Abstract:
An electrically small, dual polarized, hemispherical coverage, multi-element adaptive array antenna exhibiting low VSWR and operational over an octave of bandwidth is flush-mounted with the airframe structure of high performance aircraft. The antenna is configured as a cavity type structure in the shape of a regular polygon and has a plurality of radiation elements mounted on an insulative support board, each of the elements being is connected to a respective output port for use in a multi-channel adaptive array system. The array provides broadband capability of nulling interfering sources having arbitrary polarization and spatial direction.

Description:
The present invention was developed under a contract with the U.S. Air Force, AFWAL Contract No. F33615-81-C-1552. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to dual polarized, flush-mounted antennas, and is particularly directed to a multi-element adaptive array antenna for use in high performance aircraft that is compact, broadband, exhibits low VSWR and the radiation profile of which provides hemispherical coverage. 
     BACKGROUND OF THE INVENTION 
     Antenna design is generally based on intended performance and space constraints associated with its deployment, which typically include operational characteristics such as required bandwidth, radiation profile coverage, polarization and gain, as well as physical limitations such as size and weight. In an adaptive array, the elements must be closely spaced (usually about one-half wavelength) to avoid spurious (grating) nulls which may be inadvertently generated in the direction of the desired signal when nulling an interfering signal. Also, the array elements must be electrically small, in order that the radiation profile does not change appreciably with frequency. In addition the elements must be identically shaped and exhibit low VSWR, so that their performance tracks one another with frequency, thereby insuring broadband nulling of interference sources. These characteristics, when combined with octave band performance, are not compatible with conventional antenna configurations. 
     SUMMARY OF THE INVENTION 
     Pursuant to the present invention,, there is provided an electrically small, dual polarized, hemispherical coverage, flush mounted, multi-element adaptive array antenna exhibiting low VSWR which is operational over an octave of bandwidth. The antenna of the present invention is especially suited to be incorporated with the airframe structure of high performance aircraft. For this purpose the antenna is configured as a cavity type structure in the shape of a regular polygon and having a plurality of radiation elements mounted on a dielectric support board, each of the elements being connected to a respective output port for use in a multi-channel adaptive array system. Such an array requires closely spaced, low VSWR, electrically similar elements which track each other with frequency so as to provide broadband capability of nulling interfering sources having arbitrary polarization and spatial direction. The above-mentioned, normally incompatible antenna requirements of small electrical size, broad (octave) bandwidth and low VSWR are achieved in the antenna of the present invention with a somewhat lower antenna efficiency by coupling to adjacent array elements. This loss of efficiency can be tolerated in an adaptive array since the interfering signal is reduced in the same proportion as the desired signal, thereby resulting in the same signal-to-interference ratio at each array element. (Downstream of the array this ratio will be significantly improved by the adaptive array signal processing electronics.) 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top or plan view of the adaptive array antenna structure according to the present invention; 
     FIG. 2 is a sectional view of the antenna structure of the present invention taken along line I-I&#39; of FIG. 1; and 
     FIG. 3 is a Smith Chart plot of the performance of the antenna configuration of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, an embodiment of the antenna array of the present invention is illustrated in FIGS. 1 and 2, FIG. 1 being a top or plan view of the multi-element array, while FIG. 2 is a side view taken along section line I-I&#39; of FIG. 1. 
     As shown in FIG. 1, the geometry of the antenna configuration is that of a square metallic cavity comprised of four contiguous side walls 11, 12, 13 and 14, and a bottom wall 16 integral with the side walls. The tops of the side walls are integral with a flange 51 which is adapted to be affixed to a ground plane, such as the conductive skin 52 of an aircraft, so that there is effectively provided a flush-mounting of the antenna with the surface of the airframe. The top surface of the antennas and the radiating elements need not be flat, but may be curved to match the contours of the aircraft. In those applications where a flush mounting is not required, the cavity is unnecessary and the radiating elements of the antenna can be fed directly against the ground plane. 
