Patent Publication Number: US-2005134521-A1

Title: Frequency selective surface to suppress surface currents

Description:
BACKGROUND  
      Currently the United States Federal Aviation Administration (FAA) prohibits the use of intentional radiators (e.g., cellular phones, WLANs, two way pagers) at any time that the aircraft is in flight or preparing for flight. Unintentional radiators (e.g., personal computers, PDAs) may be used at the discretion of the pilot when the aircraft is 10,000 feet or more above ground level. This is due in part to possible issues of interference caused to aircraft systems by these electronic devices. Accordingly, manufacturers of electronic devices and aircraft operators are motivated to find ways to alleviate this potential problem. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The present invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:  
       FIG. 1  is a diagram illustrating a wireless structure in accordance with one embodiment of the present invention;  
       FIG. 2  is a top view illustrating a portion of a frequency selective surface structure in accordance with an embodiment of the present invention;  
       FIG. 3  is a cross-sectional view of the structure of  FIG. 2  through line  3 - 3 ;  
       FIG. 4  is a cross-sectional view of a portion of a frequency selective surface structure in accordance with an embodiment of the present invention;  
       FIG. 5  is a top view illustrating a portion of a frequency selective surface structure in accordance with an embodiment of the present invention;  
       FIG. 6  is a cross-sectional view of the structure of  FIG. 5  through line  1 - 1 ;  
       FIG. 7  is a bottom view illustrating a portion of a frequency selective surface structure in accordance with an embodiment of the present invention;  
       FIG. 8  is a cross-sectional view of the structure of  FIG. 7  through line  2 - 2 ;  
       FIG. 9  is a top view illustrating a portion of a frequency selective surface structure in accordance with an embodiment of the present invention; and  
       FIG. 10  is block diagram illustrating a portion of a system in accordance with an embodiment of the present invention. 
    
    
      It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.  
     DETAILED DESCRIPTION  
      In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.  
      In the following description and claims, the terms “include” and “comprise,” along with their derivatives, may be used, and are intended to be treated as synonyms for each other. In addition, in the following description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other.  
      The terms “over” and “overlying,” may be used and are not intended as synonyms for each other. In particular embodiments, “overlying” may indicate that two or more elements are in direct physical contact with each other, with one on the other. “Over” may mean that two or more elements are in direct physical contact, or may also mean that one is above the other and that the two elements are not in direct contact.  
      The following description may include terms, such as over, under, upper, lower, top, bottom, etc. that are used for descriptive purposes only and are not to be construed as limiting. The embodiments of an apparatus or article of the present invention described herein can be manufactured, used, or shipped in a number of positions and orientations.  
       FIG. 1  is a diagram illustrating a wireless structure  10  in accordance with one embodiment of the present invention. Wireless structure  10  may include a base  20 , a frequency selective surface (FSS)  30 , and an antenna  40 .  
      In one embodiment, antenna  40  may be an aircraft very high frequency (VHF) antenna. VHF is the radio frequency range from 30 megahertz (MHz) (wavelength 10 meters) to 300 MHz (wavelength 1 m). In one example, antenna  40  is an aircraft VHF communications antenna having a frequency of operation ranging from about 118 MHz to about 137 MHz. In other words, antenna  40  may be a VHF communications antenna coupled to receive radio frequency (RF) signals having a carrier frequency ranging from about 118 megahertz (MHz) to about 137 MHz. The VHF communications antenna may be used in an aircraft&#39;s VHF communications system which is used for air traffic control communications. In another example, antenna  40  is an instrument landing system (ILS) aircraft antenna or a VOR aircraft antenna having a frequency of operation ranging from about 108 MHz to about 118 MHz. Both the ILS and VOR antennas may be receive only antennas coupled to ILS and VOR navigation and landing aid systems of an aircraft. VOR may refer to Very High Frequency Omnirange that allows the range to a ground based beacon to be determined. In these embodiments, antenna  40  may be a monopole antenna made of aluminum and may be triangular or trapezoidal-shaped.  
