Abstract:
Controlled phase shift in the operation of an RF antenna element is achieved by selectively choosing between different modes of antenna element resonance. Additional phase control resolution is achieved by combining this with other types of phase shifters. Such phase shifters are especially useful within a phased array of RF antenna radiator elements.

Description:
RELATED APPLICATIONS 
     This non-provisional application claims the priority benefit under 35 U.S.C. §119(e) of provisional application 60/507,515 filed Oct. 2, 2003, the entire content of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of antennas. Specifically, it relates to phase-controlled antennas including phased arrays. 
     2. Related Art 
     Array antennas refer to the class of antennas which provide radiation formed by phase-coherent combining of outputs from (or inputs to) multiple antenna elements. The antenna characteristics are determined by the spatial position of individual radiators and the amplitudes, phases, and time delays of their respective excitation(s). Advantages provided by array antennas include the ability to control the radiation and reception pattern of an antenna by changing the excitation across the array aperture. For example, the antenna main beam can be very rapidly scanned without having to mechanically reposition the antenna. It also provides the ability to modify the pattern to suppress interference or to otherwise enhance the spatial coverage which the antenna is to provide. 
     In many array applications, the relative phase response at each element is controlled via a device called a phase shifter. Different types of phase shifters rely on various physical mechanisms to effect a change in phase response. At microwave frequencies, phase shifters are typically implemented as switched lengths of transmission line (e.g., strip line) or resonant circuits, the former implementation having a larger bandwidth than the latter. 
     In the past, switching between different antenna element feed ports has been used to select elements of multi-element antennas, to control element gain, to adjust the antenna element impedance, and to change the polarization response of the antenna. To our knowledge, switched feeds on the same antenna element have never been used to achieve controlled phase shifts nor has varactor-based selection between plural modes of antenna resonance been used to implement controlled phase shifts. The most closely related prior art in our present view (U.S. Pat. No. 5,434,575) describes a relatively complex, multi-element antenna that switches among pairs of antenna elements for selectively changing the sense of circularly polarized antenna operation. 
     It is assumed that the reader will have a general background in RF circuits and antennae. However, if not, for general background information reference may be had to texts such as, for example:
         Hansen, R. C.,  Phased Array Antennas , John Wiley &amp; Sons, 1998   Kraus, J. D., and Marhefka, R. J.,  Antennas for All Applications , McGraw-Hill, 2002   Pozar, D. M.,  Microwave Engineering , Addison Wesley, 1993.       

     BRIEF SUMMARY OF THE INVENTION 
     This invention achieves controlled phase shifted antenna element operation by selectively choosing between different modes of antenna element resonance. 
     An exemplary embodiment implements a very simple phase shift with minimal insertion loss and minimal circuit complexity. The exemplary embodiment may be referred to as a one-bit (180 degree) phase shifter since the controllable phase states for a given antenna element are separated by 360°/2 n , where n is the number of bits in the digital word used to command a particular phase state (e.g., from a phased array controller). Though the present exemplary embodiment is limited to one-bit phase resolution at each antenna element, its use in combination with switched-transmission-line (or other) phase shifter designs reduces average loss over phase states by eliminating the longest (0°/180°) path (or element) otherwise required in a standard phase shifter (which longest path has the highest loss) and by reducing the size and number of circuit elements in the accompanying combined conventional phase shifter(s) required to implement a complete digitally controlled phase shifter of arbitrary resolution. 
