Patent Publication Number: US-7589686-B2

Title: Small ultra wideband antenna having unidirectional radiation pattern

Description:
This application claims priority, under 35 U.S.C. § 119(a), from Korean Patent Application Nos. 10-2005-0005078 filed Jan. 19, 2005 and 10-2005-0101159 filed on Oct. 26, 2005 in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Apparatuses consistent with the present invention relate to a small ultra wideband (UWB) antenna, and more particularly to a small UWB antenna designed to have a unidirectional radiation pattern by combining a loop radiator and a dipole radiator. 
   2. Description of the Related Art 
   All antennas are used to convert an electric signal into a specified electromagnetic wave to radiate the converted electromagnetic wave to free space, or to convert a received electromagnetic wave into an electric signal. UWB technology means a wireless transmission technology that directly transmits and receives an impulse signal without using an RF carrier. A UWB antenna is an antenna that can transmit and receive an impulse signal using a frequency band in the range of 3.1 to 10.6 GHz. 
   This UWB technology refers to a communication method that can achieve a high-speed data transmission using an ultra low power as it uses a very wide frequency band, unlike the existing narrow-band communication method. Accordingly, it can be applied to portable communication appliances that have been rapidly developed. 
   An antenna having been used in currently developed portable communication devices is required to satisfy the following conditions: being capable of performing UWB signal transmission/reception, having unidirectional radiation pattern, and being subminiature. The radiation pattern means the shape of an effective region where an antenna can radiate or sense electromagnetic waves. Since communication is possible in the case where the radiation pattern is formed in the direction of a base station, a portable communication appliance requires a unidirectional radiation pattern. 
     FIG. 1  is a view illustrating the structure of a Vivaldi antenna known in the art. Referring to  FIG. 1 , the antenna includes a power feeding part  11 , an excitation part  12 , a slot  13 , a dipole radiator  14 , and a substrate  15  that supports the above-mentioned components. The structure of such a Vivaldi antenna is disclosed in U.S. Pat. No. 5,428,364. When an external electromagnetic energy is supplied through the power feeding part  11 , the excitation part  12  is excited. Accordingly, the electromagnetic energy transmitted along the power feeding part  11  is transferred to the slot  13  the width of which is gradually widened. The transferred electromagnetic energy is converted into an electromagnetic wave in the air at a right end part of the slot  13 , and the electromagnetic wave is radiated in one direction as indicated by an arrow in  FIG. 1 . 
   This Vivaldi antenna can perform UWB signal transmission/reception and has a unidirectional radiation pattern. However, it requires an impedance matching in order to secure the radiation characteristic of the desired whole frequency band and to transmit electromagnetic energy provided from an external source without loss. In order to achieve the impedance matching, the size of the antenna should be increased as the wavelength of the wave is lengthened. 
   Consequently, in order to perform a low frequency band communication, the size of the antenna should be increased, and this causes a difficulty in miniaturization of the communication appliance. 
     FIG. 2  is a view illustrating the structure of a substrate type dipole antenna. Referring to  FIG. 2 , the substrate type dipole antenna includes a substrate  21 , a first radiator  22 , second radiators  23   a  and  23   b , a feeder  24 , and a signal supply part  25 . The antenna structure of  FIG. 2  is disclosed in U.S. Pat. No. 6,642,903, the detailed explanation thereof will be omitted. 
   In the substrate type dipole antenna of  FIG. 2 , the first radiator  22  and the second radiators  23   a  and  23   b , which are prepared as wide plane conductors, are laminated on the substrate  21  to implement a wideband antenna. The electromagnetic energy supplied from the signal supply part  25  is applied to the feeder  24 . The feeder  24  and separations  26   a  and  26   b  formed on the right and left of the feeder  24  constitute a feed region  30 . The fed electromagnetic energy is converted into electromagnetic waves by the first radiator  22  and the second radiators  23   a  and  23   b , and the converted electromagnetic waves are radiated in the direction of an arrow. This substrate type dipole antenna has the advantage in that it can transmit a UWB signal and can be fabricated with a relatively small size, but has the problem that it cannot have a unidirectional radiation pattern. 
