Patent Publication Number: US-2021167507-A1

Title: Antenna device and inverted f antenna

Description:
TECHNICAL FIELD 
     The present invention relates to a compact, low-profile antenna device having an inverted F antenna. 
     BACKGROUND ART 
     As a vehicle-mounted antenna device having an inverted F antenna for Long Term Evolution (LTE), for example, the antenna device disclosed in Patent Literature 1 is known. The antenna device is a vehicle-mounted antenna device suitable for installation on the roof of an automobile, and is configured such that three antennas for 3rd Generation (3G)/Long Term Evolution (LTE), digital audio broadcasting (DAB), and the Global Positioning System (GPS) are accommodated in a radome. Of these antennas, the 3G/LTE antenna is an inverted F antenna. 
     The inverted F antenna disclosed in Patent Literature 1 includes a planar part standing on a ground plate that acts as a grounding surface, and a short-circuit part. A portion of the planar part acts as a feeding point. The antenna device is made to operate in both the low-frequency band from 761 MHz to 960 MHz and the high-frequency band from 1710 MHz to 2130 MHz of LTE. 
     PRIOR ART DOCUMENTS 
     Patent Literature 
     [PTL 1] Japanese Patent Laid-Open No. 2013-219757 
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     Recently, demand for LTE has risen, and the d-bound frequency of the low-frequency band has been extended to 699 MHz. Also, the upper-bound frequency of the high-frequency band has also been extended up to the 5 GHz band. 
     Although the antenna device disclosed in Patent Literature 1 is usable in the low-frequency band and the high-frequency band of LTE, according to the disclosed voltage standing wave ratio (VSWR) characteristics, favorably transmitting and/or receiving a signal in the low-frequency band of LTE is difficult. 
     Meanwhile, in the high-frequency band, it is difficult to maintain stable reception of a signal over a wide band. 
     Solution to the Problems 
     An antenna device according to the present invention includes: an inverted F antenna which includes a planar part having a face opposing a grounding surface with a predetermined interval therebetween, a feeding part disposed in a plane forming a predetermined angle with respect to the grounding surface, and a short-circuit part for grounding a portion of the planar part, wherein each of the planar part and the feeding part has a plate shape, and is physically separated from each other; and wherein the planar part and the feeding part are electrically connected each other at a frequency less than or equal to a predetermined frequency. 
     An inverted F antenna according to the present invention includes: a planar part having a face opposing a grounding surface with a predetermined interval therebetween; a feeding part disposed in a plane forming a predetermined angle with respect to the grounding surface; and a short-circuit part for grounding a portion of the planar part, wherein each of the planar part and the feeding part has a plate shape, and is physically separated from each other; and wherein the planar part and the feeding part are electrically connected each other at a frequency less than or equal to a predetermined frequency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of an inverted F antenna in an antenna device according to a first embodiment. 
         FIG. 2  is a schematic diagram illustrating an exemplary configuration of a first switch circuit. 
         FIG. 3A  is an explanatory diagram illustrating the size of a component of the inverted F antenna. 
         FIG. 3B  is an explanatory diagram illustrating the sizes of components of the inverted F antenna. 
         FIG. 3C  is an explanatory diagram illustrating the sizes of components of the inverted F antenna. 
         FIG. 4  is a perspective view of an inverted F antenna according to a Comparative Example. 
         FIG. 5  is a graph comparing the VSWR characteristics of an Example and the Comparative Example. 
         FIG. 6A  is a schematic diagram of an inverted F antenna having only a single filter. 
         FIG. 6B  is a graph comparing the VSWR characteristics between Examples. 
         FIG. 7A  is a schematic diagram of an inverted F antenna having a short filter interval. 
         FIG. 7B  is a graph comparing the VSWR characteristics between Examples. 
         FIG. 8A  is a schematic diagram of an inverted F antenna having an elongated feeding part. 
         FIG. 8B  is a graph comparing the VSWR characteristics between Examples. 
         FIG. 9A  is a schematic diagram illustrating a state in which a short-circuit part is selected. 
         FIG. 9B  is a schematic diagram illustrating a state in which a short-circuit part is selected. 
         FIG. 9C  is a schematic diagram illustrating a state in which a short-circuit part is selected. 
         FIG. 10  is a graph comparing the VSWR characteristics when each of short-circuit parts  15  to  17  is selected. 
         FIG. 11  is a perspective view of an inverted F antenna according to a second embodiment. 
         FIG. 12  is a schematic diagram illustrating an exemplary configuration of a second switch circuit. 
         FIG. 13  is a graph comparing the VSWR characteristics when one of paths p 1  to p 3  is selectively closed. 
         FIG. 14A  is a schematic diagram illustrating a modification of the short-circuit parts and the second switch circuit. 
         FIG. 14B  is a schematic diagram illustrating a modification of the short-circuit parts and the second switch circuit. 
         FIG. 15A  is an external view of an inverted F antenna according to a third embodiment. 
         FIG. 15B  is an external view of an inverted F antenna according to the third embodiment. 
