Abstract:
A two-dimensional (2-D) beam steerable phased array antenna is presented comprising a continuously electronically steerable material including a tunable material or a variable dielectric material, preferred a liquid crystal material. A compact antenna architecture including a patch antenna array, tunable phase shifters, a feed network and a bias network is proposed. Similar to the LC display, the proposed antenna is fabricated by using automated manufacturing techniques and therefore the fabrication costs are reduced considerably.

Description:
[0001]    A two-dimensional (2-D) beam steerable phased array antenna is presented comprising a continuously electronically steerable material including a tunable material or a variable dielectric material, preferred a liquid crystal material. A compact antenna architecture including a patch antenna array, tunable phase shifters, a feed network and a bias network is proposed. Similar to the LC display, the proposed antenna is fabricated by using automated manufacturing techniques and therefore the fabrication costs are reduced considerably. 
       STATE OF THE ART 
       [0002]    This invention relates to a phased array antenna. More specifically, the invention relates to an electronically steerable phased array antenna based on voltage tunable phase shifters whose low loss dielectric material can be tuned with an applied voltage. 
         [0003]    In recent years, demand for steerable antennas increased dramatically for mobile terminals due to the rapid development of broadcast satellite services. Wireless internet, multimedia and broadcasting services are provided from satellites, which operate at L-band, Ku-band or K/Ka-band by steerable antennas, e.g. to a moving vehicle such as an automobile or airplane or ship or even other portable devices like mobile TV or GPS. 
         [0004]    A steerable antenna can change its main beam direction in order to ensure that the main beam is continuously pointing towards the satellite. Most of the steerable antennas in the market are mechanically controlled. By the help of mechanical systems, which are driven by motors, the orientation of the antenna is adjusted in the elevation and azimuth planes. Some other types of antenna systems utilize hybrid methods like electronically steering in the elevation plane and mechanical adjustment in the azimuth plane. These kinds of mobile terminals are bulky, have relatively slow beam steering speed, i.e. 45°/s, sensitive to the gravitational force and require high maintenance costs since the mechanical systems are used. They are mainly used in military application and not preferred for a mobile terminal for which the aesthetic appearance is a critical requirement, i.e. for automobile industry. 
         [0005]    A phased array antenna is one of the well-known types of the electronically steerable antennas (ESA) which is fast, compact, reliable and easy to maintain compared to mechanically steerable ones. It consists of RF feed/distribution network, electronically tunable phase shifters, transmit/receive modules (for active arrays) and radiating elements. The phase of each radiating element or group of radiating elements is/are adjusted to predefined phase values by the electronically tunable phase shifters in order to tilt the radiated phase front in a specified direction. These antennas are low-weight and low-profile whereas the challenge is high price of the respective terminal due to its expensive electronics. 
         [0006]    Electronically tunable phase shifters play an essential role concerning the performance, cost, and dimensions of the ESA. The common parameter for quantifying the RF performance of a tunable phase shifter is a frequency dependent figure-of-merit (FoM) of the phase shifter. It is defined by the ratio of the maximum differential phase shift and the highest insertion loss in all tuning states. In general, the aim is to achieve the highest possible differential phase shift accompanied by the lowest insertion loss which leads to a high FoM. In art, technological approaches for electronically tunable phase shifters include micro-electromechanical systems (MEMS), semiconductors and continuously tunable dielectrics such as barium strontium titanate (BST) and liquid crystal (LC). These technologies have been compared in terms of different aspects such as tunability, power consumption, response time and cost. The state of the art FoM of MEMS based phase shifter is about 50°/dB to 100°/dB. Semiconductor based monolithic microwave integrated circuit (MMIC) phase shifters have FoM around 40°/dB to 70°/dB at microwave frequencies &gt;20 GHz. Similarly, BST based phase shifters have relatively high performance (FoM is about 40°/dB to 90°/dB) for frequencies up to 10 GHz. 
         [0007]    Liquid Crystal (LC) is another possible tunable dielectric which can be used for high micro and millimeter-wave applications. LC is a continuously tunable material with low dielectric losses. In practical application, its tenability can be controlled, i.e. applying a bias voltage with low power consumption. Its tunability is defined as the fractional change in the dielectric constant with an applied voltage. Effective dielectric constant of LC depends on the orientation of the molecules with respect to the RF-field. Desired orientation of the molecules, i.e. parallel or perpendicular to the RF-field, can be accomplished by using surface treatments or electrostatic field. The FoM of a microstrip line based LC phase shifter of the state of the art is about 110°/dB and of a partially filled waveguide based LC phase shifter is 200°/dB at 20 GHz. 
         [0008]    A low-profile, two dimensional steerable array can be fabricated in “tile” architecture where the electronically tunable phase shifters are mounted on another layer which is parallel to the radiating elements. For such a large array, i.e. with 16×16 radiating elements, compactness of the electronically tunable phase shifters become an issue. Each phase shifter or group of phase shifters has to be fabricated on a limited area. Moreover, they have to be biased individually in order to steer the antenna main beam both in elevation and azimuth planes. MEMS or semiconductor based phase shifter needs more than one bias line depending on its differential phase shift resolution. For instance, a 3-bit phase shifter has to be biased with three bias lines. On the other hand, only one bias line is required when a tunable dielectric based phase shifter is used. However, compact design of an electrically tunable phase shifter which has a 360° differential phase shift, is still challenging. 
         [0009]    Additionally, due to a compact design of a large ESA, coupling between the electronically tunable phase shifters and other components has to be prevented in order not to reduce the antenna performance. In US20090091500 possible usage of LC for antennas is given. However, practical problems such as biasing the tunable phase shifters individually and feeding the RF signal to the antenna have not been discussed. Additionally, particular attempts have been done within the scope of the present invention in order to design compact phase shifters and to prevent undesired coupling between the radiating elements and feed network. Similarly, other variable dielectric based antenna arrays are discussed in U.S. Pat. No. 6,759,980, U.S. Pat. No. 6,864,840, however, there the individual phase shifters for each antenna element have to be mounted element by element to different substrates. The present invention integrates the phase shifters in the uniform substrates and furthermore allows the use of liquid tunable dielectrics. 
         [0010]    U.S. Pat. No. 7,361,288 and WO 2011/036243 disclose Components for High-Frequency Technology utilizing liquid crystals as steearable dielectrics. However, this is not a planar device. Such phase shifters as described in these patent documents can not be used in order to fabricate a low profile antenna. 
         [0011]    Special liquid crystals developed for application in high-frequency technology are disclosed e.g. in WO 2011/009524 and WO 2011/035863. 
       Advantage of the Invention 