   In addition to the Vivaldi antenna and the substrate type dipole antenna as described above, “Microstrip Patch Antenna,” by Weigand et al, IEEE Trans. Antennas Propagat. vol. 51, no. 3, March 2003, is known. Although this microstrip patch antenna has unidirectional radiation pattern and can be subminiaturized, it has the problem that it has a narrow bandwidth. 
   SUMMARY OF THE INVENTION 
   Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above. An aspect of the present invention is to provide a small UWB antenna designed to have a unidirectional radiation pattern by using a loop radiator and a dipole radiator. 
   In order to achieve the above-described aspects of the present invention, there is provided a UWB antenna, according to an exemplary embodiment of the present invention, which comprises a substrate, a power feeding part, provided on an upper surface of the substrate, for receiving a supply of an external electromagnetic energy; a dipole radiator excited by the electromagnetic energy fed through the power feeding part and radiating electromagnetic waves in one and the other directions of the substrate; and an active loop radiator excited by the electromagnetic energy fed through the power feeding part, respectively enhancing and canceling the electromagnetic fields produced in one or the other directions of the substrate by the dipole radiator. 
   The UWB antenna may further comprise a delay part, provided to connect the power feeding part with the dipole radiator on the upper surface of the substrate, for delaying a time point where the electromagnetic energy is supplied to the dipole radiator. 
   The UWB antenna may further comprises at least one passive loop radiator excited by an induced electromagnetic energy induced by the dipole radiator and the active loop radiator, respectively enhancing and canceling the electromagnetic fields produced in one or the other directions of the substrate by the dipole radiator. 
   The active loop radiator, the dipole radiator, the delay part and the passive loop radiator may be positioned on the same plane as the power feeding part on the upper surface of the substrate. 
   In this case, the power feeding part, the active loop radiator, the dipole radiator, the delay part and the passive loop radiator may be produced by patterning a single metal film deposited on the upper surface of the substrate. 
   The power feeding part may comprise a signal terminal, provided on the upper surface of the substrate, for receiving the supply of the electromagnetic energy, and first and second ground terminals arranged on both sides of the signal terminal to form a coplanar waveguide structure on the upper surface of the substrate. 
   The active loop radiator has one end connected to the signal terminal and the other end connected to the first ground terminal. 
   The dipole radiator may comprise a first pole arranged on the upper surface of the substrate to slope at a predetermined angle to one side of the substrate, and a second pole arranged on the upper surface of the substrate to slop at a predetermined angle to the first pole. 
   The dipole radiator may have a structure in which the first pole is connected to the signal terminal and the second pole is connected to the second ground terminal. 
   In another aspect of the present invention, there is provided a UWB antenna, which comprises a substrate; a power feeding part, provided on an upper surface of the substrate, for receiving a supply of an electromagnetic energy; a dipole radiator excited by the electromagnetic energy fed through the power feeding part and radiating electromagnetic waves in specified directions; and a loop radiator for making the electromagnetic waves radiated by the dipole radiator have a unidirectional radiation pattern by interfering the electromagnetic waves. 
   The power feeding part may include a signal terminal, provided on the upper surface of the substrate, for receiving the supply of the electromagnetic energy, a first ground terminal arranged apart for a specified distance from the signal terminal on the upper surface of the substrate, and a second ground terminal, arranged in a direction opposite to the first ground terminal on the basis of the signal terminal on the upper surface of the substrate. 
   The UWB antenna may further include at least one slot for intercepting current flowing backward to the first and second ground terminal. 
   In this case, the dipole radiator may include a first pole connected to the signal terminal, a second pole connected to the second ground terminal, and a first slot line for exciting the dipole radiator. 
   One end of the first slot line may be connected to the power feeding part, the other end of the first slot line may form an input part of the dipole radiator, and a space between the first pole and the second pole may be gradually widened, starting from the input part. 
   The loop radiator may include an active loop radiator having one end connected to the signal terminal and the other end connected to the first ground terminal, excited by the electromagnetic energy fed through the signal terminal, enhancing the electromagnetic waves radiating in one direction from the dipole radiator, and canceling the electromagnetic fields produced in the other direction from the dipole radiator; and at least one passive loop radiator excited by an induced electromagnetic energy induced by the dipole radiator and the active loop radiator, enhancing the electromagnetic waves radiating in one direction from the dipole radiator, and canceling the electromagnetic fields produced in the other direction from the dipole radiator. 
