Patent Publication Number: US-7215288-B2

Title: Electromagnetically coupled small broadband monopole antenna

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. § 119 to applications entitled “Electromagnetically Coupled Small Broadband Monopole Antenna”, filed in the Korean Intellectual Property Office on Sep. 8, 2003 and assigned Serial No. 2003-62835, and filed in the Korean Intellectual Property Office on Sep. 2, 2004 and assigned Serial No. 2004-70113, the contents of both of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an antenna, and more particularly to a small broadband monopole antenna including a shorted patch and a probe with a strip line that are electromagnetically coupled with each other. The probe with the strip line has a length of about λ/4, where λ is a wavelength. 
     2. Description of Prior Art 
     Recently, the wireless communication system has been diversely and rapidly developed into a cellular phone, a personal communication service (PCS), an international mobile telecommunication-2000 (IMT-2000), and a personal digital assistant (PDA) and its market also has been enlarged to provide services at a high speed. In the IMT-2000, which is also called a third generation mobile communication system, and to which a great deal of research and development have been done, diverse communication services are available not only for voice and low speed data but also for high speed multimedia data. Together with the developments of such a variety of mobile communication systems, many efforts have been also made to develop small personal portable communication terminals with a high performance. For the miniaturization of the communication terminals, it is commonly regarded that the embedded type small antenna is essential. 
     Commonly, the prior communication terminals widely used an external type retractable antenna such as a helical antenna or a monopole antenna. However, the external type retractable antenna is disadvantageous for the miniaturization of the communication terminals. A planar inverted F antenna (PIFA) and a short-circuit microstrip antenna are suggested as a small embedded antenna to replace the external type retractable antenna. 
     These antenna structures have a benefit of a simple design, but unfortunately have a narrow bandwidth. In order to improve the narrow bandwidth problem of the PIFA and the short-circuit microstrip antenna, several types of antennas are suggested such as a 2-lines type normal mode helical antenna (NMHA), a meander line antenna consisting of two strips, a double line PIFA antenna, and a PIFA with stacked parasitic elements. These antennas are detailed in the following: 1) K. Noguchi, M. Misusawa, T. Yamaguchi, and Y. Okumura, “Increasing the Bandwidth of a Meander Line Antenna Consisting of Two Strips,”  IEEE AP - S Int Symp. Digest , pp. 2198-2201, vol. 4, Montreal, Canada, July 1997; 2) K. Noguchi, M. Misusawa, M. Nkahama, T. Yamaguchi, Y. Okumura, and S. Betsudan, “Increasing the Bandwidth of a Normal Mode Helical Antenna Consisting of Two Strips,”  IEEE AP - S Int Symp ., pp. 782–785, vol. 2, Atlanta, USA, June 1998; 3) M. Olmos, H. D. Hristov, and R. Feick, “Inverted-F Antennas with Wideband Match Performance,”  Electron. Lett ., vol. 16, no. 38, pp. 845–847, August 2002; and 4) S. Sakai and H. Arai, “Directivity Gain Enhancement of Small Antenna by Parasitic Patch,”  IEEE AP - S Int. Symp ., pp. 320–323, vol. 1, Atlanta, USA, June 1998. Among these antennas, the meander line antenna can have wider bandwidth than that of the 2-lines type NMHA or the PIFA by offsetting a balanced mode (transmission line mode) with an unbalanced mode (radiation mode). 
     Other solutions for obtaining a wide bandwidth include a method of attaching a patch with a shorting wall to an L-strip feed or an L-prove feed and a method of electromagnetically coupling the PIFA with the shorted parasitic patch. These solutions are detailed in the following: 1) C. L. Lee, B. L. Ooi, M. S. Leong, P. S. Kooi, and T. S. Yeo, “A Novel Coupled Fed Small Antenna,”  Asia - Pacific Microwave Conf ., pp. 1044–1047, vol. 3, Taipei, Taiwan, December 2001; 2) Y. X. Gou, K. M. Luk, and, K. F. Lee, “L-Probe Proximity-Fed Short-Circuited Patch Antennas,”  Electron. Lett ., vol. 24, no. 35, pp. 2069–2070, November 1999; and 3) Y. J. Wang, C. K. Lee, W. J. Koh, and Y. B. Gan, “Design of Small and Broad-Band Internal Antennas for IMT-2000 Mobile Handsets,”  IEEE Trans. Microwave Theory Tech ., vol. 49, no. 8, August 2001. These antenna structures can satisfy with a bandwidth of 30% or more, but has have some restrictions in reducing antenna size since because the L-strip structure and a shorted patch should have a resonance length of about λ/4. 
     For example, U.S. Pat. No. 6,452,558 entitled “Antenna Apparatus and a Portable Wireless Communication Apparatus” discloses a diversity antenna constructed by contacting a planar inverted F antenna (PIFA) with a monopole antenna. The diversity antenna uses two receiving antennas to create two paths for receiving electromagnetic waves in order reduce a fading phenomenon. 
     As another example, U.S. Pat. No. 5,289,198 entitled “Double-Folded Monopole Antenna” discloses an antenna that is constructed by folding a wire monopole antenna. This antenna has a total length equal to 1.0 λ of a resonance frequency and uses a traveling wave for its operation. The antenna does not use electromagnetic coupling with the shorted patch. 
     In addition, Korean Patent Application No. 10-2001-7000246 (with a U.S. counterpart application Ser. No. 09/112,366 filed on Jul. 9, 1998), entitled “Small Printed Spiral Type Antenna for Mobile Communication Terminals”, discloses an antenna structure of a spiral type monopole antenna and uses a method of directly connecting a grounding post to the spiral type monopole antenna to achieve an impedance matching. However, these antennas have different structures and characteristics from the antenna according to the present invention as will be described below. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a monopole antenna that can easily realize a single broadband or a dual band, and has several good characteristics such as a small electrical size, a low resonance frequency, and an impedance-matching-easy structure that does not require a separate matching circuit. 
     According to the present invention, for achieving the above and other objects, adjustments for the parallel capacitance and the series inductance of the antenna itself are used. A small broadband monopole antenna is provided that includes a shorted patch and a probe with a strip line with a length of about 0.25λ, where λ is a wavelength. Wide impedance bandwidth can be achieved through electromagnetic coupling between the shorted patch and the probe with a strip line that generate two resonances, parallel resonance from the shorted patch and series resonance from the probe with a strip line, closely spaced in frequency. 
