COMPACT ANTENNA RANGE EMPLOYING SHAPED REFLECTORS

Abstract: In the preferred embodiment the compact antenna range (10) includes two shaped reflectors, a subreflector (14) and a main reflector (16) arranged in an offset configuration. Each reflector is shaped so that the reflected wave front from the main reflector is planar with a substantially uniform amplitude and phase. Substantially all of the energy emitted by the feed (12) is reflected from the subreflector onto the main reflector in such a way that the energy incident on the reflector edges is greatly reduced, thus reducing the energy diffracted therefrom significantly. Use of the shaped reflectors also allows the main reflector to be of a smaller size and thus the tolerances associated with the main reflector can be more closely controlled.

Description: - 1 - COMPACT ANTENNA RANGE EMPLOYING SHAPED REFLECTORS FIELD OF THE INVENTION; This invention relates generally to compact antenna ranges, and more particularly, to a compact antenna range employing shaped reflective surfaces. BACKGROUND OF THE INVENTION; The testing of microwave antennas and radar cross-section requires that the antenna or unit under test be illuminated by a uniform plane wave. A uniform plane wave is one having uniform phase and amplitude across the unit under test. The various parameters measured for the unit under test include those related to the antenna pattern (e.g., sidelobes, beamwidth, cross-polarization, gain, boresight alignment, and radar cross-section). There are three common approaches for obtaining the pattern measurements. These approaches include a far-field range, a near-field range, and a compact range. The near field is that region where the angular distribution of the radiating energy is dependent on the distance from the antenna; the far field is that region where the angular distribution of the radiating energy is independent of the distance from the antenna. Using a far-field range requires the transmitting antenna to be located a predetermined distance from the unit under test to achieve what appears to be a plane wave at the unit under test. This distance is given by the following equation. Distance = 2D 2 / > /00927 - 2 - where D equals diameter of antenna under test and X equals wavelength of the test frequency. As can be seen from this equation, for extremely high frequencies (EHF) long antenna ranges are required. For example, a 4,000 foot antenna range is required to test an eighty inch antenna at 44 GHz. Due to the losses associated with the long range, a far-field range requires the use of high-power microwave sources. Other detrimental effects associated with the far-field range include those related to weather (e.g., high winds or rain) and interference from nearby buildings and other antennas producing reflections incident on the unit under test, making it difficult to measure low sidelobes (the amount of radiation emitted from or received by an antenna at angles to the left and right of the antenna center) or the extremely low scattered return needed for a radar cross-section (RCS) measurement. Another technique for testing antennas or making RCS measurements is the near-field range. The unit test data provided by a near-field range is transformed to approximate the effect of a uniform plane wave on the antenna under test. The near-field range provides only an approximate antenna pattern, limited by a practical number of data samples, measurement accuracy, and transform accuracy. The near-field range is accurate only for the main beam and near-in sidelobes and is very time consuming due to the large amount of data that is required. The most desirable test range is the compact range. The compact range provides indoor measurement of antenna patterns directly, i.e., no approximations are necessary, and the total scattering matrix required by RCS. All test equipment is located in a relatively close physical - 3 - arrangement that greatly simplifies unit testing. Also, low spatial losses allow for the use of low-level microwave sources. One prior art compact range uses a parabolic dish and a feed located at the focal point of the dish. The spherical rays emitted by the feed are collimated by the reflector and thus transformed into a plane wave, which is incident on the unit under test. Such a compact range is disclosed in U.S. Patent No. 3,302,205. Because the parabolic reflector of this prior art arrangement provides a one-to-one mapping of energy from the feed to the unit under test, a low gain feedhorn is required to provide uniform amplitude at the unit under test. But a low gain horn emits its energy in a substantially omnidirectional pattern. The disadvantages associated with the use of an omnidirectional emitter include stray radiation from the emitter, scattering of the radiation by any objects in proximity to the compact range, back