Abstract:
A radio frequency (RF) coupler for passing RF energy through a dielectric such as glass, includes first and second circuit boards, each board having disposed thereon an electrical conducting material, the first and second circuit boards being arranged opposite each other on opposing sides of the dielectric, the first circuit board having a first ground element that defines a first aperture, and a first exciter strip disposed within the first aperture, the second circuit board having a second ground element that defines a second aperture, and a second exciter strip disposed within the second aperture, wherein one of the first and second exciter strips is longer than the other.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to radio frequency (RF) components. More particularly, the present invention relates to couplers that couple RF signals, including ultra high frequency signals, through a medium such as air, glass or other dielectric. 
     2. Background of the Invention 
     Through-glass couplers, as explained in, e.g., U.S. Pat. No. 5,565,877 to Du et al., are employed to RF couple two antenna modules that are mounted, respectively, on the outside and inside surfaces of window glass, such as automobile glass, to transmit signals through the window glass between the opposing modules. The outside antenna module might include a vertically extending antenna element, while the inside antenna module typically contains a connector or transmission feedline, which leads to a device such as a telephone, pager, facsimile machine, radio receiver, or the like, inside the vehicle. In a radio receiver implementation, the inside antenna module receives RF energy through the glass from the outside antenna module. 
     Loss occurs in glass mount antennas due to the dielectric material interposed between the inside and outside modules, as well as impedance mismatching. Therefore, a window glass mount antenna typically has lower gain compared to a non-through-glass antenna. However, conventional (i.e., non-through-glass coupled) antennas are less desirable because there must be a physical connection that extends through the body of a vehicle, between inside and outside antenna modules. 
     Conventional through-glass couplers employ capacitive coupling to transmit RF signals through the glass. In capacitively coupled antennas, two metal plates are positioned opposite each other on opposing surfaces of the window glass. These metal plates cooperate and act as a capacitor to transmit RF energy through the intervening window glass. However, to achieve better responses, especially at relatively higher frequencies, microstrip antennas have been adopted in certain applications, as exemplified by U.S. Pat. No. 5,565,877 to Du et al. There are many variations to microstrip antenna designs, as exemplified by, e.g., U.S. Pat. No. 4,130,822 to Conroy, U.S. Pat. No. 4,197,545 to Favaloro et al., U.S. Pat. No. 4,792,809 to Gilbert et al. and U.S. Pat. No. 5,793,263 to Pozar, but because of the wide array of applications for which microstrip antennas can be used, there is significant room for improvement in microstrip antenna design, particularly in specialized applications. 
     FIG. 1 illustrates a typical application for which a through-glass coupler is employed. In the case of, for example, a radio receiver implementation (although the same principles apply to a radio transceiver implementation) an antenna  10 , receives a broadcast signal, which is applied to an outside module  200  of a through glass coupler  12 . Outside module  200  is positioned against glass  14  and opposite inside module  100  on the opposite side of the glass  14 . In some applications, a matching circuit  16  is preferably provided to match impedance values of the two complementary modules  100 ,  200 . A radio frequency (RF) cable  18 , e.g., coaxial cable, typically connects matching circuit  16  to a low noise amplifier (LNA)  20 , which feeds receiver  22 . 
     Of the known methods of transferring RF energy through glass, capacitive coupling, slot coupling, and aperture coupling represent the most common. However, an inherent drawback of all these coupling methods is that they increase the system noise due to relatively high RF coupling loss. To reduce coupling losses, the methods listed above need to be implemented on expensive circuit board ceramic material (i.e., Rogers 3003, 4003, 3010, etc.). The price of these materials, however, is significantly higher than that of, e.g., standard FR-4 printed circuit board. Thus, using low-loss type boards would make a consumer product very expensive. 
     Also, a typical slot coupler, as shown in FIG. 2, includes a circuit board  50 , a microstrip feed line  52  and a slot  54  that exposes the underlying microstrip feed line  52 . Such a device requires elaborate construction techniques, and may require the use of relatively expensive multi-layer boards. There is a need, therefore, for providing a less expensive coupler, yet one that provides the performance that matches or even exceeds known devices that are constructed using higher cost materials. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a low cost yet capable through glass coupler. 
