Abstract:
An antenna includes a first electrical conductor that is shaped to form a spiral between its first and second ends that remain electrically unconnected such that the first electrical conductor so-shaped is maintained as an unconnected single-component open-circuit having inductance and capacitance. In the presence of a time-varying electromagnetic field, the first electrical conductor so-shaped resonates to generate a harmonic electromagnetic field response having a frequency, amplitude and bandwidth. A second electrical conductor includes a loop portion overlapping at least a portion of the spiral. The second electrical conductor is electrically isolated from the first electrical conductor. A radio frequency transceiver capable of transmitting and receiving electromagnetic energy is electrically coupled to the second electrical conductor.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION(S) 
       [0001]    This patent application claims the benefit of and priority to U.S. Provisional Application Ser. No. 61/895,066, filed on Oct. 24, 2013, the contents of which are hereby incorporated by reference in their entirety. In addition, this application is related to co-pending patent applications titled “MULTI-LAYER WIRELESS SENSOR CONSTRUCT FOR USE AT ELECTRICALLY-CONDUCTIVE MATERIAL SURFACES” and “PLASMA GENERATOR USING SPIRAL CONDUCTORS” filed on the same day and owned by the same assignee as this patent application. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The invention described herein was made in the performance of work under a NASA contract and by employees of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. §202, the contractor elected not to retain title. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Recently, a new class of wireless sensing systems have been developed that use open-circuit, electrically-conductive spiral trace sensors. Details of these sensors and sensing systems are described in U.S. Pat. No. 8,430,327. Briefly, the described wireless sensing system includes a sensor made from an electrical conductor shaped to form an open-circuit, electrically-conductive spiral trace having inductance and capacitance. In the presence of a time-varying magnetic field, the sensor resonates to generate a harmonic response having a frequency, amplitude and bandwidth. A magnetic field response recorder wirelessly transmits the time-varying magnetic field to the sensor and wirelessly detects the sensor&#39;s response. 
         [0004]    The above-described wireless sensing technology provides a new technical framework for designing, powering, and interrogating sensors. These unique sensors can detect physical changes in the environment or any material placed within the near field (i.e., millimeters to tens of centimeters) of the sensor. Detected changes are generally associated with a localized change in a material&#39;s permittivity, permeability, and/or conductivity. The material may be any state of matter, plasma, gas, liquid, or solid. Changes to a material&#39;s state cause disturbances in the wireless sensor&#39;s magnetic field that can be sensed by a magnetic field response recorder. Since the sensor&#39;s magnetic field is limited to the near field, the recorder&#39;s antenna must also be in the sensor&#39;s near field, thereby limiting the number of applications that can use this technology. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention is an antenna that includes a first electrical conductor having first and second ends. The first electrical conductor is shaped to form a spiral between its first and second ends that remain electrically unconnected such that the first electrical conductor so-shaped is maintained as an unconnected single-component open-circuit having inductance and capacitance. In the presence of a time-varying electromagnetic field, the first electrical conductor so-shaped resonates to generate a harmonic electromagnetic field response having a frequency, amplitude and bandwidth. A second electrical conductor includes a loop portion overlapping at least a portion of the spiral. The second electrical conductor is electrically isolated from the first electrical conductor. A radio frequency transceiver capable of transmitting and receiving electromagnetic energy is electrically coupled to the second electrical conductor. 
         [0006]    One embodiment of the invention further includes a third electrical conductor having first and second ends. The third electrical conductor is shaped to form a second spiral between its first and second ends that remain electrically unconnected such that the third electrical conductor so-shaped is maintained as an unconnected single-component open-circuit having inductance and capacitance. In the presence of a time-varying electromagnetic field, the third electrical conductor so-shaped resonates to generate a harmonic electromagnetic field response having a frequency, amplitude and bandwidth. The loop portion of the second electrical conductor is disposed between the spiral and the second spiral. 