      In the embodiment where antenna  40  is an aircraft antenna, base  20  may be the fuselage of the aircraft, wherein FSS  30  and antenna  40  are coupled to the fuselage. As is shown in  FIG. 1 , FSS  30  may be circular. In addition, FSS  30  may be curved or conformal to the surface of the fuselage. In one embodiment, FSS  30  may include a plurality of conductive patches arranged over a top surface of a dielectric material in a cyclical pattern. In this embodiment, FSS  30  may also include a ground plane over a bottom surface of the dielectric material, wherein the conductive patches are coupled to a ground plane by a conductive via.  
      According to some reports, it is possible that electronic devices such as FM radios, cellular phones, personal digital assistants (PDA), or portable personal computers (PCs) operated within an aircraft may provide interference to aircraft ILS, VOR, and VHF communication systems. Emissions from the electronics devices within an aircraft may couple through the windows to the external surface of the fuselage, thereby creating RF surface currents. These surface currents may also be referred to as inhomogeneous plane waves, and may cause interference problems with the external avionic communication and navigation antennas of the aircraft. In accordance with an embodiment of the present invention, FSS  30  may be coupled to the fuselage adjacent to antenna  40 , and may suppress undesirable surface currents, thereby mitigating or eliminating interference problems and allowing the use of electronic devices within the aircraft by passengers.  
      Examples of FSS  30  are discussed below. Generally, FSS  30  is a structure that may conduct direct currents (DC) but may reduce or suppress alternating currents (AC) within a particular frequency range. In other words, FSS  30  may be formed or manufactured in a way to prevent propagation of radio frequency (RF) surface currents within a frequency band gap. This band gap frequency range may be referred to as a “forbidden frequency band.” The band gap of FSS  30  may also be referred to as the resonant frequency of FSS  30 . In some applications, FSS  30  may also be referred to as a high impedance surface or an artificial magnetic conductor (AMC).  
      Generally, the band gap or forbidden frequency band of FSS  30  may be altered by altering the size of FSS  30 . In particular, altering the thickness of FSS  30  or the size of some of the components of FSS  30  may alter the band gap of FSS  30 .  
      FSS  30  may be positioned adjacent to antenna  40  to lessen or suppress RF surface currents in the VHF band from propagating along the conductive back plane of FSS  30 . In one example, FSS  30  may be spaced apart from antenna  30  by about ?? cm. Placing FSS  30  adjacent to antenna  40  may reduce or eliminate interference from electronic devices located within the aircraft.  
      Surface current mitigation may be used to achieve a high impedance surface at the frequency of interest. Surface currents may propagate on smooth metal surfaces until they are scattered by discontinuities in the surface texture. By creating a high impedance surface near an antenna, the intrusive surface currents may not propagate, thereby ceasing to cause interference to the antenna. Several techniques may be used to isolate antennas from these surface currents. For example, choke rings or corrugated slabs may be used to suppress or mitigate surface currents, however, these structures may be relatively large in size since they must be a quarter-wavelength (λ/4) thick to effectively suppress surface currents. For VHF antennas, this implies that the choke rings or corrugated slabs be about 0.5 meters (m) thick to meet the quarter-wavelength requirement. Such a relatively large structure attached to the fuselage of an aircraft may not be practical due to the drag it would create for the aircraft. A choke ring is a structure comprised of a plurality of concentric rings.  
      FSS  30  may have a relatively small profile and may be much smaller than λ/4. Examples discussed below provide FSS structures that may be used with VHF antennas and have thicknesses ranging from about 0.5 centimeters (cm) to about 1.3 cm. An FSS having a thickness ranging between about 0.5 cm to about 1.3 cm may be coupled to the fuselage of an aircraft and present negligible drag and may reduce surface currents by up to about 30 dB.  
      An embodiment of FSS  30  is illustrated in  FIG. 2 .  FIG. 2  is a top view illustrating a portion of FSS  30  in accordance with an embodiment of the present invention. In this embodiment, FSS  30  may include a plurality of conductive patches  45 , conductive vias  50 , and a ground plane  55 .  
