Abstract:
A shield for shielding radio frequency emissions being emitted from a communications antenna. The shield has a first layer of material having the physical property of generally absorbing radio frequency electromagnetic emissions and a second layer of material having the physical property of generally reflecting radio frequency emissions. The first layer of material is positioned between the second layer of material and the communications antenna. Therefore, the first layer of material absorbs a portion of the radio frequency emissions from the communications antenna, and the second layer of material reflects back the remaining emissions to the first layer of material. Therefore, the first layer absorbs a further portion of the remaining emissions. A layer of absorbing material is placed between the combined first &amp; second layers and a material that is transparent to radio frequency emissions and through which the communications antenna radiates radio frequency energy. The purpose of the absorbing material between the transparent material and the combined first &amp; second layers is to minimize escape of radio frequency energy along the transparent material. The radio frequency energy could otherwise escaped around the barrier of the first &amp; second layers due to reflection and refraction of radio frequency energy within the body of the transparent material.

Description:
BACKGROUND OF INVENTION 
     FIELD OF THE INVENTION 
     The present invention relates to shielding of radiating radio frequency electromagnetic emissions and more particularly to shielding a source of such emissions so as to protect from excessive, prolonged exposure to such emissions any people and objects that might be injured or damaged by such exposure, while still facilitating the efficient and unobstructed emission from the source, for its intended purpose. 
     Shields for shielding people and objects from radio frequency electromagnetic emissions have long been known and have a number of uses. In recent years there has been a very significant increase in the use of mobile telephones and paging devices. As their use has increased, more communications towers have been built for radio frequency transmissions for communication devices, such as mobile telephones, pagers and the like. Also, it has become increasingly common for radio frequency communications of this type to be transmitted from antennae located on and in buildings and at other locations close to large numbers of people, both inside and outside of the building. The increased amount of transmission near concentrations of people has led to an increased need for a simple, economical, and compact shield to protect people and the environment from stray radio frequency emissions. 
     Accordingly, there is a need to provide a shield for electromagnetic radio frequency emissions, which is simple, economical, and compact, and which is an efficient means for protecting people and the environment from radio frequency emissions from communications antennae transmitting to mobile telephones and pagers. 
     There is also a need to provide shielding of a radio frequency antenna for environmental protection while minimizing the reflective or refractive transmission of radio frequency energy around the radio frequency shielding. 
     There is an additional need to provide or permit physical access to a radio frequency antenna without providing an escape path for radio frequency energy through shielding provided for the antenna. 
     There is a further need to minimize visibility and visual obviousness of a radio frequency antenna and its shielding. 
     SUMMARY OF INVENTION 
     The present invention involves placing a layer of radio frequency-energy-reflecting material between an antenna and people or objects near the antenna, that might be harmed by prolonged exposure to excessive amounts of radio frequency electromagnetic energy. A layer of radio frequency-energy-absorbing material is then placed between the reflecting material and the antenna, thereby absorbing a portion of the emitted energy that would otherwise pass to people or energy-sensitive objects near the antenna. The reflective layer then reflects energy that passes through the absorbing layer, further preventing the radio frequency energy from reaching people or energy-sensitive objects. The energy that is reflected by the reflective layer again passes through the absorbing layer, where another portion of the energy is absorbed. In this way, only a tiny portion of the original magnitude of transmitted energy finds its way back to the antenna and thus minimizes the amount of reflected back-scatter that might otherwise mix with and thus distort the transmission patterns of the signals issuing from the antenna. 
