Abstract:
A choke ring apparatus for attenuation of electromagnetic waves in a mobile platform fuselage includes a ground plane mounted on a surface of the fuselage. The choke ring has an axial circular window and a series of concentric circular ring segments on the ground plane arranged coaxially about the axis of the window. The circular ring segments extend from the ground plane. The ring segments defining at least one groove therebetween. The ring segments have a flat ridge at the edge, and each groove has a depth defined by a pair of adjacent ring segments. The width of the flat ridge surfaces and a width of the groove between adjacent ring segments are approximately equal. The depth of the groove is determined based on a predetermined resonant frequency, such that the choke ring apparatus selectively attenuates electromagnetic waves in a region of the resonant frequency when propagating through the window.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 11/608,265 filed Dec. 8, 2006, now U.S. Pat. No. 7,375,688, which is hereby incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention is directed to a method and apparatus for controlling electromagnetic interference in mobile platforms, and more particularly to choke ring structures mounted in a mobile platform fuselage to attenuate or eliminate interference caused by portable electronic devices carried onboard by passengers. 
   BACKGROUND OF THE INVENTION 
   There is concern in the aviation industry that portable electronic devices (PEDs) can interfere with mobile platform electronics systems (also referred to as avionics in mobile platform and space vehicle electronic applications). Mobile platforms as used herein include aircraft and space vehicles, as well as land-based and nautical transportation vehicles. Measurements of radiated energy levels in PEDs have been known to exceed earlier mobile platform equipment qualification standards, which afford less protection than current equipment standards and mobile platform certification requirements. This, combined with the increasingly widespread use of cell phones, could pose a threat to air safety. 
   There are two types of PEDs. First, there are those PEDs that intentionally transmit a signal, known as intentional transmitters. Intentional transmitters transmit a signal in order to accomplish their function. Intentional transmitters include cell phones; pagers; two-way radios; and remote-control toys. The second type of PED is the non-intentional transmitter. Non-intentional transmitters do not have to transmit a signal in order to accomplish their function. However, like most electrical devices, they emit some level of radiation. Examples of non-intentional transmitters include compact-disc players; tape recorders; hand-held games; laptop computers and personal digital assistants (PDAs); and laser pointers. 
   The Federal Aviation Administration (FAA) and other international aviation regulatory agencies have expressed concern that PEDs may interfere with navigational instruments aboard the mobile platform. There have been numerous anecdotal reports of incidents in which the use of PEDs apparently created anomalous or erroneous instrumentation signals in passenger mobile platform. The PEDs most frequently reported as being a source of interference are laptop computers. The most frequent mobile platform systems reportedly affected by a suspected PED interference source are the navigation systems. The FAA has implemented rules restricting the use of PEDs on commercial airlines. Such rules prohibit operation of a PED on an airplane unless the airline has determined that the device will not cause interference with the navigation or communication systems of the mobile platform. There are some exceptions, for example, portable voice recorders, hearing aids, heart pacemakers, and electric shavers, which may be used, and the rules do not apply at all in some cases, e.g., private planes flying under visual flight rules. 
   The FAA also recommends that the use of PEDs be prohibited during the takeoff and landing phases of flight below 10,000 feet, in order to avoid potential electronic interference with aircraft systems, and to avoid the potential for passengers to miss safety announcements. In response to the incidents and government regulations, airlines have attempted to restrict the use of portable electronic devices. Airline policies generally divide PEDs into three categories: those that may never be used, those that may always be used, and those that may be used only at certain times. PEDs such as hearing aids, pacemakers, electronic watches, and one-way pagers may generally be used at any time during flight. Conversely, most airlines prohibit certain PEDs at any time, e.g., AM/FM radios, television sets, two-way pagers, and CB radios. A third category of PEDs may be operated at specified times, i.e., prior to departure and after the mobile platform has reached an altitude of 10,000 feet. In particular, when the mobile platform is descending all PEDs in this category must be turned off. The PEDs subject to these restrictions include CD players, laptop computers, electronic video games, and GPS navigation sets. The pilot must be notified that all PEDs have been turned off before departure and/or descent. As for the use of cellular phones, many airlines permit passengers to place and receive calls onboard while the mobile platform is still at the gate. Otherwise, cell phones may not be used during airline takeoff and landings, or during flight. 
