Abstract:
The present invention relates an antenna arrangement ( 23, 330, 430, 530, 630 ) comprising a fit layer ( 331, 431, 531, 631 ) consisting of a dielectric material and a second reflective layer ( 335, 435, 535, 640,  The dielectric material has variable dielectric characteristics. An electromagnetic radiation ( 50 ) passing through said first layer ( 331, 431, 531, 631 ) and at least partly reflected by said second layer ( 335, 435, 535, 640 ) is modulated by varying said variable dielectric characteristics of said first layer.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to a reflective antenna arrangement, specially an antenna arrangement comprising a first layer consisting of a dielectric material and a second reflective  
           [0002]    The invention also concerns a communication arrangement comprising said antenna arrangement.  
         BACKGROUND OF THE INVENTION  
         [0003]    In U.S. Pat. No. 4,353,069, an absorptive coating for reduction of the reflective cross section of a metallic surface is provided which includes a layer of “IN” doped material adjacent the metal surface, the N doped material having a characteristic of increasing semiconductor conductivity from the outboard surface junction of the material to the boundary of the metallic surface, a second layer of “P” doped material having a characteristic of increasing semiconductor conductivity from its outboard surface boundary to its junction with the N doped material inboard of it. In the preferred embodiment, a third layer of P material is placed outboard of the second layer. The first and second layers further have electrical connections operatively associated with them, so that an applied voltage may be utilized to vary the electrical characteristics of the coating. The invention uses semiconductor p-n junction to control its resistivity under the applied electric signal (e.g. DC bias).  
           [0004]    Disadvantages with the purposed solution are:  
           [0005]    For large areas of the antenna the bias currents may be intolerably high  
           [0006]    The thickness required for achievement of substantial absorption way be too large (1 inch) for practical applications, especially in onboard and/or large antennas  
           [0007]    The speed of changing the resistivity of the absorption layer is extremely small due to the extremely small thickness of the p-n junction and hence its extremely large capacitance.  
           [0008]    The antenna is useful for controllable absorption only. The reflected power levels are small.  
           [0009]    In German Patent Application No. DE 43 32 042, an interference type electrically controllable reflector antenna is disclosed, which is based on an electrochemical cell with transparent conducting plates serving as mirrors for Fabry-Perot resonators This antenna is of reflective type, where the reflection coefficient is enhances due to the interferences in the Fabry-Perot etalon, where the distance between the mirrors is chosen to be λg/4 at operation frequency, f. λg is the wavelength in the Fabry-Perot etalon:  
           λ                 g     =       (       c   0          ɛ       )     f       ,                         
 
           [0010]    where ∈ is the dielectric constant inside the etalon  
           [0011]    Disadvantages with the proposed solution are:  
           [0012]    The antenna is an absorptive type device, where the resistivity, hence the reflection and transmission coefficients al controlled due to the electrical control of the resistivity inside the Fabry-Perot etalon (Resonator). Due to the inherent resonant name of the Fabry-Perot etalon this antenna is narrow band, and may produce control only the amplitude of the transmission or reflected electromagnetic waves.  
           [0013]    This device is inherently low speed due to the Red-Ox-Reaction used in the device for controlling the resistivity,  
           [0014]    The magnitude of the control (leakage) is large, especially for large area antennas.  
           [0015]    Other reflective anemias are known: EP 232 011 for example, discloses a Fonder, which receives signals from a reader, modulates them, and reflects them back to a reads to pass the information contained in the transponder to the reader. First conductive material is disposed on the first surface of the dielectric member at a first end of the member. Second conductive material on the second opposite surface of the dielectric member at the second end of the member defines a dipole with the first material. The second material is preferably triangular in configuration. An electrical circuitry on the dielectric member produces reflected signals modulated at a particular frequency from the signal transmitted by the reader to pass information contained in the transponder to the reader. The dipole is electrically coupled with the conductive material and enhances an impendance match between the dipole and electrical circuitry. The conductive material has a first low impendance portion split into two parts connected in parallel to provide an extended effective length in a relatively small distance, and has a second portion, preferably a “pigtail”, of substantially higher impendance than the first portion connected in series with the first portion. The first portion converts the antenna impendance to a low value and the second portion converts the low impendance to the impedance of the electrical circuitry module.  