         FIG. 15C  is an external view of an inverted F antenna according to the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, exemplary embodiments will be described for the case of applying the present invention to an antenna device capable of transmitting and/or receiving signals in the low-frequency band (699 MHz to 960 MHz) and signals in the high-frequency band (1.7 GHz to 2.7 GHz) of LTE. The antenna device can be accommodated in the accommodating space of a radio wave-permeable housing and used as a low-profile vehicle-mounted antenna device. 
     One object of the embodiments indicated below is to provide a compact, low-profile antenna device and inverted F antenna that enable signals to be transmitted and/or received stably at a low VSWR over a range from near the lowest frequency in the low-frequency band to near the highest frequency of the high-frequency band of LTE, for example. 
     First Embodiment 
       FIG. 1  is a perspective view of an antenna device according to the first embodiment. The antenna device is provided with an inverted F antenna  1  as a main component. The inverted F antenna  1  includes a planar part  11 , a feeding part  12 , short-circuit parts  15 ,  16 , and  17 , and a first switch circuit  18 , which are provided above a substrate  10  having a metal face, the surface of the metal face being at ground potential during operation (hereinafter referred to as the “grounding surface”). For the substrate  10 , a metal-plated resin may be used, but the substrate  10  may also be a metal plate such as a copper plate. 
     The planar part  11  and the feeding part  12  are physically separated plate-like elements. In the example illustrated in the diagram, the two parts have mutually different shapes and sizes, but are not always required to be configured in this way. The short-circuit parts  15 ,  16 , and  17  as well as the first switch circuit  18  are components for selectively grounding a portion of the planar part  11  and thereby switching the low-frequency band of LTE among three frequency bands. In the present embodiment, the three frequency bands are referred to as the first sub-band, the second sub-band, and the third sub-band for the sake of convenience. The first sub-band is the frequency band from 699 MHz to 803 MHz. The second sub-band is the frequency band from 791 MHz to 894 MHz. The third sub-band is the frequency band from 880 MHz to 960 MHz. 
     The planar part  11  is a rectangular plate having a metal face (hereinafter referred to as the “back face”) opposing the grounding surface with a predetermined interval therebetween. The feeding part  12  is a metal plate disposed in a plane forming a predetermined angle (for example, approximately 90 degrees) with respect to the grounding surface. The feeding part  12  has an edge (hereinafter referred to as the “proximity edge”) that does not contact, but is in close proximity to, one of the edges of the planar part  11 , while the remaining portions are fin-shaped. 
     The term “fin-shaped” refers to a shape in which at least one of the corner areas of the metal plate is arc-shaped, or alternatively, a shape in which two adjacent corner areas are arc-shaped. In this embodiment, the metal plate is elongated, and among the corner areas, the two corner areas near the grounding surface are arced with different radii of curvature. A feeding terminal  121  is formed at the portion where the arc with the larger radius of curvature begins. The two portions of the feeding part  12  may also be arced with the same radius of curvature, or the feeding part  12  may have a single arced portion. 
     The reason why the proximity edge of the feeding part  12  is longer than the fin-shaped edges is to secure a long filter interval for filters  13   a  and  13   b  as described later. 
     The planar part  11  and the feeding part  12  are metal plates such as copper plates, but because these parts are used in frequency bands where a surface effect is obtained, a metal-plated resin may also be used. 
     The planar part  11  is designed such that the electrical length, which is the sum of the lengths of the long and short edges in the case of the present example, is a length approximately ¼ of a wavelength XL of the lowest frequency (699 MHz) in the low-frequency band of LTE. On the other hand, the feeding part  12  is designed such that the electrical length, which is sum of the lengths of the edges in the case of the present example, is a length approximately ¼ of a wavelength λ H  of the lowest frequency (1.7 GHz) in the high-frequency band of LTE. By setting the planar part  11  and the feeding part  12  to the above sizes, signals in a frequency band above the lowest frequency of the low-frequency band and below the highest frequency of the high-frequency band can be made to resonate in a useful spectrum space. 
     The physically separated planar part  11  and feeding part  12  are electrically connected via the two filters  13   a  and  13   b . The term “electrically connected” does not mean a state in which any slight amount of current flows, rather, the term is directed to a state in which the planar part  11  and the feeding part  12  function as an antenna element at a frequency in use which is lower than or equal to a predetermined frequency. If only a tiny amount of current flows, a substantial electrical connection is not formed. Each of the filters  13   a  and  13   b  operates as a high-frequency cutoff filter which cuts off frequencies exceeding the lowest frequency (1.7 GHz) of the high-frequency band of LTE. Additionally, each of the filters  13   a  and  13   b  is electrically connected at a frequency equal to or lower than a predetermined frequency, namely the highest frequency of the low-frequency band of LTE (in this example, 960 MHz in the third sub-band), and each operates as an antenna element. 