     In another aspect of the present invention, an absorbing layer is placed between the combination absorbing &amp; reflective layers and a radio frequency-energy transmitting or transparent layer through which the radio frequency energy is intended to be transmitted. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     A more complete understanding of the present invention will be had from the following detailed description when considered in connection with the accompanying drawings, wherein the same reference numbers refer to the same or corresponding items shown throughout the several figures, in which: 
     FIG. 1 is a perspective illustration of a portion of the windows of a building, showing a typical installation location of a shield in accordance with an embodiment of the present invention; 
     FIG. 2 is a simplified, partial sectional view of the upper portion of a typical window and false ceiling and blind cove inside the window of the building depicted in FIG. 1, the section taken as shown by the arrows of the line  2 - 2  of FIG. 1; 
     FIG. 3 is a view of the same cross section as shown in FIG. 2 but with the original window treatment removed and the first portion of an embodiment of the present invention shown mounted on or attached to the interior surface of the window; 
     FIG. 4 is a view of the same cross section as shown in FIG. 3 but with a radio frequency antenna and shield in accordance with an embodiment of the present invention shown installed in the blind cove between the window and the false ceiling: 
     FIG. 5 is a detailed sectional view of an access door of a shield in accordance with an embodiment of the present invention, showing some of the details of the door&#39;s construction; 
     FIG. 6 is a sectional view, of the same section shown in FIG. 4 but with an access door in place and a substitute window treatment shown below the shield in accordance with an embodiment of the present invention, the section taken as shown by the arrows of the line  6 - 6  of FIG. 1; 
     FIG. 7 is a partial sectional illustration of a top view of the shield in accordance with an embodiment of the present invention, taken in the direction of the arrows  7 - 7  of FIG. 6; 
     FIG. 8 is a more detailed partial sectional illustration, as in FIG. 7, showing more of the details of construction and support of the shield in accordance with an embodiment of the present invention; 
     FIG. 9 is an elevational, front view of the shield in accordance with an embodiment of the present invention, taken in the direction of the arrows  9 - 9  of FIG. 6; and 
     FIG. 10 is an elevational front view of the shield in accordance with an embodiment of the present invention, taken in the same general direction as in FIG. 9 but shown in perspective and with the door. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description of preferred embodiments refers to the accompanying drawings which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the present invention. 
     Referring now to the drawings and more particularly to FIG. 1, a typical window system of an urban office building is shown in a generalized elevational perspective view of a bay of windows  20 . Four glass windows  22  are fully shown in FIG.  1 . The four windows  22  are separated by three vertical, side mullions  24 , which are usually metallic. The two leftmost windows  22  (as seen in FIG. 1) serve one partitioned space in the building and the two rightmost windows  22  serve another partitioned space. Each partitioned space has a false or dropped ceiling  26 . As shown in the cross sectional view of FIG. 2, an open space or blind cove  28  is kept open between the end  30  of the false ceiling  26  and the window  22 . The blind cove  28  provides space for full-length window coverings or treatments (not shown in FIG.  2 ), such as drapes, shades, or blinds. However, a top frame  32  for a blind is shown in FIG. 2, for illustration. 
     Referring now to FIG. 3, when a transmitting antenna is to be placed in the blind cove  28 , in order to transmit radio frequency electromagnetic emissions through the window  22 , the portion of the window treatment that occupies the blind cove  28  is removed. The glass of a typical window, being an electrically-insulating material, is almost transparent to radio frequency electromagnetic energy. Any metallic or other radio frequency-reflecting film should be removed from the window  22  in the area of the blind cove  28 , where the radio frequency antenna is to be located, extending substantially from one vertical mullion  24  (FIG. 1) to another, across the width of the window or windows  22 . 
     Radio frequency-energy-absorbing shielding material  34 , for absorbing electromagnetic radio frequency energy, is first applied to the inside of the glass, near the top of the window  22 , just beneath a horizontal, top mullion  36  of the window. More radio frequency-energy-absorbing material  38  is also applied to the inside of the glass of the window, approximately at the height of the bottom of the false ceiling  26 . A second piece of radio frequency-energy-absorbing material  39  is placed over the radio frequency-energy-absorbing material  38  but does not extend down as far as the radio frequency-energy-absorbing material  38 . Radio frequency-energy-absorbing material (not shown) is also arranged in a vertical direction and is attached to the glass in a location near the outer, side edges of the windows  22 . The reason for and function of the energy-absorbing material attached to the inside of the window  22  will be explained below, in connection with FIG.  6 . 
     The radio frequency-energy-absorbing material  34 ,  38 ,  39 , and all of the other radio frequency-energy-absorbing material used and described in connection with the illustrative embodiment of the present invention may be a product of Cuming Corporation of Avon, Massachusetts, U.S.A. The Cuming radio frequency-energy-absorbing material is referred to by the manufacturer by the designation C-RAM MT-30 FR PSA, RF Absorber panel. It is available in 24×24 panels, preferably in thicknesses of ½ and ⅛. Both thicknesses are available with a pressure-sensitive adhesive backing, for easy application. 