     As shown in FIG. 1, for a square cavity geometry, the array may consists of four elements 21, 22, 23 and 24, each of which is substantially hexagonally shaped and is formed of a thin layer of copper foil atop a printed circuit board 18. The sides of the board 18 may be affixed to the flange 51 at the top end of the walls 11-14 for support. Two adjacent corner edges of each element which are closest to the corners of the cavity, for example edges 21c and 21d for element 21, are spaced apart from the ground plane side walls (for example, walls 14 and 11 as shown in FIG. 1) by a prescribed separation distance ΔC and ΔD. Thus, edges 22c and 23c of elements 22 and 23 are spaced by distance ΔC from side wall 12, whereas edges 23d and 24d of elements 23 and 24 are separated by a distance ΔD from side wall 13. Similarly, edge 22d and edge 24c, respectively, of element 22 and 24 are separated by distances ΔD and ΔC from side walls 11 and 14 of the cavity 17, proper. 
     For purposes of describing an exemplary embodiment, the antenna configuration shown in FIG. 1 may operate over a bandwidth of 1,200-2,000 MHz. For this purpose, the cavity may be 4&#34; by 4&#34; square with a depth of 13/4&#34;. The copper foil of which the elements are comprised may be very thin and electro formed on printed circuit board 18, as noted previously. The thickness of board 18 may be on the order of 0.032 inches. 
     On the opposite side of the printed circuit board adjacent the corners of the cavity and aligned with the respective copper foil antenna elements 21-24 are respective capacitive coupling elements 25-28. Each of these elements has a respective tab 35, 36, 37 and 38 with a respective feed through hole 45, 46, 47 and 48 for receiving the center conductor of a signal feed cable, such as a standard 50 ohm coaxial feed. As shown in FIG. 2, this may comprise a coaxial feed cable 31, the outer ground sleeve 32 of which is connected to the ground plane side wall 11, while the center conductor 33 is electrically connected at feed point 45 for coupling element 25. Similarly, coaxial feed cable 41 has its outer ground sleeve 42 connected to the cavity ground plane and its center conductor 43 joined to feed element 28 at feed through hole 48. In addition to being separated from the side walls of the cavity by respective distances ΔC and ΔD, at their corner edges, each of the elements is separated from an adjacent element by a prescribed linear separation along the opposite corner edges of the elements at the center portion of the pattern. Namely, adjacent intersecting corner edges 21a and 21b of antenna element 21 are separated from edge 24a of element 24 and edge 22b of element 22 by respective separation distances ΔA and ΔB, respectively. The same holds true for the other elements, as shown in FIG. 1. 
     These respective separations ΔA, ΔB, ΔC and ΔD as well as the size and shape of the elements relative to each other are adjusted to provide a desired input impedance (VSWR) for each element when all of the other elements are terminated in an impedance equivalent to that of the electronics to which the adaptive array will be eventually coupled (usually a 50 ohm resistive termination). 
     The size, shape and location of the elements is selected on an emperical basis with capacitive and inductive coupling between the elements iteratively adjusted, as necessary. Obviously, the separation will not be the same for every antenna structure, as the size, shape, location as well as the bandwidth and desired VSWR, will be tailored to the needs of the user. For purposes of the present exemplary embodiment and providing a general set of guidelines for implementing the present invention for changes in parametric values, the following procedure may be carried out. 
     The length of each element, namely the separation between effectively opposite corners taken along a lengthwise direction, such as along line 21R shown for element 21 in FIG. 1, and the capacitive coupling C A  (proximity) of the edges of the element to adjacent elements (namely separations ΔA and ΔB) are adjusted to provide the desired input resistance level at the low end of the frequency band. Next, the shunt capacitance of the element to the grounded side of the feed circuit (namely the capacitance C D  across the separation between the corner edges of the element and the side cavity walls (separations ΔC and ΔD) is adjusted to provide the same input resistance level at the high frequency end of the band of interest. As noted above, this shunt capacitance is primarily controlled by the spacing (ΔC and ΔD) between the element and the cavity wall, and to a lesser extent by the width of the element. For the 4&#34;×4&#34; layout shown in FIG. 1, operational over a frequency band of 1,2000-2,000 MHz, the separations ΔC and ΔD between the edges of the antenna elements 21-24 and the respective side walls of the cavity may be on the order of 0.062 inches. Also the separations ΔA and ΔB between the edges of adjacent elements may be on the order of 0.312 inches. As shown in FIG. 1, the width of element 22 is the separation along line 22w between side parallel edges 22e and 22f. The same width measurement applies to the remaining elements 21, 23 and 24. 