   In this case, the active loop radiator may include a second slot line exciting the active loop radiator, and a loop connected to the second slot line and having remaining sides except for a side connected to the second slot line, which are closed sides. 
   The dipole antenna, the power feeding part and the loop radiator are formed in a manner that a metal layer deposited on the surface of the substrate is patterned in a specified form, and the surface of the substrate that corresponds to an area between the first pole and the second pole, an area between the signal terminal and the first ground terminal, an area between the signal terminal and the second ground terminal, a loop area of the active loop radiator and a loop are of the passive loop radiator is exposed. 
   The at least one slot may include at least one first slot formed by patterning a specified area of a side metal layer in which the active loop radiator is formed on the basis of the dipole radiator, and at least one second slot formed by patterning a specified area of a side metal layer in which the passive loop radiator is formed on the basis of the dipole radiator. 
   In the exemplary embodiments of the present invention as described above, the substrate may be produced in the form of a rectangular flat board of which vertical sides are longer than its horizontal sides. 
   In this case, the power feeding part may be positioned at an edge of the vertical side of the substrate, and the dipole radiator may be arranged in a direction toward the side opposite to the vertical side where the power feeding part is positioned to radiate the electromagnetic waves in the same direction as a feeding direction. 
   The power feeding part may be positioned at an edge of the horizontal side of the substrate, and the dipole radiator may be arranged in a direction toward the vertical side of the substrate to radiate the electromagnetic waves in a direction perpendicular to a feeding direction. 
   The substrate may be a rectangular flat board having a horizontal side of 0.2 λmin and a vertical side of 0.3 λmin if a minimum frequency in an available frequency band is fmin and a free-space wavelength corresponding to the minimum frequency fmin is λmin. 
   The characteristic impedance of the second slot line may be three or four times the characteristic impedance of the first slot line. 
   The width of the second slot line may be wider than the width of the first slot line to improve the characteristic impedance. 
   An area of the substrate in which the second slot line is formed may be etched to increase the characteristic impedance of the second slot line. 
   The difference between an electric length of the first slot line and an electric length of the second slot line in the minimum frequency state may be 0.15 λmin if a minimum frequency in an available frequency band is fmin and a free-space wavelength corresponding to the minimum frequency fmin is λmin. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above aspects and features of the present invention will become more apparent by describing certain exemplary embodiments of the present invention with reference to the accompanying drawings, in which: 
       FIG. 1  is a view illustrating the structure of a conventional Vivaldi antenna; 
       FIG. 2  is a view illustrating the structure of a conventional substrate type dipole antenna; 
       FIG. 3  is a view illustrating the structure of a UWB antennal according to an exemplary embodiment of the present invention; 
       FIGS. 4 and 5  are exemplary sectional views illustrating the antenna of  FIG. 3 ; 
       FIG. 6  is a view explaining the principle of the unidirectional radiation pattern that the UWB antenna of  FIG. 3  has; and 
       FIGS. 7 ,  8  and  9  are views illustrating the structure of a UWB antenna according to another exemplary embodiment of the present invention; 
       FIG. 10  is a graph explaining the voltage standing wave ratio (VSWR) characteristic of a UWB antenna of  FIG. 9 ; 
       FIG. 11  is a graph explaining the antenna gain characteristic of a UWB antenna of  FIG. 9 ; 
       FIGS. 12 and 13  are views illustrating the structure of a UWB antenna with a slot added thereto according to still another exemplary embodiment of the present invention; 
       FIG. 14  is a graph explaining the voltage standing wave ratio (VSWR) characteristic of a UWB antenna of  FIG. 13 ; and 
       FIG. 15  is a graph explaining the antenna gain characteristic of a UWB antenna of  FIG. 13 . 
   

   DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
   Certain exemplary embodiments of the present invention will be described in greater detail with reference to the accompanying drawings. 