         [0007]    Another embodiment of the invention is an antenna that includes a first electrical conductor having first and second ends. The first electrical conductor is shaped to form a spiral between its first and second ends. The spiral lies in a first plane. The first electrical conductor&#39;s first and second ends remain electrically unconnected such that the first electrical conductor so-shaped is maintained as an unconnected single-component open-circuit having inductance and capacitance. In the presence of a time-varying electromagnetic field, the first electrical conductor so-shaped resonates to generate a harmonic electromagnetic field response having a frequency, amplitude and bandwidth. A second electrical conductor includes a loop portion lying in a second plane that can be parallel to the first plane. The loop portion overlaps at least a portion of the spiral. The second electrical conductor is electrically isolated from the first electrical conductor. A radio frequency transceiver capable of transmitting and receiving electromagnetic energy is electrically coupled to the second electrical conductor. 
         [0008]    These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic view of an antenna having far field transceiving capabilities in accordance with an embodiment of the present invention; 
           [0010]      FIG. 2  is an isolated schematic view of an embodiment of an electrically-unconnected spiral conductor used in the antenna of the present invention; 
           [0011]      FIG. 3  is a schematic view of an antenna having elements that form a one-piece structure in accordance with another embodiment of the present invention; 
           [0012]      FIG. 4  is a plan view of the antenna&#39;s spiral conductor and loop portion in accordance with an embodiment of the present invention; 
           [0013]      FIG. 5  is a side view of the spiral conductor and loop portion taken along line  5 - 5  in  FIG. 4  illustrating the antenna&#39;s spiral conductor and loop portion arranged in parallel planes in accordance with an embodiment of the present invention; 
           [0014]      FIG. 6  is a plan view of the antenna&#39;s spiral conductor and loop portion with their geometric centers aligned in accordance with another embodiment of the present invention; 
           [0015]      FIG. 7  is a generalized graph of field impedance illustrating the far field propagation of time-varying electromagnetic energy; 
           [0016]      FIG. 8  is a schematic view of an antenna of the present invention paired with a wireless sensor to thereby increase the read range of the wireless sensor in accordance with another embodiment of the present invention; and 
           [0017]      FIG. 9  is a schematic view of an antenna of the present invention paired with a wireless sensor with elements thereof formed in a one-piece structure in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood, that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions, relative dimensions, and/or other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
         [0019]    Referring now to the drawings and more particularly to  FIG. 1 , an antenna having far field transceiving capabilities in accordance with an embodiment of the present invention is shown and is referenced generally by numeral  10 . In general, antenna  10  is capable of transmitting and receiving radio frequency energy in accordance with a far field energy pattern referenced by numeral  100 . The term “far field” as used herein refers to distances on the order of a meter to tens of meters. By way of an illustrative example and as will be explained further below, antenna  10  can be used to increase or boost the read range of wireless sensors of the type described in the above-describe mentioned U.S. Pat. No. 8,430,327, whose interrogation or reading has previously been limited to “near field” read ranges on the order of millimeters to tens of centimeters. 
         [0020]    Antenna  10  includes an electrically unconnected, open-circuit spiral conductor  12 , an electrically conducting loop  14  electrically isolated from spiral conductor  12 , and a radio frequency (RF) transceiver  16  electrically coupled/connected to conducting loop  14 . Transceiver  16  is any device/system capable of transmitting time-varying electromagnetic energy to loop  14  and measuring electromagnetic energy received by loop  14 . Such RP transceiver devices/systems are well understood in the art. 
