Abstract:
A dual band antenna array comprises a pair of coplanar antenna elements, the first antenna element excitable at a frequency within a first frequency band, the second antenna element excitable at a frequency within a second frequency band. A single transmission feed line in a second plane has an input for receiving a signal, the feed line dividing at a branch point into a first line segment for communicatively coupling the first antenna element with the input at a first feed point, and a second line segment for communicatively coupling the second antenna element with the input at a second feed point. The first and second line segments have lengths adapted for impedance matching at the first and second frequency bands, respectively, relative to the feed line input, to selectively allow energy transmission in one of the first and second line segments while reflecting energy in the other line segment according to the input signal frequency, whereby the activated antenna elements are passively switched based on the input signal frequency.

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates generally to antenna systems and more specifically to a dual band array antenna system.  
       BACKGROUND  
       [0002]     Antennas are useful in a variety of data communications applications, including, for example, line-of-sight (LOS) communications applications, satellite communications (SATCOM), cellular telephony and digital personal communications systems (PCS). Antenna systems are often implemented as phased arrays of individual antennas or subarrays of antennas that are excited to cumulatively produce a transmitted electromagnetic wave that is highly directional, in order to provide a signal gain in a desired direction or to reject unwanted signals from other directions. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front or cumulative wave front of electromagnetic energy radiating from all of the antenna elements in the array travels in a selected direction. Phase or timing differences among the antenna activating signals determines the direction in which the cumulative beam from all of the individual antenna elements is transmitted, and the characteristics of the radiation pattern of the array. Analysis of the amplitudes and phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.  
         [0003]     In communication systems, radar, direction finding and other broadband multifunction systems having limited aperture space, it is often desirable to efficiently couple a radio frequency transmitter and receiver to an antenna having an array of broadband radiator elements. For many antenna array applications, it is further desirable that the radiating antenna elements have low losses (e.g. low RF loss), operate across a wide frequency band of interest, and be inexpensive to fabricate. Several concepts have been investigated to provide radar or communications coverage over more than one frequency band using a single integrated array antenna.  
         [0004]     With dedicated radar bands such S-Band and C-Band; C-Band and X-Band, for example, many of which are separated by nearly an octave or more of bandwidth, the selection of a radiating element becomes problematic due at least in part to difficulty in providing a suitable impedance match and element radiating pattern over both bands and the scan volume. A single element type may be impedance matched over one frequency band or the other band, but cannot adequately cover both bands. Similarly, a single element type may provide an adequate element radiating pattern over one frequency band, but exhibits nulls (i.e. minima) in the element radiating pattern in the other frequency band that represents a “blind” scan angle for the array.  
         [0005]     Moreover, variations in dipole construction have not yielded a structure that provides both sufficient bandwidth and physically configurable within an element grid of a conventional radar array. While it is known that certain broadband elements such as notch antennas having flared or tapered notch antenna elements may be useful in forming wideband antenna arrays, dual band antenna array systems often require active switching, multiple apertures, and complex impedance matching that undesirably affect performance and usefulness of the device.  
         [0006]     A system and method which overcomes the aforementioned difficulties is highly desired.  
       SUMMARY OF THE INVENTION  
       [0007]     According to an aspect of the present invention, there is provided a plurality of integrated antenna elements configured on a substrate and each associated with a corresponding frequency band, that are passively switched during operation so as to provide a dual band array antenna system.  
         [0008]     An array of antenna elements excitable at a lower frequency band are spaced in the E-Plane to provide grating lobe-free radiation at an upper frequency band. Interspersed between these elements are smaller antenna elements configured for excitation at an upper frequency band. The upper frequency band may be separated from the lower frequency band by approximately an octave. A single feed line feeds a reactive power divider that, in turn, feeds a pair of the antenna elements, one for the lower band and one for the upper band. From the power divider, the transmission feed lines are constrained by an open circuit feeding the lower band element, which is spaced an integral number of half wavelengths from the power divider at the upper band to prevent energy from entering that leg of the feed line at the upper band. The open circuit feeding the upper band element is spaced an integral number of half wavelengths from the power divider at the lower band to prevent energy from entering that leg of the feed line at the lower band. At each of the two elements, sufficient transmission is provided between the element and the power divider to accommodate an impedance matching transformer.  