     The filters  13   a  and  13   b  can be configured with only inductive reactance in a simple configuration. In this case, the inductance of the filters  13   a  and  13   b  is set to approximately 7.5 nH in consideration of factors such as the floating capacitance. The arrangement interval of the two adjacent filters  13   a  and  13   b  is set to a predetermined interval or greater, namely a distance at which the operations of the filter components do not influence each other. This arrangement interval (hereinafter referred to as the “filter interval”) is preferably as large as possible. 
     The short-circuit parts  15 ,  16 , and  17  are provided to selectively receive the above three sub-bands in the low-frequency band of LTE. One end of each of the short-circuit parts  15 ,  16 , and  17  is joined to respective positions at different distances from a site close to the feeding part  12  on the face orthogonal to the feeding part  12  on the back face near one of the short edges of the planar part  11 . In other words, electrical continuity with the planar part  11  is achieved at the above positions. The other ends of the short-circuit parts  15 ,  16 , and  17  are selectively grounded by the first switch circuit  18 . In the following description, the three short-circuit parts are referred to as the first short-circuit part  15 , the second short-circuit part  16 , and the third short-circuit part  17  in order from the end near the feeding part  12  along the short edge of the planar part  11 . 
       FIG. 2  is a schematic diagram illustrating an exemplary configuration of the first switch circuit  18 . The other ends of the first short-circuit part  15 , the second short-circuit part  16 , and the third short-circuit part  17  are electrically connected to one end, respectively, of three switching elements  181 ,  182 , and  183  of the first switch circuit  18 . The other ends of the switching elements  181 ,  182 , and  183  are a common terminal in electrical continuity with the grounding surface. The switching elements  181 ,  182 , and  183  are controlled such that only one has electrical continuity (i.e., closed), according to an external signal transmitted from an electronic device in the vehicle, for example. 
     Exemplary sizes of the components of the inverted F antenna  1  will be described.  FIG. 3A  is a top view of the inverted F antenna  1 , while  FIG. 3B  is a side view of the inverted F antenna  1  from the direction of the feeding part  12 , and  FIG. 3C  is a side view of the inverted F antenna  1  from the direction of the short-circuit parts  15 ,  16 , and  17 . 
     The planar part  11  is a rectangular plate having a short edge W 11  of 30 mm, a long edge W 12  of 42.5 mm, and a thickness t 1  of 10μ (microns). The proximity edge of the feeding part  12  is of the same size as the long edge W 12  of the planar part  11 , and the feeding part  12  has a width W 21  of 23.5 mm and a thickness t 2  of 10μ (microns). 
     For this reason, in the case of accommodating the inverted F antenna  1  in a housing, the height from the grounding surface to the housing can be set to 25 mm. 
     The feeding terminal  121  projects outward slightly in the direction of the grounding surface, but this projection can be avoided by bending the feeding terminal  121 . 
     Also, in the case of disposing a resin onto the substrate  10  and attaching the planar part  11  and the feeding part  12  to the resin, the size of each component is appropriately modified according to the effective wavelength that is shortened by the effective dielectric constant considering the dielectric constant of the resin. 
     The short-circuit parts  15 ,  16 , and  17  are square columns (having a square cross-section) with respective widths t 3 , t 4 , and t 5  of 1 mm. However, circular columns or some other cross-sectional shape may also be used. Starting from the end near the feeding part  12  along the short edge of the planar part  11 , a distance D 1  to the first short-circuit part  15  is 1 mm, a distance D 2  to the second short-circuit part  16  is 6 mm, and a distance D 3  to the third short-circuit part  17  is 21 mm. 
     One characteristic of the inverted F antenna  1  of the first embodiment is that the planar part  11  and the feeding part  12  are physically separated plates, which are electrically connected at a frequency lower than or equal to a predetermined frequency, such as a frequency lower than or equal to the highest frequency of the low-frequency band of LTE, for example. To investigate the effects of such a configuration, the inventors created an inverted F antenna  41  illustrated in  FIG. 4  as a Comparative Example. The inverted F antenna  41  of the Comparative Example has the same material, the same shape and size, and the same configuration as the inverted F antenna  1  of the first embodiment, except that a planar part  411  and a feeding part  412  are cast in a single piece. The material, the sizes of the long and short edges, and the thickness of the planar part  411  are the same as the planar part  11 . The material, the sizes of the long and short edges, and the thickness of the feeding part  412  are the same as the feeding part  12 . 
       FIG. 5  is a graph comparing the voltage standing wave ratio (VSWR) characteristics of an Example and the Comparative Example of the inverted F antenna  1 , and illustrates results calculated by a predetermined simulator. The solid line represents the VSWR characteristics of the inverted F antenna  1  according to the Example (hereinafter referred to as “Example 1”), while the dashed line represents the VSWR characteristics of the inverted F antenna  41  according to the Comparative Example (hereinafter referred to as “Comparative Example 41”). The relationship (excerpt) between the frequency (MHz) and the VSWR is as follows. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Frequency (MHz) 
                 Comparative Example 41 
                 Example 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 747.5 
                 8.67 
                 8.21 
               