         [0021]    Antenna  10  includes an electrically unconnected, open-circuit spiral conductor  12 . Spiral conductor  12  and its attributes are described in detail in U.S. Pat. No. 8,430,327, the entire contents of which are hereby incorporated by reference. Briefly, and with reference to  FIG. 2 , spiral conductor  12  is made from an electrically-conductive run or trace. More specifically, spiral conductor  12  is a spiral winding of conductive material with its ends  12 A and  12 B remaining open or unconnected. Accordingly, spiral conductor  12  is said to be an open-circuit. Techniques used to construct or deposit spiral conductor  12  on a substrate material can be any conventional metal-conductor deposition process to include thin-film fabrication techniques. En the illustrated embodiment, spiral conductor  12  is constructed to have a uniform trace width throughout (i.e., trace width W is constant) with uniform spacing (i.e., spacing d is constant) between adjacent portions of the spiral trace. However, it is to be understood spiral conductor  12  is not limited to a uniform-width conductor spirally wound with uniform spacing as illustrated in  FIG. 2 . 
         [0022]    Conducting loop  14  is essentially a loop formed by an insulated or uninsulated electrical conductor where the two ends  14 A and  14 B of the loop are electrically connected to transceiver  16 . Conducting loop  14  is electrically isolated from spiral conductor  12  by air or some other dielectric material. When loop  14  is excited by electromagnetic energy from transceiver  16 , the electromagnetic energy is coupled to spiral conductor  12  thereby exciting spiral conductor  12  into resonance to generate radiation pattern  100 . Structural factors affecting the frequency and power of radiation pattern  100  include the attributes of spiral conductor  12 , the attributes of loop conductor  14 , the input provided by transceiver  16 , and the physical relationship between spiral conductor  12  and loop  14 . These structural factors impact one or more of a number of electrical factors to include impedance, resonant frequency, VSWR (Voltage Standing Wave Ratio), efficiency, bandwidth, gain, radiation pattern, and polarization. Each of these electrical factors as they relate to the present invention will be discussed briefly below. 
         [0023]    Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength (i.e., inverse of frequency) in use. The impedance is most commonly adjusted at the antenna (i.e., spiral conductor  12  in the present invention) by means of changing the electrical length of spiral conductor  12 , the capacitance (gap width) of spiral conductor  12 , the inductance (trace width) of spiral conductor  12 , or combinations of such changes. The impedance of spiral conductor  12  can also be matched to the feed (i.e., loop  14  in the present invention) and the source (i.e., transceiver  16  in the present invention) by adjusting the impedance of loop  14  via changes in the diameter and circumference of loop  14  thereby essentially using loop  14  as an impedance transformer. The impedance may also be adjusted by varying the permittivity value and/or thickness of a dielectric (see  FIG. 3 ) between loop  14  and spiral conductor  12 . Finally, the impedance of transceiver  16  can be adjustable by electronic means. 
         [0024]    Resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Electrical resonance occurs at the fundamental resonant frequency when the total impedance of the system that contains the transceiving elements of antenna  10  matches the source impedance of transceiver  16 . At the fundamental resonant frequency, a standing wave is presented along spiral conductor  12 . The standing wave has current minimums and voltage maximums at the end-points of spiral conductor  12  and current maximum and voltage minimum approximately half-way between spiral conductor  12  and its end-points. The voltage minima are centered in the vicinity of the feed-point for loop  14 , thus presenting lower impedance than at other frequencies. Also, the large current and small voltage are in phase at that point resulting in a purely resistive impedance allowing for maximum energy transfer from and/or to transceiver  16 , whereas away from the design frequency the feed-point impedance rises and becomes reactive and impedes energy transfer. 
         [0025]    Standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at a maximum to the amplitude at an adjacent minimum along an electrical transmission path, The most common case for measuring and examining SWR is when installing and tuning antennas. When a transmitter is connected to an antenna by a feed line, the impedance of the antenna and feed line must match exactly for maximum energy transfer from the feed line to the antenna. The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio. In general, antenna  10  should have an impedance that is resistive and near the characteristic impedance of the transmission path from transceiver  16  to spiral conductor  12  in order to minimize the standing wave ratio (SWR) and the increase in transmission path losses it entails, in addition to supplying a good match at transceiver  16 . Accordingly, SWR is used as an efficiency measure for transmission paths that conduct radio frequency signals from transmitters and receivers to their antennas. 