     For the exemplary embodiment of the present invention described herein the width of each element (e.g. width 22w of element 22) is on the order of 1.20 inches. Also the area of each capacitor coupling element relative to that of the radiation element may be on the order of 0.25 square inches. Thus, capacitive coupling element 25 has dimensions of 0.50 inches, along the lengths of edges 21c and 21d of radiation element 21, which may be on the order of 0.85 inches, and equal to those of edge 21a and 21b at the opposite end of element 21. The same size measurement relationships apply to elements 22, 23 and 24. 
     After establishing the capacitive coupling at the end of the element, the input element capacitance and the shunt capacitance, the inductive reactance of an element over the entire band is cancelled by a series capacitance at the feed point. As shown in FIG. 2, this series capacitance is formed by the size and shape of the feed elements 25 and 28 and their separation through the printed circuit board to their adjacent antenna elements 21 and 24. It should be observed that in some applications this series capacitance may not be necessary and the antenna element may be directly connected to the feed wire center conductor at the feed point. 
     Once the above adjustment of impedance matching parameters has been completed to establish desired capacitance coupling and inductive reactance, a minor readjustment of each parameter may be carried for an optimum impedance match for the particular cavity size and frequency band desired. A near optimum adjustment exists when there is one loop in the input impedance curve (as plotted on a Smith Chart such as shown in FIG. 3 and referenced to the element feed point) and this loop is centered about the center of the chart with the low and high ends of the frequency band being nearly coincident. 
     For the exemplary embodiment described here, a VSWR of less than 1.5:1 is obtainable over nearly an octave of frequency band, using a four element array shown in FIGS. 1 and 2, in a cavity that is approximately 0.4 wavelengths square at the low frequency. 
     It should be observed that the coupling to adjacent elements represents a loss in antenna efficiency since energy is dissipated in the resistive termination on these adjacent elements. This coupling has been measured and the lost energy was found to be less than 3 dB over the full frequency range. Of course, the amount of energy lost due to this coupling (antenna efficiency) is a function of the number of adaptive elements in the array, cavity size in terms of wavelength, and the antenna bandwidth. For changes in the configuration of the antenna array, these various factors must be taken into account. 
     While the embodiment of the invention shown in FIGS. 1 and 2 is a square metallic cavity having four array elements, other configurations may be employed. For example, the cavity may be a circular cavity having a plurality of identical, equally spaced elements distributed therein. Similarly, the cavity may be in the shape of a regular polygon other than a square with the number of sides corresponding to the number of elements of the array. In each instance, the elements may be fed from the corners of the cavity (it being noted that there are no corners in the circular configuration) or the side walls. The radiating elements need not be etched on a printed circuit board but may be made of any electrical conductor. Also, they need not be flat, but may be curved to fit any desired contour. 
     When deployed in a high performance aircraft, the antenna of the present invention is generally used for receive applications. When used as an adaptive transmitting antenna, isolators may be used at each antenna port to provide the required input impedance for each adaptive transmitter source. 
     As described above, for the embodiment of the invention shown in FIGS. 1 and 2, the Smith Chart shown in FIG. 3 illustrates a VSWR of less than 1.5:1 over nearly an octave of frequency band (i.e. 1,200-2,000 MHz). Using substantially the same design shown in FIGS. 1 and 2, a VSWR of approximately 2.0:1 was obtained over a full octave (950-1,900 MHz) frequency band for a four inch square cavity with slightly less antenna efficiency (3.4dB maximum loss). It will be readily appreciated, therefore, that the present invention offers a substantial performance improvement over conventional antenna matching techniques and structures heretofore proposed. 
     While I have shown and described an embodiment in accordance with the present invention, 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 I 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.