Patent Publication Number: US-6218991-B1

Title: Compact planar inverted F antenna

Description:
This application claims the benefit under 35 U.S.C. § 119(e) of the U.S. provisional patent application Ser. No. 60/151,274 filed Aug. 27, 1999. 
    
    
     TECHNICAL FIELD 
     The present invention relates to antennas and more particularly to a compact isotropic planar inverted F antenna. 
     BACKGROUND ART 
     Planar inverted F antennas generally include at least one planar radiating element and a ground plane in a plane parallel to the radiating element. A short tab at one end of the radiating element that extends transverse to the radiating element is connected to the ground plane. A coaxial cable extends through the ground plane at a selected location and the center pin of the coaxial cable is connected to the radiating element. This type of antenna is called an inverted F antenna because the side profile is shaped like the letter F with the radiating element forming the long portion, the tab forming top transverse leg and the center pin of the coaxial cable forming the other transverse leg. In prior known planar inverted F antennas, the radiating element is located relatively near the ground plane so that the length of the tab is less than 10% of the length of the radiating element. 
     Planar inverted F antennas are used in wireless communications. As the number of wireless applications increases and the physical size of wireless devices decreases, antennas for these applications and devices are needed. Prior known planar inverted F antennas have been limited by the required size of the radiating element, the required size of the ground plane, limited bandwidth and limited isotropic characteristics. 
     DISCLOSURE OF THE INVENTION 
     A compact planar inverted F antenna having a circuit element and a ground plane is disclosed. The circuit element is conductive sheet, preferably brass, and includes a planar portion with a tab. The tab is narrower than the planar portion and extends perpendicular to the plane of the planar portion from a first edge. The length of the tab is about 25% and greater of the length of the planar portion. The planar portion has a reactance window extending inward from an edge that is adjacent to the first edge. The ground plane includes a substrate, a conductive planar first portion on a first side of the substrate, and conductive planar second and third portions on a second side of the substrate. The second and third portions are connected to the first portion through vias near a first end of the substrate and near a second end opposite the first end, respectively. The second and third portions are separated by a gap extending across the substrate parallel to the first end. The tab of the circuit element is soldered to the first portion of the ground plane. A feed pro is soldered to the circuit element and to a plated pad on the first portion of the ground plane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Details of this invention are described in connection with the accompanying drawings that bear similar reference numerals in which: 
     FIG. 1 is a perspective view of an antenna embodying features of the present invention. 
     FIG. 2 is a top plan view of the antenna of FIG.  1 . 
     FIG. 3 is a side elevation view of the antenna of FIG.  1 . 
     FIG. 4 is an end elevation view of the antenna of FIG.  1 . 
     FIG. 5 is a top plan view of the ground plane of the antenna of FIG.  1 . 
     FIG. 6 is a bottom plan view of the ground plane of the antenna of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, the compact planar inverted F antenna embodying features of the present invention includes a circuit element  10  and a ground plane  11 . The circuit element has a planar portion  13  which is the main radiating element and a tab  14 . The planar portion  13  is generally rectangular with a first edge  15 , a spaced second edge  16  opposite the first edge  15 , and spaced, opposed third and fourth edges  17  and  18  extending between and transverse to the first and second edges  15  and  16 . 
     The planar portion  13  also has a reactance window  20 . The reactance window  20  is generally rectangular and extends inward from third edge  17  towards fourth edge  18  parallel to and spaced a selected distance from the first edge  15 . The tab  14  is rectangular with a first edge  22  that is connected to the first edge  15  of the planar portion  13  and a spaced second edge  23  opposite the first edge  22 . Preferably the tab  14  has a length from the first edge  22  to the second edge  23  that is equal or greater than 25% of the length of the planar portion  13  from the first edge  15  to the second edge  16 . The tab  14  extends in a plane transverse or perpendicular to the plane of the planar portion  13  and the tab  14  is narrower than the first edge  15  of the planar portion  13 . The tab  14  is shown midway between the third edge  17  and the fourth edge  18 , however the tab  14  may be located at any position along the first edge  15 . An aperture  25  is located in the planar portion  13  intermediate the first edge  15  and the reactance window  20  and intermediate the third and fourth edges  17  and  18 . 
     The ground plane  11  includes a substrate  27  that is generally rectangular with a planar first side  28  and a spaced second side  29  opposite the first side  28 , and a first end  30  and a spaced second end  31  opposite the first end  30 . A conductive first portion  33  is attached to and substantially covers the first side  28  of the substrate  27 , and has a first end  34  adjacent to the first end  30  of the substrate  27  and a second end  35  adjacent to the second end  31  of the substrate  27 . Conductive second and third portions  37  and  41  are attached to the second side  29  of the substrate  27 . The conductive second portion  37  has a first end  38  adjacent to the first end  30  of the substrate  27  and extends inward on second side  29  of substrate  27  to a spaced second end  39  opposite the first end  38 . The third conductive portion  41  has a first end  42  adjacent to the second end  31  of the substrate  27  and extends inward on second side  29  of substrate  27  to a spaced second end  43  opposite the first end  42 . The second end  39  of the second portion  37  and the second end  43  of the third portion are separated on the second side  29  of the substrate  27  by a uniform gap  45 . 