           [0016]    A system for discovering objects of different kinds, for finding victims of avalanches, ship wreckage etc., for warning of risky actions et., consisting of a transmitter and a transponder is disclosed in WO 92/09906. A signal emitted from the transmitter is reflected by the transponder in the form of an overtone of the emitted signal. The transponder, which can be double-sided with a reflector in-between, consists of an antenna with one or several semiconductors, a reflector and an intervening dielectricum dimensioned to give the reflected output signal maximum strength. The dielectricum can be m integral part of an article of clothing or an object  
         SUMMARY OF THE INVENTION  
         [0017]    The main object of the present invention is to provide a reflector antenna, preferably an controllable reflector antenna, which:  
           [0018]    is based on low loss dielectric materials providing not only magnitude, but also phase/polarization control of the reflected signals with minimum absorption of the electromagnetic waves (i.e. low loss reflective antenna),  
           [0019]    has small switching (control) time, which allows high-speed modulation of the reflected power, and hence possibility to provide a useful signal (information) on top of the reflected signals,  
           [0020]    due to the good dielectric properties the control (leakage) currents and powers are small, which is desired for remote antennas, and antennas operating without maintenance and power supply for longer period of time.  
           [0021]    Yet another object of the invention is to provide a communication arrangement employing an antenna according to the invention.  
           [0022]    For these reasons, in the initially mentioned antenna arrangement said dielectric material has a variable dielectric constant. An electromagnetic radiation passing through said first layer and at least partly reflected by said second layer is modulated by varying said variable dialectic constant of said first layer.  
           [0023]    According to one aspect of the invention the antenna arrangement further comprises a fist electrode layer, a second electrode layer, a third layer and a third electrode layer. Said first layer is a plate made of an electrically tile dielectric material. The plate consists of one of ferroelectrics, ceramics, polymers or crystallines. The first and second electrode layers are made of a material transparent to said electromagnetic radiation, allowing the radiation to pass towards the second layer. In one embodiment, the first and second electrode layers are arranged on opposite sides of said first layer. In another embodiment, the first and second electrode layers are arranged inside said first layer. Thus, a modulation signal is applied to said first and second electrode layers to changes said variable dielectric characteristics of said first layer. According to one embodiment said second layer is a plate arranged as an electromagnetic radiation sensor. The second layer at one side is provided with said second layer being a non-transparent electrode layer and at an opposite side with said th electrode layer being a parent electrode layer. The second layer has a larger thickness than said first and second electrode layers. Moreover, the third layer consists of a semiconductor plate arranged with an Schottky barrier. Thus, said third layer is abed to transform said incident electromagnetic radiation into low frequency or DC electric signals, which are extracted from said second layer and third electrode layer. According to one said first electrode layer consists of conducive strips, which reduces the capacitive coupling between the electrode layers. It is also possible to arrange said first and second electrode layers consisting of grids of elegizes comprising thin wire electrodes imbedded in said first dielectric layer, which offers reduced voltage of the modulation signal, and smaller capacitance between electrodes.  
           [0024]    According to another aspect of the invention, said first layer is a dielectric plate mechanically attached to said second layer consisting of a metallic layer. The plate is sensitive to temperature and/or mechanical pressure. It is possible to allow temperature variations vary said dielectric characteristics of said plate. Change of said dielectric characteristics is exerted though mechanical actuation. Applying alternating forces on said plate or a frontal plate in communication with said plate could also produce the mechanical tension.  
           [0025]    In one preferred embodiment the antenna arrangement comprises a frontal layer, which is arranged to couple electromagnetic radiation into and out of said first layer. The frontal plate has a thickness of  
         λ     4          ɛ   2           ,                         
 
           [0026]    where ∈ 2 ={square root}{square root over (∈ 1 )} is the dielectric constant of a said second ( 332 ,  432 ,  532 ), and ∈ 1  is the dielectric constant of said first layer.  
           [0027]    The invention also relates to a communication arrangement for receiving, modulating and transmitting electromagnetic radiation. The arrangement comprises a communication module, a transmitter/transreceiver, and a receiver, said communication module comprising an antenna arrangement comprising a fast layer consisting of a dielectric material and a second reflective layer. The dielectric material has a variable dielectric characteristics and an electromagnetic radiation passing through said first layer and at least partly reflected by said second layer is modulated by varying said variable dielectric characteristics of said first layer due to output signals from said electric module. The communications module mainly comprises an electronic module, a microwave senor, said antenna arrangement and a power supply unit. The electrical unit is arranged to generate low frequency modulation signals. The microwave sensor transforms an incoming electromagnetic radiation signal into low frequency or DC electric signals and transmits the signals to the electronic module.  