       [0012]    Low-cost, lightweight, electronically steerable phased arrays which can be fabricated by using automated manufacturing techniques are of interest for mobile terminals such as for automobiles, airplanes and radars. The antennas main beam direction can be continuously steerable in order to provide the services, e.g. wireless internet or broadcasting, simultaneously on moving vehicles via satellite. Planarity and aesthetic appearance of the antenna with low-profile has to be maintained since these are other critical issues, i.e., for automobile industry. Such an antenna requires compact, low loss, electronically tunable phase shifters which can be integrated to the radiating elements and feeding network. A biasing network is necessary by which all phase shifters can be biased individually. Such an electronically steerable antenna is subject of the invention. 
       SUMMARY OF THE INVENTION 
       [0013]    This invention provides a low profile, electronically steerable, planar phased array antenna whose main beam can be continuously steerable in one or two dimensions. The antenna comprises an input, feed network, at least one power divider (combiner), at least one electronically tunable phase shifters, a biasing network and at least two radiating elements. The electronically steerable phased array antenna comprises a stack of at least three dielectric substrates, preferred uniform dielectric substrates, at least two of which are solid and can carry a plurality of electrodes. An individual element of the array antenna comprises at least an electronically tunable phase shifter, a biasing network and a radiating element. The phase shifter electrodes are grouped in order to form the plurality of individual antenna elements whereas a single uniform substrate can carry electrodes for any number of antenna elements. The substrates may furthermore carry electrodes for the feed network. A continuously variable dielectric being either liquid or solid is sandwiched by two of the aforementioned solid dielectric substrates. Electronically tunable phase shifters utilizing the variable dielectric substrate are thereby integrated into the antenna. The dielectric constant of the variable dielectric substrate and therefore the electrical characteristic of the phase shifters are controlled continuously in order to achieve a desired differential phase shift between the radiating elements for a continuous beam steering, so that the antenna can be adjusted in elevation and azimuth planes. 
         [0014]    In an embodiment the antenna comprises a plurality of power dividers and/or a plurality of electronically tunable phase shifters and/or a plurality of radiating elements. The electronically steerable phased array antenna is built as a stack of at least three dielectric materials. These materials are a front dielectric substrate (solid), a variable dielectric (solid or liquid) and back dielectric substrate (solid). One of the major advantages of the invention is that the phase shifter and all the other components are not prefabricated and assembled into a large one when an antenna is built; instead they are fabricated on large simultaneously on the three mentioned substrates. 
         [0015]    Electronically tunable phase shifters based on planar transmission lines, preferably microstrip lines, are integrated to the antenna. The dielectric properties of the variable dielectric material, and therefore the electrical characteristics of the phase shifter can be changed by applying a bias voltage. 
         [0016]    According to another aspect of the invention, instead of the microstrip lines, loaded lines can be used as transmission lines. Using a loaded line phase shifter, the LC layer thickness can be reduced to a few micro meters and therefore the response time is improved considerably. The planar transmission lines are also called the phase shifter electrodes or electrodes of the phase shifter. 
         [0017]    A preferred example of an antenna constructed in accordance with the invention has 4 (2×2) radiating elements. It is a planar antenna with low profile. The antenna utilizes liquid crystal (LC) material as a variable dielectric substrate. Similar to the LC display technology, LC is sandwiched between the front and back dielectric substrates. A LC material having a maximum loss tangent of 0.05 is preferred as for example nematic LC. Other types can be used as well but performance will be poor. 
         [0018]    According to other aspects of the present invention, the radiating elements can be grouped in order to form a sub-array. Such a sub-array comprises an input, feed network, an electronically tunable phase shifter and plurality of radiating elements. The biasing complexity of a large array antenna is reduced and antenna reliability is increased since only one phase shifter is required for each sub-array. 
         [0019]    According to further aspects of the present invention, a low profile active phased array antenna including low noise amplifiers or transmit/receive modules can be constructed. 
         [0020]    The demand for steerable antennas increased dramatically for mobile terminals due to the rapid development of broadcast satellite services. The invention can be used for wireless internet, multimedia and broadcasting services are provided from satellites, which operate at high frequencies of e.g. about 1-2 GHz in the L-band, or even at frequencies higher than 10 GHz as for example in the Ku-band or K/Ka-band, to a moving receiver, e.g. in a portable device or in a vehicle such as an automobile or airplane or ship by the steerable antennas. However, the antenna can be scalable for other operation frequencies as well. 
         [0021]    BST is preferred for frequencies up to 10 GHz. LC is preferred for frequencies higher than 10 GHz due to the lower dielectric loss. Especially for high frequency operations like 77 GHz or W-band application LC is preferred according to the invention. 
         [0022]    For a 2-D steerable antenna, if the radiating elements are grouped, only one phase shifter is required for each group. Otherwise, one phase shifter is required for one radiating element. 
         [0023]    The challenge for the geometry of the electrodes of the phase shifter is to reduce the coupling between the electrodes, if the electrodes are meandered. Meandering the electrodes is necessary where the area where the phase shifters are fabricated is limited. Different shapes can be used theoretically. However, the preferred geometry is the spiral geometry since it improves the performance. With spiral geometry the output port is in the middle. This is an advantage when the phase shifter is integrated to the antenna. 
         [0024]    In addition the preferred geometry of the corners of the spiral phase shifters are rounded in order to reduce the metallic losses. 
         [0025]    A phase shifter is device which changes the signal phase and has a flat phase response over the frequency. LC based phase shifters usually have frequency dependent phase response, however it is also possible to integrate flat phase response into a LC based phase shifter and use this type in an antenna according to the invention. In another embodiment of the invention the phase shifter is a time delay unit. A time delay unit is a structure that provides a specific time delay, or programmable time delay, using a multi-path structure. Also in time delay units the preferred geometry of the delay lines is spiral geometry. 
         [0026]    The length and the width of the antennas are independent of the technology and therefore they are more or less constant depending on the frequency. Theoretically, the distance between two radiating elements is λ/2 where λ is the wavelength of the radiation emitted resp. received. If there is a number of “N×N” radiating elements, with “N” being an integer, preferably in the range from 10 to 100 the size of the antenna is N(λ/2)×N(λ/2) for the length and width. However, its thickness depends on the technology. Using LC according to the invention one can easily build a thin antenna array. This is similar to the LC displays or monitors. 
         [0027]    The length and the width of the antennas are related with the antenna gain. Table 1 shows possible antenna sizes and the corresponding antenna gains of a microstrip patch antenna operating at 20 GHz. The theoretical values are given in parentheses and the ones without the parentheses are the practical values. Latter is more than the former because some space is need for the sealing, LC filling, bias pads. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Exemplary embodiments 
               
