HIGH GAIN-AREA-PRODUCT ANTENNA DESIGN

Abstract: A high gain-area-product antenna using a dual mode feel element is designed to provide substantially uniform gain coverage over an angular region. The primary pattern (6) emitted at the antenna feed (2) is modified to make the H-plane distribution approximate the E-plane distribution (3). Both the main lobe and side lobes of the primary pattern are projected onto the reflector (4) in order to establish a secondary projected pattern (8) which approaches an ideal sector beam.

Description: HIGH GAIN-AREA-PRODUCT ANTENNA DESIGN Field of the Invention The present invention generally relates to sector beam antennas, and more particularly, to a method and apparatus for obtaining a high gain-area-product and a substantially uniform gain coverage over a regional area by utilizing both the main and side lobes of the electromagnetic signal emitted from a dual mode antenna feed element. Background of the Invention Since the advent of satellite communications sys¬ tems, ways have been sought to provide signal coverage over angular sectors of the Earth, to increase the projected signal strength, and to improve the uniformity of coverage over the regional area of interest. For example, it is obviously advantageous to establish a single telecommunications satellite in a geosynchronous orbit such that one antenna projects a uniform signal beam over the entire continental United States. Parabolic reflector antennas for communication satellites are normally used to provide a sector beam, viz. gain coverage over an angular region of the surface of Earth. Reflector antennas for most other applications (e.g., Earth stations, terrestrial relays, etc.) are used in an on-axis fashion where only the boresight gain is of importance. In order to achieve a high gain factor along the line- of-sight, or boresight, axis the energy distribution at the antenna reflector conventionally is dimensioned so as to include only a portion of the main lobe of the aperture distribution pattern (Fourier transform) emitted from the feed element. Almost all of the reflector antenna analysis available in the literature addresses maximizing the peak gain along the projection axis. It is known in the art to use dual mode feed elements, particularly the Potter horn, to equalize the E and H-plane beam widths. Basically, a Potter horn feed 2 element uses a dual mode to modify the electric plane distribution to look like the magnetic plane distribution. See "A New Horn Antenna with Suppressed Sidelobes and Equal Beamwidths," P.D. Potter, The Micro¬ wave Journal, June 1963, pp. 71-78, for the principles of *L Q operation of a dual mode horn in this manner. When these standard techniques are used to design antennas intended to provide a sector beam having gain over an angular coverage area, the resulting performance falls far short of a theoretically optimum design. •^5 A useful figure of merit for an antenna which provides gain over an angular region is the gain-area-product (GAP). This quantity is the product of the angular coverage area (in square degrees) and the minimum antenna gain (in numeric) provided over the 20 coverage region. For the theoretically ideal sector beam-a beam with uniform gain within the coverage region and with no gain outside the coverage region-the GAP is 2 41,253 deg . In contrast to this value, the Antenna Engineering Handbook by H. Jasik (pages 2-14) gives a 2 relationship between gain and beamwidth which results in a value of GAP of 10,600 deg for antennas of traditional design. This agrees fairly well with current antenna practice, which achieves coverage beams with value of GAP 2 ranging from 10,000 to 15,000 deg . Thus, current 30 practice produces antenna beams with GAP values of 25% to 35% of the maximum achievable gain-area-product. In general, area coverage is currently accomplished by using the conventional design techniques and then creating an array of overlapping beams. Techniques for 35 accomplishing such an array require elaborate systems, such as coordinating a cluster of multiple geosynchronous satellites, i.e. one for each section of the entire regional area of interest. See e.g., U.S. Patent 4,375,697 to Visher. Another common solution is to provide a single antenna system with a multiple feed array shaped roughly in proportion to the region intended to .be covered. The electromagnetic signal energy is apportioned among the feed elements. The reflector then projects a set of overlapping beams in order to attempt to achieve full coverage of the regional area with approximately the same gain factor over the entire area. These systems are complex, using computerized assistance to select the optimum arrangement of amplitudes and phases needed to coordinate the excitations. Such systems often have high power demands difficult to achieve given the current limits of space technology. Accordingly, it is an object of the present invention to provide a method for improving the gain-area-product of a sector beam antenna. it is another object of the present invention to provide an antenna having a substantially uniform gain over an angular coverage regional area. It is a further object of the present invention to increase the gain-area-product of an antenna using a single feed element. An additional object of the present invention is to provide a sector beam antenna compatible with the practical considerations involved in current satellite configurations and launch vehicle compatibilities. Summary of the Invention The present invention, in a broad aspect, is a method and apparatus for