US3011168A - Frequency independent unidirectional antenna - Google Patents

Description

Nov. 28, 1961 D. E. ISBELL 3,011,168

FREQUENCY INDEI ENDENT UNIDIRECTIONAL ANTENNA Filed Oct. 20, 1958 2 Sheets-Sheet 1 Fig. 2

INVENTOR. Dwight E. lsbe/l Merriam, Larch 8 Smifl;

A TTOfi/VEYS Nov. 28, 1961 Filed Oct. 20, 1958 D. E. ISBELL FREQUENCY INDEPENDENT UNIDIRECTIONAL ANTENNA 2 Sheets-Sheet 2 Fig. 6

INVENTOR.

Dwig/rf E. lsbel/ Merriam, Larch 8 Smith AT T OR/VE Y5 United States Patent 3,011,168 FREQUENCY INDEPENDENT UNTDIRECTIONAL ANTENNA Dwight E. Isbell, Urbana, Ill., assignor to The University of Illinois Foundation, a non-profit corporation of Illinois 1 Filed Oct. 20, N58, Ser. No. 768,297 3 Claims. (Cl. 343-908) This invention relates to antennas and more particularly it, relates to antennas-having unidirectional radiation patterns that are essentially independent of frequency over wide bandwidths.

It is 'known that an antenna whose geometry is described completely byangles, such as an infinite biconical antenna, wouldmake an ideal broadband radiator sinceits operation is theoretically completely independ ent of frequency; The theoretical performance of the infinite biconicalantenna 'is not achieved in practice, however, since'such an antenna must be of finite length, and the end effect, -i.e., the effect of finite rather than infinite length, leads to radiation characteristics showing considerable variation-with frequency. I

A planar antenna closely related to the biconical antenna, the bow-tie, antenna, is likewise theoretically frequency-independent when infinite in size. The endefiect of an actual bow-tie antenna, however, limits the range of frequencies for which the radiation pattern is essentially constant to a bandwidthof 2 or 3 to 1.

It is known that bow-tie antennas can be modified in a way which reduces the end-effec to such an extent as to permit bandwidths of to 1' or more to be achieved with structures of practical size. In general, this result is achieved by introducing periodic discontinuities along the marginal edges of the bow-tie antenna, the geometry of the discontinuities being such that all dimensions involved are directly proportional to the distance from the feed point of the antenna, i.e., the vertex or the narrowest portion of the bow-tie. Planar antennas of this type have bidirectional radiation patterns extending perpendicularly to the .plane of the antenna.

It has now been found that the above-describedmodified planar bowtie antennas can be further modified to exhibit unidirectional radiation patterns while maintain-ing the broadbandwidths which such antennas possess. The unidirectional radiation patterns are achieved by rotatingone half of the.bow -tie about an axis passing through the vertexin the plane of the antenna perpendicular to the line which bisects both halves of the antenna, so that the antenna is no longer planar. In the 3,011,168 Patented Nov. 28, 1961 and OD) contain a plurality of similar slots and teeth bounded by straight lines (e.g., E0 and A0) and arcs The radii r r bear a constant relationship to radii R R which is defined by =a constant less than 1.

a= =a constant greater than T and less than 1.

The value of dteeth.

It can be seen that, because of the geometry of the antenna as defined above, the dimensions of any tooth (or slot) are directly proportional to its distance from the feed point.

Referring to FIGURE 2, a side view of the antenna of FIGURE 1, it can be seen that an'angle 1,9 (less than 180) exists between the elements of the antenna. This configuration is produced from the similar planar antenna by rotating one of the elements about an axis lying in the plane of the antenna, said axis passing through the feed point perpendicular to the line bisecting the vertex angles of the elements. I

Infinite antenna structures of the type ofthe invention have the property that, when energized at the vertex, the fields at a frequency f will be repeated periodically at all other frequencies given by m where u may take on any integral value. The parameter 1- determines .what may be considered the bandwidth of a period of operation. That is,

thus determines the widthof the slots and f1 where f and f are two frequencies exactly one period apart (f f The shape of the antenna'structures of preferred embodiments of the invention, the angle formed at the vertex by'the planes of the'halves of the antenna FIGURES 3, 4, 5 and 6 are typical radiation patterns 1 .for the antenna of FIGURE 1.