             
          
           
               
                 Antenna 
                   
               
             
          
           
               
                 No. of Elements 
                 Size 
                 Gain 
               
               
                   
               
               
                 8 × 8 
                 10 cm × 10 cm 
                 21 dB 
               
               
                   
                 (6 cm × 6 cm) 
                   
               
               
                 16 × 16 
                 15 cm × 15 cm  
                 27 dB 
               
               
                   
                 (12 cm × 12 cm) 
                   
               
               
                 32 × 32 
                 30 cm × 30 cm  
                 35 dB 
               
               
                   
                 (24 cm × 24 cm) 
                   
               
               
                   
               
             
          
         
       
     
         [0028]    These antennas have a preferred thickness of, but not limited to, 1.5 mm and can e.g. be reduced to 0.7 mm. 
         [0029]    The advantages of the invention are the cost-efficiency, the high geometry efficiency based on the spiral geometry of the phase shifter electrodes, and the high compactness and low profile of the antenna, which is continuously steerable. 
         [0030]    The antenna according to the invention consists of at least 3 substrate layers: 
         [0000]    a uniform front dielectric substrate carrying electrodes on both sides;
 
a plurality of radiating elements on the top side of the front dielectric substrate;
 
a ground electrode with a plurality of openings covering the bottom side of the front dielectric substrate;
 
a plurality of planar transmissions line integrated to the ground electrode;
 
a uniform variable dielectric being either liquid or solid;
 
a back dielectric substrate having an electrically conductive layer on the top side;
 
a plurality of electrically conducting electrodes with different conductivities on the top side of the back dielectric substrate.
 
