Isolating Antenna Elements

Abstract

A conducting shelf is situated between adjacent levels of dipoles fed in the same or nearly the same phase. The shelf is dimensioned to reduce the intensity of the coupling between adjacent dipoles while adding partial images which tend to cancel the remaining coupling and thereby improve the SWR of each dipole over a relatively wide frequency range in the presence of other energized dipoles.

Description

BACKGROUND OF THE INVENTION

The present invention relates in general to antennas and more particularly concerns an improved array of stacked dipoles characterized by a relatively low SWR over a relatively wide range of frequencies and in certain arrangements a desired high degree of directivity in the H-plane.

In high frequency antenna systems a common approach for increasing the gain of the antenna system involved stacking a number of like elements. For example, it is desirable in many cases to feed dipoles at adjacent levels in the same relative phase or at least not in too widely different relative phases. However, there is coupling between adjacent dipoles having the effect of increasing the standing wave ratios which can be achieved over a given bandwidth. The coupling problem is large enough so that it is either difficult or impossible to obtain a really satisfactory standing wave ratio at the lower television channels with such coupling.

One approach to avoiding direct coupling between adjacent levels or dipoles might contemplate placing a large horizontal conducting sheet midway between the levels of the adjacent dipoles. All direct coupling would then stop; however, a dipole at the lower level would have its own negative image in the sheet instead of the real dipole in the level above it. The coupling with the negative image would be just as strong as the coupling with the dipole in the level above without the conducting sheet (providing the current flowing in the dipoles in the two adjacent levels were approximately equal).

An important object of this invention is to provide an improved stacked antenna array.

It is another object of the invention to provide a stacked antenna array in accordance with the preceding object characterized by improved impedance characteristics.

It is another object of the invention to provide a stacked array in accordance with the preceding object characterized by a narrower pattern in the H-planes of the stacked dipoles.

It is another object of the invention to achieve one or more of the preceding objects with relatively simple inexpensive structure.

SUMMARY OF THE INVENTION

According to the invention, conducting means are located between adjacent levels of antenna elements dimensioned large enough to appreciably reduce direct coupling between adjacent levels while being small enough to allow some direct coupling and yet large enough to produce some effective image so that the effective image coupling effects and direct coupling effects tend to oppose and nearly cancel each other. According to another feature of the invention, similar conducting means are located so as to sandwich the adjacent arrays of elements with the first-mentioned conducting means.

For a vertical array of radiating elements, it was found that satisfactory dimensions were slightly less than a wavelength between adjacent levels of stacked dipoles, a width of the supporting tower a little less than the distance between adjacent layers and the width of the conducting means being within the range of 0.1-.3 wavelength.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of the invention in which Delta Dipole antennas described in U.S. Pat. No. 2,973,517 are mounted about a square mast;

FIG. 2 shows another embodiment of the invention having three dipoles in each layer, each on a respective side of a triangular tower;

FIG. 3 shows another embodiment of the invention in which vertical dipoles arranged around a cylinder are decoupled by using vertical conducting means; and

FIG. 4 shows the improvement in impedance characteristics achieved with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference now to the drawing and more particularly FIG. 1 thereof, there is shown an embodiment of the invention in which commercially available Alford type 4730 delta dipole antennas are arranged in layers of four seated upon respective sides of a square tower. A square tower 11 is covered with a conducting sheet 12. An upper layer of delta dipoles such as 13, two of which are visible, and a lower layer of dipole elements such as 14, two of which are visible are mounted upon respective sides of the square tower 11. A conducting shelf 15 is midway between dipoles 13 and 14, all of which are sandwiched by an upper conducting shelf 16 and a lower conducting shelf 17. Each of the shelves 17, 15 and 16 are preferably substantially congruent of the shape shown. Each shelf may be made of solid conducting material or screen-like material as shown, preferably having a long straight side, such as 21 generally parallel to the tower face and a shorter straight side, such as 22 that is centered about a corner of the tower and perpendicularly bisected by a plane passing through the tower diagonal passing through that corner.

For a UHF frequency band of interest from 690 MHz to 810 MHz, typical dimensions for the separation a between adjacent ones of shelves 15, 16 and 17 is 14 inches, corresponding to 0.89 wavelength at the center frequency of the band; the separation b between dipoles 13 and 14 is also 14 inches, corresponding to a wavelength of substantially .875 wavelength at the center frequency of the band and the dimension c between the straight edge 21 of a shelf and a tower side being for example 31/2 inches corresponding to 0.223 wavelength at the center frequency of 750 MHz where a wavelength is 15.7 inches.

Although the shelf width as measured at right angles to the outer surface of the conducting screen 12 is of the order of a quarter wavelength, a smaller shelf, such as 0.14 wavelength wide may in some cases be advantageous for achieving a better impedance curve at the input to the antenna array plotted on a Smith Chart so as to minimuze the SWR over two separated band such as, for example, TV channels 9 and 13. Shelves wider than a quarter wavelength may also be used when it is desired to shorten the spread of the impedance curve on the Smith Chart within a given frequency band. The effects of the size of the shelf on the impedance characteristic is discussed below.