   As the use of passenger carry-on portable electronic devices (PEDs) becomes more prevalent, it may become considerably more difficult to maintain Electromagnetic Compatibility (EMC) between these devices and the mobile platform communications and navigation systems. The portability of these devices further makes it increasingly difficult to successfully implement traditional Electromagnetic Interference (EMI) solutions. The present invention provides a novel method and device with which to reduce or eliminate the potential for PED-to-mobile platform antenna coupled EMI that may occur through the coupling paths of the mobile platform fuselage window. The present invention may be implemented in new aircraft production as well as a retrofit application for aircraft in the field. 
   With the beginning of a multitude of inexpensive PEDs—i.e., electronic communications and data devices, it is likely that PEDs will consume more and more of the electromagnetic spectrum, whether by design, or unintentionally, e.g., in the form of harmonic or spurious signal emissions. In concert with an increase in the number of users and total emitter power, some of which utilize spread spectrum technology and increased power spectral content, mobile platform systems may be even more susceptible to EMI. Traditional mobile platform design does not incorporate EMI shielding in the mobile platform windows, thus allowing the possibility that electromagnetic energy can be coupled through the windows and into the externally-mounted mobile platform antennas. For example, the new Boeing 787 mobile platform design includes enlarged windows in the fuselage that may cause higher levels of PED-to-antenna coupled EMI through the windows. 
   As the quantity of PEDs in use during a flight increases, and in order to increase window size for passenger enjoyment, adequate space loss (attenuation) to mobile platform antennas may become nearly impossible. While traditional solutions, such as powering off of PEDs, may address interference at critical flight times, they do not address the potential for EMI during normal inflight conditions. 
   Choke ring ground planes have been employed in applications such as global positioning system (GPS) or various directional antennas, to reject multi-path signals from interfering with the primary signal being received by the antennas. As examples, U.S. Pat. No. 6,278,407 discloses dual-frequency choke-ring ground planes having an antenna mounted in the center of multiple grooved surfaces, and an electromagnetic filter structure which makes the depth of each groove appear to be different for each of two frequency bands, and also discloses using a groove depth which is either slightly less than a quarter-wavelength or greater than a quarter-wavelength of the second bandwidth L2. Also, U.S. Pat. No. 6,040,805 discloses a low profile ceramic choke for GPS antenna systems having concentric ring segments arranged coaxially about a circular antenna. 
   While the metallic structure of the fuselage provides shielding between internal EMI and externally mounted antennas, the windows that are positioned along the walls of the fuselage for passenger enjoyment do not adequately shield EMI from interfering with external antennas. Moreover, as mobile platform are designed to be more aesthetically pleasing to passengers, many mobile platform are designed with even larger windows. Therefore, there is a need for a means of attenuating signals that are generated within an enclosed structure such as a mobile platform, from interfering with the operation of external antenna from receiving direct signals, for example, navigation or communications signals from ground-based or satellite-based sources. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a choke ring apparatus for attenuation of electromagnetic waves in a mobile platform fuselage. The choke ring apparatus includes a ground plane mounted on a surface of the fuselage and having an axial aperture and at least one ring element attached to the ground plane arranged coaxially about a periphery of the axial aperture and extending from the ground plane. The choke ring apparatus selectively attenuates electromagnetic waves in a region of the resonant frequency when propagating through the aperture. 
