Abstract:
An antenna for receiving electromagnetic energy with multiple grid arrays includes first and second grid patterns of diodes each acting as a half-wave rectifying element when illuminated by the electromagnetic energy whereby the electromagnetic energy is efficiently converted into electrical current at an output even when the electromagnetic energy is randomly polarized. The multiple grid arrays are spaced from one another and can also be offset from one another. In accordance with an aspect of the invention, the first and second grid patterns of diodes have varying diode densities.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application represents a National Stage application of PCT/US2012/030526 entitled “Multi-Scale, Multi-Layer Diode Grid Array Rectenna” filed Mar. 26, 2012, which claims the benefit of the U.S. Provisional Patent Application Ser. No. 61/467,470 filed Mar. 25, 2011, entitled “Multi-Scale, Multi-Layer Diode Grid Array Rectenna”, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention pertains to the art of transmitting and receiving electromagnetic power using antennas to receive electromagnetic energy and, more particularly, to rectifying antennas that convert electromagnetic energy into direct electrical current. 
     Discussion of the Prior Art 
     Generally, a rectifying antenna, otherwise known as a “rectenna”, is an antenna designed to convert electromagnetic energy, preferably microwave energy, into direct (DC) electrical current an thus acts as an energy converter. An example of an early use of a rectenna can be found in a crystal radio which converts RF energy, i.e., radio waves, into an electric current which is sent to a speaker to produce sound. Perhaps one of the most simple of rectennas is formed by placing a Schottky diode between the dipoles of an antenna. Often, rectennas are formed by multiple Schottky diodes linked together. Under certain favorable conditions, such antennas have been known to convert microwave energy into electrical current with an efficiency of over 90%. 
     Because rectennas are able to transmit power over a distance with high efficiency, they are commonly employed in solar powered satellites. Additionally, proposals have been made to use rectennas as part of a system to transfer energy to flying machines. For example, U.S. Pat. No. 3,434,678 discloses a combined antenna and conversion mechanism for the reception of beamed high frequency energy. Specifically, a full-wave bridge connected diode network is used as a rectenna to provide power to a helicopter. However, such an arrangement requires that the received energy be polarized and that the orientation of the applied field be maintained parallel to the orientation of the rectenna dipoles in order to maintain high efficiency in collecting power. 
     Another broadband rectenna system is formed from an array of traces loaded by Schottky diodes and is often referred to as a “Dense Diode Grid Array.” With initial reference to  FIG. 1 , an energy converter such as a rectifying antenna or rectenna for receiving electromagnetic energy constructed is generally indicated at  10 . Rectenna  10  has a positive terminal  12  and a negative terminal  14  which collectively constitute an electrical output. Rectenna  10  also has a grid array  20  that is located along a surface  30  of a substrate  35 . Grid array  20  includes a grid pattern  40  of diodes  41 . Diodes  41  are divided into a first group, represented by diode  44 , of electrodes that extend in a first direction and a second group, represented by diode  46 , of electrodes that extend in a second direction, perpendicular to the first direction. For the sake of convenience, the first and second directions are referred to as horizontal and vertical respectively, although it should be noted that Rectenna  10  might be positioned in various different orientations. 
     Referring now to  FIG. 2 , there is shown a portion of grid array  20 . Two diodes  50 ,  51  are linearly aligned and arranged in series along a conductive horizontal trace  55  on surface  30 . Similarly, two diodes  60 ,  61  are arranged in a linear fashion along a conductive vertical trace  65 , also on surface  30 . Vertical and horizontal traces  55 ,  65  share a common node  70 . 
     As shown in  FIG. 3 , an electromagnetic field vector  100  is shown rotating by arrow  101  about an axis of rotation  104  (extending into and out of the page) and in a plane  105  extending in a vertical direction along vertical axis  106  and a horizontal direction along horizontal axis  107 . Rotating electromagnetic field vector  100  is used to represent a randomly polarized electromagnetic wave of incident RF energy impinging on array  20 . Rotating electromagnetic field vector  100  is continuously split into two orthogonal components  100 V,  100 H as vector  100  completes a cycle of rotation about axis  104 . In this case, vertical component  100 V of field vector  100  aligns with diodes  44  arranged vertically and, in a similar manner, horizontal component  100 H aligns with diodes  46  arranged horizontally. Of course, grid array  20  will harvest energy from incident RF energy even if diodes  44  and  46  are not arranged at right angles, but the efficiency of harvesting will drop. 