obtaining a substantially uniform gain in an electromagnetic signal projected over a large regional area. An electromagnetic beam is projected by emitting a signal pattern from a dual mode antenna feed using two modes to form the magnetic plane (H-plane) amplitude distribution, such that the H-plane amplitude distribu¬ tion looks like that of the plane containing the electric field vector (the E-plane). The radiation pattern from the antenna feed, including side lobes, is reflected toward the regional area to be covered by the beam. Proper selection of the specific number of side lobes of the feed horn primary radiation patterns in both the H-plane and the E-plane to be reflected along with the main lobe provides a beam of substantially uniform gain and a high gain-area-product. In the preferred embodiment, a dual mode feed horn is used to emit the primary radiation pattern. Two modes are used to form the H-plane amplitude distribution in order to make the H-plane amplitude distribution look like that of the E-plane uniform amplitude distribution obtained in single mode emission from the horn. An offset parabolic antenna reflector is used to form the secondary beam pattern which is projected toward the regional area to be covered. When applied to satellite communications using only a single feed element and reflector, the method and apparatus provides a higher gain-area-product with substantially uniform coverage over large geographic regions than is currently obtained. Hence, a stronger and more uniform beam over the coverage area is achieved while reducing the complexity and the power requirements of the system needed to obtain those features using current technology. Other objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description and the accom¬ panying drawings, in which like reference numerals designate like features throughout the Figures. Brief Description of the Drawings FIGURE 1 is a schematic elevation view of the present invention showing the electromagnetic dis¬ tribution as well as signal patterns before and after reflection. FIGURE 1(a) is a graphical representation of a circularly symmetric electromagnetic signal distribution across an ideal antenna aperture; FIGURE 1(b) is a graphical representation of the signal pattern of a beam obtained from an ideal antenna focusing device from the pattern of Figure 1(a); FIGURE 2(a) is a graphical representation of a circularly symmetric electromagnetic signal distribution across a finite antenna aperture as contemplated by the present invention; FIGURE 2(b) is a graphical representation of the signal pattern of a beam obtained from a finite antenna focusing device from the pattern of Figure 2(a); FIGURE 3(a) is a graphical representation of the results of an analytical study showing the aperture distribution function contained on a circular antenna aperture versus the scaling factor K for a normalized aperture radius condition in a circularly symmetrical distribution of the form 2J.(r)/r; FIGURE 3(b) is a graphical representation of the results of the analytical study as defined in Figure 3(a) showing antenna efficiency vs. K; FIGURE 3(c) is a graphical representation of the results of the analytical study as defined in Figure 3(a) showing gain-area-product vs. K; FIGURE 4 is a schematic drawing of the antenna feed/reflector geometry of the preferred embodiment of the present invention as shown in Figure 1; FIGURE 5 is a graphical representation of the gain line product vs. K for the E-plane distribution pattern obtained from an arrangement of the embodiment as shown in Figure 4; FIGURE 6 is a graphical representation of the gain line product vs. K for the H-plane distribution pattern obtained from an arrangement of the embodiment as shown in Figure 4; and FIGURE 7 is a computer simulated representation of the signal coverage pattern obtained with the embodiment as shown in Figure 4. Detailed Description of the Invention The general arrangement of the feed horn and reflector schematically is shown in Figure 4. Note the arbitrary association of the E-plane of the feed horn 2 with the x dimension and the H-plane with the y dimen- sion. The primary pattern selected to be seen by the reflector 4 is represented at 6 in Figure 1. The resultant secondary pattern of the projected beam is shown at 8, closely approximating the ideal sector beam as shown in Figures 1(a) and (b) . The E-plane distri- bution is chosen to be a uniform amplitude distribution function 3 across the antenna aperture feed horn 2. To make the H-plane amplitude distribution function approximate the uniform E-plane amplitude distribution, the fundamental cosine function of the H-plane is modified by the addition of one or more of its odd number harmonics. In practicality, it is adequate to use the TE Q3 mode as the harmonic in that the feed horn complexity to obtain higher modes is not justified by the increased performance level achieved. As a result of operating in this dual mode fashion, the H-plane primary and secondary signal patterns will be substantially similar to those as shown in Figure 1 for the E-plane distribution. In traditional antenna aperture theory, an ideal circular sector beam is formed when a circularly symmetric distribution of the form 2J (r)/r is put on a circular aperture of infinite extent. In this formulation, r is the radial coordinate and J,(r) is a Bessel function of order 1. This aperture distribution and the resulting beam distribution are shown in Figures la and lb, respectively. An infinite aperture has no meaning in a practical sense; however, i-t has