   In another aspect the present invention is directed to an electromagnetic interference attenuation system for a mobile platform. The attenuation system includes a hollow fuselage having an interior surface portion and an exterior surface portion. Window portions are spaced at intervals along the fuselage. Each window portion is disposed between the interior and exterior surface portions and has a choke ring surrounding an aperture supporting the window portion; and communications antennas mounted on the fuselage exterior surface for receiving electromagnetic signals for onboard mobile platform electronic systems. Each choke ring portion has a ground plane mounted on a surface of the fuselage and has an axial aperture and at least one ring element attached to the ground plane arranged coaxially about a periphery of the axial aperture and extending from the ground plane. The choke ring apparatus selectively attenuates electromagnetic waves in a region of the resonant frequency when propagating through the aperture. 
   In yet another aspect the present invention is directed to an improved mobile platform window assembly. The improvement consists of a choke ring structure surrounding a periphery of the window assembly wherein RF energy generated internally in a mobile platform fuselage is inhibited from interfering with mobile platform antennas disposed externally on the mobile platform. 
   An advantage of the present invention is significant reduction of in-band and out-of-band coupled EMI between avionics/electronics systems and PEDs. 
   Another advantage is that the implementation of the choke ring requires only minor structural modifications to a mobile platform. 
   A further advantage is that the implementation and installation of the choke rings does not affect existing radio frequency (RF) coaxial interconnection between the externally mounted system antenna and the onboard mobile platform electronics. 
   Yet another advantage is greater flexibility for passengers using PEDs. 
   Still another advantage of the present invention is the reduced risk of interference with onboard electronics systems due to PEDs. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial view of a window from inside a mobile platform, with a choke-ring structure of the present invention. 
       FIG. 2  is a cross-sectional view of the choke-ring structure taken along the lines  2 - 2  in  FIG. 1 . 
       FIG. 3  is section of exemplary fuselage employed to measure current distribution on the fuselage. 
       FIG. 4  is a graph comparing attenuation levels for various configurations of windows with and without choke ring structures. 
       FIG. 5  illustrates the current distribution in the skin of a simulated fuselage without a choke ring structure. 
       FIG. 6  illustrates the current distribution in the skin of the simulated fuselage with a choke ring structure. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIGS. 1 and 2 , a choke ring structure  10  is integrated in the interior skin  12  of a fuselage of a mobile platform. Multiple concentric circular ring segments  14 ,  16  and  18  are coaxially disposed and project outwardly from a disk-shaped ground plane  11  having a center aperture  19  for placement of a mobile platform window  34 . Note that the circular window  34  is exemplary, and the invention includes non-circular window configurations, e.g., elliptical or rectangular. If the window  34  is non-circular, the ring structure  10  conforms substantially with the geometry of the window aperture, such that the window is encircled by or contained within the choke ring structure  10 . The ground plane  11  connects the ring segments  14 ,  16  and  18  to a common surface, e.g. the interior skin  12 , although in alternate embodiments, an exterior skin or an intermediate surface (not shown) may be the common surface. The ground plane  11  is mounted on the interior skin  12 , with the ring segments  14 ,  16  and  18  projecting generally perpendicularly to the ground plane  11 , inwards to the interior of the fuselage. Adjacent ring segments  14  and  16  are separated by a groove  15 ; similarly, ring segments  16  and  18  are separated by a groove  17 . Ring segments  14 ,  16  and  18  have flat ridge surfaces  14   a ,  16   a  and  18   a . The grooves  15 ,  17  provide dielectric gaps between ring segments  14 ,  16  and  18 . The grooves are preferably air gaps, or alternately, may include a dielectric material, e.g., ceramic, mica, glass, plastics, and oxides of various metals such as aluminum. The present invention includes almost limitless possibilities of cross sectional profiles—i.e., surface contours  14   a ,  16   a  and  18   a  are shown as flat ridges, however concave, convex, waveform, pointed, and other surface contours may be employed—and dielectric combinations for the choke ring structure  10 . The dimensions of the ridge surfaces  14   a ,  16   a  and  18   a  in relation to the depth d of the adjacent grooves  15 ,  17  is predetermined by the selected resonant frequency ω for the choke ring. The resonant frequency ω has a wavelength λ t . The depth d of the choke ring is approximately determined by the following equation:
 