   In the following description, same drawing reference numerals are used for the same elements even in different drawings. The matters defined in the description such as a detailed construction and elements are nothing but the ones provided to assist in a comprehensive understanding of the invention. Thus, it is apparent that the present invention can be carried out without those defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
     FIG. 3  is a view illustrating the structure of a UWB antennal according to an exemplary embodiment of the present invention. 
   Referring to  FIG. 3 , the UWB antenna according to an exemplary embodiment of the present invention includes a power feeding part  110 , an active loop radiator  120 , and a dipole radiator  130 . 
   The power feeding part  110  is connected to an external terminal, and transfers electromagnetic energy supplied from the external terminal to the following parts. For this, the power feeding part  110  includes a signal terminal  111  and ground terminals  112   a  and  112   b . In addition, it is preferable, but not always necessary, that the power feeding part  110  is constructed to have a coplanar waveguide structure in which the ground terminals  112   a  and  112   b  and the signal terminal  111  are positioned on the same plane. This is because the coplanar waveguide structure is useful to the implementation of a monolithic microwave integrated circuit (MMIC) or a micro integrated circuit (MIC). The ground terminals  112   a  and  112   b , which are now referred to the first ground terminal  112   a  and the second ground terminal  112   b , are arranged on both sides around the signal terminal  111 . 
   The active loop radiator  120  has one end connected to the signal terminal  111  of the power feeding part  110  and the other end connected to the first ground terminal  112   a . Accordingly, the electromagnetic energy inputted through the signal terminal  111  is guided in the direction of the first ground terminal  112   a . Accordingly, an omnidirectional radiation pattern is formed around the UWB antenna. 
   The dipole radiator  130  is composed of a first pole  131  and a second pole  132 . The dipole radiator  130  radiates the electromagnetic waves of the same polarity toward one side and the other side of the UWB antenna. The polarities of electric fields produced by the electromagnetic waves radiated from the dipole radiator  130  are the same at one side and the other side of the substrate. In this case, the electric field formed at one side (e.g., the right side in  FIG. 3 ) of the substrate has the same polarity as that produced by the electromagnetic wave-radiated from the active loop radiator  120 , and thus the electric field is enhanced. By contrast, the electric field formed at the other side (e.g., the left side in  FIG. 3 ) of the substrate has a different polarity from that produced by the electromagnetic wave radiated from the active loop radiator  120 , and thus the electric field is canceled. As a result, a unidirectional radiation pattern, which corresponds to the electric field produced on only one side of the substrate, is formed. 
     FIG. 4  is a sectional view of the UWB antenna of  FIG. 3 , seen from a point ‘a’. Referring to  FIG. 4 , the UWB antenna is supported by the substrate  100 . The signal terminal  111  and the first and second ground terminals  112   a  and  112   b  that constitute the power feeding part  110  are constructed to have the coplanar waveguide structure. 
     FIG. 5  is a sectional view of the UWB antenna of  FIG. 3 , seen from a point ‘b’. Referring to  FIG. 5 , the active loop radiator  120  and the dipole radiator  130  are positioned on the same plane as the power feeding part  110  on the upper surface of the substrate  100 . In addition, the first pole  131  of the dipole radiator  130  becomes a part of the active loop radiator  120 . 
   The UWB antenna having the structure as illustrated in  FIGS. 4 and 5  may be produced by depositing a metal layer on the substrate  100  and patterning the metal layer by etching. That is, the power feeding part  110 , the active loop radiator  120  and the dipole radiator  130  can be formed at a time by inputting an etching liquid or etching gas after depositing a photoresist layer patterned as shown in  FIG. 3  on the metal layer. 
     FIG. 6  is a view explaining the principle of the unidirectional radiation pattern that the UWB antenna of  FIG. 3  has.  FIG. 6  illustrates the polarities of the electric fields produced in a far-field region that is a predetermined distance apart from the UWB antenna. Referring to  FIG. 6 , the electric fields produced in one and the other directions of the substrate  100  by the dipole radiator  130  are all directed downward. That is, electric fields having the same polarity are produced. By contrast, the electric field produced at one side of the substrate  100  by the active loop radiator  120  is directed downward while the electric field produced at the other side of the substrate  100  is directed upward. That is, electric fields having different polarities are produced. 