     The present exemplary embodiment may also be, at the elemental level, substantially simpler than the polarization control approach of U.S. Pat. No. 5,434,575 in that only a single antenna element and pair of switches is required, reducing mass (important for ultra-lightweight satellite applications), eliminating complex switch matrix and hybrid circuitry, and permitting phase shifting of arbitrarily polarized signals. The feed switches may also require less circuit area than switched delay lines or resonant phase-shifters, thus reducing cost in highly integrated, single-chip transmit/receive module designs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of this invention will become more apparent from a detailed study of the following detailed description of exemplary embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts, respectively, a cross-sectional diagram of a half-wavelength patch antenna above a ground plane, the voltage distribution across the patch and an equivalent circuit model of the patch; 
         FIGS. 2   a  and  2   b  are, respectively, cross-sectional diagrams of two probe-fed patches with equal but opposite offset locations of the feed probe (and the resulting voltage distribution indicating a phase reversal, i.e., a 180° relative phase shift) while  FIG. 2   c  schematically depicts a single patch having the offset feed points of both  FIGS. 2   a  and  2   b;    
         FIGS. 3   a - 3   d  are a set of circuit diagrams showing different possible switch configurations including, respectively, PIN diode shunt, PIN diode series, MEMS (micro electromechanical systems) switch shunt, MEMS switch series; and 
         FIG. 4  depicts a presently preferred exemplary embodiment for a one-bit (180 degree) phase shifter implemented using varactor diodes to modify the phase response of the antenna element where the design frequency for this example is 10 GHz (for other frequencies, the capacitance, capacitance swing, and bias circuitry values would be adjusted). 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring now to the drawings, which are intended to illustrate only a presently preferred exemplary embodiment and not to in any way limit the scope of this invention, a one-bit (180°) phase shifter is implemented as a switched feed to a single antenna element  10  such as a microstrip patch antenna element as shown in cross section at  FIG. 1 . Those in the art will appreciate that the patch effectively defines a 3D volume underlying a 2D conductive patch (i.e., the patch  10  has a resonant half-wavelength width W and also has a length dimension extending orthogonal to the plane of  FIG. 1 ). The response of a patch antenna can be viewed as a half-wavelength resonator with an equivalent circuit model as shown in  FIG. 1 . In response to an impinging plane wave of the proper wavelength, the voltage from the patch to the ground plane as a function of distance in the E-plane across the patch as shown in  FIG. 1 . 
       FIGS. 2   a  and  2   b  show a pair of patch radiator elements  20 ,  21  being fed via offset probes  20 ′ and  21 ′ where the offset from center of the patches is the same distance, but in opposite directions (in the E-plane). Although depicted with internal bottom feed points, those in the art will also understand that microstrip radiators may be edge fed (e.g., using microstrip feed lines in the plane of the patch). Note that the two feed points  20 ′ and  21 ′ are located symmetrically about the center of the respective patches  20 ,  21 . The impedance of the patch antenna as seen at each of the feed points is the same in both cases, but the voltage distribution phase response is changed by 180°. Selection of a particular feed (e.g., to one of two such feeds  22 ,  23  on a single patch  24  as depicted in  FIG. 2   c ) may be accomplished via a switch  25  controlled by array controller  26 . As previously noted, additional auxiliary phase shifters (e.g., of conventional switched transmission line types) may be combined with this simple 180° one-bit phase shifter at each element to achieve arbitrary phase shifting resolution as desired. 
     A variety of switching methods may be used, depending on frequency, to route the RF signal to the desired one of plural feeds. Therefore, the general technique is not frequency limited. The switching methods include, but are not limited to, varactor diodes, PIN diodes, and MEMS in different circuit configurations.  FIGS. 3   a - 3   d  show some of these switch control configurations. 
     In order to minimize RF insertion loss and digital phase shifter circuit complexity, the present exemplary embodiment may incorporate dual feed points connected via varactor diodes. Controlling the capacitance of these two varactors modifies the resonant response of the antenna resulting in the desired 180° phase shift. The required capacitance swing can be as low as 3:1 so that the varactors are not real switches in the usual sense (i.e., to effectively physically connect or disconnect electrical conductors), rather they act to modify the resonant behavior of the antenna. 
     An exemplary switching circuit is shown in  FIG. 4 . This switch architecture utilizes a pair of varactor diodes whose capacitance, capacitance swing, and bias circuitry are adjusted so that both of the diodes act as an on/off switch at the designed radio frequency of the antenna. The control bit voltage V n  for this stage of the array is applied in such a way that only one of the diode switches is in the high capacitance state at any time. 
     While the probe-fed patch antenna element discussed thus far is presently a preferred mode of practice, the present exemplary embodiments also contemplate use of other feed techniques such as aperture coupling, co-planar microstrip feeds, and strip line feeds. Similarly, other antenna elements can be substituted for the patch example described above. These other antenna elements include dipoles, flared notches, slots, and any other antenna that supports balanced 0°/180° modes. Such exemplary embodiments may be aptly described as employing a switched resonance, one-bit (180 degree) antenna phase shifter. 
     It will be appreciated that this invention may be combined with other switched antenna element control features. For example, both polarization and one-bit phase shift control can be achieved by using a pair of properly situated feed points for each 0°, 180° relative phase control thus providing two different possible polarizations for each phase shift value. 
     This invention has been described in connection with one or more exemplary embodiments. However, those skilled in the art will readily appreciate that many modifications and variations may be made to these exemplary embodiments while yet retaining novel features and advantages. Accordingly, all such modifications and variations are intended to be covered by the following claims.