 d=λ   t /3.5  Equation 1
 
   The width of the ridge surfaces  14   a ,  16   a  and  18   a  and the grooves  15  and  17  are about one quarter of the depth (d/4) of the ring segments  14 ,  16  and  18 . The quarter wavelength relationship may be more precisely optimized by iteratively adjusting the choke geometric parameters to achieve maximum coupling reduction, but the general relationship of one quarter of the wavelength is generally effective. Further, the number of rings  14 ,  16  and  18  affects the attenuation of coupled directional power. More or less ring segments may be used, however, in the example of  FIG. 1 , through the iterative adjustment process described above the inventors have determined that three ring segments are generally more effective than a single choke ring configuration (not shown). An even numbers of rings may be used as well. Further, by varying the depth of the ring segments  14 ,  16  and  18 , and the width of the grooves  15 ,  17  and ridge surfaces  14   a ,  16   a  and  18   a , the choke ring structure  10  may achieve an increased bandwidth of signal attenuation. Thus, the geometry of the choke ring structure  10  may be designed for greater bandwidth. 
   Referring next to  FIG. 3 , a simulated fuselage section  30  illustrates the principle of operation of the present invention. A source antenna  32  represents an exemplary PED as a source of EMI. The source antenna is completely surrounded by the metal skin of the fuselage  30 . The fuselage has windows  34  at intervals along the length of the fuselage, which provide a path for EMI to escape the interior of the fuselage. One or more external antennas  36  may be positioned on the exterior of the fuselage  30 . A normal passenger mobile platform includes a plurality of antennas  36  for various systems, e.g., communication and navigation systems. The antennas  36  are typically located at various locations fore and aft, and are mounted on the top or bottom centerlines of the mobile platform. For clarity,  FIG. 3  illustrates just a segment of a fuselage, having a single source antenna  32 , a single victim antenna  36  and a single window  34 . However, it will be readily understood that the present invention is applicable to multi-antennas, multi-source and multi-window arrangements such as found in a typical passenger mobile platform. 
   The choke ring structures  10  are positioned around each window  34  of the mobile platform. When EMI signals are generated by the source antenna  32 —e.g., PEDs located inside the fuselage  30 , the choke rings  10  attenuate EMI radiating through the surface of the fuselage by forming a directional pattern that is directed generally at right angles to a vertical center plane through centerlines of the windows and orthogonal to fuselage  30 . In this way, the strongest EMI is directed away from the victim antennas  36 , and the EMI signals from the source  32  diminish in strength as they propagate from the orthogonal centerline through the window  34 . Thus, while some portion of the EMI signals are received by the victim antennas  36 , the received EMI signals are greatly attenuated relative to the intended signals, and pose significantly less risk of interference with the electronics of the mobile platform than would be possible without the choke ring structures  10 . 
   While the choke ring structure  10  is incorporated into the interior skin of the mobile platform fuselage in the example shown in  FIGS. 1 and 2 , it will be understood that the CRS  10  may be installed in either or both of the inside skin  12  or the exterior skin (not shown) of the fuselage, or alternately, may be placed between the interior  12  or exterior skin. 
   The choke ring structure  10  is preferably formed of metallic, electromagnetically conductive material, such as copper beryllium, Monel®, tin plated copper clad steel, powder coated aluminum, stainless steel or similar antenna material. 
   Referring next to  FIG. 4 , a graph illustrates the results of an analysis designed to compare attenuation levels for various configurations of windows with and without choke ring structures  10 . In the configuration represented by  FIG. 3 , isolation results were determined for a cylinder or fuselage  30  having the following configuration:
         Cylinder length (l)=80 in. (approx.)   Cylinder radius (r)=42 in. (approx.)   Flat section (fs) of body=24 in.   Window ( 34 ) radius=12 in.   Resonant frequency=700 MHz   Choke ring ( 10 ) depth=d=λ t /3.5   Source antenna ( 32 )—dipole within cylinder   Victim antenna ( 36 )—simulated mobile platform blade at top centerline       

   The broken line  100  represents a response for a window configuration without the choke ring structure  10 . A solid line  102  represents a response for a choke ring structure  10  having only a single ring segment. In the simplest form in which the choke ring structure  10  includes a singular ring, a lower level of signal reduction is provided; in some instances, the single-ring configuration may be sufficient to achieve a desired level of signal attenuation. Finally, a dotted line  104  represents a response for a choke ring structure  10  having three ring segments. As indicated in  FIG. 4 , a tuned response occurred at 660 MHz, a slightly lower frequency than the designed resonant frequency. Attenuation of the EMI for the 3-ring choke ring structure  10  was approximately 20 dB greater than the configuration without a choke ring structure. There was an obvious reduction in surface current on the fuselage  30  when the EMI was predicted with the three-ring choke ring structure  10  installed around the window  34 , as opposed to when EMI was predicted without a choke ring structure  10  around the window  34 .  FIG. 5  illustrates the current distribution in the skin of the simulated fuselage  30  without a choke ring structure.  FIG. 6  illustrates the current distribution in the skin of the simulated fuselage when a choke ring structure having three ring segments was used.  FIGS. 5 and 6  were developed during the same simulation/analysis represented by  FIG. 4 .  FIGS. 5 and 6  depict the current distribution that results on the surface of the simulated fuselage. In both  FIGS. 5 and 6 , the stippled areas  106  represent areas of the fuselage surface  30  where current intensity was high. The clear regions  108  represent areas of the fuselage surface  30  having low current intensity. As is apparent from the graphic representations, the area of greater current intensity was significantly greater in the ring-less configuration than for the configuration with the three choke ring structure  10 . The results for the choke ring structure  10  having three rings  14 ,  16   18  resulted in predominantly low current intensity levels except for minor sidelobe areas in the immediate proximity of the window. 
   It is known that certain frequency bands are allocated for various aviation communications and navigation systems (e.g., GPS), and for various PEDs (cellular phones, radio and UHF broadcasts, etc.) While such frequency bands are of concern for designing the various choke ring configurations, the choke ring structure may be designed to attenuate signals in all or some of the frequency bands, depending on cost considerations, the likelihood that some PEDs are used more than others, and various other combinations. Table 1 provides a non-exclusive listing of some relevant frequency bands applicable to mobile platform communication and navigation systems. 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
                 