         [0026]    The efficiency of an antenna relates the power delivered to the antenna and the power radiated or dissipated within the antenna. The power supplied to an antenna&#39;s terminals that is not radiated is converted into heat. This is usually due to loss resistance in the antenna&#39;s conductors, but can also be due to dielectric or magnetic core losses in antennas (or antenna systems) using such components. Such loss effectively robs power from the transmitter or receiver requiring a stronger transmitter in order to transmit a signal of a given strength or amplifiers to receive small signals. In terms&#39;of the present invention, loss resistance will generally affect the feedpoint impedance of loop  14  and any dielectric losses occurring between spiral conductor  12  and loop  14  adding to its resistive (real) component. The real resistance component consists of the sum of the radiation resistance from spiral conductor  12  and the loss resistance from loop  14  and any dielectric between spiral conductor  12  and loop  14 . 
         [0027]    Bandwidth describes the range of frequencies over which the antenna can properly radiate or receive energy. An antenna&#39;s bandwidth specifies the range of frequencies over which its performance does not suffer due to a poor impedance match. Typical spiral antennas have wide bandwidths on the order of 180% while typical planer microstrip antennas have narrow bandwidths on the order of 3%. Spiral conductor  12  functions as a hybrid of these two antenna types. That is, spiral conductor  12  presents a number of periodic harmonics, each with a narrow resonance bandwidth but across a wide frequency band. 
         [0028]    Antenna gain is a parameter that provides a measure of the deuce of directivity of the antenna&#39;s radiation pattern. A high-gain antenna will preferentially radiate in a particular direction. Specifically, the antenna gain or power gain of an antenna is defined as the ratio of the intensity radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by the intensity radiated at the same distance by a hypothetical isotropic antenna. The gain of antenna  10  is a parametric governed by the geometry of spiral conductor  12  that radiates predominantly in a direction perpendicular to the plane of spiral conductor  12  to produce radiation pattern  100 . 
         [0029]    Antenna radiation pattern defines the variation of the power radiated by an antenna as a function of the direction away from the antenna. This power variation as a function of the arrival angle is observed in the antenna&#39;s far field. The radiation pattern of an antenna is plotted as the relative field strength of the radio waves emitted by the antenna at different angles. It is typically represented by a three-dimensional graph or polar plot of the horizontal and vertical cross sections. Antenna  10  radiates predominantly in a direction perpendicular to the plane of spiral conductor  12  to produce radiation pattern  100 . 
         [0030]    The polarization of an antenna refers to the orientation of the electric field (E-plane) of the radio wave with respect to the Earth&#39;s surface and is determined by the physical structure of the antenna and by its orientation. In the far field, the magnetic field of a radio wave is at right angles to that of the electric field. However, by convention, an antenna&#39;s “polarization” is understood to refer to the direction of the electric field. Polarization is predictable from an antennas geometry. In the present invention, polarization of antenna  10  is circular as it is governed by the geometry of spiral conductor  12 . 
         [0031]    The Physical relationship between spiral conductor  12  and loop  14  can be fixed. For example,  FIG. 3  illustrates an embodiment of the present invention in which a dielectric material  18  interposed between spiral conductor  12  and loop  14  is used to construct a one-piece structure with spiral conductor  12  coupled to one face  18 A of dielectric material  18  and loop  14  coupled to the opposing face  18 B of dielectric material  18 . If surfaces  18 A and  18 B are parallel to one another, spiral conductor  12  and loop  14  reside in parallel planes. The dielectric material could also be used to completely encase spiral conductor  12  and loop  14  without departing from the scope of the present invention. 