               
                 802.5 
                 6.45 
                 2.43 
               
               
                 815.0 
                 6.08 
                 1.92 
               
               
                 850.0 
                 5.24 
                 1.09 
               
               
                 887.5 
                 4.58 
                 1.97 
               
               
                 900.0 
                 4.40 
                 2.49 
               
               
                 . . . 
               
               
                 1907.5 
                 2.75 
                 2.74 
               
               
                 2050.0 
                 2.70 
                 1.98 
               
               
                 2100.0 
                 2.66 
                 1.80 
               
               
                 2200.0 
                 2.58 
                 1.51 
               
               
                 2500.0 
                 2.33 
                 1.15 
               
               
                 2600.0 
                 2.25 
                 1.20 
               
               
                 2800.0 
                 2.15 
                 1.36 
               
               
                 2900.0 
                 2.12 
                 1.89 
               
               
                 2960.0 
                 2.09 
                 2.09 
               
               
                   
               
            
           
         
       
     
     In this way, in Example 1, the VSWR is significantly less than Comparative Example 41 in both the low-frequency band (first sub-band, second sub-band, third sub-band) and the high-frequency band. In other words, it is confirmed that by configuring the inverted F antenna  1  like in Example 1, the VSWR is lowered, and an effect of transmitting and/or receiving LTE signals more easily over a wide band is obtained. 
       FIG. 6A  illustrates an exemplary configuration of an inverted F antenna according to another Comparative Example using a single filter  136  instead of the two filters  13   a  and  13   b  in Example 1. The filter  136  is disposed at the same position as the filter  13   a.    
     In Example 1, the two filters  13   a  and  13   b  are filters having an inductance of 7.5 nH, but for the filter  136  of the Comparative Example illustrated in  FIG. 6A  to achieve the same frequency cutoff effect as Example 1, a filter having an inductance of 15 nH is used. 
       FIG. 6B  is a graph comparing the VSWR characteristics of the other Comparative Example above which includes a single filter (only the filter  136 ) and Example 1 which includes the two filters  13   a  and  13   b , and illustrates results calculated in the low-frequency band by a predetermined simulator. The solid line represents the VSWR characteristics of Example 1 using the two filters  13   a  and  13   b , while the dashed line represents the VSWR characteristics of the Comparative Example using the single filter  136 . The relationship (excerpt) between the frequency (MHz) and the VSWR is as follows. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Frequency (MHz) 
                 Other Comparative Example 
                 Example 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 815.0 
                 2.68 
                 1.92 
               
               
                 825.0 
                 1.96 
                 1.59 
               
               
                 850.0 
                 1.19 
                 1.09 
               
               
                 880.0 
                 2.07 
                 1.80 
               
               
                 887.5 
                 2.34 
                 1.97 
               
               
                   
               
            
           
         
       
     
     In this way, it is confirmed that by electrically connecting the planar part  11  and the feeding part  12  via the two filters  13   a  and  13   b , like in Example 1, as compared to the case of using the single filter  136 , the VSWR in the low-frequency band of LTE can be decreased, and furthermore, the frequency band where the VSWR is less than 2 can be greatly expanded. 
     This trend is almost similar in the high-frequency band of LTE, and the relationship (excerpt) between the frequency (MHz) and the VSWR is as follows. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Frequency (MHz) 
                 Other Comparative Example 
                 Example 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1990.0 
                 1.99 
                 2.27 
               
               
                 2047.5 
                 1.82 
                 1.99 
               
               
                 2352.5 
                 1.33 
                 1.24 
               
               
                 2505.0 
                 1.39 
                 1.15 
               
               
                 2760.0 
                 1.99 
                 1.28 
               
               
                 2920.0 
                 2.72 
                 1.99 
               
               
                   
               
            
           
         
       
     
     In the Example of the inverted F antenna  1 , a case of using the two filters  13   a  and  13   b  is given as an example, but there may also be three or more filters. However, the value of the inductance needs to be altered according to the number of filters and the type of filter components. 
     In the Example of the inverted F antenna  1 , the interval between the two filters  13   a  and  13   b  is set to the length of the short edge of the planar part  11 , namely 30 mm. To investigate the effect of this configuration, the inventors created an inverted F antenna according to another Comparative Example in which the above interval is changed. An exemplary configuration of the inverted F antenna according to the other Comparative Example is illustrated in  FIG. 7A . In the example of  FIG. 7A , the filter  13   a  is left unchanged, but a filter  13   c  is disposed at a position forming an interval of 5 mm. The filter components of the filter  13   c  are similar to the filter  13   b.    
       FIG. 7B  is a graph comparing the VSWR characteristics of the other Comparative Example with a filter interval of 5 mm and Example 1 with a filter interval of 30 mm, and illustrates results calculated by a predetermined simulator. The solid line represents the VSWR characteristics of Example 1 using the filters  13   a  and  13   b , while the dashed line represents the VSWR characteristics of the Comparative Example using the filters  13   a  and  13   c . The relationship between the frequency (MHz) and the VSWR is as follows. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Frequency (MHz) 
                 Other Comparative Example 
                 Example 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 815.0 
                 2.68 
                 1.92 
               
               
                 825.0 
                 1.96 
                 1.59 
               
               
                 850.0 
                 1.19 
                 1.09 
               
               
                 880.0 
                 1.96 
                 1.72 
               
               
                 882.5 
                 2.07 
                 1.80 
               
               
                 887.5 
                 2.34 
                 1.97 
               
               
                   
               
            
           
         
       
     
     In this way, it is confirmed that by setting the filter interval between the filters  13   a  and  13   b  to 30 mm like in Example 1, compared to the case of setting the filter interval to 5 mm, the VSWR in the low-frequency band of LTE can be decreased, and furthermore, the frequency band where the VSWR is less than 2 can be greatly expanded. 
     This trend is almost similar in the high-frequency band of LTE. The relationship (excerpt) between the frequency (MHz) and the VSWR is as follows. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Frequency (MHz) 
                 Other Comparative Example 
                 Example 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 2022.5 
                 1.99 
                 2.14 
               
