Abstract:
A multilayer tunable ferroelectric antenna assembly which includes a first laminar structure that includes a tunable ferroelectric substrate positioned on top of a conducting ground plane and a copper radiating sheet on the other side of the substrate. A second laminar structure includes a single-sided copper cladded high dielectric substrate with the copper sheet acting as the radiator. The passive second laminar structure is electromagnetically coupled to the first laminar structure via an air-gap spacing. Application of a bias voltage across the first laminar structure changes the dielectric permittivity and, hence, the resonating frequency of the antenna structure.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/139,712, filed Jun. 17, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to microwave antenna and, in particular, is directed to a tunable ferroelectric stacked antenna with enhanced bandwidth and gain. 
     BACKGROUND OF THE INVENTION 
     Tunable antennas with different operating frequency bands have received increasing attention recently. However, most of them use diodes or shorting pins to achieve tuning performance. This additional circuitry adds protrusion and complexity to the circuit structure that impedes the capability of these antennas to operate in a high temperature, conformal and rugged environment. 
     The use of ferroelectric materials in phase shifters is disclosed in “Ceramic Phase Shifters for Electronically Steerable Antenna Systems”, Varadan et al., Microwave Journal, January 1992, pages 116-126. Some different configurations also appear in U.S. Pat. No. 5,561,407 and U.S. Pat. No. 5,307,033, both issued to Koscica et. al. The use of ferroelectric tunable resonators in filter circuits appears in U.S. Pat. No. 5,617,104 to Das. Ferroelectric materials have also been described for use in electronic phased scanning periodic arrays. For example, such arrays are described in U.S. Pat. No. 5,589,845 to Yandrofski et al., U.S. Pat. No. 5,729,239 to Rao and U.S. Pat. No. 5,557,286 to Varadan et. al. In such arrays, electrical scanning of an RF energy beam pattern is the main concern. 
     The common dielectric constant values for barium strontium titanate materials used in the systems disclosed in U.S. Pat. No. 5,427,988 to Sengupta et al. and U.S. Pat. No. 5,557,286 to Varadan et al. are relatively high for typical antenna applications. The challenges and difficulties to produce a low dielectric constant material with good electrical properties for antenna applications has been highlighted in “Ferroelectric Materials For Phased Array Applications”, Rao et. al., “IEEE Antennas &amp; Propagation Society International Symposium”, volume. 4, pages. 2284-2287, 1997. In trying to produce a low dielectric substrate, electrical inhomogeneity, low tunability and poor loss tangent performance are the commonly associated drawbacks. As a result, most of these ferroelectric antennas are realized on a high dielectric constant substrate. 
     Microstrip antennas with high permittivity substrates suffer from poor efficiency due to the energy loss associated with the excitation of surface wave modes. It has been found that for a single layer ferroelectric antenna with dielectric constant of around 16, the radiating output power from the antenna is lower than the power supplied to the input port. Parasitically coupled antennas may be used to increase the gain, but for these antennas, the performance is optimized at a certain discrete frequency only. 
     Accordingly, there is a need for a compact antenna that is electrically tunable. There is also a need for such an antenna with a substantial bandwidth and gain. 
     SUMMARY OF THE INVENTION 
     The present invention provides an antenna structure, which operates in a continuous tunable mode, which exhibits resonance at different tunable frequency bands and at the same time has a substantial bandwidth and enhanced radiation efficiency. 
     The antenna of the invention has a stacked assembly that includes a ferroelectric substrate that carries on one face thereof an electrically ground plane and on its opposite face an electrically conductive patch serving as an active feeder-resonator. A second dielectric layer is supported above the ferroelectric substrate. A parasitic radiator patch is disposed on top of the second dielectric layer. The resonant frequency of the stacked antenna assembly varies with the value of a DC voltage applied across the ferroelectric substrate. The tunable ferroelectric substrate has the advantage of being conformal and yet achieving the goal of a frequency hopping microwave communication system. 
     An aspect of the invention is an air gap between the ferroelectric substrate and the second dielectric layer. The air gap space provides two important useful features for the antenna structure. First, it enhances the gain of the antenna structure. Second, it allows wire connections to the feeder resonator for the coupling of the bias voltage thereto. The air gap also serves to enhance an electromagnetic coupling of electrical energy from the feeder resonator to the parasitic radiator. 