radiation from the emitter, and high edge scattering from the edges of the main reflector. To achieve the plane wave at the unit under test, the feed must have a very low gain and the main reflector must be oversized. The energy from the feed is directed at only a portion of the parabolic main reflector to produce the plane wave. For example, to produce a four foot quiet zone for a unit under test requires a twelve foot parabolic main reflector. The disadvantages associated with placing the feed at the focal point include cross polarization of the radiation, deleterious effects due to the surface inaccuracies in such a large reflector, and space attenuation of energy emitted from the feed. The latter is especially important when measurements of - 4 - large EHF antennas are considered. In general, the disadvantages associated with this prior art technique arise from the use of a low-gain feed and an oversized main reflector; these disadvantages relate primarily to high diffraction at the main dish and large amounts of reflection and diffraction from the feed and supporting structures. The broad radiation pattern of the feedhorn that is required to illuminate the large parabolic reflector produces radiation in unwanted directions, which degrades the quiet zone. The lack of control of the illumination energy produces high energy levels at the reflector edges, which in turn produces a high order of edge diffraction. The additional edge diffraction causes more stray radiation and further degradation of the quiet zone. Various techniques have been employed to reduce the order of edge diffraction including: rolling the reflector edges, attaching serrated panels to the reflector edges, or placing absorbing material around the perimeter of the reflector. Another compact range is disclosed in U.S. Patent No. 4,208,661, entitled "Antenna With Two Orthogonally Disposed Parabolic Cylindrical Reflectors". The patent discloses an antenna test range including first and second parabolic cylindrical antennas arranged in an offset configuration. The antennas are simple conic sections; they are not shaped to control the quiet zone excitation. This scheme is also discussed in an article by the patentee appearing in the February 1985 issue of Microwaves and RF, entitled "Seeing Double Improves Indoor Range." SUMMARY OF THE INVENTION A compact antenna range employing dual-shaped reflectors is disclosed. The present - 5 - invention overcomes the disadvantages discussed above by employing two shaped reflectors arranged in an offset configuration to produce a plane wave incident on an unit under test. In the present invention, a spherical wave front produced by a feedhorn is incident on an offset shaped subreflector. The wave front reflected from the shaped subreflector is incident on a shaped main reflector, and the reflected wave front therefrom is a uniform plane wave incident on the unit under test. The zone in which the unit under test is placed is referred to as the quiet zone; the uniform plane wave traverses the quiet zone. The quiet zone should be substantially free of stray radiation, which would have a deleterious effect on the measurements. Shaping of reflectors means changing the surface shape from a conic section, i.e., a parabola, paraboloid, ellipse, ellipsoid, hyperbola, or hyperboloid, to a new curvature or shape. For additional details, see "Minimum-Noise Maximum-Gain Telesceopes and Relazation Method for Shaped Asymmetric Surfaces" by Sebastain Von Hoerner, appearing in IEEE Transactions on Antennas and Propoqation, Volume AP-26, No. 3, May 1978, pp. 464-471. According to the principles of the present invention, the shaping of a subreflector and main reflector is accomplished to provide a substantially flat amplitude plane wave in the quiet zone and low excitation on the edges of the main reflector. In one embodiment, the amplitude ripple is less than 0.25 dB. The present invention also provides a substantially uniform phase distribution; in one embodiment the phase ripple is less than ten degrees at 44 GHz. Further, shaping allows the use of a 86/00927 - 6 - high-gain feed, which has lower spillover and diffraction scattering than the low-gain feed used in the prior art. Further, shaping of the subreflector and the main reflector allows for the use of a much smaller main reflector than used in the prior art, and a small subreflector. Several of the disadvantages discussed above in conjunction with the prior art, are overcome by the present invention. The stray radiation from the feed and its supports is substantially eliminated or greatly reduced by the use of a high-gain feed in the present invention. According to the present invention, the aperture illuminations of both the subreflector and the main reflector are shaped to provide very low excitation of reflector edges, which reduces scattering from the