         [0031]    In a preferred embodiment the front and back dielectric substrates comprise mechanically stable, low loss substrates for example glass substrates, fused silica, ceramic substrates and ceramic thermoset polymer composites. 
         [0032]    The front and the back dielectric substrate can be held apart for example by a punched out sheet forming cavities for the liquid dielectric material or by spherical spacers. 
         [0033]    The vertical interconnects can be made by vias through the substrates. 
         [0034]    In an embodiment the feed network can be distributed over a stack of substrates attached to the three top substrates. 
         [0035]    The geometry of the electrodes of each element can be different from element to element. The preferred phased array antenna is a patch antenna, also called a microstrip antenna or a microstrip patch antenna. In a preferred embodiment the opening on the ground electrode underlies the radiating element. 
         [0036]    Preferable the radiating element and the opening on the ground electrode are centered. 
         [0037]    The planar transmission line integrated on the ground electrode comprises microstrip line, coplanar waveguide, slotline and/or stripline. 
         [0038]    The variable dielectric substrate can be a liquid variable dielectric substrate, preferable a liquid crystal material and/or a solid dielectric material as barium strontium titanate. This means that the substrate layer can be a combination of both materials. 
         [0039]    The liquid tunable substrate may be doped with compounds like carbon nanotubes, ferroelectric or metallic nanocomponents. 
         [0040]    The bottom side of the front dielectric and/or the top side of the back dielectric can be coated fully or locally with an alignment layer in order to pre-orient the liquid variable dielectric material. 
         [0041]    The electrically conductive layer on the top of the back dielectric substrate is preferred a planar transmission line which is an electronically tunable phase shifter. 
         [0042]    The electronically tunable phase shifter may be electromagnetically coupled to the radiating elements. 
         [0043]    In an embodiment the contactless RF interconnection utilizes the electromagnetic coupling of the RF signal between identical or different transmission lines which are mounted on different layers. 
         [0044]    The electrically conductive layer can comprise high conductive electrodes including Gold and Copper. 
         [0045]    The transmission line in a preferred embodiment is a microstrip line. The microstrip line is preferable meandered regularly or irregularly and especially the microstrip line is in spiral shape. 
         [0046]    In an embodiment the dielectric constant of the variable dielectric substrate and therefore the electrical characteristics of the phase shifter are changed by applying a voltage across the planar transmission line and the ground electrode through a bias line in order to achieve a desired differential phase shift between the radiating elements for beam steering. 
         [0047]    The bias line can comprise electrically low conductive electrode material including indium tin oxide or chromium or nickel-chromium alloy. 
         [0048]    In an embodiment in addition a thin film transistor circuit is implemented on the upper side of the back substrate. 
         [0049]    The electronically tunable phase shifter can include loaded line phase shifters, wherein the planar transmission line is loaded periodically or non-periodically by the varactors, whereas the varactors can be loaded shunt or serial to the planar transmission line. Also here the planar transmission line can comprise microstrip line, coplanar waveguide, slotline and/or stripline. The dielectric constant of the variable dielectric substrate and therefore the load of the varactor can be changed by applying a bias voltage trough an electrically low conductive bias line in order to control the electrical characteristics of the loaded line phase shifter for beam forming. 
         [0050]    In a preferred embodiment the radiating elements can be grouped in order to form a sub-array. In this case the radiating elements in the sub-array can be fed through a common electrically tunable phase shifter. Especially the sub-array comprises 2×2 radiating elements. 
         [0051]    In an embodiment the antenna has a two stacked dielectric substrates having electrically conductive layers on the bottom sides instead of the front dielectric substrate wherein the solid dielectric substrates can comprise thin substrates including Kapton Folio, liquid crystal polymer and Mylar Folio. The radiating elements can be mounted on the bottom side of the thin dielectric substrate. The ground electrode with openings and a planar transmission line can be mounted on the bottom side of the second dielectric substrate. 
         [0052]    In another embodiment the antenna comprises an electrically conductive layer on the bottom side of the back dielectric substrate; a low noise amplifier (LNA) and/or a transmit/receive module (TRM) placed on the bottom side of the back dielectric substrate, wherein the radiating elements can be grouped and utilize a common LNA. The LNA can be placed either between or after the radiating element and the phase shifter. 
         [0053]    For the operation of the inverted microstrip line (IMSL) phase shifter (delay line), the LC material underlying the phase shifter electrodes  111  is required. This is the minimum requirement. In the preferred embodiment LC is filled in between two glass substrates. This works as well but it is not necessary. Wells or pools in which LC is filled are sufficient. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0054]      FIG. 1  is a block diagram of an example of a two dimensional, electronically steerable phased array antenna according to the present invention; 
           [0055]      FIGS. 2   a  and  2   b  are exploded and side views of a unit element of the electronically steerable antenna according to an embodiment of the present invention; 
           [0056]      FIG. 3  is a schematic representation of a layout of a spiral shape phase shifter; 
           [0057]      FIGS. 4   a ,  4   b  and  4   c  are schematic representations of three layouts of the steerable phased array antenna according to the embodiment of the present invention given in  FIG. 2 ; 
           [0058]      FIGS. 5   a ,  5   b  and  5   c  are photos of a realized phased array antenna according to the embodiment of the present invention given in  FIG. 4 ; 
           [0059]      FIGS. 6   a ,  6   b  and  6   c  are schematic representations of three layouts of the steerable phased array antenna according to another embodiment of the present invention; 