         [0009]     A dual band antenna array comprises a pair of coplanar antenna elements, the first antenna element excitable at a frequency within a first frequency band, the second antenna element excitable at a frequency within a second frequency band. A single transmission feed line in a second plane has an input for receiving a signal, the feed line dividing at a branch point into a first line segment for communicatively coupling the first antenna element with the input at a first feed point, and a second line segment for communicatively coupling the second antenna element with the input at a second feed point. The first and second line segments have lengths adapted for impedance matching at the first and second frequency bands, respectively, relative to the feed line input, to selectively allow energy transmission in one of the first and second line segments while reflecting energy in the other line segment according to the input signal frequency, whereby the activated antenna elements are passively switched based on the input signal frequency.  
         [0010]     According to another aspect, a dual band antenna system comprises a substrate having a first side and a second side; a conductive layer disposed on the first side and configured to form a linear array of interleaved pairs of first and second antenna elements, the first antenna elements excitable at a first frequency band, the second antenna elements excitable at a second frequency band, each pair having an associated single transmission feed line disposed on the second side of the substrate for carrying an input signal at a frequency causing excitation of one of the element pairs. Each single transmission feed line divides at a branch position into a first line segment and a second line segment, the first line segment electromagnetically coupled to the corresponding first antenna element of the pair at a first feed point, and the second line segment electromagnetically coupled to the corresponding second antenna element of the pair at a second feed point, wherein an end of the first line segment terminates in an open circuit a distance of about one quarter wavelength from the first feed point at the center of the first frequency band, and an end of the second line segment terminates in an open circuit a distance of about one quarter wavelength from the second feed point at the center of the second frequency band, and wherein the terminating ends of the first and second line segments are an integer number of half wavelengths from the branch position at the first and second frequency bands, respectively.  
         [0011]     According to another aspect, a method for operating a dual band antenna array comprises: providing a pair of coplanar antenna elements excitable at separate frequency bands; feeding the pair of coplanar antenna elements via a single transmission feed line formed in a second plane and having an input for receiving a signal, the feed line dividing at a branch point into a first line segment extending transversely over a first one of the pair of antenna elements at a first feed point, and a second line segment extending transversely over a second one of the pair of antenna elements at a second feed point; the lengths of the first and second transmission line segments adapted according to the first and second frequency bands relative to the feed line input for matching the impedance of the antenna frequency bands with the transmission line impedance to allow efficient transfer of energy to and from the antenna elements, and, selectively applying a signal frequency within one of the separate frequency bands to selectively allow energy transmission in one of the first and second line segments while reflecting energy in the other line segment, thereby communicatively coupling the transmission line input with the corresponding antenna element via the corresponding feed point, whereby the activated antenna elements are passively switched based on the applied signal frequency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts, and wherein:  
         [0013]      FIG. 1  is an exemplary illustration of prior art linear array of stripline notch antenna on a coplanar substrate.  
         [0014]      FIG. 2  is a schematic illustration of a Smith Chart Impedance Plot useful in describing the principles of the present invention.  
         [0015]      FIG. 3  is an exemplary illustration of two sets of element pairs in a linear antenna array configured for dual band operation according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding, while eliminating, for the purpose of clarity, many other elements found in radar or communications systems and methods of making and using the same. Those of ordinary skill in the art may recognize that other elements and/or steps may be desirable in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. Moreover, it is to be recognized that the passive radiating antenna elements and their feed structure are reciprocal devices, behaving analogously in a receiving mode as in a transmitting mode. Thus, while the behavior of the elements and their feed structure may be described herein in the receive mode, it should be recognized that these aspects (aside from power-handling characteristics) also apply to the transmit mode.  
         [0017]     Referring now to  FIG. 1 , there is shown a conventional (prior art) notch antenna array  10  comprising a substrate formed from a sheet of dielectric material  12  sandwiched between a conducting material  14  and a pair of feed strip transmission lines  16 ,  18 .  FIG. 1  illustrates a top view showing the antenna face of the dielectric  12 . A plurality of tapered notch antenna elements  20 ,  22  are disposed in conducting element  14 . Each notch  20 ,  22  has a broad end  20   a,    22   a  disposed on the leading edge of the substrate forming ground plane(s)  60 . Each tapered notch element  20 ,  22  is disposed transverse to the respective feed strip  16 , 18  and is electromagnetically coupled thereto. The antenna elements  20 ,  22  are spaced apart a given distance X to enable grating lobe free operation at an upper frequency band.  