Referring to FIG. 2, there is shown another embodiment of the invention having three dipoles per layer on respective sides of a triangular tower. Two of the delta dipoles 31 and 32 are shown between an upper shelf 33 and a lower shelf 34. In this embodiment only a single layer of dipoles is shown, it being understood that additional layers with a corresponding increase in shelves may be provided. It has been discovered that the use of shelves is beneficial even when there is only a single layer of dipoles because the shelves increase the gain of a single layer of dipoles arranged around the tower. This was found to be true in the case of both square and triangular towers and is believed to be true no matter what is the shape of the tower.

The addition of shelves is even more advantageous when functioning to improve the impedance match of an array having a number of layers of dipoles. The dimensions which have been found to result in approximately circular radiation patterns in the horizontal plane when the layer or layers of dipoles are mounted around a triangular tower are as follows:

Tower width approximately 0.51 wavelength;

spacing between layers of dipoles approximately 0.9 wavelength;

shelf width approximately a quarter wavelength, for example, 0.22 wavelength.

It was found advantageous to form the shelves with a long edge 35 generally parallel to the side of the tower and a shorter generally straight edge perpendicularly bisected by a plane passing through the corner and bisecting the angle at that corner.

Shelves may be made of sheet metal, fine mesh grain, a grid with rather large holes or any other suitable means. Specific dimensions are set forth in FIG. 2. A mesh with these dimensions was found to give good results in the frequency range of 690-810 MHz. This mesh structure was found to be close to and somewhat better than that of a shelf made of conducting sheet metal of the same dimensions. It was also discovered that when one of the rods of the grid was removed, thereby making the holes still bigger, the effect of the shelf was less satisfactory. The specific dimensions shown in FIG. 2 are related to the wavelength at the center frequency of 750 MHz.

It was also discovered that dipoles mounted on one face of a square tower approximately 0.9 wavelength wide has little coupling with dipoles located on adjacent sides of the tower. In fact, the arrangement acted substantially as if the array of dipoles on one face of the tower were independent of the other dipoles on the other faces of the tower.

With the small triangular tower embodiment of FIG. 2 there was some coupling between elements on the same level. The results described above are applicable to dipole arrays mounted on relatively large sheets; for example, arrays consisting of a few or many horizontal dipoles mounted one layer above another layer. The effect of the shelves would also be beneficial for controlling the effective coupling between such layers of dipoles to improve the impedance characteristics of the dipoles and thereby obtaining good impedance match over a relatively wide range of frequencies.

Referring to FIG. 3, there is shown an array of vertical dipoles 41, 42 and 43 decoupled from each other and having their impedance characteristics improved by shelves 44, 45, 46 and 47 vertically oriented.

Referring to FIG. 4, there is shown a graphical representation of the impedance as a function of frequency on a Smith Chart representing actual measurement of the embodiment in FIG. 2 made through a carefully matched rigid transmission line. The six dipoles were excited with equal power in the same phase, and a slotted line was inserted into the branch feeder supplying one of the dipoles. The division of power and the phases of the currents supplied at the six dipoles were carefully checked with the aid of an automatic transfer characteristic plotter.

Curve 1 in FIG. 4 shows the impedance characteristic of a single delta dipole on a triangular tower from 710 to 770 MHz. Curves 2 and 3 show the impedance characteristic of the middle level dipole in a three tier array on a triangular tower without and with, respectively, shelves 0.23 wavelength wide. These results demonstrate that the effect of the shelves on the impedance over about 10 percent band of frequencies effected a reduction in SWR, particularly after suitable compensation by well-known means. Furthermore, it was found that the shelves as described in connection with the embodiment of FIG. 2 increased the power gain of the two-element array from about 1.9 to approximately 2.3, an improvement of about 20 percent.

With the dimensions of the tower and shelves as shown in FIG. 2, the radiation pattern in the horizontal plane was observed to be approximately circular as the frequency was increased from 690 to 810 MHz. The most circular patterns were observed in the neighborhood of 780 MHz. At frequencies substantially below this frequency, the pattern became somewhat triangular; however, still acceptable. At frequencies above 780 MHz the patterns again gradually become more triangular but in a transition region in the vicinity of 780 MHz the pattern is hexagonal with rounded corners, all these patterns still being acceptable for omnidirectional radiation in most applications. At frequencies below the frequency of best circularity the maximum of radiation occurred in the directions of bisectors of the triangular tower cross section. At frequencies above the frequency of best circularity, the maxima of radiation occurred in directions at right angles to the sides of the tower.

