US 2998605 A
Description (OCR text may contain errors)
Aug. 29, 1961 R. A. ORLANDO ANTENNA SYSTEM Filed July 9, 1957 United States Patent ois Filed July 9, 1957, Ser. No. 670,697 3 Claims. (Cl. 343-848) General This invention relates to antenna systems and, though not limited thereto, particularly to omnidirectional antenna systems for use at UHF and microwave frequencres.
The chief problem in building omnidirectional antennas is one of obtaining a radiation pattern which is uniform in all directions. This arises from the inherent tendency of single radiator elements, even one so simple as a straight length of wire, to have directional radiation characteristics. This limitation has in the past been overcome by utilizing relatively complex arrays of radiator elements, the arrays frequently being stacked to form several tiers of such arrays. In this manner the weak points in the radiation pattern of one radiator element may be olfset by the strong points in the radiation pattern of one or more of the other radiator elements.
At higher operating frequencies difficulties are encountered in constructing complex antenna arrays. For one thing, the more radiator elements there are, the more diflicult it is to connect them to the transmission line which feeds the'energy from the transmitter. without in troducing impedance mismatches. Such impedance mismatches may cause both misphasing and unbalance of the magnitudes of the relative currents drawn by the different radiation elements which, in turn, cause undesirable alterations in the radiation pattern. It is desirable, therefore, to have an antenna system which requires only a minimum of active radiator elements.
Also, strictly as a matter of power efficiency, it is desirable to utilize as few radiator elements as possible because the fewer the number of radiator elements, the smaller is the energy loss of the antenna as a whole. For example, resistive losses due to the current flow within the radiator elements are reduced when the number of such radiator elements is reduced.
It is an object of the invention, therefore, to provide a new and improved antenna system of relatively simple construction for obtaining a radiation pattern of increased uniformity.
It is another object of the invention to provide a new and improved antenna system which has increased power efliciency.
It is a further object of the invention to provide a new and improved antenna system for obtaining an omnidirectional radiation pattern with a minimum of active radiator elements.
In accordance with the invention, an antenna system comprises an active radiator element for radiating electromagnetic energy, a surface positioned below the radiator element to form a ground plane, and a ring shaped reflector element encircling the radiator element and positioned at a height intermediate the radiator element and the ground plane, the position being chosen such that energy radiated by the radiating element towards the ground plane at a predetermined angle is intercepted and reflected by the reflector element rather than being reflected by the ground plane thereby modifying the phase of the energy so that, rather than combining in phase opposition with directly radiated energy, and producing energy null points in space, the reflected energy combines in a phase-aiding manner with directly radiated energy 2,998,605 Patented Aug. 29, 1961 "ice to produce a radiation pattern of increased uniformity in all directions.
For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.
Referring to the drawing:
FIG. 1 is a plan view of a particular embodiment of an antenna system constructed in accordance with the present invention, and 7 FIG. 2 is a cross-sectional view taken along the section line 22 of FIG. 1.
Description of antenna system Referring to FIG. 1 of the drawing, there is shown a plan view of an antenna system constructed in accordance with the teachings of the present invention. Such a system includes an active radiator element 10 for radiating electromagnetic energy. For the case of an omnidirectional antenna system, such radiator element 10 may take the form of a tri-dipole radiator as shown, in which case the radiator element includes dipole elements 11, 12 and 13 which are individually directly connected to a center position 14 of the radiator element by way of feed lines 15, 16 and 17, respectively.
The antenna system also includes a nonuniform ground plane having an upper boundary of nonuniform height for modifying the ground plane reflections of the radiated energy to obtain a radiation pattern of increased uniformity. In the case of the embodiment of the invention which is illustrated in the drawing, such nonuniform ground plane may be composed of a metallic surface 18 positioned below the radiator element 10 to form a conductive ground plane together with a. passive reflector element positioned at a height intermediate the radiator element 10 and the ground plane 18 for effectively altering the height of the ground plane over the region covered by such reflector element. As illustrated in the drawing and best seen in the cross-sectional view of FIG. 2, such passive reflector element may take the form of a ringshaped conductive element 20, which is centered about a vertical axis 21 corresponding to the center line of the radiator element 10.