           [0060]      FIGS. 7   a  and  7   b  are side views of a unit element and a unit sub-array element of an active phased array antenna according to another embodiment of the present invention; 
           [0061]      FIG. 8 : Simulated Δφ b  and FoM of the meander and spiral phase shifters without a cpw to microstrip line transitions. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0062]    In the following, a detailed description is given according to one possible embodiment of the present invention. The embodiment is not dedicated to present every features of the invention instead it provides a basic understanding of some aspects of the invention. It is a two-dimensional steerable antenna which can be used either in receiving or transmitting mode since it is a passive and reciprocal antenna. However, most of the description is given only for a receiving antenna in order to explain the invention in a clear way. The illustrations and relative dimensions may not necessarily be scaled in order to illustrate the invention more efficiently. 
         [0063]    Referring to the drawings,  FIG. 1  is a block diagram of an electronically steerable phased array antenna  100  according to the present invention. The phased array antenna includes signal input port  101  for example a RF signal input port, feeding network  102 , plurality of power combiners  103 - 109 , plurality of DC block structures  110 , plurality of electronically tunable phase shifters  111  and plurality of radiating elements  112 . 
         [0064]    In another embodiment (not shown) the feeding network is on another substrate. 
         [0065]    The feeding network  102  may include plurality of transmission lines with different electrical length and characteristic impedance in order to provide the impedance matching between the radiating elements  112  and input port  101 . The power combiners  103 - 109  may combine the power equally or unequally and deliver it to antenna unit element  200  for a desired radiation pattern. According to the antenna theory the distance between the radiating elements  112  is about 0.5 to 0.8 times of the wavelength in vacuum. A lower distance results in high electromagnetic coupling between the elements and a higher distance leads to a grating lobes in the radiation pattern. 
         [0066]      FIGS. 2   a  and  2   b  show exploded and side views of a unit element  200  of the electronically steerable antenna according to an embodiment of the present invention. The unit element  200  includes a radiating element  112 , a tunable phase shifter  111 , a DC blocking structure  110  and a bias line  201  in order to apply a bias voltage to the electronically tunable phase shifter  111 . These components are placed on three dielectric layers, namely front dielectric substrate  202 , tunable dielectric substrate  205  and back dielectric substrate  206 . 
         [0067]    A radiating element  112  is mounted on the top side of a low loss, front dielectric substrate  202 . 
         [0068]    As shown here, the radiating element  112  may be a rectangular patch antenna which can be used for different polarizations. In other embodiments the radiating element  112  is a circular, a square patch or any other kind of patch with a slot. A rectangular or square patch can also be cut from one or more corners. It is made of an electrically high conductive electrode. The bottom side of the front dielectric substrate  202  is covered with electrically conductive electrode which forms a ground electrode  203  for the radiating element  112 . The ground electrode  203  includes a slot  204  overlying the antenna element  112 . An aperture coupling is formed via the slot  204  in order to couple the RF signal between the radiating element  112  and the phase shifter  111 . The ground electrode  203  also includes a coplanar waveguide (CPW) which is a part of the DC blocking structure  110 . 
         [0069]    The preferred embodiment the signal is coupled between the different transmission lines. In another embodiment the signal is coupled capacitively. This means there are two patches, whereas one is mounted on the bottom side of the front dielectric substrate and the other is placed on the top side of the back dielectric substrate, like a parallel plate capacitor. 
         [0070]    A tunable dielectric substrate  205  is encapsulated between the front dielectric substrate  202  and a back dielectric substrate  206 . A cavity between these two dielectrics  202 ,  206  is required when the tunable dielectric substrate  205  is liquid. Such a cavity can be accomplished by using appropriate spacers. The mechanical stability of the front and back dielectrics  202 ,  206  is significant in order to maintain a uniform cavity height. The cavity height can be in the range of a 1 μm . . . 3 μm to several hundred milli-meters depending on the phase shifter topology. For a microstrip line based phase shifters a higher cavity height corresponds to a higher dielectric thickness and therefore the metallic losses are reduced. However, when a liquid crystal material is utilized, the device response time will be relatively longer due to a thick LC layer. On the other hand, the LC cavity height can reduced to 1 μm . . . 50 μm when a loaded line phase shifter is used. In the embodiment of the invention IMSL phase shifter is used. As a compromise between the metallic loss and phase shifter response time a cavity height of about 100 μm is preferred. However, the height can be reduced or increased according to the aforementioned range. If the height is reduced it lets to an increase of the metallic loss, if it is decreased it lets to a reduction of the metallic loss. 
         [0071]    In operation of a unit element  200 , the RF signal received by the radiating element  112  is coupled to the microstrip line  111 , via the aperture coupling which is formed by a slot  204  on the ground electrode  203 . The dielectric properties of the variable dielectric substrate  205 , and therefore the phase of the RF signal can be changed by applying a bias voltage across the ground electrode  203  and microstrip line  111  through a bias line  201 . The bias line  201  is an electrically low conductive electrode, compared to the electrode of the phase shifter  111 . The signal is then electromagnetically coupled to the CPW on the ground electrode  203  which is mounted on the bottom side of the front dielectric substrate  202 . After propagating along a short CPW line, the RF signal is coupled to the unit element input port  207 . By this way, a contactless RF interconnection as a DC blocking structure  110  is achieved between the phase shifter  111  and the unit element input port  207 . The variable dielectric substrate  205  is tuned only underneath the microstrip line  111  because the biasing voltage can not affect the rest of the antenna, i.e. other unit elements, due to the DC blocking  110 . 
