Abstract:
A cavity backed slot antenna comprises a conductive cavity, a conductive film carried by a thin dielectric substrate which is above the cavity. The conductive film includes one or more slots which an electric field is applied to radiate an electromagnetic energy.

Description:
BRIEF DESCRIPTION 
     This invention relates generally to a cavity-backed slot antenna and more particularly to a slot antenna having low back-scatter. 
     BACKGROUND OF THE INVENTION 
     In the design of aircraft and other vehicles with low radar cross section, the back-scatter from antennas is an important issue. Often the problem is to design antennas that function efficiently over a relatively narrow bandwidth but suppress the back-scatter at frequencies outside this band. At first glance the microstrip patch antenna appears to be an ideal candidate for solving this kind of problem. It is typically thin, making it easy to suppress structural scattering. More importantly, it has a narrow operating bandwidth with an impedance that tends toward a short circuit outside of this band. The problem is that the patch, like other transmission line components, does not resonate at a single frequency. A second resonance typically occurs somewhere between the second and third harmonic, and other resonances follow. At these higher frequencies, antenna back-scatter tends to be large and generally unacceptable. 
     One solution to this problem is to recess a patch or cavity-backed slot antenna slightly below the surrounding surface. The resulting cavity is filled with a layer of closed cell foam, or some other material with a very low dielectric constant, and then a layer of magnetic radar absorbing material (RAM) is placed on top of the foam. The RAM is brought flush with the surrounding surface, and its edges are usually tapered to provide a gradual transition to the surrounding metallic surface. In the operating band of the antenna the RAM is designed to be somewhat transparent with resulting losses usually not exceeding two or three dB. At higher frequencies the RAM is designed to be much more absorptive so that the antenna, and its back-scatter at higher order resonances, are hidden by the RAM cover material. The use of RAM for back-scatter suppression makes the design relatively large, complex and costly. It is very difficult to obtain a sufficiently sharp frequency cut-off in the RPM to avoid compromising either the radiation efficiency or the back-scatter suppression. 
     Another approach to the problem is to actually suppress the higher order resonances within the structure of the antenna. a recessed circular patch antenna which suppresses the higher order modes is shown in FIGS. 1 and 2. The antenna includes a high dielectric alumina substrate 11 having a conductive film or layer 12, such as copper, on one surface. The conductive film is etched to form a slot 13. The dielectric substrate 11 is placed in a cavity 14 formed in the support structure 16. The ground plane formed by the recessed supporting structure is electrically connected to the film 17 surrounding the circular patch 18. a coaxial connector 19 is attached to the ground plane with the center conductor 21 extending to the patch 18 and connected to the patch. The position of the connection determines the impedance presented by the antenna. The electric fields across the gap 13 radiate in an omnidirectional pattern into the half space above the ground plane. 
     The resulting resonance of the patch is determined not simply by the dimensions of the patch but also by the capacitive loading along the edges of the patch. The capacitance of the narrow slot tends to act as a lumped capacitance so that its susceptance monotonically approaches infinity as frequency increases. While this susceptance works well in combination with the susceptance of the patch to form the primary resonance, the larger values of susceptance at higher frequencies tend to short out the higher order resonances. The suppression of higher order resonances by capacitively loading slot edges is smaller, less complex, less costly and more effective than using RAM. However, this approach has required the use of a material with a high dielectric constant to achieve the required value of capacitance. Ceramics such as alumina are suitable for this purpose and are good dielectrics. Typical gap widths on alumina are 0.005 to 0.010 inch, which are quite reasonable. Nevertheless, ceramics are difficult to work with in development, and their dielectric constant varies significantly from lot to lot. Soft substrates with ceramic loading can also be used for this application, but the control of the dielectric constant is even more of a problem. Both materials tend to be relatively costly. What is needed is a way to suppress the higher frequency resonances without using special materials. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a patch antenna which suppresses higher frequency resonances using inexpensive materials. 
     It is another object of the present invention to provide a patch antenna having increased capacitance per unit length at least along the radiating portion of the slot. 
     It is another object of the present invention to provide a patch antenna having a meander slot to provide increased capacitance. 
     The foregoing and other objects of the invention are achieved by a patch antenna in which the capacitance per unit length of the radiating portion of the patch is increased by increasing the area of the capacitance per unit length. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a slot antenna in accordance with the prior art. 
     FIG. 2 is a sectional view of the antenna of FIG. 1 taken along the line 2--2 of FIG. 1. 