reflector edges. The dual reflector arrangement of the present invention provides a longer focal length, thereby lowering cross-polarization as compared to the cross-polarization encountered in the prior art technique. In one embodiment, the cross-polarization is approximately -30 dB. Shaping of the aperture illuminations can also correct for space attenuation present in the prior art. Shaping also allows the use of smaller reflectors that can be manufactured in one piece so that higher tolerances can be obtained. This aspect of the present invention overcomes the reflector surface accuracy problem of the prior art. BRIEF DESCRIPTION OF THE DRAWINGS The present invention may be better understood, and the further advantages and uses thereof more readily apparent, when considered in view of the following detailed description of exemplary embodiments, taken with the accompanying drawings in which: - 7 - Figure 1 illustrates a preferred embodiment of the dual-shaped offset compact range of the present invention; Figure 2 illustrates a second embodiment of the present invention; and Figures 3A and 3B illustrate the amplitude and phase distribution, respectively, for the dual-shaped offset compact range of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a dual-shaped offset reflector compact range 10 including a feedhorn 12, a subreflector 14, and a main reflector 16. According to the inventive principles, the subreflector 14 and the main reflector 16 are shaped, meaning that the surface geometry thereof has been changed from a true parabolic surface of revolution. As is well known in the art, antennas can be shaped to achieve various desirable objectives. Shaping techniques are disclosed and claimed in the commonly-assigned, copending U.S. patent application entitled "Technique for Fabricating Offset, Shaped Antenna Reflectors", Serial No. 06/676,924, filed, November 30, 1984, which is hereby incorporated by reference. The objectives achievable with the shaping of the subreflector 14 and the main reflector 16 will be discussed further hereinafter. By offset it is meant that the subreflector 14, the supporting structure thereof (not shown in Figure 1), and the feedhorn 12 are positioned outside the aperture of the main reflector 16, so that none of the collimated energy from the main reflector 16 is scattered by the feedhorn 12, the subreflector 14, or the supporting structure for either. The side sectional beam outline from the 27 - 8 - feed-horn 12 is defined by a ray 18 extending from an emission point 20 to an upper edge boundary 22 of the subreflector 14. The ray 18 reflects off an upper edge boundary 24 of the main reflector 16. The aperture center beam is represented by a ray 24 emitted from the emission point 20 to a center point 26 of the subreflector 14, and reflected off a center point 28 of the main reflector 16. The other extremity of the side-sectional beam outline from the feedhorn 12 is defined by a ray 30 emitted from the emission point 20 to a lower edge boundary 32 of the subreflector 14, and reflected off a lower edge boundary 34 of the main reflector 16. Reference numeral 35 shows generally the quiet zone where the rays 18, 24, and 30 form a plane wave. According to the present invention, the principle characteristic of the dual-shaped offset reflector compact range 10 is the shaping of the subreflector 14 and the main reflector 16 to transform the feed pattern from the feedhorn 12 to the desired illumination from the aperture of the main reflector 16. The control of the aperture illumination is achieved by shaping of the subreflector 14 and the main reflector 16. Although the embodiment of Figure 1 illustrates an offset (or clear aperture) geometry it is not necessary that this geometry be utilized to attain the advantages associated with the present invention. As is well known, the offset geometry of Figure 1 eliminates blockage of the main aperture by the subreflector 14, the feedhorn 12, and the supporting structures thereof. The advantages associated with the dual-shaped offset reflector compact range of Figure 1 are as follows. Since the subreflector 14 controls - 9 the main aperture illumination, the feedhorn 12 may be of a high-gain type. With a high-gain feedhorn, most of the energy radiated therefrom falls on the subreflector 14, and very little radiates in undesired directions. In the offset system of Figure 1 both the feedhorn 12 and the subreflectors 14 are positioned outside the main aperture, so that none of the collimated energy from the main reflector 16 is scattered by the feedhorn 12 or the subreflector 14. The subreflector 14 is shaped to control the illumination on the main reflector 16 to compensate for the effects of space attenuation. The longer focal length of the dual reflector geometry of the preferred embodiment of Figure 1 lowers the cross-polarization below