Energy may be supplied to the antenna system, more particularly to the active radiator element 10, by way of a transmission line conductor 22 which may also serve as the vertical support of the radiator element 10. Such support for the radiator element 10 may also include an additional support member 23 made of dielectric material and surrounding the transmission line conductor 22. The ring-shaped reflector element 20 may be supported above the ground plane 18 by means of dielectric support members which, for the sake of simplicity, have not been shown. The lower end of the transmission line conductor 22 which projects through the ground plane 18 may be connected to the transmitter by way of a suitable transmission line. Where the antenna is used for receiving instead of transmitting, such transmission line would instead be coupled to a receiver.
Explanation of operation Considering now the operation of the antenna system just described, assume the antenna is being used for transmitting purposes. The radio-frequency energy is supplied from the transmitter by way of the transmission line conductor 22 and the feed lines 15, 16, and 17 to k the individual dipole elements 11, 12, and 13. Such di- Considering first the elevational radiation pattern and assuming for the moment that the ring-shaped reflector element 20 of the present invention is not present, then the energy radiated to any given point in space 1 s made up of a direct component indicated by the ray 25 111 FIG. 2 and a reflected component indicated by the ray 26 which is reflected off the ground plane 18. As is apparent, the distance traveled by these two component rays in reaching a point out in space is different and varies as a function of the angle 8. Thus at some angles the field strength will be increased due to the in-phase addition of these two components, while at other angles the field strength will be decreased because of the out-ofphase cancellation of these components. In particular, at some angles, as seen by an observer in space, these two components will be 180 out-of-phase.
Another useful and perfectly valid 'way of looking at the matter is one of considering that instead of the ray 26 occurring due to reflection from the ground plane 18, such ray is instead produced by an imaginary radiator element 10, representing the image of the radiator element 10, which is spaced an equal distance below the ground plane 18. Looking at it in this way and considering the geometry of the matter, it will be apparent that minimums or nulls will occur in the radiation pattern at angles defined by the following relationship:
Sm (i where =the angle of the null 7\=the operating wave length d=the height of the radiator element 10 above the ground plane 18.
For the case where the radiator element 10 is spaced at a distance of one wave length above the ground plane 18, the null should appear at an angle of 30. This is the illustrative case which is intended to be represented in the drawing. As will be seen, the diflerence in path length for the rays 25 and 26 is indicated by the dimension A and corresponds to one wave length. This results in the signal components being 180 out-of-phase as seen by an observer out in space where it is remembered that the reflected ray 26 experiences an additional 180 phase shift due to reflection ofl the surface of the ground plane 18.
A feature of the present invention is the recognition of the fact that certain regions of the ground plane may be identified with radiation at different elevation angles. As a result, the ground plane regions, corresponding to elevation angles at which nulls or minimums in the radiation pattern occur, may be modified so as to increase the signal strength at these angles and thereby improve the uniformity of the radiation pattern. For the representative case illustrated in the drawings, such nonuniformity of the ground plane is introduced by placing a reflector element, namely the ring-shaped element 20, at a height intermediate the radiator element 10 and the ground plane 18 and at a position at which it can intercept energy which would otherwise be reflected from the ground plane 18 at the null angle and at a height above the ground plane which is sufiicient to modify the phase of such energy so that it is no longer in phase opposition with the direct radiation indicated by the ray 25.
In other words, for the region of the ground plane 18 covered by the ring-shaped reflectorelement 20, that element itself behaves as the ground plane and results in a new reflected ray 27 instead of the former reflected ray 26. The difference in path length for this new reflected ray 27 and the direct ray 25 is indicated by the dimension s. As is apparent from the drawing, this difference in path length s is different from the difference in path length A for the my 26. The effect of the ringshaped reflector 20 is to cause the imaginary 01' image 4 radiator 10' to appear closer to the ground plane 18 for radiation at the 30 elevation angle. As a result, the energy components represented by the rays 25 and 27 will no longer be out-of-phase and hence the field strength will have been increased.