     In the antenna, the strip line has a shape selected from a group of a spiral shape, a helix shape, and a folded shape that is made by folding a straight strip line. A wire can also be used instead of the strip line. By designing an antenna to have the shape and length as described above, the antenna can have a resonance length within a minimum space. 
     In order to achieve a small size and a wide bandwidth of an antenna, it is preferable that the shorted patch being operative as a monopole antenna of capacitive component should be electromagnetically coupled to the probe with a strip line as a monopole antenna of inductive component. 
     As a design scheme to obtain a wider bandwidth, it is preferable to position a resonance frequency of the probe with a strip line and a resonance frequency of the shorted patch at adjacent points with each other because the two resonance frequencies are adjustable. Furthermore, it is possible to design the antenna to have a dual-band by making the two resonance frequencies be different from each other. 
     The antenna suggested by the present invention is small size and has an omni-directional monopole radiation pattern. Accordingly, the antenna is applicable as an embedded antenna for mobile communication devices or a wireless local area network (LAN) because it enables data communication at any direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIGS. 1A ,  1 B, and  1 C are a top view, a side view, and a perspective view, respectively, of a monopole antenna including a shorted rectangular patch and a probe with a rectangular spiral strip line, in accordance with an embodiment of the present invention; 
         FIGS. 2A and 2B  are a top view and a side view of a monopole antenna including a shorted circular patch and a probe with a circular spiral strip line, respectively, in accordance with an embodiment of the present invention; 
         FIGS. 3A ,  3 B,  3 C, and  3 D are a perspective view, a partial detailed view, a top view, and a side view, respectively, of a monopole antenna including a shorted patch and a probe with a folded strip line, in accordance with an embodiment of the present invention; 
         FIG. 4  is an equivalent circuit of an antenna according to the present invention; 
         FIG. 5  illustrates impedance characteristics of a monopole antenna including a shorted patch and a probe with a spiral strip line; 
         FIG. 6  illustrates variation of return loss with shorting pin diameter; 
         FIG. 7  illustrates variations of impedance with the height of probe; 
         FIG. 8  illustrates variations of return loss with the spiral strip line length; 
         FIGS. 9A and 9B  illustrate return loss and variation of impedance characteristics, which are obtained by using the equivalent circuit and EM simulation, respectively; 
         FIGS. 10A and 10B  illustrate return loss and variation of impedance characteristics of a monopole antenna including a shorted patch and a probe with a circular spiral strip line; 
         FIGS. 11A and 11B  illustrate the return loss and variation of impedance characteristics of a monopole antenna including a shorted patch and a probe with a folded strip line; 
         FIGS. 12A and 12B  illustrate calculated antenna radiation patterns at 1.95 GHz in x-z plane and y-z plane, respectively; 
         FIGS. 13A and 13B  illustrate calculated antenna radiation patterns at 2.1 GHz in x-z plane and y-z plane, respectively; 
         FIG. 14  illustrates a calculated antenna radiation pattern in an x-y plane; 
         FIGS. 15A to 15D  are views illustrating antennas having shorting pins, the number of which is different according to embodiments of the present invention; 
         FIGS. 16A and 16B  illustrate differences in impedance and return losses according to changes in a number of the shorting pins connected to the rectangular patch in an antenna according to an embodiment of the present invention; 
         FIG. 17  is a view illustrating variations of an input impedance characteristic according to adjustments of a distance between a shorting pin and a feed probe in an antenna according to an embodiment of the present invention; 
         FIGS. 18A to 18C  are views illustrating electric current distributions depending on the adjustment of a distance between shorting pins in an antenna having two shorting pins according to an embodiment of the present invention; 
         FIGS. 19A and 19B  are graphs illustrating return losses and impedance variations depending to adjustment of a distance between shorting pins in an antenna structure having two shorting pins according to an embodiment of the present invention; 
         FIG. 20  is a graph illustrating return losses of antennas optimized according to a number of shorting pins, which are connected to the rectangular patch designed with parameters shown in Table 4; 
         FIGS. 21A and 21B  illustrate radiation patterns of an antenna having a single shorting pin, at frequencies of 1.8 GHz and 2.0 GHz, respectively; 
         FIGS. 22A and 22B  illustrates radiation patterns of an antenna having two shorting pins, at frequencies of 2.1 GHz and 2.4 GHz, respectively; 
         FIGS. 23A and 23B  illustrates radiation patterns of an antenna having three shorting pins, at to frequencies of 2.3 GHz and 2.7 GHz, respectively; 
         FIG. 24  is a view illustrating an antenna having three shorting pins according to yet another embodiment of the present invention; and 
         FIG. 25  is a view illustrating an antenna having four shorting pins according to still another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, detailed descriptions of preferred embodiments of the present invention will be given with reference to the attached drawings. In the following descriptions, any detailed description of known functions and configurations incorporated herein has been omitted for conciseness. 
     The present invention provides several structures of monopole antennas. In one embodiment in accordance with the present invention, a monopole antenna includes a shorted rectangular patch  10  and a probe  14  with a rectangular spiral strip line  12 , as illustrated in  FIGS. 1A ,  1 B, and IC. Preferably, the spiral strip line  12  has a rectangular shape, where its total length is l s  and its width is w s . 
     The probe  14  has a diameter Φ 1  at a height h f  from a ground plane  20 . The sum of the length l s  of the spiral strip line  12  and the probe height h f  from the ground plane  20  is equal to about 0.25λ. In general, a monopole antenna that is perpendicular to the ground plane  20  has a resonance length of about 0.25λ. Therefore, by a design scheme to construct the strip line as a spiral type, it becomes possible to design the monopole antenna to have the least volume and the longest resonance length. In addition, the probe with a spiral strip line  12  can be modeled into an equivalent circuit of series RLC, where R is a radiation resistance, L is a series inductance, and C is a capacitance  12 . However, to reduce the size of the probe with a spiral strip line  12 , its vertical height is reduced and a shape of the strip line is constructed as the spiral type, but such a design scheme may bring decrease of radiation resistance of the antenna. Therefore, the resonance frequency of the probe with a spiral strip line  12  may give a poor characteristic of resonance as compared with a vertical type monopole. 