               
                 2057.5 
                 1.87 
                 1.99 
               
               
                 2415.0 
                 1.28 
                 1.16 
               
               
                 2440.0 
                 1.28 
                 1.14 
               
               
                 2475.0 
                 1.31 
                 1.11 
               
               
                 2550.0 
                 1.38 
                 1.11 
               
               
                 2745.0 
                 1.98 
                 1.19 
               
               
                 2765.0 
                 2.15 
                 1.20 
               
               
                 2957.5 
                 1.82 
                 1.80 
               
               
                   
               
            
           
         
       
     
     In Example 1, the filter interval between the two filters  13   a  and  13   b  is set to 30 mm, but obviously the filter interval may also be 30 mm or more. 
     In Example 1, the portion of the edges other than the proximity edge of the feeding part  12  are fin-shaped, and to investigate the effect of this configuration, the inventors created an inverted F antenna according to another Comparative Example in which the shape of the feeding part is different. An exemplary configuration of the inverted F antenna according to the other Comparative Example is illustrated in  FIG. 8A . The example in  FIG. 8A  illustrates a rectangular feeding part  82  having an electrical length, or the sum of the lengths of the edges, which is the same as that of the feeding part  12  of Example 1. The material and thickness of the feeding part  82 , the planar part  11 , and the filter interval between the filters  13   a  and  13   b  are similar to Example 1. 
       FIG. 8B  is a graph comparing the VSWR characteristics of the other Comparative Example with a differently shaped feeding part and Example 1, and illustrates results calculated by a predetermined simulator. The solid line represents the VSWR characteristics in the case of using the feeding part  12  shaped like in Example 1, while the dashed line represents the VSWR characteristics in the case of using the rectangular feeding part  82 . The relationship (excerpt) between the frequency (MHz) and the VSWR is as follows. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Frequency (MHz) 
                 Other Comparative Example 
                 Example 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 815.0 
                 3.74 
                 1.92 
               
               
                 850.0 
                 1.80 
                 1.09 
               
               
                 880.0 
                 1.11 
                 1.72 
               
               
                 887.5 
                 1.21 
                 1.97 
               
               
                 912.5 
                 1.96 
                 3.17 
               
               
                 . . . 
               
               
                 2047.5 
                 2.35 
                 1.99 
               
               
                 2122.5 
                 1.99 
                 1.73 
               
               
                 2212.5 
                 1.79 
                 1.47 
               
               
                 2662.5 
                 4.49 
                 1.26 
               
               
                 2802.5 
                 2.01 
                 1.37 
               
               
                   
               
            
           
         
       
     