     Turning back to  FIG. 1 , grid array  20  includes an entire dense rectangular grid pattern of diodes  44 ,  46  connected by nodes such as node  70 . As such, numerous vertical traces  120  and horizontal traces  130  connect diodes  44 ,  46  to positive and negative terminals  12 ,  14 . When vertical and horizontal traces  120 ,  130  are so arranged in array  20 , the entire array  20  reacts to both components  100 V,  100 H of electromagnetic field vector  100 . Electromagnetic forces are combined in array  20  giving a large DC voltage between terminals  12 ,  14 , with junction capacitance minimizing any ripple voltage. Vertical and horizontal, diode-loaded, conductive traces  120 ,  130  alternately forward bias during each half cycle of rotating vector  100 , allowing free charges to store up and aid in biasing adjacent orthogonal, diode-loaded traces  55 ,  65  during the next half of the cycle. Therefore, the dense rectangular grid pattern  40  of orthogonally oriented diodes  44 ,  46  will act as a half-wave rectifying element when illuminated by the incident RF energy represented by vector  100 . 
     In general, such prior art broadband rectennas exhibit low efficiencies when harvesting RF energy from a randomly polarized illumination source. With the above in mind, there is considered to be various advantages associated with further developments for rectennas. In particular, there is seen to be a need in the art for a rectenna array that has increased power and efficiency along with a wide frequency response that is efficient at harvesting energy from a randomly polarized illumination source. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus comprising an energy converter such as a rectifying antenna or rectenna for receiving electromagnetic energy and converting the energy to DC current. The antenna includes an electrical output and a grid array including a grid pattern of diodes located substantially along a first surface and divided into a first group of serially connected diodes extending in a first direction and a second group of serially connected diodes extending perpendicular to the first direction. The grid array acts as a half-wave rectifying element when illuminated by electromagnetic energy. In addition, a second grid array is provided, including a second grid pattern of diodes located substantially along a second surface. The second grid array also acts as a half-wave rectifying element when illuminated by the electromagnetic energy. The first surface is spaced from the second surface. An electrical circuit connects the first and second arrays to the electrical output whereby the electromagnetic energy impinging on the first and second grid arrays is converted into electrical current at the output even when the electromagnetic energy is randomly polarized. In accordance with one aspect of the invention, the first grid array is offset from the second grid array. In accordance with another aspect, the number of diodes per unit area varies between the first and second grid arrays. 
     In another embodiment, grid arrays are located on opposite sides of a substrate having a low dielectric constant and additional substrates with additional grid layers, each having variable grid spacing, are mounted near the substrate. An electrical circuit connects the additional grid layers to the output in series or in parallel. The rectenna is used to power numerous devices such as a wireless detonator in a mining operation or batteries located on a soldier&#39;s vest used to power flashlights, radios and other portable gadgets. 
     Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of preferred embodiments when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a group of diode grid array elements linked together to form a planer array of diodes according to the prior art; 
         FIG. 2  shows a schematic view of a diode grid array element of  FIG. 1 ; 
         FIG. 3  shows a rotating electromagnetic field vector representing an elliptically/randomly polarized electromagnetic wave; 
         FIG. 4  illustrates an embodiment with two planer arrays of diodes in spaced relationship on a single substrate; 
         FIG. 5  shows an electrical diagram for a combination front side grid array and back side grid array disposed on a single substrate and connected in series; 
         FIG. 6  shows an electrical diagram for a combination front side grid array and back side grid array disposed on a single substrate and connected in parallel; 
         FIG. 7  shows a perspective view of a multi-layer, dual sided, diode grid array; 
         FIG. 8  is a graph of frequency response versus efficiency setting forth a grid spacing comparison; 
         FIG. 9  is an exploded and side view of a multilayer, dual sided, diode grid array with three layers constructed in accordance with the invention; 
         FIG. 10  is a cross-sectional view of a dual sided diode grid array constructed in accordance with the invention arranged to power a wireless detonator system for mining; and 
         FIG. 11  is an isometric view of a dual sided diode grid array arranged on a vest to power devices carried by a soldier. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in  FIG. 4 , a double-sided antenna  210  is shown in accordance with the invention. Double-sided antenna  210  is similar to energy converter or rectenna  10  shown in  FIG. 1  and is mounted on substrate  35 ′. In addition to grid array  20 , which is also referred to as a first grid array or a front side grid array, double-sided antenna  210  has a second grid array  220 , also referred to as a backside array. Second grid array  220  includes a positive terminal  212  and a negative terminal  214  and is mounted on an opposite surface  230 , spaced from surface  30  of substrate  35 ′. Second grid array  220  also includes a grid pattern  240  of diodes that is divided into a first group  244  of electrodes that extend in the first direction and a second group  246  of electrodes that extend in the second direction. Double sided antenna  210  extends the capability of the single-sided, one unit operation of antenna  10  to a dual-sided multiple unit module where inherent electrical parameters like output voltage, source impedance and frequency response are tuned to a user&#39;s application. Substrate  35 ′ is preferably chosen to minimize absorption and reflection to penetrating RF energy. Preferably, substrate  35 ′ is made of 20 mil thick Duroid having a relative dielectric constant of 2.2 and a loss tangent &lt;0.001, but other low loss substrates with very low reflection coefficients can also be used. Diode grid arrays  20 ,  220  implemented on opposing sides of substrate  35 ′ are spatially offset, by 12.7 mm for example, both vertically, as represented by arrow pair  300 , and horizontally, as represented by arrow pair  310 , and thus are independently illuminated by impinging RF energy. 