     Referring now to FIG. 4, a major portion of a shield  40  is shown in place in the blind cove  28 . For ease of construction, it is preferred that the shield  40  may be at least partially pre-fabricated and then placed in the blind cove  28 , as shown in FIG.  4 . However, for purposes of description, it is more understandable and more convenient to describe the shield  40  in situ, as shown in FIG.  4 . 
     The outer, supporting structure of the shield  40  does not participate in the radio frequency-shielding process; therefore, any suitable construction material can be used. The supporting structure of the shield  40  is preferably made of duct board, wood, fiberglass, or gypsum board panels. The most prominent panels shown in FIG. 4 are a top panel  41  and a rear panel  42 . 
     A radio frequency-reflecting layer  44  is placed on the inside of the panels  41  and  42 , as well as other structural panels supporting the shield  40 , which are not shown in FIG.  4 . Radio frequency-reflecting layer  44  may be electrically-conductive material, such as metal foil that reflects radio frequency energy and is used to line the inside surfaces of all of the structural panels of the shield  40 . The radio frequency-reflecting layer  44  or metal foil may be aluminum foil. For example, extra heavy duty Reynolds Wrap™ aluminum foil can be used, however, aluminum foil with an adhesive back might be easier to mount to the inside of the panels. If metal foil-covered board such as R-Matte™ manufactured by Rmax, Inc. located in Dallas, Tex., U.S.A., is used as the structural material of the panels, the reflective foil covering the panel material should be sufficient. 
     Radio frequency-energy-absorbing material  46 , preferably about ½ thick, covers the radio frequency-reflecting aluminum foil  44 , that lines the inside of the portion of the shield structure comprised of the aluminum-lined panels  41  and  42  that are shown in FIG.  4 . The insides of all of the other aluminum foil-lined panels (not shown in FIG. 4) of the structure of the shield  40  are also similarly lined with radio frequency-energy-absorbing material. A gap is formed in the radio frequency-energy-absorbing material  46  that is mounted on the rear panel  42 . That gap is filled with an antenna-mounting board  50 . 
     The antenna-mounting board  50  is nominally a 1×4 piece of lumber fully covered with a conductive material or aluminum foil. Holes are drilled through the antenna-mounting board  50  to accommodate bolts (not shown) for mounting an antenna  52  to the board  50  and supported by the rear panel  42 , that is in contact with the end  30  of the false ceiling  26 . The bolts mount the antenna  52  to the board  50  and to the rear panel  42 . The aluminum foil that is wrapped around the board  50  is thus held in intimate electrical contact with both the antenna  52  and the aluminum foil  44  that is between the rear panel  42  and the radio frequency-energy-absorbing material  46 . 
     An opening  56  may exist at the bottom (in FIG. 4) of the shield  40 . This opening is for access to the antenna  52 , inside of the shield  40 . Referring now to FIG. 5, a cross section of a door  60  is shown, for closing that bottom opening  56  of the opening  56  in the shield  40 . This door  60  extends the full width of the shield  40 , along the width of the window  22 . The door  60  is preferably made of two pieces of structural panel material. One panel-material piece  62  is the main structure of the door  60 . A second panel-material piece  64  is a step  64  that is firmly attached along one edge of the panel-material piece  62 . When in place and closing the opening at the bottom (FIG. 4) of the shield, the door  60  is held in place by the step  64  resting on top of a lip  66  (FIG. 4) of panel material. A left end  68  of the door  60  is then preferably held in place by clips or locks  102 ,  104 ,  106  and  108  shown in FIGS. 9 and 10 and described below. 