been found that truncated versions of this same aperture distribution on a finite aperture will result in beam shapes which closely approximate the ideal sector beam. A truncated aperture distribution and the resulting beam distribution are shown in Figures 2a and 2b, respectively. In general, the approximation to the ideal sector beam improves as the aperture grows radially to encompass more of the distribution function before truncation occurs. A parametric study was performed to determine the gain-area product (GAP) as a function of the amount of the distribution function, 2J 1 (r)/r, contained on the aperture. The appropriate mathematical expressions associated with this process were obtained from Microwave Antenna Theory and Design by S. Silver, and therefore no detailed derivations or discussions are presented here. Note that for mathematical convenience, the aperture radius was normalized to a value of 1, which necessitates the inclusion of a scaling parameter K in the distri¬ bution function. Hence, K is an index which is proportional to primary signal distribution across the antenna aperture. The value K=0 corresponds to a uniform distribution the value K=00 corresponds to the distribution of the ideal sector beam, while intermediate values of K correspond to various levels of approximation to the ideal sector beam. The voltage pattern g ( M ) is given by the Hankel transform normally associated with circular apertures. This function was normalized to yield a value of 1 at (°r ©) equal to 0. The normalized power pattern is merely the square of the voltage pattern and these were scaled to yield relative gain patterns. The expression for gain-area-product is the area of 2 coverage (piθ Q ) times the absolute gain at θ , where θ is the angle at which GAP is maximized. No explicit mathematical expression exists for determining θ . Therefore, for each value of K, the gain pattern was evaluated by a computer program and θ and the value of GAP at Θo Λ was determined by Λ an iterative search procedure. In this manner, the GAP was determined as a function of K (the index proportional to the amount of the distribution function contained on the aperture), and the results are shown in Figure 3. Figure 3a shows the aperture distribution function versus K; Figure 3b shows the on-axis antenna efficiency versus K; and Figure 3c shows GAP versus K. In general, the gain-area-product increases as K increases; however, several local minimums can be seen in Figure 3c. These minimums occur at the zero crossings of the distribution function, and are particularly deep for positive going zero crossings. Note that traditional antenna design would suggest operating at a value of K where the antenna efficiency is maximized. This occurs when K equals 2.8, and the value of GAP at this point is 2 about 13,000 deg . This performance can be increased by a factor of 2 by selecting a value of K in the range of 11 to 12. This results in a value of GAP ranging from 25,000 to 28,000 deg 2 . It should be pointed out that for large values of K, the reflector may grow to unacceptable size. However, for the case of K=_L1, a beam covering the contiguous 48 states could be generated at 12 GHz with a reflector diameter of about 96 inches. This reflector size would yield a GAP in excess of 25,000 deg 2 . yet would still be physically compatible with today's satellite configurations and launch vehicle envelopes. The above analysis was repeated for a rectangular reflector operating with a rectangular feed horn. This feature permits the analysis and application of the results to be separated in the vertical and horizontal dimensions. For this reflector type the GAP is formed by the product of the GLP (gain-line-product) in the X dimension and the GLP in the Y dimension. Figure 5 shows the available GLP for the E-plane as a function of Kx, the index proportional to the number of sidelobes on the reflector in the E-plane. Figure 5 indicates a generally increasing GLP with Kx. Figure 6 shows the available GLP in the H-plane (using two modes) as a function of Ky. These results can be used to design an antenna for coverage of the area of interest. Figure 6 indicates that the best H-plane GLP (150.1) can be obtained with Ky equal to 3.5. Kx equal to 5.35 with E-plane GLP of (166.7) was chosen. The resulting GAP 2 (gain-area-product) is about 25,000 deg . Using the analytical results, an antenna for creating a coverage pattern for the continental United States was designed. The reflector 4 is an offset parabolic square, 76.16 inches per side, having a focal length of 65.96 inches. The feed 2 is a rectangular, dual mode type horn, 6.78 inches long in the H-plane and 10.36 inches long in the E-plane. A horizontally polarized radiation pattern is emitted at 12 GHz having a mode content of 3.96% E Q3 and 96.04% TE Q .. The resulting simulated coverage pattern is shown in Figure 7 for a satellite antenna of this design in a stationary orbit at 100 degrees west longitude. The 2 gain-area-product for the 30 dB contour is 25,000 deg ; two gain depressions inside the east and west edges 2 reduce this value to 23,800 deg , still a major increase of GAP over conventional technology results. Multiple feed elements also can be used to create alternative designs. Many modifications and variations of the present invention are obviously possible based upon this disclosure of the preferred embodiments. Therefore, it is to be understood that the invention may be practiced otherwise than as specifically described herein, but such remains within the scope of the present invention.

