Abstract:
Embodiments relate to systems and methods for a frequency reconfigurable filtenna system. Implementations incorporate a reconfigurable band-pass filter within the feeding line of an antenna structure. The combination of the filter and the antenna may be referred to as a “filtenna”. Implementations integrate both the band-pass filter and the antenna within the same substrate, permitting easier, more efficient and more compact integration in the transceiver hardware. Moreover, by using this configuration, the biasing of the switching elements are not present in the radiating plane of the antenna. This reduces the negative effect of the biasing lines on the antenna radiation performance, as we!! as provides a tunable filtered antenna radiation characteristic.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Stage application of PCT/US2013/032482 filed Mar. 15, 2013 which claims priority to U.S. Provisional Application No. 61/611,848, entitled “Reconfigurable Filtenna,” filed on Mar. 16, 2012, by the same inventors herein, which applications are incorporated by reference in their entireties. 
    
    
     GOVERNMENT FUNDING 
     This invention was made with Government support under Contract FA9453-09-C-0309 awarded by the United States Air Force. The Government has certain rights in the invention. 
    
    
     FIELD 
     The present teachings relate to systems and methods for a frequency reconfigurable filtenna structure, in which the operating frequency of an antenna is changed without incorporating active components on the antenna radiating surface 
     BACKGROUND 
     With the advancement in cellular and other wireless communications, there is a significant demand to implement antennas that are “smart” in the sense of being able to tune their operating characteristics (frequency, polarization, radiation pattern, etc) according to the ever-changing wireless communication requirements. Using multiple dedicated antennas to cover a variety of different wireless services that may be scattered over a wide frequency bands increases the system cost, the space requirements for the antennas, and their isolation. Reconfigurable antennas are therefore potential candidates for future RF front-end solutions to minimize the number of antennas required in a particular system. 
     Reconfigurable antennas have been studied in the wireless communication industry throughout the last two decades or longer. This type of antennas requires some type of reconfiguring element to change the antenna&#39;s electrical properties for each channel or communication standard. 
     Conventional electrically reconfigurable antennas use RF-MEMS, PIN diodes, or varactors to reconfigure their structures and create the required tuning in the antenna function. The activation and de-activation of these switching elements require the incorporation of biasing lines in the radiating plane of the antenna. The switching elements can introduce interference that disturbs the antenna electromagnetic performance. The effects of that interference need to be minimized and the placement of the reconfiguring component needs to be optimized. 
     The interference effects manifest themselves, first, as unwanted resonances in the operating bands of the antenna. Second the switching interference can cause a change in the antenna radiation pattern away from the design requirements, especially if the biasing lines are not designed properly. To avoid some of these difficulties, and to satisfy the design constraints, reconfigurable antennas can be designed with external matching networks or with reconfiguring elements outside the antenna radiating plane. 
     On the other hand, some researchers have resorted to optical switches to solve the problems and limitations produced in the electrically reconfigurable antennas. For example, n-type silicon material can be used as a switching element to tune the antenna parameters. One limitation of this technique is the integration of laser diodes within the antenna structure for the switch activation mechanism which adds to the bulkiness of the structure and increases the power consumption of the whole system. Reconfigurable antennas have also been designed using a physical change in the antenna radiating structure. For example, a stepper motor has been proposed to rotate the radiating surface of a microstrip antenna, and for each rotation a different radiating structure is fed. A significant limitation of this technique is the lack of tuning speed. 
     In addition to reconfigurable antennas, reconfigurable band-pass and band-stop microwave filters have been also investigated as stand-alone components. RF-MEMs, PIN diodes and varactors have been proposed mainly to tune the bandwidth of a filter. However, the non-linearity produced by the switching elements as well as the filter&#39;s insertion loss need to be addressed. It may be desirable to provide methods and systems for reconfigurable antennas to, selectively reconfigure their operation without introducing interference, or other issues. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present teachings and together with the description, serve to explain the principles of the present teachings. In the figures: 
         FIG. 1  illustrates an overall filter structure which can be used in systems and methods for reconfigurable antenna, according to various embodiments; 
         FIG. 2  illustrates a bias tee circuit or module that can be incorporated in systems and methods for reconfigurable antenna, according to various embodiments; 
         FIGS. 3A and 3B  illustrate bandpass frequency graphs based on simulated and measured data, according to various embodiments; 
         FIG. 4  illustrates a transmission characteristic of the filtenna device using simulated data, according to various embodiments; 
         FIGS. 5A and 5B  illustrate a top layer and bottom layer of the filtenna device, according to various embodiments; 
         FIGS. 6A and 6B  illustrate reflection coefficient graphs for the reconfigurable filtenna, using simulated and measured data, according to various embodiments; and 
         FIGS. 7A and 7B  illustrate filtenna radiation pattern graphs at different operation frequencies, according to various embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present teachings relate to systems and methods for a reconfigurable combination of a filter and antenna, referred to herein as a “filtering antenna” or “filtenna,” having enhanced filtering and radiation performance. The inventive filtenna design can be implemented by integrating a reconfigurable band-pass or band-stop filter structure directly within the feeding line of a wideband antenna. The filter structure can utilize a varactor incorporated directly on the same substrate of the planar wideband antenna. The varactor is biased or driven by injecting a direct current (DC) signal into the microstrip feeding line through a bias tee circuit. Thus, the filter is tuned by varying the DC voltage supply. Accordingly the antenna tunes its frequency based on the filter&#39;s frequency tuning operation. The overall filtering antenna structure as noted combines both the reconfigurable filter and the antenna structure into the same substrate, which further allows easier integration in a complete RF front-end for cellular or other wireless applications. Implementations described herein do not resort to switching components incorporated on the antenna radiating structure that can affect the antenna total radiation pattern, or introduce other undesirable radio frequency behaviors in the wireless device. 