     Returning again to FIG. 5, a piece of aluminum foil  70  covers the top of the panel pieces  62  and  64  of the door  60  and is so constructed as to make electrical contact with the aluminum foil  44  that covers the rear panel  42  of the shield  40 . Radio frequency-energy-absorbing material  72  covers the aluminum foil  70  on top of the panel-material piece  62 . More radio frequency-energy-absorbing material  74  covers the aluminum foil  70  over the panel-material step piece  64 , overlapping the radio frequency-energy-absorbing material  72 , to prevent any gaps. The step piece  64  fits tightly into a gap  76  (FIG. 4) between the radio frequency-energy-absorbing material  46  on the rear panel  42  of the shield and the lip  66  of panel material. The radio frequency-energy-absorbing material  74  is not as long as the panel-material step piece  64  and abuts the radio frequency-energy-absorbing material  46 . 
     Referring now to FIG. 6, the sectional view of FIG. 1 is shown with the door  60  of FIG. 5 shown in place. In this view (FIG.  6 ), it will be noted that the radio frequency-energy-absorbing material  72 , of the door  60 , abuts the radio frequency-energy-absorbing material  38  and underlies the bottom of the radio frequency-energy-absorbing material  39 . The step  64  of the door  60  rests on the lip  66 , and the radio frequency-energy-absorbing material  74  abuts the radio frequency-energy-absorbing material  46  on the rear panel  42 . 
     The top frame  32  of the window treatment is then reinstalled, shown in FIG. 6 with a blind hanging from it. However, the window treatment should not be positioned so close to the door  60  that the top frame  32  prevents the door  60  from opening, unless it is intended that the window treatment, and its top frame  32  be removed any time that the door  60  is to be opened. 
     FIG. 7 is a cross-section view from the top of the shield  40 , taken in the direction of lines  7 - 7  of FIG.  6 . The rear panel  42  supports the aluminum foil  44  and the radio frequency-energy-absorbing material  46 , along with the mounting board  50  and the antenna  52 . In addition, structural side panels  86  are shown, lined with aluminum foil  88  and with radio frequency-energy-absorbing material  90  over the aluminum foil. 
     Referring now to FIG. 8, there is shown a sectional view from the same direction as FIG.  7 . However, additional parts of the structural support of the door  60  are shown. Two support arms  94  and  96 , each having an inner end  95  and an outer end  97 , are attached, for support, at their inner ends  95 , to the bottom of the rear panel  42 . The support arms  94  and  96  project into the opening  56  of the shield. These two support arms are also suspended from the top panel  41  (FIG. 4) by two dowels  98  and  100 , which are attached near the outer ends  97  of the support arms  94  and  96 . These two dowels are of an electrically-non-conducting material, preferably such as wood or fiberglass, so as to be substantially transparent to radio frequency energy and are shown and described more fully in connection with FIGS. 9 and 10. 
     The support arms  94  and  96  are engaged by rotating locks  102  and  104 . Two more rotating locks  106  and  108  engage lips  105  on the side panels  86 . The four rotating locks  102 ,  104 ,  106 , and  108  are mounted proximate to the left end  68  of the door  60  and hold the door in place, as shown more clearly in FIGS. 9 and 10. The four rotating locks can be better understood by the description (below) in connection with those latter two figures. The four rotating locks can be of a type rotatable by a screwdriver or wrench or can even be equipped with an internal key lock, in order to discourage unauthorized exploration of the antenna. 
     FIG. 9 is a front view of the shield  40  as it would be presented to the windows  22 . The dowels  98  and  100  are shown suspending the support arms  94  and  96  to prevent the weight of the door  60  from putting excessive bending stress on the attachment of the support arms  94  and  96  to the rear panel  42  (FIG.  8 ). The four rotating locks  102 ,  104 ,  106 , and  108  are also illustrated in their positions engaging the support arms  94  and  96  and the lips  105 . 
     The partial perspective view of FIG. 10 shows, in greater detail, the cooperation between the door  60  and the support arms  94  and  96 . There are gaps  112  and  114  in the radio frequency energy-absorbing material  72  and  74  to accommodate the support arms  94  and  96 . The support arms  94  and  96  are topped with layers  101  of aluminum foil and radio frequency-energy-absorbing material to cover and thus compensate for the gaps  112  and  114  in the door  60 . The rotating lock  106  is shown in its unlocked position, and the rotating locks  102  and  104  are arbitrarily illustrated in their locked positions. The layers  101  of foil and radio frequency-energy-absorbing material may be cut or notched  103  to accommodate the rotating locks  102  and  104 . 