         [0018]     Transmission line  16  has a first arm segment  16   a  in a plane parallel to slot  23  and a second arm segment  16   b  normal to first arm segment  16   a  and terminating at an end that is electromagnetically one quarter wavelength from slot  23  and is open circuited at end point B. As is understood from transmission line theory and as illustrated in  FIG. 2 , an open circuit appears as a short circuit when viewed at a distance of one quarter of an operating wavelength. Therefore, the terminating end at one quarter wavelength from slot  23  therefore reflects a short circuit between the radiating element and the transmission line conductor at point B and thus operates to transfer energy to the ground plane and excite antenna element  20  at the lower frequency band. With regard to notched element  22 , the distance d associated with notch antenna  22  of one quarter wavelength that is short circuited presents an open circuit at element feed point C, thereby exciting the element in a balanced mode of operation. Reference numeral  30  depicts an element matching transformer in this segment of feed line  18 . The aforementioned structure can have impedance matching at either the higher or lower frequency band (e.g. S-Band or C-Band); however, such structure may not be capable of performing impedance matching at both frequency bands.  
         [0019]     It is noted that transmission line theory as depicted in the Smith Chart of  FIG. 2  wherein an impedance presented by a load along a transmission line is modulo Pi repeating every one half wavelength such that an open circuit at one quarter wavelength appears as a short circuit, is applied by the present invention to enable a dual band antenna structure with radiating elements whose impedance can be matched over multiple frequency bands and passively switch operation between those bands.  
         [0020]     Referring now to  FIG. 3 , there is shown a portion of a dual band antenna array structure  300  according to an embodiment of the present invention. Two sets of element pairs in a linear array of antenna elements are shown with the transmission line ground plane and radiating element structure provided in solid lines and the element feed structures provided in dashed lines for a stripline or microstrip implementation. By way of example only, a linear array of notch antenna elements is configured for excitation (e.g. resonance) at a first frequency band (e.g. a lower band) and are spaced apart in the E-Plane to provide grating lobe-free radiation at a second frequency band (e.g. an upper band). Interspersed between these elements is a second set of notch elements (e.g. smaller notch elements) configured for resonance at the second (upper) band. A single feed line or transmission line  316  feeds a reactive power divider F that, in turn, feeds a pair ( 320 ,  321 ) of the notched elements; one for the lower frequency band F 1  and one for the upper frequency band F 2 . In an exemplary embodiment, the notch antenna elements may be configured as Vivaldi notch antennas, and may further be configured as exponential tapered or flared notch antenna elements.  
         [0021]     In the exemplary embodiment depicted in  FIG. 3 , the shape of the tapered notches depends on the desired bandwidth, size of the antenna, and matching impedance, for example. Impedance matching may be accomplished by placing conductive transmission line segments in appropriate locations with respect to the tapered notch elements, thereby affecting the electrical coupling between the transmission line and the antenna elements.  
         [0022]     The antenna array  300  of  FIG. 3  may be formed on a coplanar substrate  310 , which may, in an exemplary embodiment, be made of two sheets of dielectric material pre-clad with metallic copper or other electrically conducting sheets  312 ,  314 . The structure may be fabricated from two individual sheets with one being pre-clad on both sides and the other clad on one side with sheets  312 ,  314  from which portions of the sheet have been cut away to leave radiating elements and striplines or transmission lines  316 ,  318 . By cutting away the electrically conducting sheets on each of the dielectric in mirror image fashion to form the radiating elements, and with the center conductor formed on the dielectric sheet, the two sheets may be laminated together so that the total laminated antenna structure includes the transmission lines and dipole structures which are portions of the electrically conducting sheets  312 ,  314  comprising the exterior or “skin” of the antenna array structure. U.S. Pat. No. 3,845,490 issued Oct. 29, 1974 to Mannwarren entitled “Stripline Slotted Balun Dipole Antenna” discloses such fabrication and feed arrangement for implementation with the present invention, the subject matter thereof being incorporated herein by reference in its entirety. It is understood that while the above subject matter has been described with respect to a stripline implementation, the present invention may be embodied in other forms, including but not limited to the form of a microstrip using a single sheet with the feed line etched on one side and the radiating element etched from the ground plane on the other side.  
         [0023]     Still referring to  FIG. 3 , conductive layer  312  encompasses a linear array of flared or tapered notch antenna element pairs  320  and  321 ,  322  and  323 , etc. each having broad ends  320   a,    321   a,    322   a,    323   a  disposed on the same leading edge of the substrate or stripline ground plane, and tapering down to slot lines  323 ,  343 ,  324 ,  344 . The configuration of the tapered notch regions is a parallel, linear arrangement along the leading edge  360  of the ground plane. By way of example and not limitation, the particular broadband antenna specifications for antenna array  300  may be designed to transmit and/or receive signals in a first frequency band F 1  of about 3 Giga Hertz (GHz), and in a second frequency band F 2  of about 5 GHz. The antenna array length, element slot width, and taper are driven by requirements specific to the application of the array, including beamwidth, scan volume, operational bandwidth, and the like. In general, the spacing between the elements for the lower frequency band will vary from slightly less than about one wavelength at the upper frequency for no scan in the plane of the linear array, varying down to about one half wavelength for scans of up to 90 degrees.  