     In order to improve the resonance characteristic and bandwidth of the probe with a spiral strip line  12 , a shorted patch  10 , which is electromagnetically coupled to the probe  14  with a spiral strip line  12 , is added. Preferably, the shorted patch  10  is square shaped, where its length, width, and height from the ground plane  20  are L, W, and h, respectively. The center of the shorted patch  10  is connected to a ground plane  20  through a shorting pin  16  of diameter Φ 2 . To reduce the size of the shorted patch  10 , a high permittivity dielectric substrate  18   a  is added on the lower surface of the shorted patch  10 . A dielectric substrate  18   b  may be further added on the ground plane  20 . The distance between the probe  14  and the shorting pin  16  is d. The shorted patch  10  improves the impedance matching characteristic of the probe  14  with a spiral strip line  12  and causes a resonance due to an effect of the electromagnetic coupling with the probe  14  with a spiral strip line  12 , which functions as a disk-loaded monopole antenna having a capacitive component. In addition, the shorted patch  10  is modeled into an equivalent circuit of parallel RLC resonance circuit. Therefore, in the structure including a shorted patch  10  and a probe  14  with a spiral strip line  12 , the probe  14  with a spiral strip line  12  that generate series resonance and the shorted patch  10  that generates parallel resonance are electromagnetically coupled each other, and operate as a monopole antenna. The resonance characteristic of the antenna can be adjusted by varying values of inductance and/or capacitance of the probe  14  with a spiral strip line  12  and the shorted patch  10 . Consequently, these features amenable the designing of an antenna having such characteristics as a wide single-band or dual-band. 
       FIGS. 2A and 2B  illustrate a structure of a shorted circular patch and a probe with a circular spiral strip line in another embodiment of the monopole antenna in accordance with the present invention. In  FIGS. 2A and 2B , the total length and width of a circular spiral strip line  32  are l s  and w s , respectively. 
     Referring to  FIGS. 2A and 2B , a probe  34  with a spiral strip line  32  has a diameter Φ 1  at a height h f  from a ground plane  40 . The sum of the length l s  of the spiral strip line  32  and the height of the probe  34  from the ground plane  40  becomes about 0.25λ. A shorted circular patch  30  is electromagnetically coupled to the probe with a circular spiral strip line  32  and has a diameter of 2ρ and a height of h. The center of the circular patch  30  is connected to the ground plane  40  through a shorting pin  36  with a diameter of Φ 2 . The distance between the probe  34  and the shorting pin  36  is d. Similarly to the antenna illustrated in  FIGS. 1A ,  1 B, and  1 C, a dielectric substrate  38   a  of a high permittivity may be added to the bottom surface of the circular patch  30  and a dielectric substrate  38   b  may be added on the ground plane  40 . 
     A helix type strip line can be constructed by slightly modifying the spiral type strip line. However, even in the helix type strip line its length should be equal to about 0.25λ. 
     As another embodiment of the monopole antenna, a structure including a shorted patch  50  and a probe  54  with a folded strip line  52  is illustrated in  FIGS. 3A ,  3 B,  3 C, and  3 D. The folded strip line  52 , as illustrated in  FIG. 3A , is constructed by folding a straight strip line. The folded strip line  52  consists of an upper strip line  52   a  and a lower strip line  52   b . The upper strip line  52   a  and the lower strip line  52   b  have a width of w s  and are connected by a part of strip line to have a vertical height h f2 . 
     The probe  54  has a of diameter Φ 1  at a height h f  from a ground plane  20 . The sum of the total length of folded strip line  52  and the probe height h f1  from a ground plane  60  becomes about 0.25λ at the resonance frequency.  FIG. 3C  is a top view of the antenna in which a shorted patch  50  is electromagnetically coupled to the probe  54  with a folded strip line  52 . Preferably, the shorted patch  50  is a rectangular patch of a length L and a width W. The shorted patch  50  has a height h from the ground plane  60  and its center is connected to the ground plane  60  via the shorting pin  56  of a diameter Φ 2 . The distance between the shorting pin  56  and a vertical probe  54  is d. Similar to foregoing embodiments, a dielectric substrate  58   a  of a high permittivity may be added to the lower surface of the rectangular shorted patch  50  and a dielectric substrate  58   b  may be added on the ground plane  60 . 
     The antennas of above-described embodiments of the present invention have a common structure in that the probe with a strip line, which functions as a series RLC resonance circuit, and the shorted patch, which functions as a parallel RLC resonance circuit, are electromagnetically coupled to have the same principle of operation. 
     Herein below, design schemes and characteristics of the monopole antenna according to the present invention are described. Electromagnetic (EM) simulation for designing an antenna was performed with the equipment IE3D made by the Zeland Company. RT Duroid 6010 substrate was used as the dielectric substrate  18   a  applied beneath the patch  10 , where the relative permittivity ε r1  and the thickness h 1  of the dielectric substrate  18   a  were ε r1 =10.2 and h 1 =1.27 mm, respectively. The RT Duroid 4003 substrate was used as the dielectric substrate  18   b  applied on the ground plane  20 , where the relative permittivity ε r2  and the thickness h 2  of the dielectric substrate  18   b  were ε r2 =3.38 and h 2 =0.813 mm, respectively. The simulation was carried on an infinite-ground plane. The advanced design system (ADS) equipment made by the Agilent Company was used for the simulation to realize an equivalent circuit model of the antenna. 
     The antenna structure illustrated in  FIGS. 1A  to C can be represented as an equivalent model illustrated in  FIG. 4 . In the antenna, the probe with a spiral strip line  12  or  80  operates as a monopole antenna of λ/4 and can be modeled into an equivalent circuit of series RLC resonance circuit. Assuming that the rectangular spiral strip line  12  or  80  is a straight strip line, an initial design value of inductance L strip  (nH) of the strip line can be obtained as shown in Equations (1) and (2). Detailed explanations on the following equations are described in “C. S. Walker,  Capacitance, Inductance, and Crosstalk Analysis , Boston: Artech House Inc., 1990”. 
     
