularly with asymmetrically fed or unbalanced-line-fed excited or driven elements, it has been observed that as additional directors are added, the impedance presented by the antenna will have discontinuities and dips in the response, and the corresponding performance at different frequencies in the desired band will have gaps and aberrations. Startingly, if the unsymmetrically fed driven element is provided with director element(s) having the unsymmetrical characteristics hereinafter described, this sensitivity to number of director elements and aberration in performance is remarkably minimized, the antenna performing in a stable substantially predictable manner, without deleterous effects as additional elements may be added, and without the dips in performance before mentioned. It has also been found, moreover, that the spacing between directors is not then as critical as is the case with prior systems.

An object of the invention is thus to provide a novel Yagi-type or similar antenna having the improved performance above discussed and a novel method of achieving the same.

A further object is to provide an improved antenna of more general character; and further objects are later discussed and delineated in the appended claims.

While the precise theory of operation is not required for the practice of the invention, it being sufficient to describe the structures necessary reliably to produce the novel results of the invention, and while there is no intention to predicate the invention upon any particular theory of operation, it may be helpful in understanding the phenomena involved to consider one of the possible theoretical explanations for these vastly improved results.

The typical driven element used for Yagi-type antennas fed by a coaxial line, employs a matching device, such as a gamma match, to match the impedance of the transmission line to that of the dipole. Usually, a single side or leg or portion of the driven element is fed by the inner conductor of the coaxial transmission line, the so-called "hot" side. The other side or leg or portion of the dipole is connected to the outer grounded coaxial line conductor and may be considered as an undriven side coupled magnetically (that is field-coupled or air-coupled) to the driven side of the dipole, and also electrically, in the case of folded dipoles, through the direct physical connection, via the folded element. This undriven side will thus have less energy induced along it than the side directly connected to the inner "hot" transmission line conductor, resulting in the production of an asymmetrical antenna pattern. When parasitic conductive directors are then placed in such a non-uniform electrical field, the performance of the directors is no longer predictable, because the extent and precise nature of the field are unknown. In accordance with the invention, it is endeavored to correct the asymmetrical field of the driven dipole by placing in front of the dipole a parasitic director having a longer transverse portion (physically and/or electrically longer) in the proximity of the undriven side of the dipole, preferably parallel thereto, to assist in restoring the strength of the field on that side and thereby re-establishing a symmetrical pattern. Correspondingly, the length of the transverse director portion parallel and in proximity to the driven side of the dipole is made shorter (physically and/or electrically), to reduce the energy directed from that side. This has been experimentally determined to introduce parasitic element interaction to compensate for the original unsymmetrical character of the dipole pattern and thus to render the antenna pattern substantially symmetrical; and, in addition, as further directors were added, no substantial dips in performance or deleterous effects accompanying previous antennas of this type were encountered.

In summary, from one of its broad aspects, the invention relates to an improved Yagi-type antenna having, in combination, coaxial-line-fed antenna means having one portion connected to the inner line and the other portion connected to the outer line of the coaxial line and thus producing an unsymmetrical pattern; and parasitic conductive means spaced from but near the said antenna means and comprising portions corresponding to and extending substantially parallel to the portions of the said antenna means, the parasitic conductive means portion corresponding to the said other portion of the said antenna means being of greater dimension that the portion corresponding to said one portion of the said antenna means to introduce compensatory unsymmetrical parasitic interaction to render the pattern symmetrical. Preferred details are hereinafter presented.

The invention will now be described in connection with the accompanying drawing,

FIG. 1 of which is an isometric view illustrating the invention in preferred form;

FIG. 2 is a similar view of a modification adapted for circular polarization; and

FIG. 3 is a fragmentary view of a reflector modification.

The Yagi antenna of FIG. 1 is shown disposed upon a conductive boom 1, provided with a folded dipole element D, having side portions D.sub.1 and D.sub.2 and a remote side D.sub.3 folded there-below. The driven side D.sub.1 is directly fed by the inner conductor 4 of the coaxial line 2-4, and the undriven side D.sub.2 is connected at its inner end to the outer grounded line conductor 2. This feed produces the unbalanced or unsymmetrical phenomenon before discussed. Rearwardly, a conventional parasitic reflector R is mounted on the boom 1. Forwardly of the dipole D, the director parasitic elements are mounted at spaced longitudinal positions in-line with the boom 1, the first element of which is shown at 3--3', and which embodies the asymmetrical configuration underlying the invention. The first director 3--3' is shown positioned on the boom 1 approximately two-tenths of the mean wavelength of the desired band forward of the dipole D; and, in accordance with the present invention, the transverse extension 3 of the director 3--3', adjacent the undriven portion D.sub.2 of the dipole, is made slightly longer than the extension 3' adjacent the driven portion D.sub.1, in order to attain the results of the invention, as before explained. Subsequent directors, such as 5 and 7 are shown mounted on the boom 1 at successive approximately two-tenths of a wavelength longitudinal spacings, but have symmetrical-length transverse extensions on each side of the boom 1, as in conventional fashion. The dipole antenna D and parasitic elements R, 3--3', 5 and 7 are substantially disposed in a horizontal plane and parallelly arranged to operate for linear horizontal polarization.