       FIG. 3  is a cross-sectional view of the structure illustrated in  FIG. 2  through section line  3 - 3 . As is illustrated in  FIG. 3 , vias  50  may be coupled at one end to ground plane  55  and at the other end to conductive patches  45 . FSS  30  may further include an electrically insulating or dielectric material (not shown in  FIGS. 2 and 3 ) sandwiched between ground plane  55  and conductive patches  45 . Examples of the dielectric material may include a fiber reinforced polymer or a copper laminate epoxy glass (e.g., FR4). In another embodiment, the dielectric material may be a dielectric layer that incorporates ionizing particles. For example, an ionizing material may be formed within a dielectric layer. In this embodiment, the ionizing material may become ionized in the event of a lightning strike, and conduct current to ground since conductive vias  50  alone may not be sufficient to carry the high current.  
      Conductive vias  50  may also be referred to as posts, poles, pillars, or columns, and ground plane  55  may also be referred to as a conductive back plane. Conductive patches  45  may also be referred to as conductive elements, plates, or pads. In the embodiment illustrated in  FIG. 2 , conductive patches  45  may be substantially square-shaped, although the scope of the present invention is not limited in this respect. In other embodiments, conductive patches  45  may be substantially rectangular, triangular, hexagonal, circular or irregularly shaped.  
      As is illustrated in  FIG. 3 , FSS  30  may effectively be considered a lumped circuit element modeled by a second order LC resonance circuit. A capacitive element or capacitor may be formed using conductive patches  45  and ground plane  55 . For example, conductive patches  45  may form the upper plate of a capacitor and ground plane  55  may form the lower plate of the capacitor. As may be appreciated, at least four capacitors are illustrated for FSS  30  in  FIG. 2 , wherein ground plane  55  serves as a common lower plate of these four capacitors. These capacitors may be referred to as printed capacitors since their upper and lower plates may be formed by patterning a conductive material such as, for example, copper.  
      Conductive patches  45  may be coupled to ground plane  55  by inductive vias  50 . The LC resonance of FSS  30  may enable a zero degree phase shift at its resonant frequency. This effectively emulates free space, where surface currents are not supported. Because of its ability to suppress surface currents, FSS  30  may be effective in mitigating interference at a particular frequency of interest, e.g., in the VHF band.  
      Referring to  FIGS. 2 and 3 , in one embodiment, FSS  30  may be formed by forming a layer of a conductive material such as, for example, copper, overlying a top surface of a dielectric material. The conductive layer may be bonded to the top surface of the dielectric material using, e.g., an adhesive. The conductive layer may be patterned using, for example, an etch process to form the plurality of conductive patches  45 . Similarly, a layer of conductive material such as, for example, copper, may be formed overlying and adhesively bonded to a bottom surface of the dielectric layer to form ground plane  55 .  
      In one embodiment, after patterning the conductive layer on the top surface of a dielectric layer to form conductive patches  45 , holes (not shown) may be formed in the dielectric layer. These holes may be filled or plated with an electrically conductive material such as, for example, copper, to form conductive vias  50 . Alternatively vias may be formed by aluminum rivets attaching the FSS material to the aircraft fuselage. Vias  50  may be formed at least between the top and bottom surfaces of the dielectric material, and may be formed so that one end of a via  50  is planar with an exposed surface of conductive patch  45  and so that the other end of via  50  is planar with an exposed surface of ground plane  55 . Vias  50  may also be formed at the geometric centers of conductive patches  45  or may be formed off-center.  
      One embodiment of an FSS  30  that may be placed on a aircraft fuselage adjacent to aircraft ILS, VOR, or VHF communications antennas is discussed as follows. In this embodiment, FSS  30  may have a thickness ranging from about 0.5 cm to about 1.3 cm. Vias  50  may have a length approximately equal to the thickness of the dielectric material, e.g., the length of conductive via  50  and the dielectric material may range from about 0.5 cm to about 1.3 cm. The diameter of conductive via  50  may be about 0.16 cm.  
      The thickness of ground plane  55  may be about 0.005 cm and the thickness of conductive patches  45  may also be about 0.005 cm. The length and width of conductive patches  45  may be about 3.8 cm to form a 3.8 cm×3.8 cm square, and conductive patches  45  may be spaced apart from each other by about 0.05 cm.  