       
         
           
             
               
                 
                   
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     In Equations (1) and (2), w s  and l s  are width and total length of the rectangular spiral strip line  12 , respectively. In addition, K g  represents a correction factor and h f  represents the height of the strip line  12  from the ground plane. Assuming that the probe is a column made with a conductor such as a conductor pin, an inductance L probe  (nH) of the probe  14  can be calculated as shown in Equations (3) and (4). For more specific details on Equations (3) and (4), please refer to the descriptions in “M. E. Goldfard and R. A. Pucel, ‘Modeling Via Hole Grounds in Microstrip’,  IEEE Microwave Guided Wave Lett ., vol. 1, no. 6, pp. 135–137, June 1991”. 
     
       
         
           
             
               
                 
                   
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     In Equations (3) and (4), Φ 1  represents the diameter of the probe  14  and h f  represents the height of the probe  14 . Therefore, the total inductance L se  of the probe  14  and the spiral strip line  12  can be represented as the sum of L strip  and L probe . 
     The shorted patch  10  or  70 , as a monopole antenna of a capacitive component being coupled to the probe  14  with a strip line  12 , operates as a parallel RLC resonance circuit. The inductance of the shorting pin  16  can be calculated by Equation (3). Assuming that the space between the shorted patch  10  and the ground plane  20  is a free space with the permittivity of ε r =1, the initial design values for the capacitance C p  (pF) of the patch  10  in the parallel RLC resonance circuit and the capacitance C pe  (pF) of external of the patch  10  can be acquired by using the Equations (5) and (6). For details on these equations, please refer to “C. H. Friedman, ‘Wide-band matching of a small disk-loaded monopole’,  IEEE Trans. Antennas Propagat ., vol. AP-33, No. 10, pp. 1142–1148. October 1985.” and “H. Foltz, J. S. McLean, and L. Bonder, ‘Closed-Form Lumped Element Models for Folded, Disk-Loaded Monopoles’,  IEEE AP - S Int. Symp ., pp. 576–579, vol. 1, 2002”. 
     
       
         
           
             
               
                 
                   
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     Initial design values of the series inductance of the probe with a spiral strip line  12  can be determined from Equation (4) and the parallel capacitance of the shorted patch  10  can be determined from Equations (5) and (6). However, the initial designing equations leave some matters, e.g., variation of the permittivity between the patch  10  and the ground plane  20 , and a coupling effect between the probe with a spiral strip line  12  and the shorted patch  10 , out of consideration. Therefore, it may be difficult to determine a precise result from only these equations and accordingly optimization through a number of simulations is needed. 
     The antenna structures illustrated in  FIGS. 2A–2B  and  3 A– 3 D follow the same operation principle with that of the antenna structures illustrated in  FIGS. 1A–1C  and thus, have a common equivalent circuit. In the foregoing embodiments of the present invention, the total length of the probe and the strip line is about 0.25λ in accordance with a design scheme of the antenna. A preferable design characteristic can be obtained when the length is determined within about 0.24λ˜0.26λ. It should be noted, however, that an ideal value of the length is 0.25λ. 
       FIG. 5  illustrates impedance characteristics of a monopole antenna including a shorted patch and a probe with a spiral strip line. In  FIG. 5 , impedance characteristics of the antenna illustrated in  FIGS. 1A–1C , i.e., including a probe with a rectangular spiral strip line  12  only, and impedance characteristics of the antenna with the shorted patch  10  that is coupled to the probe  14  with a spiral strip line  12  are illustrated. In  FIG. 1A , the length l s  of the rectangular spiral strip line  12  is l s =37.2 mm, the height h f  of the probe  14  is h f =7.5 mm. The shorted patch  10  has a dimension of length L=11.0 mm, width W=11.0 mm, and height h=11.0 mm and the probe  14  and the shorting pin  16  have a diameter Φ 1  of 0.86 mm and a diameter Φ 2  of 1.6 mm. Distance d between the probe  14  and the shorting pin  16  is d=3.6 mm. The probe  14  with a rectangular spiral strip line  12  functions as a monopole antenna of which resonance frequency is 2.0 GHz. From impedance variation of the probe with a rectangular spiral strip line  12  represented with a solid line, it can be known that even though it is possible to reduce the dimension of the monopole antenna structure, because it can have the maximum physical resonance length within the minimum volume by making the strip line into a spiral shape, the resonance characteristics of the probe with a spiral strip line itself may not acceptable because a radiation resistance is decreased due to the low height of the probe as compared to the wavelength of the resonance frequency. 
     From an observation on the impedance variation, when the shorted patch  10  is added to the probe with a rectangular spiral strip line  12 , the series resonance of the probe with a spiral strip line  12  and the parallel resonance of the shorted patch  10 , which are combined with each other to produce a double-resonance, can be determined. That is, in the resonance of a spiral strip line, the loop of the impedance locus is largely rotated one time, to thereby produce a single-resonance. However, as described above, when the resonance of the shorted patch and the resonance of a spiral strip line are combined, a double-resonance is produced, which shows in the form of a loop of a small circular locus as shown in  FIG. 5 . Such a form is called a double resonance. 
       FIG. 6  illustrates variations of return loss with the diameter of the shorting pin  16  illustrated in  FIG. 1A , while all other design parameters are fixed. As the diameter of the shorting pin  16  increases in turn of 1.4 mm, 1.6 mm and 1.8 mm, a low resonance frequency f L  moves from 1.83 GHz to 1.95 GHz and a high resonance frequency f H  is kept around 2.1 GHz. The shorted patch  10  and the probe with a spiral strip line have the resonance frequencies of f L  and f H , respectively. As the diameter of the shorting pin  16  for the patch  10  increases, the capacitance in the shorted patch decreases. Therefore the resonance frequency of the shorted patch  10  increases and thus, the resonance frequency f L  of the shorted patch  10  is shifted into a higher frequency. 
       FIG. 7  illustrates variations of impedance of the antenna with the change of the height of the probe, which is connected to the spiral strip line  12 , illustrated in  FIG. 1A . All other parameters are fixed. If the height h f  of the probe  14 , where the spiral strip line  12  is connected, is raised from 6.5 mm to 8.5 mm, the inductance of the probe increases. In addition, the coupling area between the shorting pin  16  and the probe  14  increases and the distance between the shorted patch  10  and the spiral strip line  12  is shortened. Therefore, the coupling of the shorted patch  10  and the probe with a spiral strip line  12  becomes enhanced. In the result, the loop of the impedance locus enlarges and moves upwards on the Smith chart as the height of the probe increases. 
       FIG. 8  illustrates return losses of an antenna with the change of the length of the rectangular spiral strip line  12  illustrated in  FIG. 1A . When all other parameters are the same as the previous case, the length l s  of the spiral strip line  12  is changed from 35.2 mm to 39.2 mm. As a result, by increasing the length of the spiral strip line  12 , its inductance also increases and the resonance frequency f H  decreases from 2.19 GHz to 2.05. 
     As illustrated in  FIGS. 6 ,  7 , and  8 , the resonance frequencies f L  and f H  can be adjusted by varying design parameters of the shorted patch  10  and the probe  14  with a spiral strip line  12  to change the inductance and the capacitance. It should be noted that a wide single-band can be obtained by positioning the resonance frequency of the spiral strip line  12  and the resonance frequency of the shorted patch  10  nearer with each other, while a dual-band can be obtained by positioning the two resonance frequencies at different positions with each other (farther apart). 
     In  FIGS. 9A and 9B , return loss and impedance variation of an optimized antenna are illustrated, which are obtained from an equivalent circuit and EM simulation for the antenna illustrated in  FIGS. 1A˜1C . Table 1 shows examples of the design parameters of the optimized antenna. 
     Referring to  FIGS. 9A and 9B , when a calculation result by the equivalent circuit is compared with EM simulation, it can be seen that the two calculated values are similar with each other. In the EM simulation, the antenna has a bandwidth from 1.835 GHz to 2.17 GHz, which is about 16.5% with respect to Voltage Standing Wave Ratio (VSWR)≦2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Exemplary design parameters of the monopole antenna including a 
               