     Because the feeding part  12  is designed to be a size which resonates in the high-frequency band of LTE, as the above Comparative Example of the VSWR characteristics in the high-frequency band clearly demonstrates, the difference in the shape exerts a large influence in the high-frequency band of LTE. In other words, it is confirmed that by making the edges of the feeding part  12  that point toward the grounding surface fin-shaped, compared to the Comparative Example, the VSWR in the high-frequency band of LTE can be decreased significantly, and furthermore, the band where the VSWR is less than 2 can be expanded stably. This trend is almost similar in the low-frequency band of LTE. 
     Next, the way in which the electrical characteristics of the inverted F antenna  1  are influenced by the selective arrangement of the short-circuit parts  15 ,  16 , and  17  will be described. Herein, the VSWR characteristics are cited as an example of the electrical characteristics. 
     The first switch circuit  18  includes the three switching elements  181  to  183 , as described earlier.  FIG. 9A  illustrates the operating behavior of the first switch circuit  18  when a portion of the planar part  11  is grounded via the first short-circuit part  15 . In the first switch circuit  18 , when only the first switching element  181  is closed, the second switching element  182  and the third switching element  183  are open. For this reason, only the portion which is within the 1 mm distance D 1  from the end of the planar part  11  is in electrical continuity with the grounding surface. 
     Similarly,  FIG. 9B  illustrates the operating behavior of the first switch circuit  18  when a portion of the planar part  11  is grounded via the second short-circuit part  16 . In the first switch circuit  18 , only the second switching element  182  is closed, while the first switching element  181  and the third switching element  183  are open. For this reason, only the portion which is within the 6 mm distance D 2  from the end of the planar part  11  is in electrical continuity with the grounding surface. 
     Similarly,  FIG. 9C  illustrates the operating behavior of the first switch circuit  18  when a portion of the planar part  11  is grounded via the third short-circuit part  17 . In the first switch circuit  18 , only the third switching element  183  is closed, while the first switching element  181  and the second switching element  182  are open. For this reason, only the portion which is within the 21 mm distance D 3  from the end of the planar part  11  is in electrical continuity with the grounding surface. 
       FIG. 10  is a graph comparing the VSWR characteristics when each of the short-circuit parts  15  to  17  is selected, and illustrates results calculated by a predetermined simulator. The dashed line represents the VSWR characteristics for the case where the distance from the end of the short edge of the planar part  11  to the grounded site is D 1  (1 mm: 1/30 the length of the short edge), while the solid line represents the case where the distance is D 2  (6 mm: ⅕ the length of the short edge), and the dotted line represents the case where the distance is D 3  (21 mm: approximately ⅔ the length of the short edge). 
     In the case where the distance D 1  is selected, the minimum value of the VSWR is 2.16 (frequency 922.5 MHz). Also, the VSWR is less than 5 from 857.5 MHz to 985.0 MHz (bandwidth 127.5 MHz), the VSWR is less than 4 from 870.0 MHz to 975.0 MHz (bandwidth 105 MHz), and the VSWR is less than 3 from 885 MHz to 975.5 MHz (bandwidth 90 MHz). In other words, the above demonstrates that in the case of transmitting and/or receiving signals in the third sub-band (880 MHz to 960 MHz) of the low-frequency band of LTE, it is sufficient for the first switch circuit  18  to close only the first switching element  181 . 
     When only the first switching element  181  is closed, the VSWR in the high-frequency band of LTE is less than 3 (2.99) at 1905 MHz, less than 2 (1.99) at 2085 MHz, and approximately 1.16 from 2492.5 MHz to 2520 MHz. 
     Also, from 2037.5 MHz to 3000.0 MHz, the VSWR is at most 2.22 (bandwidth 962.5 MHz or more). In other words, it is possible to transmit and/or receive signals stably not only in the low-frequency band but also in the high-frequency band of LTE. 
     In the case where the distance D 2  is selected, the minimum value of the VSWR is 1.09 (frequency 850.0 MHz). Also, the VSWR is less than 5 from 770.0 MHz to 932.5 MHz (bandwidth 162.5 MHz), the VSWR is less than 4 from 780.0 MHz to 922.5 MHz (bandwidth 142.5 MHz), and the VSWR is less than 3 from 885 MHz to 975.5 MHz (bandwidth 90.5 MHz). In other words, the above demonstrates that in the case of transmitting and/or receiving signals in the second sub-band (791 MHz to 894 MHz) of the low-frequency band of LTE, it is sufficient for the first switch circuit  18  to close only the second switching element  182 . Particularly, with the distance D 2 , the VSWR is less than 1.1 at 850.0 MHz as well as several dozen MHz before and after 850.0 MHz, and maximum performance (transmitting and/or receiving capability) in the low-frequency band of LTE can be exhibited. 
     When only the second switching element  182  is closed, the VSWR in the high-frequency band of LTE is less than 3 (2.99) at 1867.5 MHz, less than 2 (1.99) at 2047.5 MHz, and approximately 1.15 from 2482.5 MHz to 2530 MHz. 
     Also, the VSWR is less than 2 from 2047.5 MHz to 2920.0 MHz (bandwidth 872.5 MHz). In other words, high performance is exhibited not only in the low-frequency band but also in the high-frequency band of LTE. 
     In the case where the distance D 3  is selected, the minimum value of the VSWR is 3.19 (frequency 790.0 MHz). Also, the VSWR is less than 5 from 735.0 MHz to 845.0 MHz (bandwidth 110.0 MHz), and the VSWR is less than 4 from 752.5 MHz to 827.5 MHz (bandwidth 75.5 MHz). In other words, the above demonstrates that in the case of transmitting and/or receiving signals in the first sub-band (699 MHz to 803 MHz) of the low-frequency band of LTE, it is sufficient for the first switch circuit  18  to close only the third switching element  183 . 
     When only the third switching element  183  is closed, the VSWR in the high-frequency band of LTE is less than 2 (1.99) at 1752.5 MHz, less than 1.2 (1.19) at 1937.5 MHz, a minimum (1.03) at 2017.5 MHz, and less than 1.09 from 1975.0 MHz to 2065 MHz. 
     Also, the VSWR is less than 2 from 1752.5 MHz to 3000.0 MHz (bandwidth 1247.5 MHz), and the VSWR is less than 1.1 from 1975.0 MHz to 2065.0 MHz (bandwidth 90.0 MHz). In other words, in the low-frequency band of LTE, the VSWR is slightly higher than the case where the distance D 1  or D 2  is selected, but in the high-frequency band of LTE, maximum performance is exhibited. 
     The results of comparing the inverted F antenna according to each Comparative Example in the first embodiment and Example 1 of the inverted F antenna  1  are summarized as follows. 
