Abstract:
A conformal antenna having a microstrip patch centered below a slot in a  undplane and covered by a dielectric window and coupled to a stripline feed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to antennae and more particularly, to stripline and microstrip antennae. 
     Two types of antennae are presently in use in conformal arrays: the stripline slot and the microstrip patch. The stripline slot antenna is inherently unstable in those applications where exposure to environmental stresses such as diurnal variations in the ambient temperature cause changes in the dimensions of the slot or cavity. The microstrip patch antenna has unshielded feed lines that tend to radiate and couple with other feed lines and radiators mounted on the same circuit board, unpredictably influencing radiation patterns and impedance characteristics. 
     The noun &#34;stripline,&#34; as used here, is a contraction of the phrase &#34;strip type transmission line&#34;, a transmission line formed by a conductor above or between extended conducting surfaces. A shielded strip-type transmission line denotes generally, a strip conductor between two ground planes. The noun &#34;groundplane&#34; denotes a conducting or reflecting plane functioning to image a radiating structure. 
     SUMMARY OF THE INVENTION 
     A hybrid stripline-microstrip microwave antenna with a radio-frequency source coupled to an external connector. A stripline coupled to the connector lies sandwiched between a pair of parallel dielectric layers clad with exposed groundplanes and feeds a microstrip patch radiator. A dielectric window fills a cavity in one groundplane adjoining the patch, and covers the patch. 
     It is an object of the present invention to provide an antenna that is free from variations in performance due to environmental stresses. 
     It is a second object to provide an antenna free of unpredictable variations in its radiation pattern. 
     It is another object to provide an antenna free of variations in its impedance characteristics. 
     It is yet another object to provide an antenna suitable for use in a conformal array. 
     It is still another object to provide a lightweight, easily made, radio frequency antenna element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily enjoyed as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like numbers indicate the same or similar components, wherein: 
     FIG. 1 is a top view of one embodiment made according to the present invention. 
     FIG. 2A is an exploded front view of the embodiment shown in FIG. 1. 
     FIG. 2B is an exploded front view of the embodiment shown in FIG. 1, taken along line 2B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and in particular to FIGS. 1, 2A and 2B, where there is shown respectively, a top, an exploded front view, and an exploded sectional view, of a stripline patch antenna 10. A nearly square microstrip patch radiator 17 is supported on a dielectric layer 16 surrounded by a square cavity in an adjacent dielectric layer 26; one surface of patch radiator 17 is coplanar with one surface of the layer 16. The dielectric layers 16, 26 and the patch radiator 17 are sandwiched between two parallel, electrically conducting groundplanes 14, 15. A nearly square cavity 19 also extends through the groundplane 14 adjoining the coplanar surface of layer 26 and is centered around patch radiator 17 to form an aperture. Cavity 19 is larger in area than patch radiator 17. A window 18 of a dielectric material completely fills cavity 19. A coaxial external connector 11 of conventional design is mounted along one of the sides of antenna 10 formed by the edge of groundplanes 14, 15 and dielectric layer 16. A stripline 12, preferably having a length equal to one quarter of the carrier frequency wavelength, is also embedded in the coplanar surface of dielectric layer 16, and electrically couples the external connector 11 with the patch radiator 17. Mode suppression screws 13 extend through dielectric layers 16, 26 and between opposite ground planes 14, 15 to form opposite rows along, and parallel with, the length of stripline 12. These screws 13 maintain an equipotential between groundplanes 14, 15 and prevent spurious modes from being induced into dielectric layers 16, 26. When connector 11 is coupled to a radio frequency generator, energy travels along stripline 12 to patch 17, and is radiated through dielectric window 18 into the surrounding environment, typically the atmosphere. 
     It is apparent from the details of this description that the disclosed structure provides an improved antenna. Since a dielectric window completely fills the cavity, there is no tuned slot, and changes in ambient temperature will not cause a change in the effective area of the patch radiator. Also, the stripline 12 (i.e., the &#34;feedline&#34;) over which energy travels between patch radiator 17 and coaxial external connector 11 is shielded by ground planes 14, 15 and mode suppression eyelettes 13, thereby preventing the feedline from causing unpredictable variations in radiation patterns and impedance characteristics. 
     Although the stripline patch antenna is described as an antenna for radiating electromagnetic energy, it can also be used to receive electromagnetic energy. In either utility, several of the stripline patch antennae can be arranged and, with the ancillary switching and phase shifting circuitry, operated as a cylindrical array. The embodiment described may be made with two or more stripline feed elements 12, with each different in pathlength by one quarter of the wavelength of the carrier signal. Alternately, stripline 12 may serve as a quarter wavelength transformer between two phase shifting sections. 
     Several characteristics of the stripline patch antenna require consideration and the exercise of judgment by one endeavoring to practice the teachings set forth in the preceeding paragraphs of this description. For example, the dimensions of patch radiator 17 are determined by the value of the carrier frequency selected. In one embodiment, the length of patch radiator was empirically set at 0.49 of one wavelength of the carrier signal in the dielectric window, while the width (i.e., the dimension normal to the width), was empicirally set at less than 0.49 of the same wavelength. Patch radiator 17 may also have a circular perimeter, in which instance dielectric window 18 and cavity 19 will be annular. The dimensions of cavity 19 exceed those of patch 17 by one eighth of the dielectric wavelength of the carrier signal. The dielectric wavelength of the carrier signal is the quotient of the dielectric constant, ε w , for the window material, into the free space wavelength, λ c , of the carrier signal. Typically window 18 is made of a dielectric material having a low coefficient of thermal expansion, such as teflon or fiberglass. Although more susceptible to thermal deformation, polyethylene may also be used. The width of stripline 12 is inversely proportional to its resistance, and is determined by the antenna impedance required. 
     The stripline patch antenna may be made either by using discrete groundplanes 14, 15 or by using as groundplanes the copper clad exposed sides of two dielectric circuit boards of the well known teflon-fiberglass or perhaps, Mylar, bonded together to produce a sealed module in the manner taught by U.S. Pat. No. 4,021,813. The thickness, d, between the outside surfaces of the groundplanes of the assembled antenna 10 is set at less than one eighth of the dielectric wavelength of the carrier signal in order to avoid monopole radiation. Stripline 12 and patch 17, in comparison to dielectric layers 16, 26, have negligible thickness. In the latter structure, aligned holes through the two circuit boards plated with solder could be used in lieu of the mode suppression screws 13 to prevent spurious modes from being induced in the circuit boards between the groundplanes.