Abstract:
An octave bandwidth conformal cavity-backed slot antenna includes a ground plane with a number of different length slits that come together at the central feedpoint. The slit length varies from one-half a wavelength at the highest frequency at which the antenna is to operate for the short side to one wavelength at the highest frequency for the long side, with the proximal ends of the slits having a common feedpoint. Such slot antennas may be arrayed in a quad configuration. Because the trapezoidal envelope of the antenna induces the phase-center to shift with frequency, when two are arrayed with short sides adjacent, the spacing between them results in a phase center from one antenna to the next that is effectively within half a wavelength at all frequencies.

Description:
RELATED APPLICATIONS 
     This Application claims rights under 35 USC §119 (e) from U.S. Provisional Application Ser. No. 61/527,760 filed Aug. 26, 2011, the contents of which are incorporate herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention claimed in this patent application was made with U.S. Government support under contract no. F33657-91-C-0006 awarded by the Aeronautical Systems Center. The U.S. Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to conformal antennas, slot antennas, antenna arrays, and more specifically to a quad-array of multi-arm slot antennas. 
     BACKGROUND OF THE INVENTION 
     Aircraft and other vehicles are commonly provided with cavity-backed slot antennas which in general involve a slot through a ground plane. These cavity-backed slot antennas are by their very nature narrow banded and it is only with difficulty that one can increase the bandwidth of the slot antenna so that it may be used over a wide frequency range for detection of multiple transmitters. Wide bandwidth slots support, for instance, direction finding involving angle of arrivals (AOA) determinations, and radar-warning systems. Unlike horn and spiral antennas, slots can be spaced within a half wavelength to allow unambiguous phase determination, beamforming and sidelobe control. 
     Moreover, it is desirable to provide an S band or an L band conformal slot antenna for high power communications. In general slots may be scaled dimensionally to support systems of various mission needs. Combining ultra-wide bandwidth with the scalability and phase control of slot arrays allows them to be used with the most demanding radar and communications signal receiving electronic-warfare receivers and transmitters. 
     The slot is cut or etched into a metallic ground plane, which may be shaped to conform to the smoothly curving surface of an aircraft or other platform, thus being described as conformal. 
     In the past one way to obtain greater bandwidth was to attempt to increase the width of the slot. However, the result is a wide open wave guide cavity. The problem with such a wide slot is an intolerable radar cross section that would cause a stealthy platform to be susceptible to illumination by enemy radars. 
     Another application that was attempted was to create an array of four slots in a square arrangement so that the resulting antenna would not only have a broader bandwidth, but also would behave like a monopole extending out of the surface of the ground plane. If the four slots were fed together in phase, while one would achieve a monopole behavior, the bandwidth would nonetheless be limited by the bandwidth associated with the slots. Other feed arrangements for the square array allow diversity of polarization or direction finding. However, these also would be of limited application if the slot bandwidth could not be extended. 
     The challenge was to come up with a way to make a fat slot but with the fat slot mostly covered up so as not to present a large structural radar cross section. Moreover, there needed to be a way to fit the slots in a square array without the wide fat slots overlapping. 
     In short, a topology needed to be developed that would provide a 2:1 or 3:1 bandwidth without significantly increasing the platform&#39;s radar cross section and to do so with conformal antenna apertures usable on the skin of aircraft or other vehicles. 
     In summary, there is a conflict between close spacing and minimum length and width for slot antennas in a quad array. Close half-wavelength spacing or less is required element-to-element at highest frequencies (i.e., short wavelengths), but the length of elements must be half-wavelength at low frequencies (long wavelengths) for efficient performance. Also, each element must be wide enough to achieve bandwidth. Additionally, the radar cross section of the conformal aperture must be minimized. A need therefore exists for wide instantaneous bandwidth (3:1) conformal slot apertures capable of handling high power and of being arrayed in a quad configuration for 360 degree azimuthal coverage. 