     In accordance with another aspect of the invention, a DC bias pad is positioned along the centerline of the feeder resonator. The centerline lies on the symmetry plane that bisects the feeder resonator patch into two equal halves. DC voltage is then applied via a DC block to the bias pad. 
     Another aspect of the invention is a cascaded of multi-stage feed network is designed and optimized on the ferroelectric tunable substrate. The tunable feed network provides a frequency variable impedance matching function for the antenna structure over different frequency bands. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the accompanying drawings, in which like reference characters denote like elements of structure and: 
     FIG. 1 is a perspective view of the antenna of the present invention. 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG.  1 . 
     FIG. 3 is a perspective view of the first ferroelectric laminar with the feeder-resonator deposited on it. 
     FIG. 4 is a schematic diagram that includes the tunable matching ferroelectric substrate and some external biasing circuits. 
     FIG. 5 is another perspective view of the layered antenna structure. 
     FIG. 6 is a graph depicting the enhanced gain and S11 input reflection layered structure of the invention. 
     FIG. 7 is a graph showing the optimized S11 performance being tuned to a different frequency band. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, the tunable antenna of the present invention includes a first substrate layer  10  that is spaced apart from an overlying second dielectric layer  30  via an air gap  20 . First substrate layer  10  is disposed on a ground plane  1 . A feeder-resonator  11  is located in air gap  20  and is disposed on the top of first substrate layer  10 . An electrically conductive sheet  31  is disposed on the top of second dielectric layer  30 . Conductive sheet  31  and second dielectric layer  30  together form a parasitic radiator that derives its energy via electromagnetic coupling from feeder-resonator  11 . 
     First substrate layer  10  is formed of a ferroelectric material, such as barium strontium titanate or any other low loss perovskite and paraelectric films. Second substrate layer  30  has a low loss and low dielectric material available, for example, under the Duroid™ brand from Rogers Corporation of Chandler, Ariz. First substrate layer  10 , ground plane  1  and feeder-resonator  11  form a stacked assembly and are adhered to one another by any suitable technique, such as adhesive bonding or microwave joining. Similarly, second dielectric layer  30  and conductive sheet  31  are joined together by similar techniques. 
     Ferroelectric substrate  10  has a thickness H that separates feeder-resonator element  11  from highly conductive ground plane  1 . The permittivity of second substrate layer  30  is designed to be higher than that of layer  10 . In a preferred embodiment, feeding resonator element  11  is designed with a length approximately equal to a quarter wavelength (λ/14) of a desired center frequency at which resonance will occur. This resonance phenomenon is characterized by a minimized reflection at an input port  13 , shown in FIG.  3 . The S11 value used in the design is about −24 dB, while a VSWR figure of less than about 2 is also used as a guideline. 
     Referring to FIGS. 1 and 3, a variable voltage source  16  is connected to apply a bias voltage between feeder resonator  11  and ground plane  1 , thereby changing the dielectric constant and the resonating frequency of the entire antenna device. Tunability may then be defined to be the derivative of the new resonating frequency and the designed center frequency, with the antenna performance being constant or kept to a slight variation. A feed  9  feeds received RF energy from RF input port  13  to feeder resonator  11 . 
     Referring to FIG. 3, a DC bias pad  12  is positioned along a centerline of feeder-resonator  11 . The centerline lies on the same orientation as the input feed and bisects feeder-resonator  11  into two equal halves. This location is chosen so as to minimize interference caused by the excitations of other higher wave modes. In addition, bias pad  12  is positioned near the edge opposite the input feed to ensure that DC feed line  17  does not impede the antenna performance. 
     Referring to FIG. 4, a DC capacitor block  13  prevents high DC voltage from destroying the RF signal sources. A resistance and inductor element  18  prevents the RF signal from leaking into DC source  16 . 
     Due to the high dielectric constant of the ferroelectric material, the microstrip line feed  9  on ferroelectric substrate  10  has an impedance typically less than about 10 ohms. The impedance of the antenna is a function of the substrate properties. Hence, when the applied bias voltage varies, the dielectric constant changes and the input impedance of the antenna changes. Impedance mismatch arises between the fixed feeding structure of a pair of signal feed elements  14  and  15  (FIG. 4) and the varying input impedance. 