Similar phenomena were observed in the case of a square tower. In this case, the best circularity of the pattern was obtained when the tower width was in the vicinity of 0.85 wavelength when the pattern became octagonal with rounded corners. Wider towers result in squarish patterns in which the signal maxima (along the diagonals) are directed at right angles to the faces of the tower. Towers which have sides narrower than 0.85 wavelengths result in patterns which result in increasingly squarish patterns which have maxima in the directions of the diagonals of the tower. These phenomena are similar to those observed with triangular towers except that in that case the optimum tower width is approximately 0.53 wavelength where the patterns are hexagonal instead of octogonal as in the case of the square tower. With sides less than 0.53 wavelength one obtains triangular patterns with maxima in the direction from the center of the tower through the corners of the tower. With wider towers, the patterns again become triangular but with maxima in the directions at right angles to the sides. These measurements were made using the delta dipoles.

In the above description, it was assumed that the sides of the tower are covered with conducting panels made of metal net or of parallel metal bars spaced preferably 0.1 wavelength or closer. In some cases, the tower size may be too small to obtain optimum circularity of patterns, in which case a triangular or a square screen may be installed around a triangular tower or a square screen or triangular screen around a square tower. The dimensions of these artificial towers should be preferably chosen as described above, that is, 0.53 wavelengths on the side for triangular screens and 0.85 wavelength for square screens.

Measurements were made to determine the action of the shelves in the cases when the dipoles on one face of the tower are fed less power than the dipoles on other faces of the tower in order to obtain a directional pattern rather than substantially circular pattern in the horizontal plane. It was found that the optimum dimensions of the shelves were still approximately the same as described above. Furthermore, smoother directional patterns are obtained when the dimensions of sides of the triangular tower is in the vicinity of 0.53 wavelength and when the sides of the square tower are in the vicinity of 0.85 wavelength.

Shelves of approximately the same widths in terms of wavelength may be used with beneficial results on towers of other cross sections such as, for example, hexagonal, octagonal or circular. In summary, the invention improves SWR or impedance characteristics, power gain and retains acceptable omnidirectionality over a relatively wide range of frequencies.

It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims.

Claims

1. An antenna system comprising; a tower having means defining at least one electrically conducting surface,

an antenna array having dipole radiators mounted on the tower at spaced locations, adjacent dipoles of the array being parallel and being directly coupled,

conducting means mounted on the tower intending in a perpendicular direction with relation to the conducting surface, the conducting means dispose, between adjacent dipoled, the dipoles extending parallel to the plane of the conducting means and the conducting means being dimensioned to reflect images of the dipoles while reducing the direct coupling between the dipoles to cause the image coupling effect to oppose the

2. An antenna system as set forth in claim 1 wherein said antenna array comprises,

a first plurality of dipole radiators forming a line thereof,

a second plurality of dipole radiators forming a line

said conducting means being disposed between said lines of first and second

3. An antenna system as set forth in claim 2 and further comprising,

second and third ones of said conducting means sandwiching said first and

4. An antenna system as set forth in claim 1 wherein the separation between

5. An antenna system as set forth in claim 1 wherein said tower is of

6. An antenna system as set forth in claim 1 wherein said tower is of

7. An antenna system as set forth in claim 1 wherein said means defining at least one electrically conducting surface is of square cross-section with the length of each square side substantially 0.85 wavelength at the center

8. An antenna system as set forth in claim 1 wherein said means defining at least one electrically conducting surface is of triangular cross-section with the length of each triangle side substantially 0.53 wavelength at the

9. An antenna system as set forth in claim 1 wherein the separation between said conducting means and an adjacent dipole is substantially 0.45 wavelength and the width of said conducting means is substantially a

10. An antenna system as set forth in claim 2 wherein said conducting means are midway between adjacent ones of said lines,

the separation between adjacent ones of said lines being substantially 0.9 wavelength and the width of said conducting means is substantially a

11. An antenna system as set forth in claim 10 wherein said means defining at least one electrically conducting surface is of square cross-section with the length of each square side substantially 0.85 wavelength at said

12. An antenna system as set forth in claim 10 wherein said means defining at least one electrically conducting surface is of triangular cross-section with the length of each triangle side substantially 0.53

13. An antenna system as set forthin claim 4 wherein each of said conducting means includes generally symmetrical trapeqoidal segments having a long parallel adjacent side spaced from said long parallel side

14. An antenna system as set forth in claim 12 wherein each of said conducting means includes generally wherein each of said conducting means includes generally symmetrical trapeqoidal segments having a long parallel side adjacent to a triangle side and short parallel side spaced from said

15. An antenna system as set forth in claim 1 comprising a second of said conducting means wherein the separation between said conducting means is

16. An antenna system as set forth in claim 4 wherein the conducting means has a long dimension in the direction o f the dipole radiator of at least 0.5 wavelength and a shorter dimension orthogonal to the longer dimension

17. An antenna system as set forth in claim 1 whereby the input impedances of the dipoles are mor nearly matched to the characteristic impedances of

18. An antenna system as set firth in claim 1 wherein said conducting means includes a conducting sheet or screen