     SUMMARY OF INVENTION 
     According to the present invention, a wide bandwidth conformal cavity-backed slot antenna is comprised of multiple arms in the form of slits that connect at a feedpoint to form a multi-armed slot that behaves as a single slot antenna. In one embodiment each multi-arm slot antenna includes two opposed multi-slit back-to-back pitchforked shapes, with each pitchfork having at least three slits of decreasing length, with the proximal ends of the slits connected at a feedpoint. The ends of the slits define a trapezoidal envelope due to their decreasing lengths across the width of the antenna. The slits loosely resemble either a spider or pitchfork tines that extend from a lateral support. 
     It has been found that the width of the pitchfork antenna from short slit to long slit must be no greater than λ/4 at the highest frequency for which the antenna is designed. If this width is over λ/4 then the performance is severely degraded to point of inoperability. Moreover, for a 2:1 bandwidth the overall length of the shortest slits is λ/2 for the highest frequency, whereas the overall length of the longest slits are one wavelength at the highest frequency. Thus the ratio of the lengths of the short side to the long side is 2:1. 
     In summary, the pitchfork embodiment the slits or arms increase in length from one side of the antenna element to the other, with the overall length of the longest slits corresponding to one wavelength at the highest frequency at which the antenna is to operate, and with the overall length of the shortest slits corresponding to one-half wavelength at the highest frequency. The multi-arm slot is designed to have a phase center that moves toward the shortest arms with increasing frequency. Without loading, this multi-arm slot configuration achieves a 2:1 bandwidth. 
     According to an improved embodiment of the multi-arm slot antenna, the distal ends of the slits are loaded in order to prevent higher level modes and the slits are tapered wider toward the distal ends. Further, a balun feed structure is added within the cavity to match or offset reactive impedances inherent to the slot. This combination of enhancements enables extension of the RF bandwidth to 3:1. 
     To establish monopole-like performance, four multi-arm slot antennas are arrayed at 90 degrees relative to each other in a square to form a quad-slot array. This square arrangement, made possible due to the trapezoidal envelope of the antenna elements, may be altered into a rhombus to reduce the tendency of the square array to reflect higher-frequency incoming radar signals back to their source. The trapezoidal envelope of the array element slit-lengths may be canted to better fit into the rhombus array configuration to align with platform edges of an aircraft. 
     In summary, an octave bandwidth conformal cavity-backed slot antenna includes a ground plane with a number of different length slits that come together at the central feedpoint. The slit length varies from one-half a wavelength at the highest frequency at which the antenna is to operate for the short side to one wavelength at the highest frequency for the long side, with the proximal ends of the slits having a common feedpoint. Such slot antennas may be arrayed in a quad configuration. Because the trapezoidal envelope of the antenna induces the phase-center to shift with frequency, when two are arrayed with short sides adjacent, the spacing between them results in a phase center from one antenna to the next that is effectively within half a wavelength at all frequencies. Furthermore, also due to the trapezoidal envelope, four multi-arm slots may be arrayed in a square configuration without exceeding the half-wavelength array spacing requirement for the phase centers. Extended low frequency bandwidth is provided by either magnetically or resistively loading the distal ends of selected slits and the use of an ultra-wideband balun. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the subject invention will be better understood in connection with the Detailed Description, in conjunction with the Drawings, of which: 
         FIG. 1  is a diagrammatic illustration of a prior art cavity packed conformal slot antenna; 
         FIG. 2  is a diagrammatic illustration of a multi-arm slot antenna; 
         FIG. 3  is a more stylized diagram of the multi-arm slot antenna of  FIG. 2 . 
         FIG. 4  is a diagrammatic illustration of a quad slot array utilizing four of the multi-arm slot antennas of  FIG. 2 ; 
         FIG. 5  is a series of graphs showing the elevation patterns of an L-band quad-slot array; 
         FIG. 6  is a series of graphs showing the azimuth patterns of an L-band quad-slot array; 
         FIG. 7  is a graph of the mean L-band and S-band quad-slot swept gain; 
         FIG. 8  is a series of graphs showing Phase vs. AOA for a L-band quad-slot aperture at 1 GHz. 
         FIG. 9  is a graph of the calibrated and uncalibrated AOA error of an L-band quad-slot array. 