Another important feature of the invention is that the uniformity of the radiation pattern is further increased by means of additional energy which is reflected ofi the inner edge of the back side of the ring-shaped element 20. This energy reflected from the inner edge of the back side of the element 20 is indicated by the ray 28 which, as indicated, is thence reflected off the ground plane 18 and radiated out into space in the same direction as the rays previously considered. In order that the energy reflected from the back side of the ring-shaped element 20 may add to or increase the uniformity of the radiation pattern, it is necessary that the radius from the center axis 21 to the inside edge of the element 20 be proportioned so that the energy path represented by ray 28 will not be 180 out-of-phase with the energy following the direct ray 25.
Considering the resultant energy which will reach the various points in space, where reflection is occurring off the ring-shaped element 20, the energy seen by an observer out in space represents the vector sum of the direct component 25 and of the two reflected components 27 and 28. For other angles where reflection is occurring primarily off of the ground plane 18 instead of the ringshaped element 20, the energy reaching the observer will correspond to the vector sum of the direct component, the
rflected component from the back side of the ring, and the reflected component reflected off of the ground plane 18. Due to the physical thickness of the different elements, there will be regions intermediate these two regions where the energy seen by the observer will represent the vector sum of the direct component and portions of all three of the possible reflected components.
Another feature of the invention is that the energy reflected off the inner edge of the back side or opposite side of the ring-shaped element 20, as illustrated by the ray 28, also serves to increase the uniformity of the azimuth radiation pattern. In other words, assuming the observer is at such an azimuth that the field strength would be a minimum due to direct radiation, then the energy reflected from the back side of the ring-shaped reflector element 20 may be made to correspond to energy which would have been directed towards a maximum or peak of the an'muth patern in the opposite direction.
While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.
What is claimed is:
1. An omnidirectional antenna system which operates near a ground plane, the system comprising: an active radiator element for radiating electromagnetic energy generally in all directions, the energy being radiated in a given direction on a first side of the radiator element undesirably being of reduced strength; and a passive reflector element positioned at a height intermediate the radiator element and the ground plane but on the opposite side relative to the first mentioned side of the radiator element for causing additional phase-aiding energy to be reflected back in said direction on the first side of the radiator element to obtain a radiation pattern of increased uniformity.
2. -An omnidirectional antenna system comprising: an active radiator element for radiating electromagnetic energy generally in all directions; a conducting surface positioned below the radiator element to form a ground plane; some of the energy radiated by the radiator element approaching the ground plane at a predetermined angle therewith, which, if reflected by such ground plane, would combine in phase opposition with energy proceeding directly from the radiator element and thus cause energy null points to be created at certain regions in space; and a ring-shaped conductive element positioned at a height intermediate the radiator element and the ground plane and centered about a vertical axis which passes through the center of the radiator element, such that the ring-shaped element intercepts energy which would be reflected at said predetermined angle and modifies the phase of such energy so that it is not in phase opposition with energy radiated directly to said certain regions in space, thereby to obtain a radiation pattern of increased uniformity, the uniformity of the radiation pattern being additionally increased in a given direction by the reflection of additional phase-aiding energy from the portion of the ring-shaped element on the opposite side of the radiator element with respect to the given direction.
3. An omnidirectional antenna system comprising: an active radiator element; a surface positioned below said radiator element to form a ground plane; and a ring shaped reflector element encircling said radiator element and positioned at a height intermediate said radiator element and said ground plane, said position being chosen such that energy radiated by said radiating element towards said ground plane at a predetermined angle is intercepted and reflected by said reflector element rather than being reflected by said ground plane thereby modifying the phase of said energy so that, rather than combining in phase opposition with directly radiated energy and producing energy null points in space, said reflected energy combines in a phase-aiding manner with directly radiated energy to produce a radiation pattern of increased uniformity in all directions.
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