   As a result, if the UWB antenna  300  is implemented by combining the active loop radiator  120  and the dipole radiator  130 , the electric field produced at one side of the substrate is enhanced and the electric field produced at the other side is canceled. Accordingly, a unidirectional radiation pattern is formed at one side of the substrate. 
     FIG. 7  is a view illustrating the structure of a UWB antennal according to another exemplary embodiment of the present invention. Referring to  FIG. 7 , the UWB antenna further includes a passive loop radiator  240  and a delay part  250  in addition to the power feeding part  210 , the active loop radiator  220  and the dipole radiator  230 . 
   The passive loop radiator  240  is formed in a metal layer part connected to the second ground terminal  212   b . Accordingly, the passive loop radiator cannot receive the electromagnetic energy from the power feeding part  210 , but can receive the induced electromagnetic energy induced when the active loop radiator  220  and the dipole radiator  230  are excited. Accordingly, the passive loop radiator  240  also radiates the electromagnetic wave in an omnidirectional radiation pattern. By adjusting the size and position of the passive loop radiator  240 , the radiation pattern of the UWB antenna can be optimally adjusted. That is, the electromagnetic field produced by the passive loop radiator  240  enhances and cancels the electromagnetic fields produced in one and the other directions of the substrate by the dipole radiator  230 . In  FIG. 7 , only one passive loop radiator  240  is illustrated. However, a plurality of passive loop radiators may be implemented according to exemplary embodiments of the present invention. 
   On the other hand, the first pole  231  that constitutes the dipole radiator  230  is connected to the signal terminal  211 , and the second pole  232  is connected to the second ground terminal  212   b . In this case, the region where the first pole  231  and the second pole  232  are branched is a predetermined distance apart from the power feeding part  210  to form a delay part  250 . Accordingly, the delay part  250  serves to delay the time point of supplying the electromagnetic energy being supplied to the dipole radiator  230 . As a result, by matching the phase of the electromagnetic field produced by the active and passive loop radiators  220  and  240  to the phase of the electromagnetic field produced by the dipole radiator  230 , the electromagnetic field enhancement and cancellation can be performed. 
     FIG. 8  is a view illustrating the structure of a UWB antennal according to still another exemplary embodiment of the present invention. According to the UWB antenna of  FIG. 8 , the shapes and positions of a power feeding part  310 , an active loop radiator  320 , a dipole radiator  330 , a passive loop radiator  340  and a delay part  350  are different from those of the UWB antenna of  FIG. 7 . By changing the pattern of the metal layer, the UWB antenna can be produced to have the structure as illustrated in  FIG. 8 . Referring to  FIG. 8 , the passive loop radiator  340  is not connected to the second ground terminal  312   b  of the power feeding part  310 , but is formed on the side of the first ground terminal  312   a . The passive loop radiator  340  is formed on an upper part of the dipole radiator  330 . Since the operation of the UWB antenna of  FIG. 8  is the same as that of the UWB antenna of  FIG. 7 , further explanation thereof will be omitted. 
     FIG. 9  is a view illustrating the structure of a UWB antenna according to still another exemplary embodiment of the present invention. The UWB antenna of  FIG. 9  includes a power feeding part  410 , an active loop radiator  420 , a dipole radiator  430 , and a passive loop radiator  440 . The respective constituent elements may be formed by patterning the metal layer deposited on the substrate. That is, parts except for parts marked with slanting lines in  FIG. 9  represent the upper surface of the substrate. Accordingly, the respective constituent elements in  FIG. 9  are separately formed on the metal layer of the first pole side  433  of the dipole radiator  430  and on the metal layer of the second pole side  434  of the dipole radiator  430 . Referring to  FIG. 9 , the active loop radiator  420  is formed on the metal layer of the first pole side  433 , and the passive loop radiator  440  is formed on the metal layer of the second pole side  434 . 
   The power feeding part  410  includes a signal terminal  411 , a first ground terminal  412   a  and a second ground terminal  412   b . Although not illustrated in  FIG. 9 , the power feeding part  410  is provided with a connector in which a power feeding cable can be mounted. In  FIG. 9 , parts indicated as the signal terminal  411 , the first ground terminal  412   a  and the second ground terminal  412   b  mean parts connected to the signal line and ground lines of the connector. 