         FIG. 10  is a diagrammatic representation of one possible configurations of a multi-arm slot array; 
         FIG. 11  is a diagrammatic representation of another possible configuration of a multi-arm slot array; 
         FIG. 12  is a diagrammatic representation of a spider-like arrangement for the slits of the cavity-backed slot antenna; 
         FIG. 13  is a diagrammatic representation of a quad arrangement for the spider antenna embodiment of  FIG. 12  showing a common feedpoint of the antenna elements; and 
         FIG. 14  is a diagrammatic illustration of a balun for use in feeding the conformal multi-arm slot antenna of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     A multi-arm slot implementation is disclosed that can achieve wide instantaneous bandwidth 3:1 using conformal antenna apertures. These conformed multi-arm antenna are capable of handling high power and of being arrayed in a quad configuration for 360 degree azimuthal coverage. This coverage can support Electronic Warfare, Direction Finding (DF), Communications and other applications. The conformal surface is particularly useful for airborne and other mobile platforms. According to one embodiment, the multi-arm slot may result in high effective radiated power in a transmit configuration and increased electronic surveillance measurement sensitivity, while being compatible with low-complexity arctangent DF implementations. In other embodiments, the slots may be arrayed for increased gain, beam-forming and side-lobe control. 
     According to one embodiment, the array uses multiple narrow slits fed in parallel to broaden bandwidth relative to a single narrowband slot design. The multi-arm geometry is chosen such that the radiating portion of the antenna is close to half-wavelength for efficient operation, maintaining a nominal cosine radiation pattern. Narrow slits minimize structural scattering of incident radar signals. Each multi-arm slot in a quad array may optionally have its own cavity-backing thus allowing greater structural support. 
     Referring now to  FIG. 1 , the prior art a typical cavity backed slot antenna  10  includes a slot  12  through the surface  14  of for instance the skin of an aircraft. It is noted that a cavity shown in dotted outline  16  is placed in back of the slot for containing radiation that is projected into the aircraft or behind the skin of the vehicle and controlling the slot&#39;s electrical parameter of input impedance. It will be appreciated that the slot antenna shown in  FIG. 1  is a narrow band antenna. The bandwidth is proportional to the width of the slot shown by double ended arrow  18 . As mentioned hereinbefore, while the slot may be fattened, the problem for some platforms is intolerable radar cross section which exposes a low-observable aircraft or other vehicle to detection by enemy radar. 
     Referring now to  FIG. 2 , in order for the provision of a slot antenna of for instance a 2:1 bandwidth, the slot antenna of  FIG. 1  is converted into a multi-armed slot antenna in which antenna  20  is composed of a ground plane with connecting slits, with the slits forming multiple arms all fitting within a trapezoidal envelope afforded by the decreasing length of the slits in the illustrated embodiment. 
     As illustrated, the longest arms  22  and  22 ′ have an overall length  43  of one wavelength at the highest frequency. Alternatively, for a 2:1 bandwidth, the overall length of the longest arms can be set to one-half wavelength at the lowest frequency of operation. Intermediate arms  24  and  24 ′ are shorter than arms  22  and  22 ′ corresponding to an intermediate frequency. In order to accommodate the highest frequency, arms  26  and  26 ′ are shortened such that their overall length  41  corresponds to a half the wavelength at the highest frequency. 
     It will be seen that all of the arms are connected together by backbone slits  28  and  28 ′, with a feedpoint  30  being in the form of a slit that runs between slits  28  and  28 ′. It is this feedpoint slit that is driven diagrammatically by a coaxial cable  32  which has its center conductor  34  coupled to one side of slit  30  and with its exterior shield grounded to the ground plane, here illustrated at  36 . 
     What will be appreciated from viewing antenna  20  is that the cavity-backed slot antenna is formed within a ground plane by slits  22 ,  22 ′;  24 ,  24 ′; and  26 ,  26 ′. These slits are interconnected by slits  28 ,  28 ′ and  30 . Moreover, it will be appreciated that the slits that form the arms may be tapered wider towards the ends of the arms to provide for better input impedance versus frequency. 