               
                 rectangular shorted-patch and a probe with a rectangular spiral strip line 
               
            
           
           
               
               
               
            
               
                   
                 Design parameters 
                 Length (mm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Probe with a rectangular 
                 l s   
                 37.2 
               
               
                   
                 spiral strip line 
                 w s   
                 0.5 
               
               
                   
                   
                 a 
                 1.3 
               
               
                   
                   
                 b 
                 1.3 
               
               
                   
                   
                 d 
                 3.6 
               
               
                   
                   
                 h f   
                 7.5 
               
               
                   
                   
                 φ 1   
                 0.86 
               
               
                   
                 Rectangular shorted 
                 L 
                 11.0 
               
               
                   
                 patch 
                 W 
                 11.0 
               
               
                   
                   
                 h 
                 11.0 
               
               
                   
                   
                 h 1   
                 1.27 
               
               
                   
                   
                 h 2   
                 0.813 
               
               
                   
                   
                 h 3   
                 8.917 
               
               
                   
                   
                 φ 2   
                 1.6 
               
               
                   
                   
               
            
           
         
       
     
       FIGS. 10A and 10B  illustrate variations of impedance and return loss, which are obtained by an EM simulation, of an optimized antenna as illustrated in  FIGS. 2A and 2B . Table 2 illustrates examples of design parameters for an optimized antenna. In the return loss illustrated in  FIG. 10A , the antenna has a 17.4% bandwidth from 1.965 GHz to 2.34 GHz with respect to VSWR≦2.  FIG. 10B  illustrates the impedance variation in a Smith chart. From comparisons between the graphs illustrated in  FIGS. 9A–9B  and the graphs illustrated in  FIGS. 10A–10B , it can be known that the antenna with the circular patch and the circular spiral strip line has a similar characteristics as the antenna with the rectangular patch and the rectangular spiral strip line. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Exemplary design parameters of the monopole antenna including a 
               
               
                 circular shorted-patch and a probe with a circular spiral strip line 
               
            
           
           
               
               
               
            
               
                   
                 Design parameters 
                 Length (mm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Probe with a circular 
                 l s   
                 31.5 
               
               
                   
                 spiral strip line 
                 w s   
                 0.4 
               
               
                   
                   
                 a 
                 1.3 
               
               
                   
                   
                 b 
                 1.3 
               
               
                   
                   
                 d 
                 3.4 
               
               
                   
                   
                 h f   
                 8.0 
               
               
                   
                   
                 φ 1   
                 0.86 
               
               
                   
                 Circular 
                 2 ρ   
                 11.0 
               
               
                   
                 shorted patch 
                 h 
                 11.0 
               
               
                   
                   
                 h 1   
                 1.27 
               
               
                   
                   
                 h 2   
                 0.813 
               
               
                   
                   
                 h 3   
                 8.917 
               
               
                   
                   
                 φ 2   
                 1.6 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 11  illustrates variations of impedance and the return loss of an optimized antenna acquired from the EM simulation with respect to the folded strip line illustrated in  FIG. 3A . Table 3 illustrates examples of the design parameters of the optimized antenna. In the return loss illustrated in  FIG. 11A , the antenna has a 16.5% bandwidth from 1.835 GHz to 2.165 GHz with respect to VSWR≦2.  FIG. 11B  illustrates the impedance variation in a Smith chart. Accordingly, the folded strip line antenna has a similar characteristic with the rectangular spiral strip line antenna. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Exemplary design parameters of the monopole antenna including 
               
               
                 a rectangular shorted-patch and a folded strip line 
               
            
           
           
               
               
               
            
               
                   
                 Design Parameters 
                 Length (mm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Probe with a 
                 l s1   
                 6.1 
               
               
                   
                 folded strip line 
                 l s2   
                 6.5 
               
               
                   
                   
                 l s3   
                 6.2 
               
               
                   
                   
                 l s4   
                 2.45 
               
               
                   
                   
                 w s   
                 0.3 
               
               
                   
                   
                 a 
                 1.3 
               
               
                   
                   
                 b 
                 1.3 
               
               
                   
                   
                 d 
                 2.6 
               
               
                   
                   
                 h f1   
                 9.1 
               
               
                   
                   
                 h f2   
                 1.2 
               
               
                   
                   
                 φ 1   
                 0.86 
               
               
                   
                 Rectangular 
                 L 
                 11.0 
               
               
                   
                 shorted patch 
                 W 
                 11.0 
               
               
                   
                   
                 h 
                 11.0 
               
               
                   
                   
                 h 1   
                 1.27 
               
               
                   
                   
                 h 2   
                 0.813 
               
               
                   
                   
                 h 3   
                 8.917 
               
               
                   
                   
                 φ 2   
                 1.6 
               
               
                   
                   
               
            
           
         
       