     Reference will now be made in detail to exemplary embodiments of the present teachings, which are illustrated in the accompanying drawings. Where possible the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     An overall filter structure  100  according to implementations of the present teachings is shown in  FIG. 1 , The microstrip feeding line  132  of the filter structure  100  is composed of three sections. The two outer sections are illustratively shown as having a length of 9.6 mm and a width of 5 mm, which corresponds to an impedance of 50 ohms. At a first end and a second end, a port  104  (Port  1 ) and a port  106  (Port  2 ) are respectively configured. A hexagonal slot  134  is etched in the center of the third and middle section of the microstrip feeding line  132 , in the substrate  102  of the filter structure  100 . A varactor  108  is incorporated inside the hexagonal slot  134 , to achieve a variable capacitive connection between the two terminals in the slot of the middle section of the microstrip feeding line  132 . The middle section is separated from the two outer sections of the microstrip feeding line  132  by two gaps, having illustrative widths of 0.4 mm ( 112 ) and 0.6 mm ( 110 ) respectively. These gaps contribute a fixed capacitance to the overall microstrip feeding line  132 , and allow the filter structure  100  to have the desired band-pass operation. Thus different gap dimensions result in different band-pass behavior. By supplying different voltage levels to the varactor  108  using the biasing line  114 , the total capacitance of the filter structure  100  changes accordingly, allowing the filter structure  100  to be tuned to various operating frequencies. 
     According to implementations, the filter structure  100  and related elements are printed on a commercially available Taconic TLY substrate available from Taconic, Petersburgh, N.Y., as the substrate  102 , with a dielectric constant of 2.2 and a thickness of 1.6 mm, although it will be appreciated that other materials and dimensions can be used for an alternative performance. The total dimensions of the illustrative filter structure  100  are 30 mm×30 mm, although it will again be appreciated that the dimensions are merely exemplary, and others can be used for other frequency ranges. The reconfigurability of the filter structure  100  is achieved by incorporating the varactor  108  directly within its structure, as an integrated element. The varactor  108  in turn can be biased while eliminating the need for external DC wires attached to the filter structure  100 , through the use of an external bias tee  120  at input port  104  of the filter structure  100 . 
     The purpose of the bias tee  120  is to feed the filter structure  100  with the desired RF signal, while also providing the required DC voltage to drive the capacitance value of the varactor  108 . Since the outer section of the filter structure  100  where the DC voltage is fed is separated from the inner section where the varactor  108  resides by the 0.4 mm gap, a biasing line  114  is needed to provide a connection between the two sections and allow the DC voltage to be supplied to one end of the varactor  108 . Biasing line  114  (labeled Biasing line  1 ) shown in  FIG. 1  has an illustrative width of 0.1 mm, which corresponds to a high impedance line. The biasing line  114  has an illustrative length of 13.56 mm, which corresponds to λg/2 at f=7.45 GHz. Moreover in order to have a continuous voltage path through the varactor  108 , the other end of the varactor  108  should be grounded. Biasing line  116  (labeled as Biasing line  2 ), shown in  FIG. 1 , connects the second end of the varactor  108  to the ground plane  118  of the filter structure  100 . The biasing line  116  has an illustrative width of 0.1 mm and a length of 12.5 mm, which corresponds λg/2 at f=8.1 GHz. The connection to the ground  118  can be done by soldering a wire from the biasing line  116  to the ground of the filter. An illustrative commercially available varactor that can be as varactor  108  is the SMV 1405 from Skyworks Solutions Inc., Woburn, Mass., while an illustrative commercially available bias tee  120  is the BT-V000-HS from United Microelectronics Corp. Sunnyvale, Calif. 
       FIG. 2  illustrates an internal structure of the bias tee  120  that can be used in implementations of the present teachings. The bias tee can be connected to port  104  (Port  1 ) of the filter structure  100 . At the input of the bias tee  120 , the RF signal  122  is fed. The DC voltage is supplied at the bias input  124 . At the output of the bias tee the RF and the DC signals are present simultaneously in output signal  130 , which is fed to port  104  of the filter structure  100 . The bias tee is also composed of a capacitor  126  to block the DC voltage to go to  122 , and an inductor  128  to block the RF signal to leak to the DC power supply. The path of the voltage that is responsible to change the capacitance of the varactor  108 , and hence tune the operating band of the filter structure  100 , travels into port  104  via bias tee  120 , across bias line  114  and ultimately to ground  118  via biasing line  116 . 