FIGURE 7 is a sketch identifying theco-ordinate system used in FIGURES 3, 4, 5 and 6. l

As shown in FIGURE 1, the antennas of the present invention comprise two identical substantially V-shaped,

electrically conducting elemen-ts each of which is partiallydefined by intersecting straight lines (e.g., A0 and I, CD) the apexes of said elements almost meeting (i. e., no electrical connection) at -point ;O (the'feed point of the antenna). The marginal edges of each element (e.g., A0

1 its complement are fitted together theycover the whole:

the invention is such that the variation of the radiation pattern and impedance is small over one period, and because of the periodically repeating nature of the fields, the same will be true for all periods, the result being an extremely broadband antenna. For the finite structures of the invention it has been found that the fields along the antennas decay very rapidly after passing a point where a resonant discontinuity, such as a tooth one quarter wavelengthlong, exists. This decay of the field causes the end effect of the antennas of'the invention to be small, so that Wide bandwidths arereadily obtained with structures of finite size. I a

The lower and upper-limits of the frequency band in which the radiation patterns are independent of frequency are determined by the longest and the shortest teeth, respectively, in'the antenna. The low frequency limit is that for which the longest teeth are wavelength long. Likewise, the high frequency limit is that for which the shortest teeth are wavelength long. It can therefore be seen that the bandwidth of the antenna can be adjusted as desired by making the shortest and the longest teeth in the antenna correspond to wavelength of the desired frequency limits.

It .is known that for planar antennas, including the rime versions of the antennas of this invention (i.e., 0:180"), any antenna whichh'a'stlie same shape as its complement has [a constant irripedancej which is' inde .pendent of frequency.complem' ent of a planarantenna is defined as the portionof theplane which is' not covered by the original antennaz'when an antenna and plane without overlapping. It is preferred that the antennas of the invention be self-complementary in the plane in order to minimize the variation of impedance with frequency. This can be accomplished in the antenna of FIGURE 1, for example, by making the sum of angle a and angle [3 equal to 90.

Although the particular embodiment of FIGURE 1 (i.e., in which the slots and-teeth are defined by circular arcs) is preferred, the invention is not limited thereto. Other types of discontinuities can be used along the edges of the antenna, such as trapezoids, triangles, etc, provided that the dimensions of all such discontinuities are proportional to the distances from the feed point. Regardless of the actual configuration used for the discontinuities, it is preferred that all discontinuities in a given antenna have similar shapes.

It can be seen that the antenna of FIGURE 1 possesses no axis of symmetry but'is symmetrical about the feed point only when the elements are coplanar, i.e., when =180. Symmetry about the feed point is defined as a configuration such that for every point in the antenna falling on a straight line passing through the feed point, there exists a corresponding point on the same line'at an equal distance on the other side of the feed point. Although antennas made in accordance with the invention which possess one or more axes of symmetry exhibit some broadband characteristics, the bandwidths thereof are in general inferior to those of antennas which are symmetrical about the feed-point only, and which are therefore preferred.

In the preferred embodiment the same arcs which define the teeth and slots at one marginal edge of one of the elements of the antenna also define the teeth and slots on the other marginal edge of the same element. However, in order to avoid symmetry about any axis, the slots and teeth on the edges are arranged such that each tooth has a corresponding slot and each slot has a corresponding tooth on the opposite edge of the element at the same distance from the feed point, the corresponding slot or tooth being a mirror image of the tooth or slot to which it corresponds.

In order to demonstrate the performance of the antennas of the invention, a number of antennas of the type shown in FIGURE 1 were constructed of light gauge copper sheet, approximately ,5 inch thick. The periodic discontinuities were in the form of teeth defined by arcs of circles connected to a sector-shaped central conducting strip. In order to maintain an equi-complementary condition, the sum of angles at and ;8 was made equal to 90. A small area near the center of the antenna was left as solid conductor since continuation of the teeth in that direction requires an infinite number of teeth of'zero width in the limit. For all models tested, the ratio a, as defined above, was taken equal to the square root of the ratio 1-, also defined above, providing a ratio of tooth to slot width which is the same for all rows of teeth. This, however, is not a necessary condition'since can assume any value greater than 1 but less than 1.