     Broadband rectenna  210  has a center frequency determined by the spatial period of grid array  20  and should be approximately 1/10th of the free space wavelength (λ 0 /10) of the RF energy. Therefore, given a center frequency-of-interest, the overall physical size of grid array  20  will scale with the frequency such that there is an inverse relationship of decreasing physical size with increasing frequency. Diode density determines overall efficiency and is also scaled with center frequency, with an upper limit of approximately 160 diodes per wavelength squared. 
     The vertical and horizontal orientation of diodes  44 ,  46  determines the polarity of the output dc voltage at terminals  12 ,  14 . Likewise, the orientations of backside diodes  244 ,  246  determine the polarity of the output voltage of backside array  220  at terminals  212 ,  214 . Each array  20 ,  220  of  FIG. 4  is preferably modeled as a separate battery  401 ,  402  with its source impedance as shown in  FIGS. 5 and 6 . Specifically, front side array  20  is modeled both by a battery  401  having a front side voltage V(fs) and a resistor  405  having an impedance value Rs. Similarly, back side array  220  is modeled both by a battery  402  having a back side voltage V(bs) and a resistor  410  having an impedance value Rs. In  FIG. 5 , a circuit  420  connects front side array  20  and backside array  220  in series to an output terminal  430 , while in  FIG. 6  a circuit  420 ′ connects front side array  20  and backside array  220  in parallel to an output terminal  440 . 
     An output voltage level V(os) and source impedance R(os) at terminal  430  are given by the equation:
 
 V ( os )= K ×( V ( fs )+ V ( bs ))
 
 R ( os )= C ×(2 ×Rs )
 
     An output voltage level V(op) and source impedance R(op) at terminal  440  are given by the equations:
 
 V ( op )= K ×( V ( fs ))
 
 R ( op )= C ×( Rs/ 2)
 
     In the equations for total source impedance (Ros), R(op) and output voltage V(os), V(op) with both configurations, there are scaling constants that arise (K,C) because of possible shadowing effects, non-matched characteristics of diodes  44 ,  46 ,  244 ,  246  and minor absorption/reflection losses of substrate  35 . 
     As shown in  FIG. 7 , in order to increase harvested power and efficiency, some preferred embodiments employ multiple layers of arrays  500 ,  502 ,  504 ,  506  formed and connected in various configurations to include series, parallel and series/parallel combinations.  FIG. 7  shows an N layer, dual-sided diode grid array  508 , separated by thin insulators  509 ,  509 ′ which can be made of the same substrate as the array or can simply be constructed by an air gap  509 ″. Each layer has a front side  510  and a backside  520  (only one of each being labeled) with a front side array  530  and a backside array  540  (again only one of each shown labeled) being printed thereon and spatially offset from each other. Each layer has two terminal connections for +/− polarities, with only terminals  541 ,  542  being labeled. The layers are preferably connected by either external wiring for field adaptability or internal connections for a specific static configuration. Series, parallel and series/parallel combinations are possible, whereby output voltages and source impedances are determined and dependent upon the characteristics of an illuminating RF wave. This arrangement increases apparent efficiency while keeping the same two-dimensional footprint as arrays  20 ,  220 . Scaling factors K, C become more of a consideration as additional layers are added. In general, the fractional value of these scaling factors are estimated based on the analysis of empirical data. As several layers  500 ,  502 ,  504 ,  506  of dual-sided diode grid arrays are added, more distinct combinations are possible. 
     The standard one-sided, single plate grid array  20  exhibits a broadband frequency response whose center frequency (fo) is determined by the diode grid spacing and the relative dielectric constant of substrate  35  upon which array  35  is mounted as mentioned above. However, the use of the low-loss, thin substrates basically negates any center-frequency dependency on substrate  35 ′. Therefore, the spatial period of the grid array  20  will determine the center frequency. If all the grid arrays, for example those shown in  FIG. 7 , have the same spatial periodicity, the efficiency peaks at the center frequency and decreases somewhat abruptly within +/−30% of the center frequency as seen by trace  610  in the graph  600  shown in  FIG. 8 . 