     Referring to FIG. 4, another aspect of the invention incorporates signal feed elements  14  and  15  as a cascaded feed network fabricated on the same tunable ferroelectric substrate  10 . This network is formed on the same layer of metal that is used for feeder-resonator element  11  to assure electrical continuity. Hence, feed elements  14  and  15  and feeder-resonator  11  experience a similar tunability response. This minimizes abrupt changes in impedance as compared to that with a fixed antenna feed and a tunable antenna. Arranging feed elements  14  and  15  in a cascading manner is aimed to improve the narrow bandwidth of the high dielectric antenna. Another feature of the invention is that planar microstrip feed  9  is used instead of a probe feed method. This avoids a need to drill a hole through the ceramic ferroelectric layer  10 , which might crack, due to its brittleness, and distort the uniformity of substrate layer 
     Referring to FIG. 5, supports  21 , such as insulating standoffs (e.g., Nylon) or plastic foams, separate ferroelectric layer  10  and second dielectric layer  30 . Supports  21  are positioned in a manner that minimizes interference with the antenna performance. Air gap  20  provides room for connection of DC feed line  17  and enhances the gain of feeder-resonator  11 . The thickness of air gap  20  may be varied to optimize gain, resonating frequency and impedance matching of the layered antenna structure. However, it is found that optimization of the antenna performance requires simultaneous variation of the thickness of air gap  20  and the dielectric constant and the thickness of second dielectric layer  30 . This is done after an optimized design has been achieved for feeder-resonator  11  on ferroelectric substrate  10 . The air gap separation distance is kept around 4 times the thickness of ferroelectric layer  10 . 
     A positive value of realized gain may be obtained with the second layer  30  having a thickness similar to that of ferroelectric layer  10  and a dielectric constant at least 6.25 times that of ferroelectric layer  10 . Parasitic radiating element  31  is maintained at a similar dimension as that of feeder-resonator  11 . This gain performance is very attractive when compared to a negative gain value obtained with a single layer structure that consists of ground plane  1 , ferroelectric layer  10  and feeder-resonator  11 . The power output is smaller than the input power for such single layer structure high dielectric antenna. Realized gain G (in dB) is defined as: 
      G(dB)=20 log (power out/power input). 
     Referring to FIG. 6, the improved gain performance achieved with the multi-layer structure is depicted. By varying the dielectric constant of ferroelectric layer  10 , it can be shown that the optimized S11 and VSWR performance for the multi-layered antenna structure is repeated at other resonating frequencies, thereby demonstrating the effect of tunability. The gain performance, however, might degrade earlier when the dielectric constant is varied over a wider range. 
     By way of example, a single layer antenna is first constructed with a ferroelectric layer and a feeder-resonator. The ferroelectric layer has a dielectric constant of 16, a loss tangent of 2.82 and a thickness of 1.5 mm. The feeder-resonator has a dimension of 48 mm by 41.34 mm. The S11 has an optimized value of −44 dB at a frequency of 915 MHz. The gain is −10 dB. The tunability obtained is 2.8% with a bias voltage of 1.46 kV. 
     On the other hand, the multi-layer antenna of the invention, for this example, has an air gap separation of about 7 mm. Second dielectric layer  30  has a dielectric constant of 120 and a thickness of 1.6 mm. The dimension of conductive sheet  31  is reduced slightly compared to that of feeder-resonator  11 . The gain obtained is 3.8 dB at 848 MHz. Optimized performance is repeated over at least a 3% tunable shift in frequency. The shift in center frequency is due to second dielectric layer  30 . However, a positive gain is achieved where there is in no way possible for a single layer structure, even though the S11 and VSWR performance are optimized. 
     The entire antenna structure can operate in a continuous tunable mode that exhibits resonance at different tunable frequency bands and at the same time with enhanced radiation efficiency. Applications may include, but are not limited to, frequency hopping communications systems, adaptive antenna arrays and antennas for re-entry vehicles. 
     The present invention having been thus described with particular reference to the preferred forms thereof, it will be obvious that various changes and modifications may be made therein without departing from the spirit and scope of the present invention as defined in the appended claims.