     
       FIGS. 12A–12B  and  13 A– 13 B illustrate sectional views of radiation patterns at 1.95 GHz and 2.1 GHz, for the antenna with rectangular spiral strip line illustrated  FIG. 1C , respectively, in x-z plane and y-z plane. The radiation patterns illustrated in  FIGS. 12A–12B  and  13 A– 13 B illustrate that at 1.95 GHz and 2.1 GHz the antenna has a monopole type radiation pattern. In addition, the radiation pattern has a good linear polarization that the difference value between co-polarization and the cross-polarization with respect to a main beam direction is over 30 dB. 
       FIG. 14  illustrates an antenna radiation pattern in an x-y plane, in a direction of main beam, at 1.95 GHz and 2.1 GHz. In  FIG. 14 , E θ  has omni-directional radiation pattern with respect to an antenna plane. Antenna gain in the direction of main beam has a value over 2 dBi within a bandwidth. 
     Hereinafter, a description will be made for several monopole antennas, which have different antenna characteristics depending on the number of shorting pins according to other embodiments of the present invention. 
       FIGS. 15A to 15D  are views illustrating antennas having shorting pins, the number of which is different according to embodiments of the present invention. Antennas illustrated in  FIGS. 15A to 15C  include a rectangular patch  150  for connecting multiple shorting pins and a rectangular spiral strip line  151  to which a probe  153  is fed. 
     More specifically,  FIGS. 15A to 15C  are front views illustrating antennas in which one, two, and three shorting pins are connected to the rectangular patch  150 , respectively, and  FIG. 15D  is a side view of an antenna according to an embodiment of the present invention. The rectangular patch  150  has a length of L and a width of W and is located at a height of h. When only a single shorting pin  152  is connected to the rectangular patch  150 , the shorting pin is located at the center of the rectangular patch  150 . When two or more shorting pins are connected to the rectangular patch  150 , the shorting pins  154  and  155  are aligned in y-axis direction on the basis of the center of the rectangular patch  150  and are connected to a ground plane. The shorting pins have the same diameter of φ 1 . The multiple shorting pins are aligned in an interval of g on the rectangular patch  150 . 
     The rectangular spiral strip line  151  has a total length of l s  and a width of w s , and is fed by the probe  153  having a diameter of φ 2  at a height of h f . Because the diameter of the probe  153  is wider than the width of the rectangular spiral strip line  151 , a small square patch having sides of length a is formed at an end to connect the probe  153  to the rectangular spiral strip line  151 . Each of the shorting pin  152 ,  154 , and  155 , and the probe  153  fed to rectangular spiral strip line  151  are located at positions that are separated by a length of d on the rectangular patch  150 , thereby being electromagnetically coupled with each other. Similarly to the embodiment described with reference to  FIGS. 1A–1C , a high permittivity dielectric substrate  156   a  is added on the lower surface of the patch  150 , and a dielectric substrate  158  is added on the upper surface of the ground plane. 
     Hereinafter, antennas according to embodiments of the present invention will be described through simulation tests using the same data as those used in the simulation of  FIGS. 1A–1C . 
       FIGS. 16A and 16B  illustrate differences in impedance and return losses according to a change in the number of the shorting pins connected to the rectangular patch in an antenna according to an embodiment of the present invention. In  FIGS. 16A and 16B , the rectangular patch  150  has dimensions of L=W=11.0 mm, and the shorting pin has a diameter φ 1  of 1.0 mm. When only a single shorting pin is connected to the rectangular patch, the shorting pin is located at the center of the rectangular patch. When a plurality of shorting pins are connected to the rectangular patch, the shorting pins are aligned in an interval g of 3.0 mm in y-axis direction on the basis of the center of the rectangular patch. Also, the rectangular spiral strip line has a total length l s  of 29.68 mm and a line width w s  of 0.5 mm. The probe connected to the rectangular spiral strip line has a diameter φ 2  of 0.86 mm, a height h f  of 8.4 mm, and an interval d between the probe and the shorting pin is 3.9 mm. 
     When the number of the shorting pins increases, the area occupied by the shorting pins also increases. As a result, the capacitance of the rectangular patch decreases. Therefore, referring to return loss illustrated in  FIG. 16A , when the number of the shorting pins increases from one to three, a center frequency of the antenna increases from about 1.69 GHz to 2.19 GHz, and then to 2.51 GHz. 
     With the increase of the center frequency, both an interval between the probe and the shorting pins and an interval between the rectangular spiral strip line and the patch become more distant electrically, such that the couplings between them decrease. 
       FIG. 16B  is a Smith chart illustrating an impedance characteristic depending on an increase of the number of shorting pins in an antenna according to an embodiment of the present invention. Referring to  FIG. 16B , it can be understood that the decrease of the capacitance resulting from the increase of the number of the shorting pins in an antenna according to an embodiment of the present invention moves the loop of an impedance locus from a capacitive region to an inductive region, and the decrease of the coupling causes the size of the loop of the impedance locus to be reduced. 
     As described above with reference to  FIGS. 15A to 15D  and  FIGS. 16A and 16B , it is possible to change characteristics of the return loss and the input impedance by increasing the number of the shorting pins. Such an effect can also be obtained by changing the locations of the shorting pins, which will be described below with reference to  FIGS. 17 to 19 . 
       FIG. 17  is a view illustrating variations of an input impedance characteristic according to adjustments of the distance between a shorting pin and a feed probe in an antenna according to an embodiment of the present invention. That is,  FIG. 17  illustrates variations of an input impedance characteristic of an antenna according to adjustments of a distance d between a shorting pin and a feed probe, when two shorting pins are aligned at an interval g of 3.0 mm in a rectangular patch. In this embodiment, the dimensions of a shorted rectangular patch and the length and height of a rectangular spiral strip line feed are established as the same values as those established in the embodiment of  FIGS. 16A and 16B . The variation of the input impedance characteristic of an antenna will be described with distance d as a parameter. 
     Referring to  FIG. 17 , an electromagnetic coupling efficiency between a shorted rectangular patch and a feed probe is determined by distance d. In addition, the variation of distance d causes the input impedance of the antenna to be changed to exert an effect on bandwidth. More specifically, when distance d between a shorting pin and a probe is 1.9 mm, an electromagnetic coupling between a shorted patch monopole and a probe-fed rectangular spiral strip line monopole is very weak, such that the loop of an impedance locus is small. The more the distance between the two monopoles increases, the more the coupling between them increases. When distance d becomes 7.9 mm, the coupling is maximized to cause the loop of the impedance locus to be maximized. However, when distance d increases over 7.9 mm, the electromagnetic coupling again decreases to cause the loop of the impedance locus to be smaller and smaller as illustrated in  FIG. 17 , for distances d of 10.9 mm and 13.9 mm. 
     Therefore, an antenna can be designed to have a maximum bandwidth by changing the electromagnetic coupling through adjustment of a distance between a feed probe and a shorting pin in a rectangular patch. 
       FIGS. 18A to 18C  are views illustrating electric current distributions depending on adjustments of the distance between shorting pins in an antenna including two shorting pins according to an embodiment of the present invention. In the antenna structure having two shorting pins according to an embodiment of the present invention, the two shorting pins are connected to a rectangular patch. A rectangular spiral strip line has a total length l s  of 23.73 mm and a line width w s  of 0.5 mm. The spiral strip line is located at a height h f  of 8.5 mm, and an interval d between a probe and the shorting pin is 4.2 mm. 
     In such a structure, electric current distributions in the rectangular patch according to alignment interval g between the shorting pins are illustrated in  FIGS. 18A to 18C . That is,  FIGS. 18A to 18C  illustrate electric current distributions in rectangular patches at the respective relevant resonant frequencies when two shorting pins separated by an alignment interval of 2.5 mm, 4.5 mm, and 6.5 mm, respectively. Referring to  FIGS. 18A to 18C , little current flows in the center of the patch (i.e., between the shorting pins) but currents to flow from the edge part to the shorting pins, such that a route of current becomes short. As a result, in-phase currents flows at the two shorting pins electromagnetically connected to a feed probe, and the electric potential difference between the two shorting pins becomes “0”. 
     When the two shorting pins connected to a rectangular patch are aligned in a narrow interval, the electric current distribution of flowing uniformly to the four directions similarly to that in a case of a single shorting pin. However, as the alignment interval between the shorting pins becomes wider, electric current does not flow in the center position of the rectangular patch (i.e., in the position between two shorting pins having no electric potential difference). In this case, an electric current distribution area of the rectangular patch is reduced, and a resonant frequency of the shorted rectangular patch increases. 
       FIGS. 19A and 19B  are graphs illustrating return losses and impedance variations depending on adjustments of the distance between shorting pins in an antenna structure having two shorting pins according to an embodiment of the present invention. Referring to  FIG. 19A , when an alignment interval between the two shorting pins increases from 2.5 mm, to 4.5 mm, and to 6.5 mm, a resonant frequency of an antenna increases from about 2.05 GHz to about 2.4 GHz. More specifically, using imaginary numbers, when the alignment interval is 2.5 mm, a reactance of the antenna is shown as a capacitance component when an alignment interval is 2.5 mm, but when the alignment interval increases to 6.5 mm, the capacitance component decreases and an inductance component increases in the rectangular patch. 
     As a result illustrated in  FIGS. 16A to 19B , it can be confirmed that variations of alignment intervals and the number of shorting pins connected to a rectangular patch causes a change of a reactance values of an antenna, such that a resonant frequency can move by adjusting the shorting pins. Therefore, it is possible to design an optimized antenna using changes of characteristics according to changes in an alignment interval and/or the number of shorting pins connected to a rectangular patch. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 One 
                 Two 
                 Three 
               