Claims: What is claimed is: 1. An antenna for projecting a sector beam comprising: feed means for emitting a primary signal having an H-plane signal pattern and an E-plane. signal pattern which have substantially similar amplitude distributions; and focusing means, communicating with said feed means, for transforming said primary signal to a secondary signal such that said secondary signal forms a sector beam providing a high gain-area-product and substantially uniform gain over the span of said beam. 2. The antenna as defined in claim 1, wherein said focusing means is dimensioned to focus both the primary lobe and a predetermined number of sidelobes of said primary signal. 3. The antenna as defined in claim 1, wherein said feed means comprises: a dual mode feed element. 4. The antenna as defined in claim 3, wherein said feed element comprises: a dual mode rectangular feed horn. 5. The antenna as defined in claim 2, wherein said focusing means comprises: an offset parabolic reflector. 6. The antenna as defined in claim 4, wherein: said E-plane signal pattern has a uniform distribution; and said H-plane signal pattern has a distribution constituted by one mode being a cosine function distribution and at least one additional mode being an odd number harmonic of said cosine function. 7. The antenna as defined in claim 6, wherein: said harmonic is the third harmonic of said cosine function. 8. A high gain-area-product antenna for projecting an electromagnetic signal sector beam to provide coverage over a terrestrial region from a geosynchronous orbital satellite, comprising: dual mode feed horn means for emitting an electro¬ magnetic signal having an H-plane amplitude distribution approximating the same characteristics as the E-plane amplitude distribution pattern of said signal; and offset parabolic reflector means, communicating with said feed horn, for reflecting both main and side lobes of said electromagnetic signal to project a sector beam having a gain characteristic which provides substantially uniform coverage over said region. 9. A method for projecting a sector beam electro¬ magnetic signal of substantially uniform gain coverage, comprising: emitting an electromagnetic signal, wherein the magnetic plane distribution is substantially similar to the electrical plane distribution; and reflecting said signal to form a secondary signal such that the reflected signal forms a beam having a high gain-area-product and a substantially uniform gain distribution. 10. A method as defined in claim 8, wherein said step of reflecting said signal further comprises: reflecting both the primary lobe and a predeter¬ mined number of the sidelobes of said electromagnetic signal . 11. A method for projecting an electromagnetic sector beam to provide substantially uniform gain coverage over a terrestrial region, comprising: emitting an electromagnetic signal•from a dual mode antenna feed, wherein the H-plane amplitude distribution has the same characteristics as the E-plane amplitude distribution; and reflecting said signal using an antenna reflector, wherein the aperture of said reflector intercepts both the primary lobe and a predetermined number of sidelobes of said electromagnetic signal such that said step of reflecting projects an electromagnetic sector beam having a high gain-area-product and gain characteristic which provides substantially uniform coverage over said angular region. 12. The antenna as defined in claim 1, wherein said feed means comprises a plurality of dual mode rectangular feed horns.

Inventors: CZUBA GREGORY S, THOMPSON JAMES D, CZUBA, GREGORY, S., THOMPSON, JAMES, D.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1985
Main classification: H01Q25
IPC codes: H01Q13, H01Q19, H01Q25