      Accordingly, FSS  30  may be placed adjacent to a VHF antenna and tuned to the operating frequency of the VHF antenna. Tuning FSS  30  may refer to adjusting or sizing the thickness of FSS  30  and the surface area or volume of conductive patches  45  to alter the LC characteristics of FSS  30  to suppress radio frequency (RF) surface currents in the VHF band from propagating along ground plane  55 .  
      In one embodiment, FSS  30  may be placed adjacent to an aircraft VHF communications antenna. In this embodiment, FSS  30  may have a band gap frequency centered at about 127 MHz and ranging from about 118 MHz to about 137 MHz. In another embodiment, FSS  30  may be placed adjacent to an aircraft ILS or VOR antenna. In this embodiment, FSS  30  may have a band gap frequency centered at about 113 MHz and ranging from about 108 MHz to about 118 MHz. Although FSS  30  has been described in some embodiments as being placed adjacent aircraft antennas, this is not a limitation of the present invention. In other embodiments, FSS  30  may be placed adjacent to non-aircraft antennas.  
      In an alternate embodiment, FSS  30  may be a flexible structure attached to the fuselage of an aircraft by rivets, wherein the rivets replace the conductive vias  50  and serve as the inductive elements of FSS  30 . Using rivets in place of conductive vias  50  to attach FSS  30  to the fuselage may eliminate ground plane  50 , wherein the fuselage may serve as the ground plane of FSS  30 .  
       FIG. 4  is a cross-sectional view of another embodiment of FSS  30 . In this embodiment, FSS  30  may include conductive patches  60  and  70 , a ground plane  80 , conductive vias  85 , and a dielectric material  90 .  
      Further, in this embodiment, FSS  30  may be realized by three metal layers  60 ,  70 , and  80 , whereby the top layers  60  and middle layers  70  are shifted replicas of each other, achieving capacitive loading through overlap capacitance. This may reduce the resonant frequency of FSS  30  and may also reduce bandwidth of the bad gap frequency of FSS  30 . This structure may be fabricated at low cost using PC board manufacturing. In one embodiment, FSS  30  may have a thickness ranging from about 0.5 cm to about 1.3 cm. Alternatively the structure may be fabricated using a flexible laminate that may be easily shaped to follow the curvature of the aircraft fuselage. In this case the conductive vias may be formed by flush rivets in place of the plated holes.  
       FIG. 5  is a top view illustrating a portion of FSS  30  in accordance with an embodiment of the present invention. In this embodiment, FSS  30  may include patterned conductive materials  110  over a top surface of a dielectric material  120 , wherein each of the patterned conductive materials  110  include an inductor  130  and a conductive plate  140 , wherein conductive plate  140  is connected to inductor  130 . Conductive plate  140  may form one plate of a parallel plate capacitor.  
       FIG. 6  is a cross-sectional view of the structure illustrated in  FIG. 5  through section line  1 - 1 . FSS  30  may further include conductive vias  150  formed in dielectric material  120 . In one embodiment, vias  150  are physically separated from each other and are formed extending between at least a top surface  121  and a bottom surface  122  of dielectric material  120 . FSS  30  may further include an electrically conductive plate  160  overlying surface  122  of dielectric material  120 .  
      In one embodiment, dielectric material  120  may be a dielectric substrate. Although the scope of the present invention is not limited in this respect, dielectric material  120  may be any material suitable for a printed circuit board substrate such as a fiber reinforced polymer or a copper laminate epoxy glass (e.g., FR4). In addition, dielectric material  120  may include ionizing particles, although the scope of the present invention is not limited in this respect.  
      FSS  30  may be formed by forming a layer of a conductive material such as, for example, copper, overlying surface  122  of dielectric material  120  to form conductive plate  160 . An adhesive may be used to bond conductive plate  160  to surface  122 . Similarly, a layer of conductive material such as, for example, copper, may be formed overlying and adhesively bonded to surface  121  of dielectric material  120 . This conductive layer on surface  121  may be a single layer or multiple layers of conductive material and may be patterned using, for example, an etch process, to form inductors  130  and conductive plates  140 .  