As a specific example of an antenna used for channel 68 in the UHF television band, of the order of 795 megahertz at mid-band, the driven element D may have left-hand and right-hand side portions D.sub.1 and D.sub.2 each of approximately 3.4 inches in length. The reflector R can be symmetrically rearwardly disposed and have an over-all length of about 8 inches. The first director 3--3', in accordance with the present invention, has the unsymmetrical longer extension 3, adjacent and in parallel juxtaposition to the grounded-line-fed portion D.sub.2 of the dipole D, of length 3.718 inches; with the other transverse extension 3', parallely adjacent the inner-conductor-line-fed driven element portion D.sub.1, being a shorter 3.09 inches. The subsequent directors 5 and 7 may have symmetrical 3.125 inch equal extension lengths on each side of the boom 1, with the spacing between the directors being 3.5 inches.

With the same system in the same channel range without the unsymmetrical director construction of the present invention, dips were produced in the region of the 800 megahertz response, which were unpredictable and uncontrollable, depending upon the number of directors added. Additional increase of the spacing of the directors produced dips just under and above 800 megahertz. Attempts to fix these aberrations, including by adjustment of the reflector, proved very difficult.

By using the unsymmetrical first director of the invention, an unusually flat response occurred across the complete band without any substantial dips in the response and with great stability. It was found that by rendering other successive directors similary unsymmetrical, such as director 5, gave some additional minor improvement. In some cases, furthermore, similarly rendering the reflector element R unsymmetrical in the same sense as described in connection with the director 3--3', aided in rendering the pattern symmetrical and in achieving the stable flat response of the invention. Suitable dimensions of an unsymmetrical reflector R for these frequencies are 4.125 inches on the D.sub.2 side, and 3.875 inches on the D.sub.1 side. From 5 to 12 directors could be added, moreover, with continued overall flat response unlike the erratic effects occuring in adding different numbers of directors in the prior art antennas of this type.

A 14-element antenna of the design of FIG. 1 produced gains, moreover, that are equivalent or superior to those of a similar properly matched dipole antenna that does not use the folded feed; namely, 13dB in the channel 68 band.

In order to obtain broad-banding response, the directors, as shown in the drawings, are preferably of substantial width, illustrated in the form of rectangular strips mounted on opposite sides of the metal boom 1.

The invention, furthermore, is not restricted to linear polarized antennas such as the horizontally polarized array of FIG. 1, or the same oriented in the vertical plane to provide vertical polarization. The terms horizontal and vertical, of course, are illustrative and generically used. In FIG. 2, the techniques of the invention are shown applied to a circularly polarized arthogonal array antenna which may also be extremely useful for television reception as well as transmission. The horizontal planar array R, D, 3--3', 5, 7 is substantially as described in connection with the embodiment of FIG. 1, and a second similar, but vertical-plane oriented array, R'-D', 3"--3'", 5' and 7', is shown interleaved within the horizontal array, with the elements thereof mounted near the corresponding elements of the horizontal array and having similar element-to-element spacings, and with substantially the same unsymmetrical dimensions of the first director 3"-3'" before explained (and, if desired, subsequent director(s), and/or unsymmetrical reflector R', earlier discussed in connection with reflector R). The appropriate 90.degree. phase shift to achieve circular polarization is shown attained by the appropriate-length insulated inner feed-conductor line extension 4', wrapped clockwise around the boom 1 from inner conductor feed point 4 to the vertical folded dipole driven element D.sub.1 ', the element D.sub.2 'being connected through the boom 1 to the outer grounded feed terminal 2. With the dipole elements D.sub.1 ' and D.sub.2 ' facing to the right in FIG. 2, and the phase-shifting feed extension 4' wrapped and connected as shown, right-hand rotating circular polarization is achieved; whereas by facing the elements D.sub.1 ' and D.sub.2 ' to the left, and wrapping the feed extension 4' oppositely to connect to D.sub.1 ' on the left-side of the array, left-handed circular polarizaton is achievable.

This novel construction, moreover, appears to be highly beneficial for circular polarized arrays with symmetrical directors, as well. The construction of the invention is also well-suited for the use of stacked parasitic reflectors and the like as shown in FIG. 3, wherein only the reflector-mounting portion of the boom 1 and the reflector R of FIG. 1 is shown. A vertical boom 1' carrying upper and lower parasitic reflectors R.sub.2 and R.sub.3 may readily be bolted to one side of the boom 1 adjacent reflector R. The boom 1' may be dimensioned to provide the necessary dimensional off-set for unsymmetrical elements of R.sub.2 and R.sub.3 as previously discussed in connection with the element R.

Further modifications will occur to those skilled in this art and all such are considered to fall within the spirit and scope of the invention.