         [0032]    In order for the electromagnetic energy in loop  14  to be coupled to spiral conductor  12 , loop  14  must overlap at least a portion of spiral conductor  12  (as illustrated in the plan view shown in  FIG. 4 ) while remaining electrically isolated therefrom. For example, spiral conductor  12  and loop  14  can reside in parallel planes as illustrated by the side view shown in  FIG. 5 . Each of spiral conductor  12  and loop  14  has a geometric center  12 C and  14 C, respectively. The antenna efficiency of the present invention can be adjusted by the relationship between geometric centers  12 C and  14 C with the highest antenna efficiency being achieved when geometric centers  12 C and  14 C are aligned with one another as shown in  FIG. 6 . When spiral conductor  12  and loop  14  are in parallel planes, alignment of the geometric centers is achieved when the distance between centers  12 C and  14 C is the same as the distance between the parallel planes of spiral conductor  12  and loop  14 . 
         [0033]    The far field operational range of antenna  10  can be explained as follows. The proximity of loop  14  to spiral conductor  12  is such that electromagnetic energy can be transferred between the two elements. More specifically, a time-varying electromagnetic field has both electric and magnetic components. The electric component establishes an electric field between the conductive traces (i.e., capacitance) of spiral conductor  12  and the magnetic component establishes a magnetic field as flux loops around the conductive traces (i.e., inductance) of spiral conductor  12 . In terms of propagation through free-space (i.e., air), propagation distance is maximized by using electromagnetic energy as opposed to pure electric energy or pure magnetic energy. This is evidenced by the field impedance graph shown in  FIG. 7  where far field propagation is achieved when electromagnetic energy propagating from/to antenna  10  is impedance-matched to free-space impedance of approximately 377 ohms. Accordingly, antenna  10  relies on a time-varying electromagnetic field to assure that the radiation pattern can propagate into (and be detected from) the far field. By coupling spiral conductor  12  to source-fed loop  14 , a far field antenna is created for purposes of communication. 
         [0034]    As mentioned above, the antenna of the present invention can be used to increase the read range of wireless sensors such as those described in detail in the above-cited U.S. Pat. No. 8,430,327. Two exemplary embodiments of such use will be described with the aid of  FIGS. 8 and 9 . Referring first to  FIG. 8 , the above-described antenna  10  is paired with an electrically unconnected, open-circuit spiral sensor  22  having the same general attributes of spiral conductor  12  described earlier herein and in U.S. Pat. No. 8,430,327. Sensor  22  is electrically isolated from loop  14 . Loop  14  is disposed between spiral conductor  12  and sensor  22 , and is located close enough to sensor  22  such that loop  14  lies in the near field resonance pattern of sensor  22 . As described in detail in the above-cited patent, sensor  22  experiences resonance changes when subjected to changes in environmental changes it has been designed to detect. However, these resonance changes only propagate in the near field of sensor  22 . Antenna  10  detects the near field resonance of sensor  22  and propagates them into the far field in radiation pattern  100 . That is, radiation pattern  100  is changed/modulated in accordance with resonance changes experienced by sensor  22 . Radiation pattern  10  can then be detected by a conventional antenna (not shown). In this way, antenna  10  boosts or increases the read range of sensor  22 . 
         [0035]      FIG. 9  illustrates another embodiment in which dielectric material is used to fix the relationships between spiral conductor  12 , loop  14 , and sensor  22 , while also creating a one-piece structure. More specifically, dielectric material  18  is interposed between spiral conductor  12  and loop  14 , and dielectric material  28  is interposed between loop  14  and sensor  22 . Dielectric materials  18  and  28  can be the same or different without departing from the scope of the present invention. Dielectric materials could also encase spiral conductor  12 , loop  14  and sensor  22  without departing from the scope of the present invention. 
         [0036]    The advantages of the present invention are numerous. The antenna provides far field propagation and reception using simple, inexpensive, and low-power elements. The antenna&#39;s elements can be tuned for a variety of applications to include radio receiving antenna, a cellular phone antenna, a GPS antenna, a WiFi antenna, a military radar antenna, or any electromagnetic antenna that must be able to receive/radiate into the far field using small amounts of power. Accordingly, the present invention is well-suited to be paired with near-field-propagating wireless sensors to boost the read range associated with such sensors.