   On the other hand, a space between the signal terminal  411  and the second ground terminal  412   b  and a space between the first pole  433  and the second pole  434  form a first slot line  432 . The first slot line  432  excites the dipole radiator  430  during a power feeding. One end of the first slot line  432  is connected to the power feeding part  410 , and the other end thereof is connected to an input part  431 . The first pole  433  and the second pole  434  branch out so that a space between them is gradually widened, starting from the input part  431 . The direction in that the first pole  433  and the second pole  434  branch out is the same as the direction toward the side opposite to the side in which the power feeding part  410  is located, i.e., the direction in which the power feeding is performed. 
   A specified part of the first slot line  432 , i.e., a part bent in a direction toward the input part  431  in  FIG. 9 , may operate as delay parts  250  and  350  provided in the UWB antennas of  FIGS. 7 and 8 . 
   On the other hand, the active loop antenna  420  includes a second slot line  422  and a loop  423 . The second slot line  422  means a space between the signal terminal  411  and the first ground terminal  412   a . The second slot line  422  excites the active loop antenna  420 . One end of the second slot line  422  is connected to the power feeding part  410 . The loop  423  has the remaining sides except for the side connected to the second slot line  422 , which are closed sides. The connection part of the second slot line  422  and the loop  423  form the input part  421  of the active loop antenna. That is, the other end of the second slot line  422  forms the input part  421  of the active loop antenna. 
   The width w 1  of the first slot line  432  and the width w 2  of the second slot line  422  are in proportion to the characteristic impedance of the first and second slot lines  432  and  422 . That is, as the width of the slot line is widened, the value of the characteristic impedance is increased. Using this characteristic, the antenna characteristic can be optimized by adjusting the characteristic impedance ratio of the first and second slot lines  432  and  422 . Specifically, the widths of the first and second slot lines may be determined so that the characteristic impedance of the second slot line  422  becomes three or four times the characteristic of the first slot line  432 . 
   In order to improve the characteristic impedance of the second slot line  422 , the width w 2  may be widened. In this case, if the width w 2  is increased too much, the second ground terminal  412   a  may escape from the range of the power feeding part  410 , i.e., the part to which the connector is connected. Thus, the characteristic impedance can be improved by widening the sectional area of the second slot line  422  through the etching of the substrate area that corresponds to the second slot line  422  in a state where the width w 2  is maintained. 
   The substrate used in the UWB antenna of  FIG. 9  may be implemented by a dielectric substrate in the form of a rectangular flat board. The lengths of the horizontal and vertical sides of the dielectric substrate may be optionally set according to the use field and purpose of the UWB antenna. 
   Specifically, if the minimum frequency in an available frequency band is fmin and a free-space wavelength corresponding to the minimum frequency fmin is λmin, the length of the horizontal side of the substrate may be set to 0.2 λmin and the length of the vertical side thereof may be set to 0.3 λmin. Also, as illustrated in  FIG. 9 , if the power feeding part  410  is arranged at the end of the left vertical side and the first and second poles  433  and  434  of the dipole radiator  430  are arranged so that they are widened in a direction opposite to the position of the power feeding part  410  (e.g., to the right in the drawing), the passive loop radiator  440  is provided on the metal layer opposite to the active loop antenna  420 . It is preferable, but not always necessary, that the passive loop radiator  440  is formed at a position of the horizontal side of the substrate that is apart for about 0.05 to 0.067 λmin from the vertical side of the substrate where the power feeding part  410  is located. 
   It is preferable, but not always necessary, that the difference between the electric length of the first slot line  432  and the electric length of the second slot line  422  in the minimum frequency condition is set to about 0.15 λmin. For example, if the minimum frequency fmin is 3.2 GHz, the wavelength λmin corresponding to the minimum frequency fmin on a dielectric material is about 3.2 cm. Accordingly, the length difference between the first and second slot lines  432  and  422  is about 5 mm. 