     (1-1) Relationship Between Planar Part  11  and Feeding Part  12   
     In Example 1, the planar part  11  which is substantially parallel to the grounding surface and the feeding part  12  disposed at an angle of approximately 90 degrees with respect to the grounding surface has a plate shape are configured as physically separated plates, and are substantially electrically connected at a frequency equal to or less than the highest frequency of the low-frequency band of LTE. For this reason, it is easy to create an inverted F antenna having an expanded frequency band in which the VSWR is less than 1.1 ( FIG. 5 ) while also keeping a low profile (a height of less than 25 mm from the grounding surface). When the angle with respect to the grounding surface of the planar part  11  is less than 90 degrees, the inverted F antenna can have an even lower profile. 
     Particularly, in Example 1, the planar part  11  is rectangular, and the feeding part  12  has a proximity edge in proximity to one of the edges of the planar part  11 , while the other edges of the feeding part  12  are fin-shaped. For this reason, the inverted F antenna  1  in which the usable frequency bands in the low-frequency band and the high-frequency band of LTE are expanded and the VSWR is stably low is achieved ( FIGS. 8A and 8B ). 
     (1-2) Filters  13   a  and  13   b    
     In Example 1, two or more filters which electrically connect the physically separated planar part  11  and feeding part  12  at a frequency less than or equal to a predetermined frequency are provided, and in addition, the filter interval between the two adjacent filters is made as large as possible (equal to or greater than the size of the short edge of the planar part  11 , for example). For this reason, the spectrum space of signals which can be transmitted and/or received can be expanded while still keeping the VSWR low in a stable manner ( FIGS. 6B and 7B ). 
     (1-3) Short-Circuit Parts  15 ,  16 ,  17  and First Switch Circuit  18   
     In Example 1, for example, the first short-circuit part  15  is provided at a position 1 mm away ( 1/30 the length of the short edge of the planar part  11 ) from the end of the short edge, the second short-circuit part  16  is provided at a position 6 mm away (⅕ the length of the short edge), the third short-circuit part  17  is provided at a position 21 mm away ( 21/30 the length of the short edge), and the first switch circuit  18  is configured to selectively put one of the short-circuit parts in electrical continuity with the grounding surface. For this reason, it is possible to switch the sub-band usable in the low-frequency band of LTE simply by switching the current distribution. For this reason, it is not necessary to attain impedance matching. Furthermore, in addition to the switching of the sub-band in the low-frequency band of LTE, the VSWR is also decreased in the high-frequency band of LTE and the usable frequency band is expanded, thereby making it possible to transmit and/or receive signals over a wide band of LTE with a low VSWR. 
     Particularly, in Example 1, in the case where the second short-circuit part  16  is selected, the VSWR in the low-frequency band of LTE falls to 1.09, while in addition, the bandwidth in which the VSWR is less than 4 is expanded to 142.5 MHz. For this reason, maximum performance can be exhibited in the low-frequency band of LTE. 
     Also, in the case where the third short-circuit part  17  is selected, maximum performance can be exhibited in the high-frequency band of LTE. 
     Although technologies other than present invention that enable the transmission and/or reception of signals in a plurality of frequency bands using a single inverted F antenna exist, most are technologies that attain impedance matching by providing a matching circuit or the like on the side of the electronic circuit connected to the inverted F antenna, and matching loss caused by component insertion is unavoidable. Also, adjusting the frequency band with a matching circuit has limits in how far the bandwidth can be expanded from the low-frequency band to the high-frequency band of LTE. This is because keeping the VSWR under 5 in all frequency bands is difficult. 
     In contrast, the inverted F antenna  1  of the first embodiment adopts a configuration which changes the current distribution of the planar part  11  and the feeding part  12  as seen by the feeding terminal  121  by selectively switching to one of the three short-circuit parts  15 ,  16 , and  17 . For this reason, it is extremely easy to expand the bandwidth while keeping the VSWR at a fixed value or less, without the need to provide a matching circuit (without producing matching loss). 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described.  FIG. 11  is a perspective view of an inverted F antenna according to the second embodiment. In an inverted F antenna  2  of the second embodiment, only the configuration for switching the current distribution is different. For this reason, parts that are the same as the components indicated in the first embodiment will be denoted with the same signs, and duplicate description will be omitted. 
     The inverted F antenna  2  includes a single short-circuit part  25  and a second switch circuit  28 . One end of the short-circuit part  25  is joined at a position where the VSWR is a minimum at a specific frequency, or in other words, at a position the distance D 2  (6 mm) away from the end of one of the short edges of the planar part  11  on the back face of the planar part  11 . The short-circuit part  25  has the same material, shape, size, and disposed position as the second short-circuit part  16  of the first embodiment. 
       FIG. 12  is a schematic diagram illustrating an exemplary configuration of the second switch circuit  28 . In the second switch circuit  28 , a common terminal is electrically connected to the short-circuit part  25 . The position of the short-circuit part  25  is as described earlier. Also, the second switch circuit  28  is provided with a path p 1  of a capacitor C having one end connected to a first switching element  281  and the other end grounded, a path p 2  having one end connected to a second switching element  282  and the other end simply grounded, and a path p 3  of a coil L having one end connected to a third switching element  283  and the other end grounded. The reactance of the capacitor C is 3 pF, and the inductance of the coil L is 30 nH. 
     Each of the switching elements  281 ,  282 , and  283  is controlled such that only one has electrical continuity (i.e., closed), according to an external signal transmitted from an electronic device in the vehicle, for example. 