      In one embodiment, after patterning the conductive layer on surface  121 , holes (not shown) may be formed in dielectric material  120 . These holes may be filled or plated with an electrically conductive material such as, for example, copper, to form conductive vias  150 . Vias  150  may be formed at least between surfaces  121  and  122  of dielectric material  120 , and may be formed so that one end of a via  150  is planar with an exposed surface of inductor  130  and so that the other end of via  150  is planar with an exposed surface of conductive plate  160 . Vias  150  may also be formed at the geometric centers of conductive plates  140  or may be formed off-center. In one embodiment, via  150  may have a length approximately equal to the thickness of dielectric material  120  and a diameter of about 0.16 cm. Although the scope of the present invention is not limited in this respect, the thickness of FSS  30  in this embodiment may range from about 0.5 cm to about 1.3 cm.  
      In one embodiment, inductors  130  are substantially rectangular-shaped conductors, each having a length of about ?? centimeters and a height of about ?? centimeters. The thickness of conductive plate  160  may be about 0.005 cm and the thickness of conductive plate  140  and inductor  130  may both be about 0.005 cm. The thickness of dielectric material  120  and the length of via  150  may both range from about 0.5 cm to about 1.3 cm.  
      Conductive plate  160  may serve as a conductive ground plane. A capacitive element or capacitor may be formed using conductive plates  140  and  160 . For example, conductive plate  140  may form the upper plate of a capacitor and conductive plate  160  may form the lower plate of the capacitor. As may be appreciated, at least four capacitors are illustrated in FSS  30  illustrated in  FIGS. 5 and 6 , wherein conductive plate  160  serves as a common lower plate of these four capacitors. These capacitors may be referred to as printed capacitors since their upper and lower plates may be formed by patterning a conductive material.  
      In the embodiment illustrated in  FIG. 5 , conductive plates  140  may be substantially square-shaped, although the scope of the present invention is not limited in this respect. In other embodiments, conductive plates  140  may be substantially rectangular, triangular, hexagonal, circular or irregularly shaped.  
      Inductors  130  formed overlying surface  121  may be referred to as printed inductors, inductive strips, or strip inductors. Inductor  130  may be formed between conductive plate  140  and conductive via  150 . In addition, inductor  130  and via  150  may be formed so that a portion of inductor  130  surrounds an upper end of via  150 , although the scope of the present invention is not limited in this respect. Further, printed inductor  130  and conductive via  150  may be formed substantially at the geometric center of conductive plate  140 .  
      In the embodiment illustrated in  FIG. 5 , inductors  130  may be formed by patterning a single layer of conductive material and may be substantially rectangular-shaped, straight conductors having no turns, although the scope of the present invention is not limited in this respect. In other embodiments, inductor  130  may be a coil having at least a partial turn, e.g., one turn, or have a spiral shape as is shown in the embodiment illustrated in  FIG. 9 . Altering the shape and length of inductor  130  may alter the inductance of inductor  130 .  
      FSS  30  may be coupled or in close proximity to an antenna or multiple antennas such as, for example, VHF antennas. In this example, FSS  30  may have an equivalent circuit of multiple coupled resonant circuits formed from inductors  140 , vias  150 , and conductive plates  140  and  160 . Each resonant circuit may include an inductive element and a capacitive element, wherein the inductive element includes inductor  130  and conductive via  150 . The capacitive element may include conductive plates  140  and  160 .  
      The resonance or resonant frequency may be the frequency where the reflection phase passes through zero. At this frequency, a finite electric field may be supported at the surface of conductive plate  160 , and an antenna or multiple antennas may be placed adjacent to the surface without being shorted out. The bandwidth of the band gap frequency of FSS  30  may be altered by adjusting the inductance:capacitance (L:C) ratio of the resonant circuits. For example, the bandwidth may be increased by increasing the inductance and decreasing the capacitance.  