     The first portion  33  is conductively connected to the second portion  37  with plated through holes or vias  46  that extend through substrate  27  near the first end  30  of substrate  27 . The first portion  33  is conductively connected to the third portion  41  with plated through holes or vias  46  that extend through substrate  27  near the second end  31  of substrate  27 . The ground plane  11  is preferably made with a copper clad or copper covered suitable substrate such as FR 4  with the gap  45  etched or otherwise removed from the second side  29  of the substrate. The conductive first, second and third portions  33 ,  37  and  41  are essentially folded or wrapped around the substrate.  27  and effectively provide a ground plane almost twice the size of the first portion  33  but occupying only the space of the first portion  33 . 
     The second edge  23  of the tab  14  of the circuit element  10  is attached to the first portion  33  of the ground plane  11 , spaced from and substantially parallel with the first end  34  of the first portion, such that the planar portion  13  of the circuit element  10  is spaced from and in substantially parallel alignment over the ground plane  11 . The first portion  33  of the ground plane  11  includes an opening  47  generally aligned with the aperture  25  in the planar portion  13  of the circuit element  10 . A plated pad  48  is located inside the opening  47  with clearance around the pad  48  so that the pad  48  is electrically isolated from the first portion  33 . A feed probe  49  is attached, preferably by soldering, between the pad  48  and the aperture  25 . A coaxial cable  51  is attached with the center pin  52  of the coaxial cable  51  being connected to feed probe  49  near pad  48  and the shield cover  53  of the coaxial cable  51  being connected to the first portion  33  of the ground plane  11  adjacent to the opening  47 . Alternatively, the center pin  52  of the coaxial cable  51  can be attached directly to the aperture  25 , the shield cover  53  of the coaxial cable  51  can be attached to the first portion  33  of the ground plane  11  and the opening  47 , the pad  48  and the feed probe  49  can be eliminated. 
     The circuit element  10  is partially shorted to the ground plane  11  by the tab  14  that is narrower than the first edge  15  of the planar portion  13 , which reduces the required size of the antenna. The reactance window  20  reduces the required size of the antenna. The reactance window  20  also increases the amount of diffracted wave, thereby improving the isotropic characteristics and making the antenna sensitive to two perpendicular polarizations. Multiple tabs or reactance windows could be provided. The length of the tab  14  and feed probe  49 , and therefore the depth of the antenna, are greater than in prior known planar inverted F antennas. Prior known planar inverted F antennas have a tab length and feed probe length of less than 10% the radiating element length whereas the antenna of the present invention has a tab length and feed probe length of more than 25% the radiating element length. This longer feed probe  49  is a significant radiating element, further improving the isotropic characteristics. The first, second and third portions  33 ,  37  and  41  of the ground plane  11 , connected as described, provide a compact ground plane that is electrically large enough to avoid lowering efficiency. 
     By way of example and not of limitation, an antenna embodying features of the present invention, designed for use with Global System for Mobile Communications (GSM) systems for frequencies in the range of about 890 to 960 MHz would be dimensioned as follows. The circuit element  10  is made from brass sheet with an exemplary thickness of 0.375 mm (0.015″). The planar portion  13  has a length of 54 mm from the first edge  15  to the second edge  16  and a width of 36 mm from the third edge  17  to the fourth edge  18 . The reactance window  20  extends 20 mm inward from the third edge  17  of the planar portion  13 , is 2 mm wide and spaced 16 mm from the first edge  15 . The tab  14  has a length of 14 mm from the first edge  22  to the second edge  23  and a width of 16 mm. The aperture  25  is centered between the third and fourth edges  17  and  18  of the planar portion  13 , is spaced 8 mm from the first edge  15  and is about 1.25 mm (0.050″) in diameter. 
     The ground plane  11  is made from 1.0 mm (0.040″) thick FR 4  with 2 oz. copper on each side. The ground plane  11  has a length of 68 mm from the first end 30 to the second end 31 of the substrate  27  and a width of 40 mm and the first portion  33  has these same dimensions. The second portion  37  of the ground plane  11  has a length of 28 mm, the third portion  41  of the ground plane  11  has a length of 36 mm and the gap  45  is 4 mm. The vias  46  are nominally 0.75 mm (0.030″) is diameter. The pad  48  is 2.5 mm (0.100″) in diameter and the center of the pad  48  is spaced 12 mm from the first end 30 of the substrate  27 . The opening  47  has about 6 mm clearance from the pad  48 . The second edge  23  of the tab  14  of the circuit element  10  is spaced 4 mm from the first end 30 of the substrate  27 . 
     The antenna described is particularly suited for installation in normally unfriendly RF environments. The antenna has been found to work well within an enclosure. Examples include wireless metering applications such as vending machines and meter boxes. The antenna is also suitable as an internal antenna for portable telephone handsets. 
     Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that changes in details of structure may be made without departing from the spirit thereof.