     FIG. 10  is a graph explaining the voltage standing wave ratio (VSWR) characteristic of a UWB antenna of  FIG. 9 . In  FIG. 10 , the horizontal axis represents a frequency f[GHz], and the vertical axis represents a VSWR. If the VSWR value is less than 2, electromagnetic waves corresponding to 90% or more of the input power can be radiated. According to the graph of  FIG. 10 , the UWB antenna of  FIG. 9  can be used in the frequency band of about 2.9 to 10.8 GHz, and thus the UWB communication becomes possible. 
     FIG. 11  is a graph explaining the antenna gain characteristic of a UWB antenna of  FIG. 9 . In  FIG. 11 , the horizontal axis represents a frequency f[GHz], and the vertical axis represents a gain G[dB]. According to the graph of  FIG. 11 , an average gain in the frequency band of 3 to 10.5 GHz appears high, e.g., about 3.8 dBi. In particular, an average gain in the frequency range of 6.5 to 9.5 GHz appears more than 4 dBi. A high antenna gain means a distinct directionality of the radiation pattern. That is, according to the gain characteristic of  FIG. 11 , it can be recognized that the UWB antenna has a unidirectional radiation pattern whereby stronger electromagnetic waves are radiated in a specified direction. 
     FIG. 12  is a view illustrating the structure of a UWB antenna with a slot added thereto according to still another exemplary embodiment of the present invention. The UWB antenna of  FIG. 12  is provided with a slot  550  in addition to a power feeding part  510 , an active loop radiator  520 , a dipole radiator  530  and a passive loop radiator  540 . 
   According to the UWB antenna of  FIG. 12 , the power feeding part  510  is arranged at the end of the horizontal side of the substrate, and the dipole radiator  530  is arranged toward the left. Accordingly, the main radiation direction of the electromagnetic waves is perpendicular to the feeding direction. Although the UWB antenna of  FIG. 8  is formed so that the radiation direction is perpendicular to the feeding direction, the radiation direction of the UWB antenna of  FIG. 12  is opposite to the radiation direction of the UWB antenna of  FIG. 8 . 
   The active loop radiator  520  and the passive loop radiator  540  on both sides of the metal layer are formed on the substrate around the dipole radiator  530 . One end of the active loop radiator  520  is connected to the signal terminal  511  in the power feeding part  510 , and the other end thereof is connected to the first ground terminal  512   a  in the power feeding part  510 . In this case, current flowing along the active loop radiator  520  may flow backward to the first ground terminal  512   a  as a leak current. This leak current may cause the radiation pattern to lean to the power feeding cable. 
   Accordingly, by forming the slot  550  around the active loop radiator  520  as shown in  FIG. 12 , the backward flow of the current, which flows into the signal terminal  511  and along the metal layer at the end of the substrate, to the first ground terminal  512   a  can be intercepted in advance, and thus the current leakage can be prevented. 
   The construction and operation of first and second poles  533  and  534  constituting the dipole radiator  530 , an input part  531 , a first slot line  532 , a second slot line  522  constituting the active loop radiator  520 , a loop  523 , and the passive loop radiator  540  are the same as those of the exemplary embodiments as described above, the duplicated explanation thereof will be omitted. 
     FIG. 13  is a view illustrating the structure of a UWB antenna with slots added thereto according to still another exemplary embodiment of the present invention. The UWB antenna of  FIG. 13  is provided with a plurality of slots  650 ,  660  and  670  in addition to a power feeding part  610 , an active loop radiator  620 , a dipole radiator  630  and a passive loop radiator  640 . 
   Specifically, two slots  650  and  660  are formed around the active loop radiator  620 , and one slot  670  is formed around the passive loop radiator  640 . In the following description, the slots  650  and  660  around the active loop radiator  620  are called first slots, and the slot  670  around the passive loop radiator  640  is called a second slot. The number and length of the first and second slots  650 ,  660  and  670  may be optionally adjusted. 
   Preferably, but not necessarily, the electric lengths of the slots  650 ,  660  and  670  may be set in the range of 0.2 λmin to 0.25 λmin. 
   The construction and operation of first and second poles  633  and  634  constituting the dipole radiator  630 , an input part  631 , a first slot line  632 , a second slot line  622  constituting the active loop radiator  620 , a loop  623 , and the passive loop radiator  640  are the same as those of the exemplary embodiments as described above, the duplicated explanation thereof will be omitted. 