               
                   
                 Design 
                 shorting 
                 shorting 
                 shorting 
               
               
                   
                 Parameters 
                 pin 
                 pins 
                 pins 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Rectangular spiral strip 
                 l s   
                 40.73 
                 29.68 
                 19.08 
               
               
                 line fed to probe 
                 H f   
                 6.9 
                 8.4 
                 9.3 
               
               
                   
                 d 
                 3.7 
                 3.9 
                 4.4 
               
            
           
           
               
               
               
            
               
                   
                 w s   
                 0.5 
               
               
                   
                 a 
                 1.3 
               
               
                   
                 φ 2   
                 0.86 
               
               
                 Shorted rectangular 
                 L 
                 11.0 
               
               
                 patch 
                 W 
                 11.0 
               
               
                   
                 h 
                 11.0 
               
               
                   
                 h 1   
                 1.27 
               
               
                   
                 h 2   
                 0.183 
               
               
                   
                 g 
                 3.0 
               
               
                   
                 φ 1   
                 1.0 
               
               
                   
               
            
           
         
       
     
     Table 4 shows design parameters for an optimized antenna when the antenna includes one, two, and three shorting pins connected to a rectangular patch, respectively, under the condition that a rectangular patch has dimensions of L=W=11.0 mm, a shorting pin has a diameter φ 1  of 1.0 mm, and an alignment interval g between the shorting pins is 3.0 mm. As the number of shorting pins increases, the length l s  of a rectangular spiral strip line decreases from 40.73 mm to 19.08 mm because the capacitance of the antenna decreases according to the increase of the number of the shorting pins. Accordingly, it is necessary to also decrease the inductance of the antenna in order to facilitate generation of resonance. 
     In addition, optimized design parameters having the maximum bandwidth are determined by adjusting a height of the probe and a distance between a shorting pin and the probe. 
       FIG. 20  is a graph illustrating return losses of antennas optimized according to the number of the shorting pins that are connected to the rectangular patch designed with parameters shown in Table 4. 
     Table 5 shows characteristics of antennas optimized according to the number of the shorting pins that are connected to the rectangular patch as described with reference to  FIG. 20 . 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Center 
                   
                 Electrical 
               
               
                   
                 frequency 
                 Bandwidth 
                 Volume 
               
               
                   
                 (GHz) 
                 (%) 
                 (λ 0 ) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 One 
                 1.9 
                 1.753 GHz~2.047 GHz 
                 0.07 λ 0  × 0.07 λ 0  × 
               
               
                 shorting 
                   
                 (15.47%) 
                 0.07 λ 0   
               
               
                 pin 
               
               
                 Two 
                 2.333 
                 1.995 GHz~2.471 GHz 
                 0.082 λ 0  × 0.082 λ 0  × 
               
               
                 shorting 
                   
                 (21.32%) 
                 0.082 λ 0   
               
               
                 pins 
               
               
                 Three 
                 2.54 
                 2.197 GHz~2.897 GHz 
                 0.093 λ 0  × 0.093 λ 0  × 
               
               
                 shorting 
                   
                 (27.56%) 
                 0.093 λ 0   
               
               
                 pins 
               
               
                   
               
            
           
         
       
     