     It is another object of the present invention to provide a coupler that is simple to manufacture. 
     It is yet another object of the present invention to provide a through glass coupler that has inside and outside modules having asymmetrical configurations. 
     It is also an object of the present invention to provide components of a pair of through glass couplers on a single board. 
     It is also an object of the present invention to provide a through glass coupler that can be constructed using well-known etching techniques and low-cost copper clad circuit board material. 
     It is still another object of the present invention to provide a through glass coupler having a low profile design. 
     It is also an object of the present invention to provide a through glass coupler that not only has a low profile design, but also does not require a cavity, i.e., a slot. 
     To achieve the foregoing and other objects, an embodiment of the present invention comprises a pair of single layer double sided copper clad boards that are etched to include apertures and exciter strips that have different configurations. In a particular application for the through glass coupler of the invention, each copper clad board is etched to include components of two couplers, whereby two antennas or frequency bands can be accommodated and coupled. 
     Further in accordance with embodiments of the invention, the through glass coupler comprises a single layer design, thereby substantially facilitating the manufacture thereof. Additionally, no cavities are required, thereby achieving further savings in manufacturing costs and space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully explained in the following detailed description of the invention in conjunction with the associated drawings, in which 
     FIG. 1 illustrates a typical application for which a through glass coupler might be used; 
     FIG. 2 depicts a prior art microstrip-fed slot coupler; and 
     FIGS. 3A-3C illustrate front faces and a back face of a dual RF coupler pair embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 3A-3C illustrate an exemplary embodiment of the present invention in which two separate RF signals can be passed through a dielectric, such as glass on a vehicle. 
     Generally, in accordance with the principles of the present invention, low loss is achieved by making the opposing couplers different. For example, as shown in FIGS. 3A-3B generally, and as will be explained in more detail below, one printed exciter strip on one circuit board or module is floating, while the printed exciter strip on the other circuit board or module is shorted to ground. The length of the printed exciter strips can be adjusted for tuning to the desired frequency and minimizing coupler loss. 
     Consistent with the application shown in FIG. 1, the through glass coupler in accordance with the present invention comprises an inside module  100  and an outside module  200 . Inside module  100 , which would typically be located inside a vehicle, comprises a circuit board having a left side edge  102 , a right side edge  104 , a top edge  106  and a bottom edge  108 . In addition, the substantially rectangular inside module  100  comprises a front face  110  and a back face  112 , the latter being shown in FIG.  3 C. In accordance with the illustrative embodiment, two couplers, a first coupler  150  and a second coupler  152  are provided on the same inside module  100 . This permits two separate RF frequencies to be passed through the dielectric. The dashed line indicated by X denotes the separation between the first coupler  150  and the second coupler  152 . Of course, the present invention can be configured to have only a single coupler per module. Also, although not shown in the drawings, modules  100  and  200  preferably include a cover that encapsulates at least an exposed portion of the circuit boards when they are mounted on glass. 
     Inside module  100  (as well as outside module  200 ) is preferable constructed of well known and inexpensive copper-clad circuit board material such as FR-4. The copper cladding  114  preferably etched using well known techniques to arrive at the exemplary configuration shown in FIGS. 3A-3B. 
     More specifically, the copper cladding  114  is preferably etched such that apertures  116   a  and  116   b  are provided in each of the first and second couplers  150 ,  152 . Further, exciter strips  122   a  and  122   b  are provided within each of apertures  116   a  and  116   b . The exciter strips  122   a ,  122   b  each includes a feed point through hole  124   a  and  124   b . A ground element  118  preferably includes a ground connection area  120  that includes a plurality of relatively small through holes to ensure a secure solder joint. Also, ground element  118  preferably includes gaps  126   a  and  126   b  adjacent top edge  106 . 
     Back face  112  is the back face of inside module  100 . A similar back face is provided for outside module  200 , although, for simplicity, this back face is not shown. Back face  112  includes feed point through holes  124   a  and  124   b  as well as separate ground connection area pads.  128   a  and  128   b , which correspond, in location, substantially with the ground connection areas  120   a  and  120   b  on the front face  110 . 