           [0028]    The invention also relates to a method of modulating an incident electromagnetic radiation in an antenna arrangement comprising a first layer consisting of a dielectric material and a second reflective layer. The method comprises the steps of arranging said dielectric material with a variable dielectric characteristics and modulating said electromagnetic radiation passing through said first layer and at least partly being reflected by said secondly layer by varying said variable dielectric characteristics of said first layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    In the following, the invention will be further described in a non-limiting way with reference to the accompanying drawings, in which:  
         [0030]    [0030]FIG. 1 is a block diagram of a communication arrangement according to the invention,  
         [0031]    [0031]FIG. 2 illustrates a more detailed block diagram of the communication arrangement of FIG. 1,  
         [0032]    [0032]FIG. 3 is a cross-section though a section of an antenna arrangement, according to a first embodiment of the invention,  
         [0033]    [0033]FIG. 4 a  is a cross-section through a section of an antenna arrangement, according to a second embodiment of the invention,  
         [0034]    [0034]FIG. 4 b  is a frontal cross-section through a section of the antenna arrangement, according to FIG. 4 a,    
         [0035]    [0035]FIG. 5 a  is a cross-section through a part of a section of an antenna arrangement, according to a third embodiment of the invention,  
         [0036]    [0036]FIG. 5 b  is a frontal cross-section through a part of the antenna arrangement, according to FIG. 5 a,  and  
         [0037]    [0037]FIG. 6 is a cross-section through a section of an antenna arrangement, according to a fourth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0038]    A general concept of a communications system  10 , employing an arrangement according to the invention is schematically illustrated in FIG. 1. The communications system comprises a communication module  20 , a transmitter/transceiver  30 , and a receiver  40 . A microwave carrier  50  or other electromagnetic radiation is transmitted from the (powerful) transmitter/receiver  30 . The microwave carrier can be modulated or not modulated (Continues Wave, CW). A remote antenna unit (not shown) is in communication with the communications module  20 . The antenna and the communication module do not contain microwave sources. Instead it uses the incident microwave power, modulates it, and reflects the microwave back to the original transmitter/receiver module or to another receiver module(s). The reflected microwave  51  can be amplitude and/or phase modulated.  
         [0039]    The communications module  20 , as shown in FIG. 2, mainly comprises an electronic module  21 , a microwave sensor  22 , an antenna  23  and a power supply unit  24 . The electrical unit  21  contains sensors, memory etc., and it is arranged to generate low frequency modulation signals. The microwave sensor  22  transforms incoming microwave signals  50  into low frequency or DC electric signals and transmits the signals to the electronic module  21 . The power supply  24  can be an along life battery for a remote module or any other suitable conventional power supply.  
         [0040]    The key component of the arrangement, according to the invention, is the antenna. It allows modulation and reflection of the incident microwave power. It has high reflection and consumes small DC power to modulate the reflected microwave signals.  
         [0041]    One embodiment of an antenna  330  according to the invention is illustrated in FIG. 3. The antenna  330  consist of a first layer  331 , a second layer  332 , a first electrode layer  333 , a second electrode layer  334 , a third electrode layer  335 , a fourth electrode layer  336  and a frontal layer  337 .  
         [0042]    The first layer is a plate  331  arranged as a modulating plate and made of an electrically tunable dielectric material, such as ferroelectrics, ceramic, polymer or crystalline, e.g. BaTiO 3 , The dielectric constant of this material is alterable (controlled) by an applied modulation sign, generated in the electronic module  21 . The first and second electrode layers  333  and  334 , respectively, are made of a material transparent to the microwave signals e.g. conductive, semiconducting or metal layers, with a thickness  
         δ   ≈     1         2      π                 f                 σ                        ,                         
 
         [0043]    where ƒ is the electromagnetic radiation frequency, σ is the conductivity constant of the layer.  