     The inside of the windows  22  that cover the antenna  52  and the shield  40  are preferably covered with an electrically non-conducting opaque or translucent film  120  (FIG.  1 ). The purpose of the opaque or translucent film is to avoid disrupting the esthetic appearance of the building or calling the attention of passers-by to the presence of a radio frequency antenna. The antenna is high enough and directional enough to keep excessive radio frequency radiation away from passers-by at sidewalk level. The principle purpose of the shield  40  is to protect occupants of the building whose work locations are proximate the antenna. 
     Theory of Operation 
     When the antenna  52  is emitting radio frequency energy, the preferred direction of emission is directly out through the windows  22 . 
     To that end, any radio frequency electromagnetic emissions that do not go out through the windows  22  will pass through the radio frequency-energy-absorbing material on the inside of the shield and suffer substantial attenuation. Any radio frequency electromagnetic energy that passes through the radio frequency-energy-absorbing material on the inside of the shield reflects off of the aluminum foil, back through the radio frequency-energy-absorbing material, in the opposite direction. That reflected radio frequency electromagnetic energy is further attenuated by the radio frequency-energy-absorbing material on its return journey. That twice-attenuated radio frequency electromagnetic energy then has a low enough energy level to be harmless as it re-enters the inside of the shield  40 . That low energy level is inadequate to disrupt the desired radio frequency emissions and certainly inadequate to be injurious if a minute amount of it should exit through the windows  22 . 
     As radio frequency electromagnetic energy passes through the glass of the windows  22 , a slight amount is reflected back into the interior of the shield  40 . Any such radio frequency energy that is reflected directly back to the antenna  52  has an effect on the antenna standing wave ratio and the efficiency of propagation through the glass, but does not effect the shielding. However, a percentage of the antenna emissions does not strike the glass at a right angle to the surface of the glass. This is the purpose of the radio frequency-energy-absorbing material  34 ,  38 , and  39  that is located against the windows  22  (see FIGS. 3,  4 , and  6 ). Also, additional radio frequency-energy-absorbing material (not shown) is attached to the windows  22  in the regions of the side panels  86 . 
     Radio frequency electromagnetic emissions that strike the glass windows at an oblique or acute angle to the surface of the glass reflect away from the glass and are absorbed by the radio frequency-energy-absorbing material that lines the interior of the shield  40 . However, some of that energy is also refracted as it enters the glass and reflects off of the outside surface of the glass, back into the interior of the glass. That radio frequency energy that obliquely reflects and refracts within the pane of the glass window can travel inside of the pane of the glass until it passes through the interior surface of the glass beyond the control of the shield  40 . That escaping radio frequency energy might, over the course of a working year, provide an undesirable amount of exposure to any person whose work location is proximate the windows  22 . 
     In order to protect any person who might spend a working career near a radio frequency antenna, the radio frequency-energy-absorbing material  34 ,  38 , and  39  and additional radio frequency-energy-absorbing material (not shown) to which the side panels  86  abut—has been placed directly in contact with the inside surface of the windows  22 . This absorbing material that is attached directly to the inside surface of the window has a substantial length of its contact with the window, along the path that the energy would have to take as it refracts and reflects within the body of the glass window. That part of the absorbing material that extends along the window in a direction generally toward the antenna maximizes the angle at which the radio frequency energy strikes the interior surface of the glass. Therefore, the obliqueness of the angle at which the energy strikes the glass is minimized. Minimizing obliqueness of the angle of incidence of the energy as it strikes the glass also minimizes the refraction of the energy within the glass. Minimizing the obliqueness of the angle of incidence and the resulting refraction also minimizes the obliqueness of the angle of reflection of the energy as it exits the glass at the exterior surface of the glass. 
     A percentage of the energy that reflectively travels within the body of the glass exits through the interior and exterior surfaces of the glass at each reflection. By extending the radio frequency-energy-absorbing material, e.g.  34 ,  38 , and  39 , along the interior surface of the glass, transmission of that energy traveling within the glass through the interior surface of the glass and into the interior of the building proximate the glass is minimized. 
     Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art appreciate that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown and that the invention has other applications in other environments. This application is intended to cover any adaptations or variations of the present invention. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described herein.