         [0024]     In one configuration, identical elements  320 ,  322  are spaced apart or separated from one another a predetermined distance X 1  to enable grating lobe free operation of the antenna array at a given frequency band. In the exemplary embodiment illustrated in  FIG. 3 , the notch antenna elements  320 ,  322  are configured for excitation at a first lower frequency band F 1 . The notch antenna elements  320 ,  322  have substantially identical dimensions, including notch length d, taper, and slot width w.  
         [0025]     Disposed between each of the low frequency band notch antenna elements  320 ,  322  is a corresponding smaller notched antenna element  321 ,  323 , etc. configured for operating at a relatively higher frequency band. Like elements  320 ,  322 , notch elements  321 ,  323  also have broad ends  321   a,    323   a  disposed on the same leading edge of the substrate or stripline or microstrip ground plane, and tapering down to slot lines  343 ,  344 . The configuration of the tapered notch regions is a parallel, linear arrangement along the leading edge of the ground plane.  
         [0026]     The smaller tapered notch antenna elements extend from the leading edge a distance d 2 , which is less than the distance d of tapered notch antenna elements  320 ,  322 . The smaller tapered notch antenna elements also have a smaller slot width w 2 . Like the lower band antenna elements, the higher band elements  321 ,  323  are configured to be substantially identical to one another, but have a different configuration or dimension (e.g. including notch length, taper, and slot width w associated with their frequency of operation) from that associated with the lower band elements  320 ,  322 .  
         [0027]     A single transmission line or feed line  316  feeds a reactive power divider F that, in turn, feeds a pair (e.g.  320 ,  321 ) of the notched elements; one for the lower band and one for the upper band. In the exemplary embodiment shown in  FIG. 3 , transmission line  316  having input feed terminal  316   i  comprises a first common arm or line segment  316   a  in a plane parallel to slots  323 ,  343 , and a second arm segment  316   b  normal to first arm segment  316   a  and terminating at an end that is electromagnetically one quarter wavelength at the center of the lower operating frequency band from slot  323  and is open circuited at end D. It is understood that second arm segment  316   b  need not be normal to, but simply connected in a manner that is impedance matched to first arm segment  316   a.  A third arm segment  316   c  comprises a step-like arrangement (by way of example only) extending first in a direction normal to first segment  316   a,  then in a direction parallel thereto, and finally terminating at an end normal to segment  316   a  that is one quarter wavelength at the center of the upper operating frequency band from slot  343  and is open circuited at end G. The width of the segments of transmission lines  316  and  318  are determined by the dielectric constant and thickness of the substrate material, and the transmission line impedances for matching the impedances of the radiating element feed points of  320  and  321  to the input of the structure at  316   i.  The notch antenna structure and transmission line configuration described above and as illustrated in  FIG. 3  repeats itself in the E-plane for antenna element pairs  320 ,  321 , and elements  322 ,  323 , which elements are fed by corresponding transmission line  318  and may repeat in the H-Plane to populate a phased array. Transmission line  318  is configured identically to transmission line  316  for feeding antenna element pair  322 ,  323  and includes input feed terminal  318   i  for receiving or transmitting signals of a frequency within one of the frequency bands associated with elements  323 ,  323 . A first common arm or line segment  318   a  branches at power divider F into a second arm segment  318   b  terminating at an end that is electromagnetically one quarter wavelength at the center of the lower operating frequency band from slot  324  and is open circuited at end D 2 . Third arm segment  318   c  terminates at an end normal to segment  318   a  that is one quarter wavelength at the center of the upper operating frequency band from slot  344  and is open circuited at end G 2 .  
         [0028]     The total number of antenna array elements may vary according to the particular application but include at least one pair (e.g.  320 ,  321 ) of integrated antenna elements (e.g. notch antenna elements) within a single aperture A and configured to be responsive to different excitation frequencies. A single transmission line includes a power divider for branching line segments transversely to each of the antenna elements.  
         [0029]     From the power divider F, the transmission feed lines are constrained by the requirement that the open circuited end D feeding the lower band notch antenna element  320  be an integral number of half wavelengths away from the power divider at position F at the upper frequency band to present an open circuit at the higher frequency band for preventing energy from entering leg  316   c  of the feed line at the upper band.  
         [0030]     Further, the open circuited end G of the transmission line feeding the upper band notch antenna element  321  is an integral number of half wavelengths apart from the power divider F at the lower frequency band to present an open circuit at the lower band for preventing energy from entering leg  316   b  of the feed line at the lower band.  