     Using different spatial periodicities for each array, a broader and flatter frequency response results as indicated in  FIG. 8  by the “Variable Grid Spacing” trace  650  in the graph. The response represented by trace  650  is typical for a two layer, dual-sided (4 arrays total), diode grid array antenna designed with each array having a different spatial periodicity. The maximum efficiency peaks “wash-out” due to the scattered, but close proximity center frequencies. Since, in theory, the number of layers N could be very large, the broadening of the frequency response would correlate with the number of center frequencies desired. As an example, the two layer grid array mentioned above could be designed where S mn  is the spatial periodicity for the m th  layer, n th  side, and S 0  is the overall nominal center frequency. In such a case, the spatial periodicities are as follows: 
     S 11 =λ 11 /10=0.8*λ 0 /10 or 80% of the nominal center frequency (S 0 ) 
     S 12 =λ 12 /10=0.9*λ 0 /10 or 90% of the nominal center frequency (S 0 ) 
     S 21 =λ 21 /10=1.1*λ 0 /10 or 110% of the nominal center frequency (S 0 ) 
     S 22 =λ 22 /10=1.2*λ 0 /10 or 120% of the nominal center frequency (S 0 ) 
     The frequency responses for these four arrays would overlap, thus broaden the overall response of the module. For narrower, but higher peak efficiencies, the fractional difference in grid spacing between arrays is minimized. To this end, a multi-layer, multi-scale diode grid array rectenna  700  is illustrated in  FIG. 9 . Specifically, rectenna  700  has a first layer  710  with 12 diodes, a second layer  720  with 125 diodes, and a third layer  730  with 544 diodes. This multilayered, varied diode density approach increases the physical size only in the third dimension and maintains a constant two dimensional footprint which results in high efficiencies and a widening of the frequency response, all controlled by the grid configuration. 
       FIG. 10  shows a series of an energy converters or rectennas  700  employed in an overall mining system  800  used to power wireless detonators  805 ,  805 ′,  805 ″ in a mining operation. More specifically, rectenna  700  is attached to a wireless detonator controller  806  placed at the top of a bore hole  807  and electrically connected to a wire  810  that runs down bore hole  807 . Wire  810  is then either electrically connected or, preferably, inductively connected to a detonator module  820  positioned part way down bore hole  807 . Detonator module  820  has no battery or other power source and is therefore safe to handle during an explosives loading operation. After bore hole  807  and bore holes  807 ′,  807 ″ are all loaded, blasting personnel vacate the mining area and then remotely activate an overall blast controller  825  to begin broadcasting a signal  830  on an appropriate frequency to couple with wireless controller  806 . 
     Wireless controllers  806 ,  806 ′,  806 ″ are provided at bore holes  807 ,  807 ′,  807 ″ respectively. Wireless controllers  806 ,  806 ′,  806 ″ are preferably able to collect and provide power to detonator modules  820 ,  820 ′,  820 ″,  860  and are also able to provide an RF signal  830  for programming a firing delay time for each detonator module  820 ,  820 ′,  820 ″. This is particularly important for vibration control in blasting operations for mining applications. In this application, overall blast controller  825  performs dual functions of powering detonator modules  820 ,  820 ′,  820 ″,  860  remotely while using the same RF signal  830  for 2-way data communications. 
     The ability to remotely provide both power and communication signals to wireless controllers  806 ,  806 ′,  806 ″ without the use of surface wires represents a significant advancement in mining technology and efficiency. The wireless arrangement of system  800  is designed to free personnel from the hole-to-hole wiring required by prior art systems. This feature offers a significant time advantage over all other systems where wiring can consume significant labor costs. In addition, the wireless arrangement of system  800  leaves the surface free from the clutter of wiring networks. It also eliminates the potential for wiring mistakes, as well as the potential of entanglement with personnel and blasting equipment used during the loading process. 
       FIG. 11  illustrates another potential use for rectenna  700 . That is, as shown, rectenna  700  is mounted on a vest  900  adapted to be worn by a soldier in order to wirelessly provide power to various electrically powered devices, such as a flashlight  910  or a radio  920 . More specifically, electromagnetic fields are transmitted from a network of transmitting antennae to rectenna  700  on the soldier&#39;s vest  900 . Output from rectenna  700  is employed to recharge battery powered devices  910 ,  920  carried by the soldier. 
     Although described with reference to preferred embodiments of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. For instance, the invention relates to a rectenna device that may be used in a wireless battery charging device for use in a wide range of commercial applications. The rectenna is an enabling technology across many markets, for example: wireless sensors and actuators for buildings, machinery, and engines; heavy equipment diagnostics; safety and security monitoring for: roads, bridges, rail, and mass transit; gas, oil, and electric transmission lines and equipment; long life MASINT and HUMINT sensors; data exfiltration; surveillance devices; electronic equipment such as laptops, e-books, mobile phones, calculators, toys, electronic car keys, and electronic apparel; and medical components associated with implants, ingestible diagnostic sensors, disposable testers, drug delivery and the like. In any event, the invention is only intended to be limited by the scope of the following claims.