that found in prior art compact ranges. As discussed above, the prior art arrangement wherein a large reflector is illuminated by a low-gain horn placed at the focal point thereof results in high-energy levels on the edges of the reflector, in turn producing a high order of edge diffraction. In the preferred embodiment of Figure 1, the subreflector 14 is shaped to greatly decrease the energy incident on the edges of the main reflector 16, thus reducing the diffracted energy significantly. For antenna testing at extremely-high frequencies, accuracy of the reflector surfaces is extremely important. A large reflector required by the prior art compact range makes the achievement of necessary surface tolerances very difficult. Because shaping closely controls the amplitude distribution, a significantly smaller reflector can be used with the attendant ability to fabricate such a smaller reflector to very close tolerances. Figure 3A and 3B illustrate the amplitude 86/00927 - 10 - and phase distribution of a compact range constructed according to the present invention. According to Figures 3A and 3B, there is a substantially flat amplitude and phase distribution in the quiet zone where the antenna or unit under test is to be placed, with a degradation in the characteristics outside the quiet zone. Stated somewhat differently. Figure 3A illustrates that there is very little stray radiation outside the quiet zone of the preferred embodiment of the present invention. Figure 2 shows a second embodiment of the present invention. Components of Figure 2 are identical in structure and function to the components bearing identical reference characters in Figure 1. A feed arrangement is shown in more detail in Figure 2, including an X-Y table 36, a signal generator 38, a polarization selector 40, and the feedhorn 12. In the embodiment of Figure 2, the subreflector 14 is a threefoot shaped subreflector, and the main reflector 16 is a twelve foot shaped main reflector. As actually constructed, the main reflector 16 is a best-fit surface of revolution approximating a shaped surface, fabricated in one piece to a tolerance of 0.002 inches RMS for close errors and 0.006 for wide-spaced errors. The quiet zone, designated by reference character 44 is surrounded by absorbing material 42 to reduce reflections of the collimated energy from the main reflector 16. Absorbing material 42 is also placed as shown to control diffraction from the edges of the subreflector 14 and the main reflector 16. In this embodiment, the quiet zone 44 is large enough to accept an eighty inch antenna or other unit under test. The quiet zone 44 lies in a shadow of the subreflector 14 so that little interference from the feedhorn 12 is - 11 - encountered. The shaped amplitude distribution and the highly accurate surface of the main reflector 16 provide a dynamic range improvement of at least twenty dB over the prior art compact ranges, i.e., a dynamic range of approximately ninety dB. While several embodiments in accordance with the present invention have been shown and described, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.

Claims: 0927 - 12 - What is Claimed is: 1. A compact antenna range for providing a planar wave front of substantially uniform illumination, said compact antenna range comprising: means for emitting a wave front; a first shaped surface; a second shaped surface; wherein said wave front emitted by said means for emitting a wave front is incident upon said first shaped surface and reflected therefrom onto said second shaped surface; and wherein said first shaped surface is shaped to provide substantially uniform illumination onto said second shaped surface and substantially zero energy on the edges of said second shaped surface; and wherein said second shaped surface has a shape such that a wave front reflected therefrom has a substantially constant amplitude and phase in the near field. 2. The compact range of Claim 1 wherein the means for emitting a wave front includes a high-gain feed having a narrow radiation pattern. 3. The compact antenna range of Claim 2 wherein the wave front emitted by the high-gain feed falls substantially on the first shaped surface. 4. The compact antenna range of Claim 1 wherein the first and the second shaped surfaces are positioned such that the wave front reflected from the first shaped surface and the wave front reflected from the second shaped surface are substantially unobstructed. 5. The compact antenna range of Claim 4 including absorbing material placed to the rear of the first shaped surface and to the rear of the second shaped surface for absorbing stray radiation.

Inventors: ARMSTRONG CRAIG L, CONN JAMES K, GANS LAWRENCE S, ARMSTRONG, CRAIG, L., CONN, JAMES, K., GANS, LAWRENCE, S.
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1986
Main classification: H01Q19
IPC codes: H01Q17, H01Q19