         [0031]     At each of the two slot positions E and H associated with notch elements  320 ,  321 , respectively, sufficient transmission lengths are provided between the element notch slot and the power divider F to accommodate an impedance matching transformer T. A conventional impedance matching transformer T such as a stepped impedance transmission line transformer, may be coupled at slot position E and configured at the low band to provide an impedance match between E and F at the low frequency band. Similarly, an impedance matching transformer T 2  may be coupled at slot position H and configured at the high band to provide an impedance match between H and F at the high frequency band. Impedance matching transformers T and T 2  are designed such that, when combined at the power divider F, an input impedance is presented with respect to the input  316   i  to allow maximum power transfer to the appropriate radiating element at each of the operating frequency bands.  
         [0032]     The operating frequency bands should be sufficiently separated in frequency to maintain the quality of the open circuits to ensure passive switching of the antenna array between the upper and lower frequency bands. In a preferred embodiment, the frequency band difference is on the order of one or more octaves.  
         [0033]     In operation, the dual mode antenna functions such that when a lower band signal F 1  is carried by transmission line  316  (via input  316   i ), the open circuit at position D at one quarter wavelength away from feed point position E of antenna element  320  appears as a short circuit at the feed point position D. This causes the transmission line to thereby energize the antenna element  320 . At the lower frequency band, the open circuit position G is an even number of quarter wavelengths (i.e. an integer number of half wavelengths) away from power divider position F so that it appears as an open circuit at F at the lower band. In this case, segment  316   c  is reflective and no signal (no energy) is propagated in this line segment  316   c  at the lower frequency band.  
         [0034]     For operation at the higher frequency band, the dual mode antenna functions such that transmission line  316  carries the upper band frequency signal F 2  via input line  316   i.  The open circuit at position G at one quarter wavelength away from feed point position H of antenna element  321  appears as a short circuit at the feed point position G. This causes the transmission line to thereby energize the antenna element  321 . At the higher frequency band, the open circuit position D is an even number of quarter wavelengths (i.e. an integer number of half wavelengths) away from power divider position F so that it appears as an open circuit at F at the higher band. In this case, segment  316   b  is reflective and no signal (no energy) is propagated in this line segment  316   b  at the higher frequency band.  
         [0035]     In this manner, the configuration performs a self-switching based on the input frequency to automatically excite either the lower band or the upper band antenna elements according to the frequency at the input feed. Thus, the present invention may operate to minimize insertion losses, eliminate active circuitry, and automatically switch operating bands without requiring intervention.  
         [0036]     According to an aspect of the present invention, the common leg segment  316   a  of the power divider may feed an active Transmit/Receive (T/R) module, or may be combined into subarrays using a transmission line feed network, such as a corporate feed, series, tandem-series, Blass network, and the like. Furthermore, the spacing between the leading edge of the notch elements and the ground plane behind the elements should be set to provide acceptable broadside element pattern performance for each element at its active frequency band.  
         [0037]     The present invention may be embodied in a dual mode antenna array consisting of many thousands of antenna elements configured as described herein and arranged in stacks of substrates or sub-array lattice structures, as may be understood by one of ordinary skill in the art. The antenna array may be configured as a phased array antenna for dual band operation wherein the bands are separated by about at least one octave. Such phased array antennas are believed suitable for phased array radar systems, satellite communications arrays, and data communications systems (such as cellular telephony), for example. Implementation of separate but integral antenna elements for each different band within a same aperture along with separate matching transformers for each element, in combination with a feed method that passively and automatically selects the proper element for excitation associated with the band of interest, eliminates the need for electronic or electromechanical switches for switching between modes, while reducing circuit complexity and control requirements.  
         [0038]     While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description.  
         [0039]     For example, the invention may be implemented in other forms or combinations of transmission lines such as coaxial lines or coplanar waveguides. Furthermore, while flared notch antenna or Vivaldi antenna elements have been described herein as exemplary antenna elements, it is understood that the invention may be applicable to other antenna element types, including for example, slot-fed or aperture-fed patch antenna elements. Still further, the co-planar substrate structure and fabrication method, transmission line and feed method disclosed herein represent non-limiting examples of application of the present invention. For example, the substrate may be formed of a material such as FR-4 or RT-Duroid and fabricated from a material such as PFTE or fiberglass; low density foam and air dielectric stripline or microstrip are also applicable. The conductive layer formed on the substrate may comprise a copper, silver, or other conductive alloy or conductive material and may be applied by etching a plated substrate or by electroplating, for example.  
         [0040]     It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.