         [0072]    In operation of a unit element  200  for a transmitting mode, the transmitting signal received from the array feed network is first electromagnetically coupled from the unit element input port  207  to the CPW on the ground electrode  203 . After propagating along a short CPW line, the signal is coupled to the microstrip phase shifter  111 . By this way, a contactless RF interconnection as a DC blocking structure  110  is achieved between the phase shifter  111  and the unit element input port  207 . The dielectric properties of the variable dielectric substrate  205 , and therefore the phase of the transmitted signal can be changed by applying a bias voltage across the ground electrode  203  and microstrip phase shifter  111  through a bias line  201 . The bias line  201  is an electrically low conductive electrode, compared to the electrode of the phase shifter  111 . After propagating along the microstrip line  111 , the signal is coupled to the radiating element  112  by which it is radiated. The coupling between the phase shifter  111  and the radiating element  112  is accomplished via the aperture coupling which is formed by a slot  204  on the ground electrode  203 . 
         [0073]    The DC blocking structure  110  utilizes the electromagnetic coupling between the similar or different transmission lines mounted on the different layers. It has to be mentioned that the coupling between CPW and microstrip line according to the embodiment is an example of one of the aspects of the present invention. Such a structure can also be optimized so that it may work as a RF filter. The challenge is to suppress the undesired radiation which can affect the antenna radiation characteristic and this can be solved by using an electromagnetic solver. 
         [0074]    Electrically tunable phase shifter  111  is fabricated in, but not limited to, inverted microstrip line topology. A microstrip line  111 , preferably in spiral shape, is mounted on the top of the back dielectric substrate  206 . Its ground electrode  203  is mounted on the bottom side of the front dielectric substrate  202 . The electrical properties of such a transmission line can be changed since its dielectric material is a tunable dielectric substrate  205 . 
         [0075]    Liquid crystal (LC) material can be used as a tunable dielectric substrate  205  at micro- and milli-meter wave frequencies. LC is an anisotropic material with low dielectric losses at these frequencies. Effective dielectric constant of LC for RF field depends on the orientation of the molecules. This property can be exploited to control the wavelength, and thus the phase of an electromagnetic wave, by changing the orientation of LC. The orientation of the molecules can be varied continuously by using an external electric or magnetic field, using a surface alignment of liquid crystal or a combination of these methods. 
         [0076]    In another embodiment (not shown) the antenna might consist of a stack of more layers, including more than one LC layer substrates which are separated with at least one layer of solid substrates. 
         [0077]    A tunable phase shifter having a differential phase shift of 360° has to be designed in a limited area which is the area of one unit element. The maximum achievable phase shift is frequency dependent and requirements can be adjusted by setting the length of the phase shifter. Due to the limited area, the phase shifter has to be meandered in order to achieve a desired length. Meantime, the coupling between the transmission lines has to be prevented. According to the present invention, the phase shifter is implemented in spiral shape as shown in  FIG. 3 . Such a phase shifter has 5 to 15% more differential phase shift compared to a meander transmission line, when identical design rules are used and when it is integrated to a radiating element. Additionally, due to the spiral shape, the coupling of RF signal between the phase shifter and the radiating element is accomplished in the centre of the unit element. When the phase shifter  111  is flipped along the axis  301 , the unit element input port  207  shifts to the other side whereas the coupling point  302  is still in the centre. This allows flipping the phase shifters in order to design a compact feeding network. Simultaneously, the distance between the radiating elements is kept constant which is crucial for the antenna radiation characteristic. The shape of the phase shifter is not limited to the spiral shape. Its shape can be optimized in order to design compact, high performance phase shifters which can be integrated the antenna array. 
         [0078]    According to another aspects of the invention, loaded line phase shifters can be integrated to the antenna array. Within this approach, a non-tunable transmission line is loaded periodically or non-periodically with varactor loads. The varactors can be loaded either serial or shunt to the transmission line. 
         [0079]      FIG. 4  illustrates three layouts of a two dimensional, electronically steerable phased array antenna according to the embodiment of the present invention given in  FIG. 2 . The antenna includes, but not limited to, 16 (4×4) radiating elements  112  which are mounted on the top of the front dielectric  202 . 
         [0080]    The bottom side of the front dielectric substrate  202  is covered with ground electrode  203  which includes the CPW line segments  110  and the slots  204  for DC blocking structure and aperture coupling, respectively. 
         [0081]    The RF signal input port  101 , feeding network  102 , plurality of power combiners  103 , plurality of electronically tunable phase shifters  111 , plurality of bias lines  201  and plurality of biasing patches  402  are placed on the top side of the back dielectric substrate  206 . A tunable dielectric which is not shown here is in contact with the ground electrode  203  and the top side of the back dielectric substrate  206 . The layers can be aligned accurately by using complementary alignment marks  401 . The back dielectric layer  206  is enlarged compared to the front dielectric layer  202  from the sides where contacts for RF input port  101  and biasing patches  402  are required. 
         [0082]      FIG. 5  illustrates the top, side and bottom view photos of a two dimensional, electronically steerable antenna prototype according to the embodiment of the present invention given in  FIG. 4 . 