     FIG. 3 is a plan view of a meander slot antenna in accordance with the preferred embodiment of the present invention. 
     FIG. 4 is a sectional view of the antenna of FIG. 3 taken along the line 4--4 of FIG. 3. 
     FIG. 5 is an enlarged view taken along the line 5--5 of FIG. 4. 
     FIGS. 6A-D show typical radar cross section data for an antenna constructed in accordance with FIGS. 3-5. 
     FIG. 7 is a plan view of a rhombic patch antenna with a meander slot. 
     FIG. 8 is a plan view of a single meander slot cavity-backed antenna. 
     FIG. 9 is a plan view of another cavity-backed slot antenna having increased capacitance area per unit length of the slot. 
     FIG. 10 is an enlarged view of section 10--10 of FIG. 9. 
     FIG. 11 is a plan view of still another cavity-backed slot antenna having increased capacitance area per unit length of the slot. 
     FIG. 12 is an enlarged view of the section 12--12 of FIG. 11. 
     FIG. 13 is a plan view of a surface mount slot antenna in accordance with the invention. 
     FIG. 14 is a sectional view taken along the line 14--14 of FIG. 13. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 3-5 a slot antenna including increased capacitance per unit length of the radiating portion of the patch is shown. The antenna is formed over a cavity 23 formed in a conductive structure 24 which serves as the ground plane. The patch antenna 26 is defined by etching a meander slot 27 in the conductive film 28, such as copper, carried by a thin dielectric substrate 29. The capacitance is increased per unit length of the slot in a direction perpendicular to the E fields. The outer or surrounding film 31 is connected to the ground plane or structure whereby when voltages are applied to the film via the coaxial connectors 32 and 33, electric fields are set up across the slot and radiate electromagnetic energy omnidirectionally. The cavity is preferably filled with a foam material 34. In one example the slot was 0.0075 inches wide, with a meander length of 0.12 inches, and a meander repetition rate of 28.65 per radian, formed in a copper film 0.001 inches thick, carried by a dielectric substrate 0.010 inches thick. The copper and dielectric laminate substrate can be purchased from Rogers Corporation. It is of course apparent that any laminated substrate having a conductive upper surface can be used to form the patch antenna. It is noted that by adding length in the direction parallel to the E fields of the slot makes a capacitance that is no longer a perfect lumped element. However, it has been found that a meander slot with a radial dimension of approximately 0.2 inches has good back-scatter suppression at frequencies as high 18 GHz. The use of the backup foam in the main body of the cavity reduces the antenna&#39;s susceptibility to variations in dielectric constant. An antenna was constructed and placed over a cavity 1.750 inches in diameter with the a radial slot variation of approximately 0.210 inches. The radar cross-sectional data for the antenna shown in FIGS. 3-5 is shown in FIGS. 6A-6D over the frequency range from 2-18 GHz. The solid line curve is for an elevation of 10° while the dotted line curve is for an elevation of 20°. Thus, it is seen that the antenna has a very low radar cross section throughout the frequency range. 
     While the antenna described was implemented in a circular patch structure, it is clear that the present invention is applicable to any narrow band, cavity backed antenna. For example, the rhombic patch 36 of FIG. 7 might be used when the maximum allowable radar cross section is lower in some azimuthal direction than others. FIG. 8 shows a single linear slot 37 cavity backed antenna. The slot of FIG. 8 could be combined with a second (not necessarily orthogonal) slot to form a cross-slot. Although not shown an antenna can be constructed with a circular patch structure of the type described concentric within a second larger circular patch structure, thereby creating a dual band antenna. 
     A second means of obtaining the increased capacitance per unit length of the patch is shown in FIGS. 9 and 10. A substrate 41 having a conductive film or layer 42 on each surface is etched on the upper surface to form a linear slot 43. The lower surface is etched to leave a ring 44 opposite the slot 43. This creates a parallel plate structure with an increased value of capacitance per unit length of the patch by forming capacitance on both sides of the slot. The effective capacitance may be further increased in another embodiment when the ring 44 is physically connected to one side of the slot by plated through-holes 46 as shown in FIGS. 11 and 12. 
     Although the antenna has been described with respect to cavities formed in a conductive support structure or ground plane, the antenna may be constructed so as to be surface mounted. Referring particularly to FIGS. 13 and 14 a circular patch antenna 47 including a meander slot 48 is shown formed on a cup-shaped conductive cavity 49 which can be mounted on the surface of an airplane or the like. 
     Thus there has been provided a low radiation cross section antenna which is simple and inexpensive in construction.