         [0044]    The modulation signal from the electronic module  21  is applied to terminals  338 . The second layer is a plate  332  arranged as a microwave sensor. It is provided with a thick and to microwaves non-transparent electrode layer (fourth layer)  336  and at transparent electrode layer (third layer)  335 . The third electrode layer may consist of, e.g. metal or other conductive material. The thickness, i.e. the level of non-transparency, of the third electrode layer  335  is larger than the thickness of the electrode layer  333  and  334  and it reflects most of the microwave power. Only a small portion of the power is transmitted through the electrode layer into the second layer  332 . The second layer  332 , which can consist of, e.g. a semiconductor plate with a Schottky barrier, transforms the microwave signals into low frequency or DC electric signals, which is extracted from terminals  339  corrected to electrodes  335  and  336 , and applied to the electronic module  21 . Upon appearance of an incident microwave power, the generated signals activate the electronic module  21 , which generates modulation signals, i.e. useful signals that are saved and/or generated in the electronic module to be used for modulation of signals S 1  transmitted back. These signals are applied to the terminals  338  connected to electrodes  333  and  334 , resulting in the modulation of the dielectric constant in the plate  331 . The modulation of the dialectic constant in plate  331  changes (modulates) the phase velocity of microwave signals. In other words, the reflected microwave signal is phase (and/or amplitude) modulated according to the information to be transmitted  
         [0045]    The additional frontal layer  337  is a plate used for more efficient coupling of microwave signals in and out of the plate  331 . The thickness of the plate is  
         λ     4          ɛ   2           ,                         
 
         [0046]    where ∈ 2 ={square root}{square root over (∈ 1 )} is the dielectric constant of the plate  332 , and ∈ 1  is the dielectric constant of the plate  331 .  
         [0047]    An alternate embodiment of an antenna  430 , according to the invention, is illustrated in FIGS. 4 a  and  4   b,  wherein FIG. 4 a  is a section through the antenna and FIG. 4 b  is a frontal view through layer  437 . The similar reference signs refer to some structural details as in FIG. 3. In this embodiment, the first electrode layer  433  consists of narrow conducive strips arranged to reduce the capacitance between the electrode layers  433  and  434 . Hence, the time constant τ=RC of the antenna is decreased leading to increased operation speeds. This design is preferred for high-speed operation of the antenna.  
         [0048]    [0048]FIGS. 5 a  and  5   b  show a further modification of the antenna, denoted  530 . FIG. 5 a  is section through the antenna and FIG. 5 b  is a frontal view along line through layer  537 . The first and second electrode layers consist of grids of electrode layers  533   a - 533   c  and  534   a - 534   c  comprising thin wire electrodes imbedded in the dielectric layer  531 . This design offers reduced voltage of the modulation signal, and smaller capacitance between electrodes  533  and  534 , which results in high operation speed. FIG. 5 b  illustrates the electrode configuration in one of the electrode layers. Number of such electrode layers can be more than two. FIG. 5 a  shows an antenna  530  having three electrode layers.  
         [0049]    Yet another embodiment of an antenna  630  is illustrated in FIG. 6, which corresponds to a very simple design of the antenna. In this embodiment no electronic or electrical components are used in the system. The antenna  630  comprises a dielectric plate  631 . It is mechanically attached to a metallic layer  640 . The plate  631  is sensitive to temperature, mechanical pressure (e.g. ferroelectics) or other mechanical actuations etc. Changes in the temperature, for example, will result in change in the dielectric constant of the plate  631 . Additional change can be exerted, e.g. by means of mechanical tension, which appears due to the difference in thermal expansion coefficients of plate  631  and metal  640 . The mechanical tension may also be produced by applying alternating forces  641  and/or  642  on the plates  631  or  637 , respectively. The plate  637  is a coupling transformer, as in the previous cases. Microwave signals entered in the plate  631  will be phase modulated in accordance with the changes of the dielectric constant experienced by the plate  631  due to the changes in the temperature or mechanical pressure. Modulated microwave signals will then be reflected from the metallic plate  640  and transmitted back, carrying the modulated information.  
         [0050]    The position of the reflecting layer is not limited to one face of the dielectric layer; it can also be placed inside the dielectric layer.  
         [0051]    The antenna and the communication system according to the invention are particularly suitable in applications in which the system can operate without any or a special power source. Such applications may include:  
         [0052]    Wireless computer networks in which the antenna is arranged as a part of the network transceiver card inside (or in communication with) the computer,  
         [0053]    Part of base station transceiver in communication networks (cellular/non-cellular),  
         [0054]    Antenna arrangement in a mobile station,  
         [0055]    Passive communication arrangements e.g. for railroads, arranged in the railroad tracks,  
         [0056]    Passive transponder for tracking objects,  
         [0057]    Etc.  
         [0058]    Especially in a wireless communication system, in which a base station is arranged to transmit with a power, the communication arrangement according to the invention can be a part of the mobile station. Consequently, the need for a power source for transmissions in the mobile station can be reduced or eliminated.  
         [0059]    The invention is not limited to the disclosed embodiments. It can be varied in a number of ways without departing from the scope of the appended claims, and the arrangement and the method can be implemented in various ways depending on application, functional units, needs and requirements etc.