         [0083]    The antenna includes four radiating elements. Overall height of the prototype is 1.5 mm including the front, tunable and back dielectric substrates. 
         [0084]      FIG. 6  illustrates a unit sub-array element of a phased array antenna according to another embodiment of the present invention. The unit sub-array element  700  includes, but not limited to, 2×2 radiating elements  112  on the top side of the front dielectric substrate  202 . The ground electrode  203 , slots  204  and the DC blocking structure  110  are mounted on the bottom side of the front dielectric substrate  202 . An electrically tunable phase shifter  111 , a power combiner  103  and a bias line  201  are fabricated on the top side of the back dielectric substrate  206 . A tunable dielectric which is not shown here is in contact with the ground electrode  203  and the top side of the back dielectric substrate  206 . 
         [0085]    In operation, the RF signal received by the radiating elements  112  is coupled to the power combiner  103  via the aperture coupling  204 . The power combiner  103  delivers the signal to the phase shifter  111  which surrounds the power combiner  103 . The electrical characteristics of the tunable dielectric substrate and therefore the phase of the RF signal are controlled by applying a bias voltage. 
         [0086]    Such a bias voltage is applied through the bias line  201  across the ground electrode  203  and the phase shifter  111 . The RF signal is then coupled the sub-array input port  207  via the DC blocking structure  110 . 
         [0087]    Required numbers of phase shifter and biasing lines are reduced by a factor of radiating element number in the sub-array architecture since all radiating elements are fed through one electronically tunable phase shifter. Similarly, an active phased array antenna requires less number of amplifiers. Due to that, the antenna becomes cost effective and reliable. Concerning the antenna radiation pattern, a differential phase shift between the radiating elements has to be satisfied in order to tilt the radiated phase front. In case of sub-array architecture, this requirement is accomplished for each sub-array. According to the antenna theory the distance between the sub-arrays is about 0.5 to 0.8 times of the wavelength in vacuum. 
         [0088]    This reduces the spacing between the radiating elements and therefore, the antenna aperture efficiency is increased. However, the mutual coupling between the radiating elements increases as well. For such an antenna, an optimization process is necessary between the antenna radiation characteristic and cost effectiveness, reliability and biasing complexity when defining sub-array architecture, i.e. radiating element number. 
         [0089]      FIGS. 7   a  and  7   b  illustrate the side views of a unit element and a unit sub-array element of an active phased array antenna according to another embodiment of the present invention. A low noise amplifier (LNA)  210  is mounted on the bottom side of the dielectric substrate  206 . The RF signal received by the radiating element  112  is coupled to a transmission line  211  which is located on the top side of the back dielectric substrate  206 . The signal is then coupled to a LNA  210  which is placed on the bottom side of the back dielectric substrate  206 . After amplifying, the RF signal is coupled to the tunable phase shifter  111  which has a tunable dielectric substrate  205 . By this way, the noise of the components affecting the antenna noise figure is suppressed and therefore antenna noise level is reduced. 
         [0090]    The invention has been described in details by means of embodiments. Any changes and modifications of the embodiments are limited by the scope of the following claims. 
         [0091]    The realization of an embodiment is explained here: 
         [0092]    Realization of a LC based inverted microstrip line (IMSL) phase shifter is shown in  FIG. 2 . A seed layer made of chromium/gold layer is evaporated on a low loss dielectric substrate. The chromium (Cr) layer has a thickness of 5 nm and is utilized as an adhesive layer between the substrate and the 60 nm thick gold layer. A photoresist (PR) is applied on the seed layer which is then exposed and developed. The electrodes of the structures are formed by electroplating of 2 μm thick gold. After the plating, the PR is removed and the seed layer is etched and therefore only the plated electrodes exist on the substrate. The substrate is diced precisely, i.e. ±5 μm, into two pieces. Each piece is coated with an alignment layer and rubbed mechanically in order to form grooves on the surface. The substrates are then aligned using alignment marks and bonded using glue. LC is filled between the substrates and therefore, appropriate spacers, i.e. micro pearls, are developed on the substrates after the rubbing. Finally, LC is filled and the structure is sealed by which the material is encapsulated between the two substrates. The mechanical stability of the substrates is significant in order to maintain a uniform cavity height. Hence, a low loss glass or ceramic dielectric substrate is preferred for the fabrication. 
       An Embodiment is Described Here 
       [0093]    A microstrip patch antenna is mounted on the top side of the front dielectric. The ground electrode of the patch antenna is mounted on the bottom side of the same dielectric. The ground electrode includes a slot overlying the patch ( FIG. 5   c ) which form an aperture coupling between the patch antenna and the phase shifter. The strip electrode of the IMSL phase shifter is mounted on the top side of the back substrate. The LC material is encapsulated between the two substrates. It forms the dielectric of the IMSL and has thickness of 100 μm. In operation of a receiving antenna, the received RF signal is first coupled to the phase shifter. After propagating along the phase shifter, the RF signal is electromagnetically coupled to a coplanar waveguide (cpw) which is located on the ground electrode. The signal propagates along a short cpw line, and then it is coupled to the unit element input port which is placed on the top side of the back dielectric. By this way, a contactless RF interconnection as a dc blocking structure is accomplished between the phase shifter and the unit element input port. 
       More Detailed Information about Further Embodiments are 
       [0094]    The unit element is integrated with a LC based tunable phase shifter. The phase shifter has to satisfy a desired differential phase shift Δφ b , i.e. 360°, for an optimum beam steering. The differential phase shift of the IMSL is calculated as 
         [0000]    
       