     FIGS. 14 and 15  are graphs illustrating the measured characteristics of the UWB antenna of  FIG. 13 . In  FIGS. 14 and 15 , experimental results of a UWB antenna are illustrated, in which the lengths of horizontal and vertical sides and thickness of the substrate are set to 20 mm, 30 mm and 1.27 mm, respectively, the difference between the electric length of the first slot line  632  and the electric length of the second slot line  622  is set to about 0.15 λmin, and the electric lengths of the respective slots are set in the range of 0.2 λmin to 0.25 λmin. 
     FIG. 14  shows a graph representing the VSWR characteristic of the UWB antenna of  FIG. 13 . Referring to  FIG. 14 , VSWR appears less than 2 in the frequency band of 3.0 to 10.7 GHz. Accordingly, it can be recognized that the antenna of  FIG. 13  can be used in the UWB frequency band. 
     FIG. 15  shows a graph representing the antenna gain characteristic of the UWB antenna of  FIG. 13 . Referring to  FIG. 15 , an average gain appears about 3.8 dBi in the frequency band of 3.0 to 10.7 GHz. Accordingly, it can be recognized that the UWB antenna of  FIG. 13  has a unidirectional radiation pattern. 
   As exemplary embodiments of the present invention, a UWB antenna may be produced by combination of the active loop radiators  120 ,  220 ,  320 ,  420 ,  520  and  620  and the dipole radiators  130 ,  230 ,  330 ,  430 ,  530  and  630 . The frequency characteristics of the respective radiators are as follows. The dipole radiators  130 ,  230 ,  330 ,  430 ,  530  and  630  operate like capacitors in a low frequency band, and if the frequency exceeds a specified frequency f 1 , they radiate the electromagnetic waves. That is, they operate as antennas only in a frequency band that exceeds f 1 . By contrast, the active loop radiators  120 ,  220 ,  320 ,  420 ,  520  and  620  operate like inductors, and if the frequency exceeds a specified frequency f 2 , they radiate the electromagnetic waves. According to the exemplary embodiments of the present invention, the dipole radiators  130 ,  230 ,  330 ,  430 ,  530  and  630  and the active loop radiators  120 ,  220 ,  320 ,  420 ,  520  and  620  are combined, and then the size of at least one of them is adjusted so that the threshold frequencies coincide with each other (i.e., f 1 =f 2 ). Accordingly, in the frequency range of f&lt;f 1 =f 2 , the capacitance components of the dipole radiators  130 ,  230 ,  330 ,  430 ,  530  and  630  and the inductance components of the active loop radiators  120 ,  220 ,  320 ,  420 ,  520  and  620  are canceled each other. Thus, even in the frequency range of f&lt;f 1 =f 2 , the electromagnetic waves are radiated. In this case, by additionally providing the passive loop radiators  240 ,  340 ,  440 ,  540  and  640  as illustrated in  FIGS. 7 ,  8 ,  9 ,  12  and  13 , the radiation characteristics can be tuned. Also, as illustrated in  FIGS. 12 and 13 , by additionally providing the slots  550 ,  650 ,  660  and  670 , the UWB antenna can be designed whereby the radiation pattern is not distorted. 
   As a result, since the antenna can operate in a low frequency band although the size of the antenna is not increased, the UWB communication becomes possible. Accordingly, if the UWB antenna according to the present invention is used, a gain improved as much as 3 dB at maximum can be obtained in comparison to that of the conventional UWB antenna having a similar size. 
   As described above, the antenna according to exemplary embodiments of the present invention has a unidirectional radiation pattern, makes a UWB communication possible, and can be miniaturized. Accordingly, the antenna according to exemplary embodiments of the present invention can be applied to various kinds of portable communication appliances being presently developed. In addition, since the antenna according to exemplary embodiments of the present invention can be produced by depositing a single metal layer on the substrate and then patterning the metal layer, its production process is simplified. In particular, the antenna according to the present invention has an improved antenna gain in comparison to the conventional UWB antenna having the same size. In addition, by adding at least one slot, the current leakage is prevented, and thus the distortion of the radiation pattern can also be prevented. 
   The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Also, the description of the embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.