     The simulated and the measured |S11| (dB) of the filter structure  100  for different voltage levels (11 V-27 V) are shown in  FIGS. 3A and 3B , respectively. From this plot, it can be concluded that the filter structure  100  acts as a re-configurable band-pass filter for different voltage values (different adjusted capacitances). The filter structure  100  can thus be used to reconfigure the operating frequency of an antenna structure of a smart phone, or other wireless device. The measured data of the filter structure  100  shows an illustrative tuning range from 6.16 GHz to 6.6 GHz. The tuning in the operating band of the structure is due to the change in the total capacitance of the filter structure  100 , and this is achieved by adjusting the varactor  108  that resides in the middle of the microstrip line  132  of the filter structure  100 . It will be noted, however, that the filter structure  100  can tune over a wider band of frequency as desired, using higher or lower capacitance values. As shown in  FIG. 4 , the insertion loss of the filter (|S21| (dB)) for different voltage levels is almost −1.5 dB. From this plot, one notices that the filter structure  100  provides very adequate out-of-band rejection performance for cellular or other wireless applications. While illustrated as a band-pass filter, it will be noted that filter structure  100  can be implemented as other band-limited filters, such as a band-stop filter. 
     In terms of incorporation into a completed RF antenna assembly, as shown in  FIGS. 5A and 5B , the overall filtenna structure  140  incorporating the tunable filter structure  100  can in implementations consist of a dual-sided Vivaldi antenna, which in general is a wideband structure and a reconfigurable band-pass filter. The filtenna structure  140  can be fed via a 50 ohms microstrip feeding line  132  which corresponds to a width of 5 mm. The Filtenna is made frequency reconfigurable by incorporating the band-pass filter structure  100  discussed above directly or integrally in the antenna microstrip feeding line  132 . The technique of implementing an overall reconfigurable filtenna structure  140  provides multiple advantages in comparison with the conventional approach of switch incorporation into the antenna radiating patch. In fact, the negative effects of the biasing lines on the antenna behavior are minimized since they no longer reside in the radiating surface of the antenna. Also, by tuning the operating frequency of the filter structure  100 , the filtenna structure  140  is able to maintain the same radiation pattern and a constant gain since the Filtenna surface&#39;s current distributions are not disrupted. 
     The top and bottom layers of the filtenna structure  140  are shown in  FIGS. 5A and 5B , respectively. The filtenna structure  140  has a partial ground in the bottom layer, as shown in  FIG. 5B . This ground plane  144  of the filtenna structure  140  has illustrative dimensions of 30 mm×30 mm. The structure can for instance be printed on a Taconic TLY substrate of dimension 59.8 mm×30 mm. The inner and outer contours of the antenna radiating surface are designed based on an exponential function. The top layer contains a top side antenna radiating surface  142 , as well as the microstrip feeding line  132  where the reconfigurable filter structure  100  is located. On the bottom layer of the design resides the ground plane  144  of the filtenna structure  140 , connected to the second (bottom) radiating part  146  of the Vivaldi antenna. While a Vivaldi type radiating antenna is illustrated as the radiating element in the filtenna structure  140 , it will be appreciated that in implementations, other types or constructions of the radiating element can be used for different purposes. 
     In terms of the reflection coefficient characteristics, the simulated and the measured filtenna reflection coefficients are shown in  FIGS. 6A and 6B , respectively. The filtenna structure  140  is able to tune its operating frequency based on the mode of operation of the integrated filter structure  100 . It may be noted that based on both simulated and measured data, the filtenna structure  140  produces a reflection coefficient above −10 dB outside the operating bandwidth of the filter structure  100 . It will be noted that the tuning in the operating frequency of the filtenna structure  140  is achieved by using the same voltage characteristics as with the tuning of the filter structure  100 . 
     In terms of radiation patterns,  FIG. 7  shows the normalized total radiated electric field at f=6.16 GHz (11 V) and f=6.47 GHz (27 V) in the φ=0° and φ=90° planes. The Filtenna radiation pattern remains almost the same for the different voltage levels. The Filtenna gain at θ=0° and φ=0° is 5.72 dB (6.17 GHz) and 6.77 dB (6.47 GHz), respectively. 
     The foregoing description is illustrative, and variations in configuration and implementation may occur to persons skilled in the art. For example, while embodiments have been described in which the filter structure  100  interacts with one radiating element in the overall filtenna structure  140 , it will be appreciated that in implementations, multiple radiating elements and/or filtennas, for example for diversity purposes, can be used. Other resources described as singular or integrated can in embodiments be plural or distributed, and resources described as multiple or distributed can in embodiments be combined. The scope of the present teachings is accordingly intended to be limited, only by the following claims.