Example I An antenna of the type of FIGURE 1 was constructed with the following values afiixed to the variables:

R =9 inches The antenna was fed across the vertex with a coaxial line having its outerconductor bonded to one half of the antenna and its inner conductor extended across the vertex and connectedto the other half. The radiation patterns 'for this antenna were measured at a frequency of 1400 me. of various values of angle #1. The effect of rotating thev elements of the planar antenna out of the plane about an axis passing through the vertex was to cause it to radiate more in one direction than in the other. The front to back ratio was found to increase as the angle 0 was reduced, the change being gradual down to about with a rapid increase as the angle was further reduced. The radiation patterns for four values of t are shown in FIGURES 3, 4, 5 and 6. In FIGURES 3-6 the vertical axis at 12 oclock represents 0:0", and the radiation patterns are orientated on this basis. The designation =90 represents a constant condition applicable to each figure, indicating that the radiation patterns are those taken in the =90 plane (i.e., the YZ plane as seen in FIGURE 7). The E, variations represent the horizontally (i.e., YZ plane) polarized component of the beam while the E, variations represent the vertically (i.e., perpendicular to the YZ plane) polarized component of the beam. It can be seen that excellent unidirectional performance was obtained as a result of inclining the elements of the antenna as described.

The electric field produced by an antenna of the invention lies in the horizontal or YZ plane when the antennais positioned as shown in FIGURE 7. It is, therefore, evident that the polarization of these antennas differs from that of a dipole having its arms folded to form a V, which has its electric field in the plane determined by the elements of the folded dipole.

It was further found that the radiation patterns for this antenna were independent of frequency over a bandwidth of more than 10 to 1. In addition, the bandwidth was found to be independent of angle 1/ for the range from 30 to 100.

Several additional models identical to that of Example 1 except for values ofr were tested. Essentially no change was noted in'the radiation patterns as 'r was reduced until a value of 0.5 was reached, at which point beamwidth and pattern variations were noted for the smaller values of 4/. Although the patterns remained essentially intact, they were inferior in frequency independence to those obtained for the higher values of T, which are accordingly preferred.

Example II An antenna similar to that of Example 1 was constructed having the following parameter values:

The radiation patterns for-this antenna were found to be similar to those for the antenna of Example 1.

The input impedance of the non-planar antennas of the invention exhibit somewhat more variation with frequency than, is found in the similar planar antennas, which variation increases with decreasing values of it. The impedance characteristics of the antennas tested were found to be essentially the same for'all models, showing little variation with the values of 'r and tooth angle a. The mean resistance level, however, was found to decrease with decreasing 1p, ranging from 165 ohms at =180 to approximately 70 ohms at =30.

The two halves of the antennas of the invention are fed at the vertices thereof either with a balanced two wire line or with a coaxial line having the outer conductor bonded to the vertex of one half of the antenna and the inner conductor attached to the vertex of the clearness of understanding only, and no unnecessarylirni tations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is: r

1. An antenna comprising a pair of opposed electrically conducting substantially V-shaped elements lying in different planes, the apexes of said elements being immediately adjacent each other and together constituting the feed point of said antenna, said elements forming an angle at said feed point of about 30 to about 100", each of said elements having an outline partially defined by a respective pair of intersecting straight lines, the edges of each of said elements defined by said straight lines containing a plurality of alternating slots and teeth, the dimensions of each of said slots and teeth being proportional to its distance from said feed point.

2. A non-planar antenna comprising a pair of opposed planar, electrically conducting, substantially V-shaped elements lying in different planes, the apexes of said elements being adjacent each other and together constituting the feed point of said antenna, the planes of said elements forming an angle at the feed point of about 30 to about 100, each of said elements comprising an imperforate central sector shaped portion and slotted sector-shaped portions of equal angular width on either side of said central portion, said slotted portions containing a plurality of alternating slots and teeth, the dimensions of 6 each of said slots and teeth being proportional to its distance from said feed point.

3. An antenna as described in claim 2 which is equal to its complement.

References Cited in the file of this patent UNITED STATES PATENTS 2,192,532 Katzin Mar. 5, 1940 2,480,154 Masters Aug. 30, 1949 2,615,005 White Oct. 21, 1952 FOREIGN PATENTS 937,360 Germany Jan. 5, 1956 1,100,801 France Sept. 26, 1955 OTHER REFERENCES Development of Fan Type TV Antenna," Radio and Television News, May 1950, pp. 66, 67, 132.

DuHamel and Isbell: Broadband Logarithmetically Periodic Antenna Structures," March 1957 IRE National

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