     Importantly, the width of the antenna from short slits to long slits is shown by arrow  45  which must be kept under λ/4 at the highest frequency. This is a critical dimension over which the antenna ceases to operate effectively, due to higher-order modes. 
     The antenna shown in  FIG. 2  is capable of providing a 2:1 bandwidth for a receiver or transmitter. However, by adding magnetic loading strips  38  to the distal ends of the shorter arms and resistive loading strips  40  at the distal ends of the longer arms, a 3:1 bandwidth is achievable with this multi-arm structure, assisted by the addition of a customized balun, which may feed the slot from within the cavity below it. 
     Note also that the bandwidth is increased markedly over the slot antenna of  FIG. 1  without having to utilize fat slots or open-ended waveguide that increase the structural radar cross section. 
     The antenna of  FIG. 2  is shown diagrammatically in  FIG. 3  oriented in a vertical plane  32  which is typically located on the side of an aircraft on the skin thereof and when arrayed can provide the equivalent of a monopole antenna extending outwardly from plane  32 , with the arms indicated by the associated reference characters. 
     In order to provide for the aforementioned monopole performance, in one embodiment a quad slot array  50  is shown in  FIG. 4  in which four multi-arm slot antennas of the type described in  FIG. 2  are set into ground plane  52 , with feedpoints  30  opposed as illustrated. The result is a quad arrangement of four cavity backed slot antennas that fit into a square. The first antenna shown here at  54  is opposed to antenna  56 , with antenna  58  opposed to antenna  60 . For convenience the coaxial cable  62  feed may come to the center of the array where its outer shield is connected to the ground plane  52 . The center conductor  64  is connected in parallel to the feedpoints  30  in parallel using strip lines  65  which run under the ground plane and are connected to respective feedpoints  30  using vias  67 . 
     It will be seen that antennas  54 - 60  are contained within a square area  52  such that the length across any square dimension is equal to or less than one-half wavelength at the lowest frequency at which the antenna is to operate. 
     Referring again to  FIG. 2 , one embodiment of a multi-arm slot geometry is shown. This geometry utilizes three slit radiators with end-loading applied to extend the basic frequency range lower to achieve a nominal 3:1 bandwidth. Thus the geometry can be selected such that the radiating portion is no more than half-wavelength for good radiation efficiency. The element loading may be incorporated across the longest and shortest arms prevents higher order modes that would be present otherwise in the extended frequency range. The higher order modes can cause poor radiation efficiency within a 3:1 bandwidth and distort the nominal cosine pattern impacting the broadband gain and phase, and thus increasing DF error. However, 2:1 bandwidth may be achieved without loading. In the embodiment shown in  FIG. 2 , the shortest arms are magnetically loaded, while the longest arms are resistively loaded. 
       FIG. 4  shows one embodiment of a quad-slot array configuration, with four of the multi-arm slot antennas of  FIG. 2  oriented at 90 degrees relative to each other. This particular configuration yields a monopole pattern and polarization when all elements are fed in phase. In this particular embodiment, the shorter arms are spaced at a half wavelength at the maximum frequency and the longer arms are spaced a distance less than or equal to a half wavelength at the minimum frequency. The antenna is designed such that the phase center moves toward the shorter arms with increased frequency so that electrical spacing of half-wavelength or less can be maintained over a 3:1 bandwidth. Such a design can be used for low complexity arctangent DF applications. Arraying the geometry in a quad configuration allows for a 3-channel DF compatibility. This configuration also results in effectively using a half-wavelength slot which has high-efficiency while maintaining an acceptable cosine pattern required for arctangent DF. 
     Referring to  FIGS. 5 and 6 , the elevation and azimuth patterns for an L-band multi-arm slot are shown respectively. These monopole radiation patterns are produced by the quad-slot array when all elements are fed in-phase. The weaker patterns of  FIG. 6  are cross-polarized. 
     Referring now to  FIG. 7 , the mean swept gain over 360 degrees azimuth, at θ=80 (10 degrees above the horizon) is shown. This mean swept gain is for an L-band and S-band multi-arm quad-slot aperture, demonstrating the gain response. In one embodiment, utilizing 50 W-capable terminations within the longest arms, one S-band multi-arm element was successfully tested to 50 W Average/100 W Peak. Thus the quad aperture is capable of 200 W Avg/400 W Peak. 