     Referring to  FIG. 20  and Table 5, when a single shorting pin is connected to a rectangular patch, an antenna has a bandwidth of a range from 1.753 GHz to 2.047 GHz on the basis of “VSWR≦2”, and has a bandwidth of 15.47% at the center frequency of 1.9 GHz. When two shorting pins are connected to a rectangular patch, an antenna has a bandwidth of a range from 0.1.995 GHz to 2.471 GHz, and has a bandwidth of 21.32% at the center frequency of 2.333 GHz. When three shorting pins are connected to a rectangular patch, an antenna has a bandwidth of a range from 2.197 GHz to 2.897 GHz and has a bandwidth of 27.56% at the center frequency of 2.54 GHz. 
     Additionally, an electrical volume of an antenna at a center frequency on the basis of a wavelength λ 0  of a free space is “0.07 λ 0 ×0.07 λ 0 ×0.07 λ 0 ” when a single shorting pin is connected to a rectangular patch, is “0.082 λ 0 ×0.082 λ 0 ×0.082 λ 0 ” when two shorting pins are connected to a rectangular patch, and is “0.093 λ 0 ×0.093 λ 0 ×0.093 λ 0 ” when three shorting pins are connected to a rectangular patch. From this, it can be understood that electrical size is small. 
       FIGS. 21A to 23B  are views illustrating radiation patterns calculated in a x-z plane and a y-z plane within a frequency range of a bandwidth when an antenna has one, two, and three shorting pins, respectively. In  FIGS. 21A to 23B , it is assumed that an antenna has a main beam at about “θ=72°” and has a monopole type of radiation pattern in which radiation is transmitted in all directions of Φ. 
     More specifically,  FIGS. 21A and 21B  illustrate radiation patterns of an antenna having a single shorting pin, with respect to frequencies of 1.8 GHz and 2.0 GHz, respectively. When the antenna has a single shorting pin, the maximum gain of the antenna is 0.7 dBi at 1.8 GHz, and 1.2 dBi at 2.0 GHz. 
       FIGS. 22A and 22B  illustrates radiation patterns of an antenna having two shorting pins, with respect to frequencies of 2.1 GHz and 2.4 GHz, respectively. When there are two shorting pins, the maximum gain of the antenna is 3.0 dBi at 2.1 GHz, and 4.0 dBi at 2.4 GHz. 
       FIGS. 23A and 23B  illustrates radiation patterns of an antenna having two shorting pins, with respect to frequencies of 2.3 GHz and 2.7 GHz, respectively. When there are three shorting pins, the maximum gain of the antenna is 3.5 dBi at 2.3 GHz, and 4.8 dBi at 2.7 GHz. 
       FIG. 24  is a view illustrating an antenna having three shorting pins according to yet another embodiment of the present invention. In  FIG. 24 , unlike an alignment structure of three shorting pins illustrated in  FIG. 15C , the shorting pins may be aligned in a triangular shape without being aligned in a straight line. In this case, a distance d between a probe and the three shorting pins and a distance g between the respective shorting pins become subjects in question. That is, in  FIG. 24 , a distance d between a probe and the three shorting pins is calculated on the basis of the center of gravity of a triangle formed by imaginary lines connecting the three shorting pins. In addition, it is assumed that the respective shorting pins are equidistant. 
       FIG. 25  is a view illustrating an antenna having four shorting pins according to still another embodiment of the present invention. More specifically,  FIG. 25  illustrates the four shorting pins aligned in a square form, without being aligned in a straight line. 
     In  FIG. 25 , a distance d between a probe and the four shorting pins is calculated on the basis of the center of gravity of a square formed by imaginary lines connected among the four shorting pins. In addition, it is assumed that the distance between the shorting pins made in rectangular sides is equidistant. 
     As described above, a plurality of shorting pins may be aligned in a line form, a triangle form, or a square form, on a rectangular patch, and consequently, the shorting pins may be aligned in a random form on a rectangular patch. When the shorting pins are aligned in a random form, parameters d and g are calculated according to a relevant form. 
     As described above, the present invention suggests a monopole antenna and its equivalent model that the probe with a strip line, where the strip line can be the spiral type or the folded type, and the shorted patch are electromagnetically coupled. The monopole antenna provides a low resonance by compensating the capacitive component of the shorted patch with the inductive component of the probe with a strip line. In addition, the monopole antenna is advantageous in realizing a wide single-band and a dual-band because the resonance frequencies of the shorted patch and the probe with a strip line are adjustable by varying the antenna design parameters. Specifically, the wide bandwidth can be obtained by electromagnetic coupling the shorted patch to the probe with a strip line, thereby combining the resonance by the probe with a strip line and the resonance by the shorted patch. Therefore, in this antenna, changing the inductance and the capacitance is available by adjusting the design parameters of the probe with a strip line and the shorted patch. As such, the resonance of the probe with a strip line and the resonance of the shorted patch can be adjusted by varying the inductance and the capacitance. Consequently, it is possible to design an antenna having a characteristic of a wideband or a dual-band by varying a resonance frequency. 
     In addition, the design scheme of the present invention enables the antenna structure to be small if a dielectric material of a high permittivity is used for the shorted patch. The probe with a strip line can have the maximum resonance length within the minimum volume by constructing the strip line as a modified type such as a spiral type, a folded type, or a helical type. Preferably, the total length of the modified strip line and the probe as such is equal to a length of about 0.25λ. In other words, the miniaturization of the monopole antenna according to the present invention can be achieved by modifying the probe with a strip line to have 0.25λ resonance length in the minimum volume. 
     Furthermore, it is also possible to adjust the impedance matching characteristic by using the electromagnetic coupling between the shorted patch and the probe with a strip line. In the antenna structure according to the present invention, it is possible to achieve, without any separate matching circuit, a wide bandwidth by improving the impedance matching characteristic because the capacitance of the shorted patch and the inductance of the probe with the strip line can be adjusted in the antenna itself. 
     According to the experimental data, both the antenna having a rectangular spiral strip line and the antenna having a folded strip line have a bandwidth of 16.5% at the center frequency 2.0 GHz, while the antenna having a circular spiral strip line has a bandwidth of 17.4% at the center frequency 2.15 GHz. The present antenna has an omni-directional radiation pattern. Therefore, it can be said that the antenna suggested by the present invention is applicable as an embedded antenna for the mobile communication terminals such as the cellular phone, the PCS phone, the IMT-2000 terminal, PDA, or WLAN applications. 
     It should be noted that although optimum embodiments have been described above, it is apparent that variations and modifications by those skilled in the art can be effected within the spirit and scope of the present invention defined in the appended claims. Therefore, all variations and modifications equivalent to the appended claims are within the scope of the present invention. 
     While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.