      The bandwidth of the band gap frequency of FSS  30  may be increased by altering the inductance of the inductive elements. In the embodiment illustrated in  FIGS. 5 and 6 , inductors  130  are serially connected to via  150 , and therefore, the length of vias  150  and/or the length of inductors  130  may be increased to increase the inductance of the resonant circuits, thereby increasing the bandwidth of the band gap. In this embodiment, the frequency of FSS  30  may also be lowered by using printed inductors to increase the value of the inductive component of the resonant circuit. Other methods for altering the frequency of FSS  30  may include altering the size of conductive plates  140  and/or altering the position of vias  150  relative to the center of capacitive plates  140 . FSS  30  may also be referred to as a photonic band gap structure or an artificial magnetic conductor.  
      Turning to  FIGS. 7 and 8 , another embodiment of FSS  30  is illustrated.  FIG. 7  illustrates a bottom view of FSS  30  and  FIG. 8  illustrates a cross-sectional view of FSS  30  through section line  2 - 2 . In this embodiment, printed inductors  180  may be formed overlying bottom surface  122  of dielectric material  120 .  
      In this embodiment, inductors  180  may be connected between via  150  and conductive plate  160 . Inductors  180  and conductive plate  160  may be formed by pattering a single layer of conductive material using, for example, an etch process. In this embodiment, vias  150  and inductors  130  and  180  form inductive elements of the resonant circuits of FSS  30 . As may be appreciated, the inductance of the inductive element may be altered by including inductors  180  and altering the length of inductors  180 .  
      Inductors  180  may be formed at substantially right angles (about 90 degrees) relative to inductors  130 . By forming inductors  130  and  180  at right angles to each other, the fields due to the inductors may not cancel each other.  
      Turning to  FIG. 9 , a top view of FSS  30  in accordance with another embodiment is illustrated. FSS  30  may include conductive plates  240  overlying a dielectric material  220 . FSS  30  may further include conductive vias  250  and inductors  230 , wherein an inductor  230  may be connected between a via  250  and a conductive plate  240 . Vias  250  may be formed in dielectric material  220  and may extend to a bottom surface (not shown) of dielectric material  220 . FSS  30  may further include a ground plane (not shown) overlying the bottom surface of dielectric material  220 .  
      In this embodiment, dielectric material  220 , inductors  230 , conductive plates  240 , and vias  250  may be composed of the same or similar materials as dielectric material  120 , inductors  130 , conductive plates  140 , and vias  150 , respectively. A single layer of conductive material may be patterned using, for example, an etch process, to form inductors  230  and conductive plates  240 . In the embodiment illustrated in  FIG. 5 , inductors  230  may be spiral-shaped.  
      In this embodiment, FSS  30  may have an equivalent circuit of multiple coupled resonant circuits formed from inductors  240 , vias  250 , conductive plates  240  and a ground plane (not shown in  FIG. 5 ). Each resonant circuit may include an inductive element and a capacitive element, wherein the inductive element is formed by inductor  230  and via  250 . The capacitive element may be formed by conductive plates  240  and the ground plane.  
      Turning to  FIG. 10 , is a block diagram illustration a portion of a system  300  in accordance with an embodiment of the present invention. In this embodiment, system  300  may include antenna  40  and FSS  30 . In addition, system  300  may include a wireless receiver  310  coupled to receive RF signals from antenna  40 . Wireless receiver  310  may be coupled to antenna  40  using, for example, a coax cable, wherein the outer mesh conductor of the coax cable is coupled to the ground plane of FSS  30 .  
      In one embodiment, system  300  may be an aircraft very high frequency (VHF) communications system. In this embodiment, antenna  40  may be an aircraft VHF communications antenna coupled to receive radio frequency (RF) signals having a carrier frequency ranging from about 118 megahertz (MHz) to about 137 MHz. Wireless receiver  310  may be part of the aircraft VHF communications system and may be coupled to receive the RF signals from antenna  40 .  
      In another embodiment, system  300  may be an aircraft navigation or landing aid system such, for example, of an aircraft instrument landing system (ILS) or an aircraft Very High Frequency Omnirange (VOR) system. In this embodiment, antenna  40  may be an aircraft ILS or VOR antenna coupled to receive radio frequency (RF) signals having a carrier frequency ranging from about 108 megahertz (MHz) to about 118 MHz. Wireless receiver  310  may be part of the aircraft ILS or VOR system and may be coupled to receive the RF signals from antenna  40 .  
      While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.