       FIG. 13  is a graph comparing the VSWR characteristics when one of the paths p 1  to p 3  is selectively closed, and illustrates results calculated by a predetermined simulator. The dashed line represents the VSWR characteristics for the case when the path p 1  is selected, the solid line represents the case for the path p 2 , and the dotted line represents the case for the path p 3 . Although there is only one short-circuit part  25 , by selectively switching to one of the paths p 1 , p 2 , and p 3  with the second switch circuit  28 , the VSWR characteristics become the same as those of the inverted F antenna  1  according to the first embodiment illustrated in  FIG. 10 . 
     In other words, in the case where the path p 1  is selected, the phase is advanced compared to the path p 2  because of the capacitor C, causing the short-circuit part  25  to operate as though the short-circuit part  25  existed at the distance D 1  (1 mm: 1/30 the length of the short edge of the planar part  11 ) in the first embodiment, and thereby resulting in the same VSWR characteristics as the distance D 1  in  FIG. 10 . 
     In the case where the path p 2  is selected, the short-circuit part  25  is directly grounded, thereby resulting in the same VSWR characteristics as the distance D 2  (6 mm: ⅕ the length of the short edge of the planar part  11 ) in  FIG. 10 . 
     In the case where the path p 3  is selected, the phase is retarded compared to the path p 2  because of the coil L, causing the short-circuit part  25  to operate as though the short-circuit part  25  existed at the distance D 3  (21 mm: approximately ⅔ the length of the short edge of the planar part  11 ) in the first embodiment, and thereby resulting in the same VSWR characteristics as the distance D 3  in  FIG. 10 . 
     In the second switch circuit  28 , because each of the paths p 1  to p 3  can be configured easily with patterning technology and component interconnects, and because only a single short-circuit part  25  is sufficient, production is simple compared to the inverted F antenna  1  of the first embodiment. There is also an advantage of an increased freedom in the layout when the inverted F antenna  2  is accommodated in the housing. 
     As a modification of the second embodiment, it is also possible to combine two short-circuit parts.  FIG. 14A  is a schematic diagram illustrating a first modification. The first modification illustrated in  FIG. 14A  is configured such that, in addition to the short-circuit part  25  illustrated in  FIG. 12 , another short-circuit part  35  is provided at a site a different distance away (in this example, the site corresponding to the above distance D 1 ) from the end near the feeding terminal  121  along the short edge of the planar part  11 . Furthermore, the second switch circuit  28  is configured to selectively put one of the two paths p 2  and p 3  having different electrical lengths from the grounding surface in electrical continuity with the short-circuit part  25 , or alternatively, instead of the short-circuit part  25 , the second switch circuit  28  is configured to put a path p 1 ′ of the other short-circuit part  35  in electrical continuity. 
       FIG. 14B  is a schematic diagram illustrating a second modification. The second modification illustrated in  FIG. 14B  is configured such that, in addition to the short-circuit part  25  illustrated in  FIG. 12 , another short-circuit part  45  is provided at a site a different distance away (in this example, the site corresponding to the above distance D 3 ) from the end near the feeding terminal  121  of the planar part  11 . Furthermore, the second switch circuit  28  is configured to selectively put one of the two paths p 1  and p 2  having different electrical lengths from the grounding surface in electrical continuity with the short-circuit part  25 , or alternatively, instead of the short-circuit part  25 , the second switch circuit  28  is configured to put a path p 3 ′ of the other short-circuit part  45  in electrical continuity. 
     According to the configurations in  FIGS. 14A and 14B , substantially the same effects as the inverted F antenna  2  of the second embodiment illustrated in  FIG. 12  can be exhibited. 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described. The first embodiment illustrates an example of the feeding part  12  in which the proximity edge is the same size as the long edge of the rectangular planar part  11  and the ends of the proximity edge exist at the same positions as the ends of the planar part  11 . However, in the third embodiment, an example of an inverted F antenna having a feeding part which is different from the feeding part  12  of the first embodiment will be described. 
       FIG. 15A  is a perspective view of an inverted F antenna according to the third embodiment.  FIG. 15B  is a top view of the planar part, while  FIG. 15C  is a side view as seen from the direction of the feeding part. In an inverted F antenna  3  of the third embodiment, the shape and the installation position of the feeding part are different from the feeding part  12  described in the first embodiment. For this reason, parts which are the same as the components indicated in the first embodiment will be denoted with the same signs, and duplicate description will be omitted. 
     For a feeding part  32  of the third embodiment, the length of the proximity edge is shorter than the long edge of the planar part  11 , and correspondingly, the radius of the arc of the fin-shaped portion is also slightly smaller than that of the feeding part  12  in the first embodiment. Also, the end of the proximity edge is disposed at a non-opposing position with respect to the planar part  11 . In other words, the end of the proximity edge is disposed at a position projecting out past the short edge of the planar part  11 . Like the feeding part  12  of the first embodiment, the electrical length (in this example, the sum of the lengths of the edges) is designed to be a length approximately ¼ of the wavelength λ H  of the lowest frequency (1.7 GHz) in the high-frequency band of LTE. 
     Making the proximity edge of a feeding part  32  shorter than the long edge of the planar part  11  is advantageous because similar effects are obtained even when it is necessary to make the planar part  11  long and thin, for example. In this case, the short-circuit parts  15 ,  16 , and  17  and the first switch circuit  18  may also be positioned on the long edge of the planar part  11 . In this case, nothing exists on the short edge of the planar part  11  while the short-circuit parts  15 ,  16 , and  17  and the feeding part  12  exist on the long edge. 
     Modifications 
     The first to third embodiments describe examples for the case of a rectangular planar part  11 , but rectangular shapes also include diamond shapes and trapezoid shapes. In addition, the planar part  11  is not necessarily required to be rectangular, and may also be circular, near-circular, elliptical, or near-elliptical. The edge in these cases corresponds to a rim that determines the electrical length. 
     As above, according to the embodiments, it is possible to provide an antenna device which enables signals to be transmitted and/or received stably at a low VSWR over a wide frequency band.