     Outside module  200  comprises a circuit board having a left side edge  202 , a right side edge  204 , a top edge  206  and a bottom edge  208 . Outside module  200  further comprises a front face  210  shown in FIG. 3B and a back face (not shown) that is similar to back face  112  shown in FIG.  3 C. Like inside module  100 , outside module  200  comprises a first coupler  250  and a second coupler  252 . 
     Apertures  216   a  and  216   b  are etched from copper cladding  214 , a ground element  218 , which extends substantially around a periphery of the circuit board, as well as exciter strips  222   a  and  222   b  are provided. Ground connection areas  220   a  and  220   b , including several pin holes that extend through the circuit board, are preferably provided, as are feed point through holes  224   a  and  224   b.    
     The separation between the two couplers  250  and  252  is indicated by the dashed line Y. In use, the front faces  110  and  210  of the inside module  100  and outside module  200  face each other on opposing sides of a dielectric such as a piece of glass. The two modules  100 ,  200  preferably have the same overall outer dimensions such that they can be aligned directly opposite each other and in registration with one another. Indeed, when the two modules oppose each other complementary pairs of feed point through holes  124   a ,  124   b ,  224   a ,  224   b , as well as ground connection areas  120   a ,  120   b ,  220   b ,  220   a  preferably align, or are in registration, with each other. Center conductors of coaxial conductors (not shown) can be soldered to the feed point through holes, and outer ground sheathing of the coaxial cable can be connected and/or soldered to the ground connection areas  120  and/or ground connection area pads  128 . 
     The exciter strip configurations of the two boards is a significant aspect of the present invention. Specifically, as shown in the FIGS. 3A and 3B the corresponding inside and outside modules have different exciter strip configurations. Specifically, it can be readily seen that exciter strips  222   a ,  222   b  extend to an upper portion of ground element  218 , and are indeed integrally formed therewith, as compared with “floating” exciter strips  122   a ,  122   b . Accordingly, one of ordinary skill in the art can readily appreciate that opposing inside and outside modules have different configurations. This aspect of the present invention is unlike well known capacitively coupled through glass couplers that employ simple metallic plates. Also, the present invention is different from prior art devices in that a simple dual side copper clad board can be employed to achieve a low loss through glass coupler without having to resort to expensive and intricate construction techniques to achieve a slot type micro strip antenna like that shown in FIG.  2 . 
     The dimensions shown in FIGS. 3A and 3B are also instructive with respect to illustrating the relative sizes of the different elements included on each of the inside and outside modules. For example, dimension A, which measures the distance between an exciter strip and its closest portion of ground element  118 , is preferably substantially the same for each coupler. Dimension B measures the distance between an edge of exciter strip  122   a ,  122   b  and an upper portion of ground element  118 , while dimensions C and D illustrate how the aperture widths of the first and second couplers  150 ,  152  can be different, thereby, accommodating different levels of loss. 
     Thus, the two modules described herein, when properly aligned on opposite sides of a dielectric, can pass RF signals of two separate antennas. The isolation between the two couplers is approximately 30 dB. In an actual application of the RF coupler of the invention, the coupler is used to couple through glass terrestrial based signals and space based signals. It is noted that while differently sized apertures have been described and shown, different applications may call for similarly sized apertures. The RF coupler described herein, however, was developed in connection with a satellite digital audio radio service (SDARS) that comprises a space based broadcast signal and a terrestrial based broadcast signal. Because in this particular application the terrestrial based signal is stronger than the space based signal (which is broadcast at a different frequency), the aperture corresponding to the terrestrial coupler is made smaller than the aperture for the space based (or satellite) signal. While, the smaller aperture will cause additional loss in the terrestrial coupler system, the SDARS system can nevertheless tolerate this loss. Based on a coupler having an overall length of 2.9 inches, an overall width of 1.1 inches, a satellite signal coupler having an aperture width of 1.55 inches and a terrestrial signal coupler having an aperture width of 1.26 inches, the coupling loss is as follows: satellite signal coupling loss: 0.5-0.6 dB, terrestrial signal coupling loss: 1.0-1.1 dB (based on 4-mm thick automotive glass). 
     The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.