               Receive Band 
             
             
                 
               System Designation 
               Transmit Band (MHz) 
               (MHz) 
             
             
                 
                 
             
           
           
             
                 
               ATC/Mode S 
               1089-1091 
               1027-1033 
             
             
                 
               DME 
               1025-1150 
                962-1213 
             
             
                 
               ELT 
               406.2-406.3 
               N/A 
             
             
                 
               FD AES 
               1626.5-1660.5 
               1530-1556 
             
             
                 
               Glideslope Capture 
               N/A 
               108-112 
             
             
                 
               Glideslope Track 
               N/A 
               329-335 
             
             
                 
               GPS L2 
               N/A 
               1217-1237 
             
             
                 
               GPS L1 
               N/A 
               1565-1585 
             
             
                 
               HF 
                2-32 
                2-32 
             
             
                 
               IFF 
               1089-1091 
               1029.5-1030.5 
             
             
                 
               Localizer 
               N/A 
               108-112 
             
             
                 
               LRRA 
               4250-4350 
               4250-4350 
             
             
                 
               Marker Beacon 
               N/A 
               74.6-75.4 
             
             
                 
               MLS 
               N/A 
               5031.1-5090.7 
             
             
                 
               TARS 
               894-896 
               849-851 
             
             
                 
               TCAS 
               1029.99-1030.01 
               1089.9-1090.1 
             
             
                 
               UHF-SATCOM 
               292.5-318.5 
               243.5-270   
             
             
                 
               UHF-TV 
               N/A 
               470-880 
             
             
                 
               UHF-AM 
                  225-399.975 
                  225-399.975 
             
             
                 
               VHF-ACARS 
               131.55 
               131.55 
             
             
                 
               VHF-AM 
                  116-151.975 
                  116-151.975 
             
             
                 
               VHF-FM 
               150-173 
               150-173 
             
             
                 
               VOR/ILS 
               N/A 
               108-112 
             
             
                 
               Weather RADAR 
               9353.8-9354.2 
               9353.8-9354.2 
             
             
                 
                 
             
           
        
       
     
   
   It should be noted that the square groove configuration shown in  FIGS. 1 and 2  is exemplary, and that different profiles may be employed depending on the design criteria, for example, various frequencies that are sought to be attenuated. Thus, the bottom of the groove may be rounded, i.e., concave or convex, or may converge to a point, i.e., a sawtooth profile. Different profiles may be employed to increase the bandwidth of the response. Similarly, surfaces  14   a ,  16   a ,  18   a  can be modified for adjusting the bandwidth. Each particular application involves the same iterative process described above, with analysis and testing. Significant geometry and/or frequency changes may result in new profiles each of which follow the same iterative process. 
   While the present invention is illustrated in the embodiment of a mobile platform window configuration to reduce EMI associated with PEDs from interference with electronics systems, the choke ring structures may be used to prevent EMI generated from PEDs in other circumstances too numerous to list here. For example, passenger trains are also susceptible to EMI produced from internally operated PEDs, and would be within the scope of the present invention, as would a stationary communications station having a metal structure with windows adjacent to antennas placed outside of the communications station. Thus, the present invention may be applied in various ground-based and non-transportation related applications, as well as in mobile platform applications. 
   While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.