         
           
             
               Δφ 
               b 
             
             = 
             
               
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   fl 
                 
                 
                   c 
                   0 
                 
               
                
               
                 ( 
                 
                   
                     
                       
                         ɛ 
                         
                           r 
                           , 
                           eff 
                         
                       
                       , 
                       
                          
                          
                       
                     
                   
                   - 
                   
                     
                       
                         ɛ 
                         
                           r 
                           , 
                           eff 
                         
                       
                       , 
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                 ) 
               
             
           
         
       
     
         [0000]    Whereas, f is frequency, I is physical length, c 0  is the speed of the light in vacuum, ∈ r,eff,⊥  relative effective perpendicular permittivity, ∈ r,eff,∥  relative effective parallel permittivity. 
         [0095]    The length of a phase shifter operating at 18 GHz with a Δφ b  of 360° is determined as 5.65λ 0  using a specific type of LC. On the other hand, the size of the unit element is set to be 0.65λ 0 ×0.65λ 0  in order to prevent grating lobes. Hence, the phase shifter has to be designed in a compact way due to the limited area of the unit element. One possible solution is to meander the phase shifter. In this case, however, the coupling between the lines becomes an issue. It can be minimized within the simulation by optimizing the gap between the lines. The total length of the phase shifter is 75 mm and the phase shifter itself (without the transitions) utilizes an area of 0.5λ 0 ×0.5λ 0  at 18 GHz. This area is less than the area of the unit element. This is due to the fact that when the unit elements are combined in order to form an array, the RF feed network and the bias network require certain amount of area as well. 
         [0096]    The performance and the compactness of the phase shifter can be improved further depending on its geometry. For this manner, the geometry, in which the microstrip line is meandered, is significant. One possible solution is to meander the phase shifter in spiral geometry. Such a phase shifter has several improvements compared to the meander line phase shifter. Both phase shifters are designed on the same size of area using the identical design rules, i.e. identical gap size between two electrodes. In  FIG. 8 , simulated Δφ b  and FoM results of the phase shifters are given. 
         [0097]    As can be seen from  FIG. 8 , the Δφ b  of the spiral phase shifter is 5% to 15% more compared to that of the meander phase shifter. Meantime, the insertion loss is kept almost constant and therefore the FoM is increased, for instance, from 95°/dB to 105°/dB at 18 GHz. Additionally, due to the spiral geometry, the coupling of the RF signal between the phase shifter and the radiating element is accomplished in the centre of the unit element. When the phase shifter geometry is flipped, the unit element input port shifts to the other side whereas the coupling point is still in the centre. This allows flipping the phase shifters in order to design a compact RF feed network. Simultaneously, the distance between the radiating elements is kept constant which is crucial for the antenna radiation characteristic. 
         [0098]    The antenna array requires a bias network in order to tune the phase shifters independently. The voltage applied across the bias pads and the ground electrode are delivered to the RF circuitry through the bias lines. The bias lines have to be implemented using a low electrically conductive material and therefore they have negligible impact on the RF signal. Possible materials are indium tin oxide (ITO), chromium (Cr) or nickel-chromium (Ni—Cr). Although having relatively high conductivity (σ=7.8×106 S/m), the Cr adhesive layer is utilized for implementing the bias lines. It has a thickness of 5 nm which results in a sheet resistance of 25:3=sq. The line width is set to be 10 μm in order to increase the bias line resistance. 
         [0099]    The 2D-antenna can also be 3D in structure, e.g. it can be wrapped around an object. 
       DESCRIPTION OF THE REFERENCE NUMBERS 
       [0100]      FIG. 1 : Block diagram of an example of a two dimensional, electronically steerable phased array antenna according to the present invention 
         [0101]      FIGS. 2   a  and  2   b : Exploded and side views of a unit element of the electronically steerable antenna according to an embodiment of the present invention 
         [0102]      FIG. 3 : Schematic representation of a layout of a spiral shape phase shifter 
         [0103]      FIGS. 4   a ,  4   b  and  4   c : Schematic representations of three layouts of the steerable phased array antenna according to the embodiment of the present invention given in  FIG. 2   
         [0104]      FIGS. 5   a ,  5   b  and  5   c : Photos of a realized phased array antenna according to the embodiment of the present invention given in  FIG. 4   
         [0105]      FIGS. 6   a ,  6   b  and  6   c : Schematic representations of three layouts of the steerable phased array antenna according to another embodiment of the present invention 
         [0106]      FIGS. 7   a  and  7   b  Side views of a unit element and a unit sub-array element of an active phased array antenna according to another embodiment of the present invention 
         [0107]      FIG. 8 : Simulated Δφ b  and FoM of the meander and spiral phase shifters without a cpw to microstrip line transitions.
     100  Electronically steerable phased array antenna     101  Signal input port     102  feeding network     103 - 109  power combiners     110  DC blocking structure     111  phase shifters electrodes     112  radiating element     200  Antenna unit element     201  bias line     202  front dielectric substrate     203  ground electrode     204  slot/aperture coupling     205  tunable dielectric substrate     206  back dielectric substrate     207  unit element input port     210  low noise amplifier (LNA)     211  Transmission line     301  flip axis     302  coupling point     401  alignment marks     402  biasing patch
         700  unit sub-array element