     Referring to  FIG. 8 , the vertical polarization, V-pol, azimuth angle-of-arrival can be found, with a 90 degree ambiguity, using a simple Arctangent algorithm by computing: 
             ϕ   =     arctan   |         (       v   1     +     v   3       )     +     i   ⁡     (       v   2     +     v   4       )             (       v   1     +     v   3       )     +     (       v   2     +     v   4       )         |           
Where v n  stands for the complex antenna voltages received at each feedpoint of the quad array.
 
     If the elements are ideal cosine patterns, this yields AOA vs. φ with a Sin(2φ) function. Practically, there is pattern distortion which can be calibrated using a simple look-up table. The measured element and sum patterns and their phase response vs. AOA for the L-Band quad slot antenna at 1 GHz are shown in  FIG. 6 . 
     Referring to  FIG. 9 , the calibrated and uncalibrated AOA error is shown at 0, 10, and 20 degrees elevation using 10 degree elevation data as the calibration data. This graph demonstrates both a 3:1 DF bandwidth and minimal sensitivity to calibration in elevation for near grazing angle incidence. 
     Many different configurations of the multi-arm conformal slot antenna are possible. Referring to  FIG. 10 , one possible configuration is shown. Here two of the multi-arm conformal slot antennas of Figure N are skewed or canted on their own separate ground planes as illustrated by antennas  70  and  72 . These face a pair of oppositely skewed antennas  74  on their own ground plane. The antennas are fed in parallel as illustrated. 
     Referring to  FIG. 11  another configuration shows slot antennas  76 - 82  skewed with respect to each other and fed in parallel. 
     Referring to  FIG. 12 , rather than using the pitchfork configuration for the arms previously illustrated, a spider pattern  90  for the slits may be used, revealing design flexibility within the trapezoidal envelope. Here a 2:1 bandwidth is achievable with the length  92  of the long side being twice that of the short side  94 . 
     Referring to  FIG. 13 , a quad array  100  of spider cavity-backed slot antennas  102 - 108  is shown. These pairs of spider antennas are shown opposed, with the feedpoints  110  fed from the center conductor  112  of the coaxial feed as shown by strip lines  114 . 
     For the slot embodiment of  FIG. 2 , improved bandwidth is accomplished through applying a slot line balun feed to the multi-arm conformal antenna and optimizing slot geometry. Referring now to  FIG. 14 , an ultra-wideband balun  120  is shown having an extension  122  from a 50 ohm coaxial input transmission line  124 . The center conductor of the coaxial cable  125  extends up through extension  122  to a junction  126  between balanced and unbalanced transmission lines. 
     This junction is located in a dielectric substrate  128  with a stripline on the inside (not shown) and a metallic slab or plating  130  on the outside of the substrate. 
     At junction  126  wire lead  125  connects to the center strip of the stripline at which an open-circuited quarter-wave stripline stub exists that is connected in series with a short circuited transmission line  132  and a tapered balanced transmission line  134  of balun  120 . It is noted that short circuited transmission line  132  comprises two slabs extending from junction  126  down to a metal end wall  136  with dielectric material removed from the gap. Short circuited transmission line  132  presents a high impedance as connected in parallel to tapered balanced transmission line  134 , causing greater signal power to flow on the tapered slotline  134 . It is noted that transmission line  134  leads up to a connection point that gets affixed to the feedpoint of the slot which is on a separate stripline board perpendicular to this balun. The tapered slot balanced transmission line  134  smoothly transfers the characteristic impedance from a nominal 100 ohms to approximately 200 ohms. 
     It is noted that the impedance of the slot feedpoint is nominal and actually varies with frequency. The off-center frequency reactance of the balun is designed to match that of the slot at the extremes of the 3:1 band. As a result balun  120  serves to provide an ultra-wideband impedance matching element for the subject antennas. 
     While the present invention is described in connection with preferred embodiments, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiments for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment.