Patent Publication Number: US-10309077-B2

Title: Manhole cover type omnidirectional antenna

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 2015-0167737, filed on Nov. 27, 2015 and No. 2016-0041101, filed on Apr. 4, 2016, the disclosures of which are incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a manhole cover type omnidirectional antenna, and more particularly, to a manhole cover type omnidirectional antenna installed in a manhole cover of a manhole horizontally disposed to correspond to the Earth&#39;s surface to remotely collect and manage various types of sensing information under the ground and configured as a wireless sensor network or a wireless wide area network for communicating with a gateway above the ground. 
     2. Discussion of Related Art 
     Generally, a manhole cover installed on the Earth&#39;s surface is installed in a metal medium such as iron, zinc or the like, and the manhole cover needs to have a structure which does not protrude from the Earth&#39;s surface to prevent damage, performance degradation, etc. due to external environment. 
     Types of antennas applicable to the manhole cover include a patch antenna in a planar type structure, a small-sized dielectric antenna, and the like. 
     In addition, as a technology that establishes a system by applying such a common antenna, for example, there is a technology disclosed in United States Patent Laid-Open Publication No. US20010011009 entitled “Underground Information Communication System and Related Manhole Cover.” 
     However, a plurality of manhole cover antennas installed at arbitrary locations need to be able to wirelessly communicate with a gateway on the ground. A manhole cover antenna requires omnidirectional characteristics from a horizontal plane. Here, long-distance communication efficiency can be relatively increased as an angle formed by a main beam direction from a vertical plane and the Earth&#39;s surface is decreased. 
     In the case when an antenna main body is installed inside a manhole apparatus without having a portion protruding from an upper surface of the manhole, the manhole apparatus manufactured of a metal influences designed radiation characteristics and radiation gain of the antenna main body. As a result, realizing a high performance antenna while having a small radiation angle with respect to the Earth&#39;s surface becomes very difficult. 
       FIG. 1  shows availability of communication depending on a difference in a main beam direction according to a conventional technology. 
     Referring to  FIG. 1 , there are a first sensor node  10  and a second sensor node  20  in an underground space U at a lower level than the Earth&#39;s surface S. The first sensor node  10  and the second sensor node  20  are electrically and respectively connected to internal antennas  13  and  23  installed in manhole covers  11  and  21 . 
     A gateway  30  and a gateway antenna  31  are installed at a predetermined location in a region in which a plurality of manhole covers  11  and  21  are located to communicate with the first sensor node  10  or the second sensor node  20 . Particularly, the gateway antenna  31  is located at a predetermined height h from the Earth&#39;s surface S. 
     In the case in which the internal antenna  13  or  23  for the first sensor node  10  or the second sensor node  20  is installed in the manhole cover  11  or  21 , the internal antenna  13  or  23  is at an equivalent level with the Earth&#39;s surface S. When the height h of the gateway  30  or gateway antenna  31  located is considered, the internal antennas  13  and  23  cannot have omnidirectional characteristics. 
     Radiation  22  performed by the internal antenna  23  of the manhole cover  21  connected to the second sensor node  20  is formed along a direction perpendicular to the Earth&#39;s surface S (for example, a right angle). 
     As a comparative example, radiation  12  performed by the internal antenna  13  of the manhole cover  11  connected to the first sensor node  10  can be formed along a direction corresponding to an inclination angle relatively smaller than the right angle with respect to the Earth&#39;s surface S. 
     Here, although distances g from the gateway  30 ) to the first sensor node  10  and to the second sensor node  20  are the same, actual communication distances L 1  and L 2  can be different depending on directions and angles of the radiations  12  and  22 . 
     Meanwhile, as a conventional technology, the most typical antenna of an omnidirectional antenna is a monopole antenna. Generally, the monopole antenna is installed perpendicular to the Earth&#39;s surface. Therefore, the monopole antenna has difficulty in being operated inside a manhole cover formed of a metal. 
     On the other hand, a patch antenna, a planar antenna, and a small-sized dielectric antenna can be easily installed in a manhole cover. 
     However, when such a patch antenna, a planar antenna, or a small-sized dielectric antenna is installed inside a manhole cover or inside a manhole, difficulties can be faced due to not obtaining omnidirectional characteristics therefrom. Accordingly, development of an antenna having a structure by which radiation characteristics of the antenna is improved while being easily applicable to a manhole cover is urgently required. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to providing a manhole cover type omnidirectional antenna having a planar-type multi-plate structure capable of being horizontally installed inside a manhole cover at an equivalent level with the Earth&#39;s surface and performing long range communication due to a relatively small angle formed between a main beam direction and the Earth&#39;s surface and omnidirectional characteristics. 
     The present invention is also directed to providing a manhole cover type omnidirectional antenna capable of implementing a radiation angle formed with respect to the Earth&#39;s surface to be relatively small and easily establishing a wireless wide area network compared to a conventional antenna with a single substrate, when the manhole cover type omnidirectional antenna is buried in a manhole through a main body serving as an antenna in a structure described below. 
     According to an aspect of the present invention, there is provided a manhole cover type omnidirectional antenna including: a manhole cover installed in a manhole in the Earth&#39;s surface; a main body installed in a cavity of an upper surface of the manhole cover and configured to convert an electrical signal into an electromagnetic wave to wirelessly communicate with a gateway separated from the manhole cover; and a radome inserted into the cavity to cover the main body. 
     The manhole cover type omnidirectional antenna may further include a connector connected to a cable which electrically connects the main body and a wireless transmitter. 
     The main body in a monopole shape thinner than a thickness of the manhole cover may achieve impedance matching using a shorting strip to have an antenna performance in which an angle of a main beam direction with respect to the Earth&#39;s surface is small, and slots may be symmetrically disposed in a direction perpendicular to an arrangement direction of the shorting strips so that the main body has omnidirectional characteristics at a horizontal plane. 
     The wireless transmitter may be connected to a plurality of sensors disposed inside the manhole and provide the electrical signal corresponding to sensing information input from the sensors to the main body via the cable and the connector. 
     The cavity may include a circular side surface disposed in the manhole cover and having a cavity diameter smaller than a diameter of the manhole cover but greater than a diameter of the main body, a lower surface horizontally connected to the side surface at a smaller depth than a thickness of the manhole cover, and a cable hole through which the connector is inserted or the cable is passed. 
     The main body may have a main body diameter formed to be smaller than the cavity diameter. 
     The main body may include a lower plate disposed on a lower surface of the cavity of the manhole cover on the basis of a cable hole of the manhole cover into which the connector is inserted and configured to serve as a ground surface, a metal pole which extends from the connector, passes through the lower plate, and extends in a vertical direction up to a height corresponding to a gap between plates, an upper plate connected to an upper end of the metal pole, maintained in parallel to the lower plate, having the same main body diameter as the lower plate, and configured to serve as a radiator, a shorting strip which connects the upper plate and the lower plate at a position spaced apart from the metal pole, and a slot formed in the upper plate to be spaced apart from the metal pole on the basis of a position not overlapping the shorting strip. 
     The upper plate may use a point at which the upper plate and the upper end of the metal pole are connected to each other as a feeding point. 
     The upper plate may be short-circuited with respect to the lower plate through the shorting strip. 
     The main body may be formed as a planar-type multi-plate structure by the upper plate and the lower plate parallel to each other with the metal pole and the shorting strip interposed therebetween. 
     The main body may convert the electrical signal received from the connector into the electromagnetic wave corresponding to a shape of the planar-type multi-plate structure to form a small angle between the main beam direction of the electromagnetic wave and the Earth&#39;s surface and to have omnidirectional characteristics. 
     The main body may form a large area information network over a network. 
     An upper portion of the shorting strip may be inserted into an upper connection hole of the upper plate and a lower portion of the shorting strip may be inserted into a lower connection hole of the lower plate, to be fixed by welding. 
     According to another aspect of the present invention, there is provided a manhole cover type omnidirectional antenna including: a lower plate installed in a cavity of an upper surface of a manhole cover; a connector installed at the lower plate and connected to a cable for a wireless transmitter; a metal pole with a lower end thereof connected to the connector which passes through the lower plate and extends in a vertical direction up to a height corresponding to a gap between plates; an upper plate connected to an upper end of the metal pole, maintained in parallel to the lower plate, and configured to serve as a radiator; a shorting strip which connects the upper plate and the lower plate at a position spaced apart from the metal pole; and a radome inserted into the cavity to cover the main body. 
     The radome may further include a coupling cavity portion having a diameter and thickness which correspond to those of the cavity and formed in the radome to accommodate the upper plate, the lower plate, the metal pole, and the shorting strip. 
     The upper plate may include a slot formed in the upper plate to be spaced apart from the metal pole on the basis of a place not overlapping the shorting strip. 
     The shorting strip may include a short circuit portion which short-circuits the upper plate and the lower plate, and a pillar portion in contact with a lower surface or an upper surface of the upper plate and an upper surface or a lower surface of the lower plate and configured to support the upper plate on the basis of the lower plate. 
     The upper plate may include a first substrate supported by the shorting strip, formed in a circular shape and configured to serve as a dielectric, and a circular patch portion attached to an upper surface of the first substrate and having a feeding pattern connected to the metal pole and a radiation pattern connected to the feeding pattern to convert an electrical signal into an electromagnetic wave. 
     The lower plate may include a second substrate disposed separately from a lower side of the upper plate by the shorting strip, and a ground surface attached to a lower surface or upper surface of the second substrate and electrically connected to the short circuit portion of the shorting strips. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic configuration view illustrating availability of communication depending on a difference in main beam direction according to a conventional technology; 
         FIG. 2  is a configuration view illustrating a wireless sensor network using a manhole cover type omnidirectional antenna according to a first embodiment of the present invention; 
         FIG. 3  is a perspective view illustrating a main body of the manhole cover type omnidirectional antenna shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along line A-A of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view taken along line B-B of  FIG. 3 ; 
         FIG. 6  is an exploded perspective view for describing a coupling configuration of a main body, a manhole cover and a radome which are shown in  FIG. 2 ; 
         FIG. 7  is a cross-sectional view taken along line C-C of  FIG. 6  in a state in which the main body, the manhole cover and the radome are coupled to one another; 
         FIG. 8  is an exploded perspective view illustrating a main body of a manhole cover type omnidirectional antenna according to a second embodiment of the present invention; 
         FIG. 9  is a cross-sectional view illustrating the main body shown in  FIG. 8 ; 
         FIG. 10  is a graph illustrating frequency characteristics of an antenna when the main body shown in  FIG. 8  is applied to a manhole cover; 
         FIGS. 11 to 14  are graphs illustrating radiation characteristics related to an antenna gain and a radiation pattern of a manhole cover type omnidirectional antenna depending on manhole diameters; 
         FIG. 15  is a graph for describing a formation shape of a radiation pattern of a conventional antenna according to a comparative example of the present invention; 
         FIG. 16  is a graph for describing a formation shape of a radiation pattern of a manhole cover type omnidirectional antenna; and 
         FIG. 17  is a three-dimensional graph resulting from a radiation characteristics experiment for a manhole cover type omnidirectional antenna installed in a manhole cover. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Advantages and features of the present invention and methods of accomplishing them will be made apparent with reference to the accompanying drawings and some embodiments to be described below. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, the embodiments are provided so that this disclosure is thorough and complete and fully conveys the inventive concept to those skilled in the art, and the present invention should only be defined by the appended claims. 
     Meanwhile, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” and/or “comprising” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Hereinafter, embodiments of the present invention will be described in detail with reference to accompanying drawings. 
     First Embodiment 
       FIG. 2  is a configuration view illustrating a wireless sensor network using a manhole cover type omnidirectional antenna according to a first embodiment of the present invention. As a more detailed description,  FIG. 2  shows a configuration of a wireless sensor network in which a main body  200  serving as an antenna is installed in a manhole cover  100  installed at the Earth&#39;s surface S. Here, the wireless sensor network may include a wireless wide area network. 
     Referring to  FIG. 2 , the first embodiment includes the manhole cover  100 , the main body  200 , a radome  300 , a connector  400 . 
     The manhole cover  100  is installed in a manhole  40  on the Earth&#39;s surface S and may be disposed at a step at an edge of an upper opened hole of the manhole  40  to cover the upper opened hole of the manhole  40  or to be openable. 
     The main body  200  refers to the manhole cover type omnidirectional antenna according to the first embodiment. 
     That is, the main body  200  is in a monopole shape whose thickness is smaller than a thickness of the manhole cover  100  and exhibits the performance of an antenna having a small angle formed between a main beam direction and the Earth&#39;s surface. 
     The main body  200  is mounted or installed in a cavity  110  of an upper surface of the manhole cover  100 . The main body  200  serves to convert electrical signals into electromagnetic waves so that the main body  200  performs wireless communication with a gateway  500  separated from the manhole cover  100 . Here, a gateway antenna  510  may be installed on or around the gateway  500  above the ground. 
     The main body  200 , by components, a structure, and connection relations which will be described below, may have a relatively small angle Q (for example, a radiation angle) formed between the main beam direction F and the Earth&#39;s surface S and exhibit omnidirectional characteristics compared to conventional antenna products. 
     By such a main body  200 , the gateway  500  may perform smooth communication with the main body  200  even when the gateway  500  is installed at a location of a relatively low height such as the Earth&#39;s surface S or the like. 
     The radome  300  may be inserted or filled in the cavity  110  to cover the main body  200 , in which the radome  300  may be maintained at the same level as the upper surface of the manhole cover  100 . Here, the main body  200  serving as an antenna is covered by the radome  300 . 
     The radome  300  may be formed of a solid dielectric of a nonmetallic substance. Here, the dielectric is a nonconductor having a dielectric constant higher than a dielectric constant of air. As the dielectric constant becomes higher, polarization with respect to a radio frequency (RF) occurs more often. The dielectric may be formed of any one of polycarbonate, acryl, ceramic, printed writing boards (PWBs), and Teflon. 
     The connector  400  may be disposed at a central position of a lower portion of the main body  200  depending on design. 
     In addition, the connector  400  may be at a different position to which the main body  200  may be connected and in a different direction. That is, the connector  400  may be connected to the main body  200  at another position or in another direction of the main body  200  besides at the central position or in the lower direction of the main body  200 . 
     A wireless transmitter  600  is positioned in an underground space  41  of the manhole  40  having a hollow-type structure. 
     The wireless transmitter  600  may be connected to a plurality of sensors  700  disposed in the manhole  40  or underground space  41 . 
     The wireless transmitter  600  may provide the main body with an electrical signal which corresponds to sensing information input from sensors  700  via a cable  800  and the connector  400 . Here, the connector  400  may be inserted into a cable hole  120  of the manhole cover  100 , connected to the cable  800 , and fixed using an adhesive, molding materials, or the like. 
     The sensors  700  refer to a plurality of sensor nodes and may be provided at sensing objects (not shown) already installed at the underground space  41 . 
     The sensors  700  are connected to the wireless transmitter  600  by wires or wireless communication. Each of the sensors  700  collects sensor information of the sensing object in charge and transfers the sensor information to the wireless transmitter  600 . 
     The wireless transmitter  600  is connected to the connector  400  of the main body  200  through the cable  800  serving as a RF channel Here, the connector  400  is connected to the main body  200  installed in the cavity  110  of the manhole cover  100 . For example, the connector  400  is disposed at a lower portion of the main body  200  and protrudes downward from the main body  200  to be connected to the cable  800  which electrically connects the main body  200  and the wireless transmitter  600 . 
     The wireless transmitter  600  may wirelessly transmit the sensing information to the gateway  500  on the ground or receive a signal from the gateway  500  via the cable  800 , the connector  400 , and the main body  200 . 
     As described above, the main body  200  may be easily installed in the manhole cover  100  in a planar-type metal-structure for an exemplary wireless sensor network or wireless wide area network as illustrated in  FIG. 2 . 
     In addition, when compared to the exemplary radiation  22  illustrated in  FIG. 1 , the angle Q formed by the main beam direction F with respect to the Earth&#39;s surface S is designed to be similar to or smaller than the angle formed by another exemplary radiation  12  illustrated in  FIG. 1  to exhibit omnidirectional antenna characteristics. 
       FIG. 3  is a perspective view illustrating a main body of the manhole cover type omnidirectional antenna shown in  FIG. 2 ,  FIG. 4  is a cross-sectional view taken along line A-A of  FIG. 3 , and  FIG. 5  is a cross-sectional view taken along line B-B of  FIG. 3 . 
     Referring to  FIGS. 3 and 4 , the main body  200  may be formed including a lower plate  210 , a metal pole  220 , an upper plate  230 , and shorting strips  240 . 
     As components of the main body  200 , the lower plate  210 , the metal pole  220 , the upper plate  230 , and the shorting strips  240  may correspond to metal portions in which a surface current flows. 
     The lower plate  210  or upper plate  230  may be formed in a circular shape but may also be formed in any one of various shapes such as a tetragonal shape, a hexagonal shape, a polygonal shape or the like depending on design, and may not be limited to a particular shape. 
     The shorting strips  240  may be formed in a pair as illustrated in the drawings and may also be formed in a plurality of shorting strips depending on design. 
     A height p of the shorting strip  240  or a distance between the lower plate  210  and the upper plate  230  may be determined in consideration of impedance matching. 
     A pair or one or more of slots  231  are symmetrically or unsymmetrically positioned in the upper plate  230  serving as a radiator and a feeding point  221  is positioned at the upper plate  230 . Here, a shape and the number of the slots  231  may be different depending on design, and although a pair of the slots  231  is illustrated in  FIG. 3  as an example, the slots  231  may be formed in plural slots, at multiple positions, and in a structure of an unsymmetrical arrangement. 
     The shorting strips  240  also are symmetrically or unsymmetrically disposed between the upper plate  230  serving as a radiator and the lower plate  210 . Power feeding to the upper plate  230  may be performed through the metal pole  220  which is a core of the connector  400 . 
     The lower plate  210  is disposed at a lower surface of the cavity  110  of the manhole cover  100  on the basis of the cable hole  120  of the manhole cover  100  illustrated in  FIG. 2  and serves as a ground surface. 
     The metal pole  220  is the core of the connector  400  as described above and may be a feeding probe. A lower end of the metal pole  220  extends from the connector  400 . Here, the connector  400  may be formed including a core portion  410  provided inside a body of the connector  400  and the metal pole  220  disposed inside the core portion  410 . 
     Even though the metal pole  220  is not necessarily at a central position of the lower plate  210  and the upper plate  230 , the metal pole  220  may play a role in power feeding as long as the metal pole  220  is at a position which may connect the lower plate  210  and the upper plate  230  depending on design. 
     The core portion  410  may serve to physically support the metal pole  220  and pass an electric current. A screw thread portion formed on an outer side of the core portion  410  of the connector  400  may be coupled to a connection portion of the cable to form a state in which an electric current may pass. 
     The metal pole  220  passes through the lower plate  210  and extends in a vertical direction up to an upper end with a height corresponding to the distance between the two plates. 
     The upper plate  230  is connected to the upper end of the metal pole  220 , maintained parallel to the lower plate  210 , and serves as a radiator. 
     The upper plate  230  may have the same main body diameter D as the lower plate  210  or may also be manufactured in a size different from that of the lower plate  210 . 
     The point at which the upper plate  230  and the upper end of the metal pole  220  are connected to each other is used as the feeding point  221 . 
     As an example, the main body diameter D refers to a diameter of the main body  200  or a diameter of the upper plate  230 , and is formed to be smaller than a diameter of the cavity  110  of the manhole cover  100  illustrated in  FIG. 6 . For example, the main body diameter D may correspond to any one size selected from a numerical range of 6 to 30 cm. 
     Here, the numerical value of the main body diameter D or a main body size may not be limited to a particular numerical value. That is, the numerical value of the main body diameter D or the main body size may be set in consideration of a wavelength of a frequency using the antenna. As an additional description, the minimum diameter size of the above numerical range may not be set only to 6 cm. That is, because frequency is inversely proportional to wavelength, for example, the main body  200  may be manufactured in a smaller size when an applicable frequency band goes up to the 2.4 GHz band. 
     In addition, the maximum diameter size of the above numerical range may not be limited to 30 cm because the maximum diameter size of the main body only needs to be smaller than or equal to a diameter of the manhole. 
     The shorting strip  240  is disposed between the upper plate  230  and the lower plate  210  and connects the upper plate  230  and the lower plate  210  at a position spaced apart from the metal pole  220 . 
     The shorting strip  240  is formed of a conductive substance or material and is electrically connected to the upper plate  230  and the lower plate  210  using soldering. 
     In addition, as illustrated in  FIG. 3 , the antenna according to the first embodiment is provided with the metal pole  220  positioned at a center of a circular patch (not shown) or the upper plate  230 , the feeding point  221  by which power feeding is performed, and the shorting strip  240  for impedance matching. 
     A planar type antenna with a conventional technology simply used one or more pieces of shorting strips (or short pins) normally without any particular layout rule for impedance matching, wherein, when a radio wave is applied by a pole of the planar type antenna with a conventional technology, a surface current is formed at an upper radiating portion of a disc of the planar type antenna with a conventional technology and a radiation shape is determined according to a distribution of the surface current. 
     In the first embodiment, to implement a radiation structure exhibiting omnidirectional characteristics, first, the shorting strips  240  which connect the upper plate  230  serving as a radiating portion and the lower plate  210  serving as a ground surface are separately and symmetrically disposed with respect to the metal pole  220  or the feeding point  221 . 
     Here, the impedance matching is achieved according to a length k 1  of the shorting strip  240  and a separation distance n 1  from the metal pole  220  or the feeding point  221  to the shorting strip  240 . That is, the impedance matching is achieved by adjusting the length k 1  of the shorting strip  240  symmetrically disposed and the separation distance n 1  between the feeding point  221  and the shorting strip  240 . 
     As described above, in the first embodiment, an angle between a radiation direction and the Earth&#39;s surface (for example, a radiation angle) may be very small by realizing an impedance matching to match characteristics of a monopole antenna in a thin shape. 
     However, with the structure described so far, radiation in an 8 shape is exhibited as the radiation shape of a horizontal plane (for example, an X-Y plane) illustrated in  FIG. 15  and omnidirectional characteristics may not be exhibited. That is, a main radiation shape  54  is formed because the main beam direction is formed along both directions perpendicular to an arrangement direction of shorting strips  53  of a conventional technology, and such a main radiation shape  54  of the conventional technology may not exhibit omnidirectional characteristics. 
     To compensate for this, in the first embodiment, the slots  231  are symmetrically disposed in a direction perpendicular to an arrangement direction of the shorting strips  240  as will be described below. 
     Here, the main body may be designed to exhibit the omnidirectional characteristics on the horizontal plane (the Earth&#39;s surface or the X-Y plane) when a separation distance n 2  from the metal pole  220  or the feeding point  221  corresponding to a center of the upper plate  230  to the slot  231  and a length k 2  of the slot  231  are adjusted. 
     As shown in  FIG. 3 or 4 , upper end portions of the shorting strips  240  are inserted into or connected to upper connection holes  232  of the upper plate  230 . Lower end portions of the shorting strips  240  are inserted into or connected to lower connection holes  212  of the lower plate  210 . Here, for the connection, a welding or any other connection method for fixing which may allow a physical connection while maintaining an electrical connection may be used and thereby a state in which an electrical current may pass is obtained. 
     An arrangement direction of the upper connection holes  232  and lower connection holes  212  may be perpendicular to an arrangement direction of the slots  231 . 
     The upper plate  230  is shorted with respect to the lower plate  210  by the shorting strips  240 . 
     In addition, the slots  231  are formed in the upper plate  230  in a direction perpendicular to the arrangement direction of the shorting strips  240  or formed to be spaced apart from the metal pole  220  at positions not overlapping the shorting strips  240 . 
     Each of the slots  231  is formed in the upper plate  230 . 
     Each of the slots  231  has a relatively small width compared to a length thereof, and the length of the slot  231  is in a range of 25 to 30 times the width of the slot  231 . 
     Here, the main body  200  is formed as a planar-type multi-plate structure by the upper plate  230  and the lower plate  210  being parallel to each other and the metal pole  220  and the shorting strips  240  interposed therebetween. 
     The main body  200  in a multi-plate structure having features of the slots  231  in an arrangement direction or shape converts electrical signals received via the connector  400  into electromagnetic waves corresponding to a shape of a planar-type multi-plate structure, and thereby the main body  200  exhibits the omnidirectional characteristics while having a small angle of the main beam direction of the electromagnetic waves with respect to the Earth&#39;s surface. 
     Accordingly, the main body  200  may establish a large area information network in a low power wireless sensor network or a wireless wide area network. 
       FIG. 6  is an exploded perspective view for describing a coupling configuration of the main body, the manhole cover, and the radome which are shown in  FIG. 2 , and  FIG. 7  is a cross-sectional view taken along line C-C of  FIG. 6  in a state in which the main body, the manhole cover and the radome are coupled to one another. 
     Referring to  FIGS. 6 and 7 , the main body  200  may be installed in the manhole cover  100 . Here, the main body  200  is inserted into a coupling cavity portion  310  of a lower surface of the radome  300  formed of a dielectric. In addition, the radome  300  having the main body  200  is inserted into the cavity  110  of the manhole cover  100  so that upper levels of the radome  300  and the manhole cover  100  may be horizontally maintained on the same plane. In addition, a ground portion of the main body  200  may be connected to a metal portion of the manhole cover  100  to be short-circuited. 
     The cavity  110  of the manhole cover  100  is disposed in the manhole cover  100 . Here, the cavity  110  may not necessarily be a center of the manhole cover  100  and may be formed at any position of an upper plane of the manhole cover  100 . 
     In addition, although a size and a diameter of the cavity  110  of the manhole cover  100  are smaller than a size and a diameter of the manhole cover  100 , the cavity  110  of the manhole cover  100  includes a circular side surface  111  having a cavity diameter greater than the diameter of the main body  200 . In addition, the cavity  110  includes a lower surface  112  which horizontally connects to the side surface  111  at a depth smaller than a thickness of the manhole cover  100 . In addition, the cavity  110  may include the cable hole  120 . Here, the above-described connector  400  may be inserted into the cable hole  120 . In addition, the above-described cable  800  may pass through the cable hole  120 . 
     The radome  300  has a diameter and a thickness corresponding to those of the cavity  110 . The radome  300  may further include the coupling cavity portion  310  formed in the radome  300 . The coupling cavity portion  310  may accommodate the upper plate, the lower plate, the metal pole and the shorting strips of the main body  200 . 
     According to such structural and configurational features, the main body  200  and the radome  300  of the manhole cover  100  may be formed not to protrude from the upper surface of the manhole cover  100 . 
     The main body  200  and the radome  300  of the manhole cover  100  may be components of a wireless sensor network or a wireless wide area network which connects an underground space and a ground space. 
     A user may wirelessly acquire sensing information associated with the manholes by the main body  200  and the radome  300  of the manhole cover  100  without needing to directly approach the manholes at locations on roads of a downtown area etc. which are not easy to approach. That is, the main body  200  and the radome  300  of the manhole cover  100  may ensure user safety. 
     Second Embodiment 
     A manhole cover type omnidirectional antenna of the present invention described in the present embodiment may be the same as or very similar to the manhole cover type omnidirectional antenna of the first embodiment except that a main body in a shape of a planar-type multi-plate structure is formed to be enhanced in durability and solidity due to a structural shape of a shorting strip. Therefore, the same or similar reference numbers will be marked for the same or corresponding components in  FIGS. 2 to 17 , and descriptions on the components herein will be omitted. 
       FIG. 8  is an exploded perspective view illustrating a main body of a manhole cover type omnidirectional antenna according to a second embodiment of the present invention, and  FIG. 9  is a cross-sectional view of the main body illustrated in  FIG. 8 . 
     Referring to  FIG. 8  or  FIG. 9 , a main body  200   a  is provided in the second embodiment, however, the main body  200   a  is installed in a cavity of an upper surface of a manhole cover and includes a lower plate  210   a  disposed at a lower surface of the cavity to wirelessly communicate with a gateway which is separated from the manhole cover. 
     The main body  200   a  includes a connector  400  which protrudes downward from the lower plate  210   a  or is disposed in the lower plate  210   a  and is connected to a cable for a wireless transmitter. 
     The main body  200   a  includes a metal pole  220 . A lower end of the metal pole  220  may be connected to the connector  400 . The metal pole  220  may vertically extend up to a height corresponding to a gap between the lower plate  210   a  and an upper plate  230   a  after passing through the lower plate  210   a.    
     The main body  200   a  includes the upper plate  230   a . The upper plate  230   a  may be connected to an upper end of the metal pole  220 . The upper plate  230  is maintained in parallel to the lower plate  210   a  and serves as a radiator. 
     The main body  200   a  may include one or more shorting strips  240   a  which connect the upper plate  230   a  and the lower plate  210   a  at positions spaced apart from the metal pole  220 . 
     The main body  200   a  of the second embodiment may also include a radome to cover the main body  200   a . Here, the radome may be inserted into the cavity and maintained at the same level as an upper surface of the manhole cover. 
     The upper plate  230   a  may include slots  231  formed in the upper plate  230   a  to be spaced apart from the metal pole  220  at positions not overlapping the shorting strips  240   a.    
     The shorting strips  240   a  may include short circuit portions  241  which short-circuit the upper plate  230   a  and the lower plate  210   a.    
     The shorting strips  240   a  may include pillar portions  242 . Here, the pillar portion  242  may be in contact with a lower surface or upper surface of the upper plate  230   a  or an upper surface or lower surface of the lower plate  210   a , and support the upper plate  230   a  on the basis of the lower plate  210   a.    
     A method of bringing an end of the pillar portions  242  into contact with the lower surface or upper surface of the upper plate  230   a  or the upper surface or lower surface of the lower plate  210   a  may be performed by a direct contact manner or welding method. 
     The pillar portions  242  may be integrated wing portions or integrated support structures which extend from the short circuit portions  241 . The pillar portions  242  may be support structures disposed at positions spaced apart from the short circuit portions  241 . The pillar portions  242  may serve to enhance durability and solidity of the main body  200   a.    
     As illustrated in  FIG. 8 , the shorting strips  240   a  are provided with upper end portions  241   a  and lower end portions  241   b  so that the short circuit portions  241  protrude more upward and downward than the pillar portions  242 . 
     Step portions  243  may be formed between the upper end portions  241   a  of the short circuit portions  241  and upper surfaces of the pillar portions  242 , or between lower end portions  241   b  of the short circuit portions  241  and lower surfaces of the pillar portions  242 . 
     Upper connection holes  232  are formed in the upper plate  230   a  for the upper end portions  241   a  of the shorting strips  240   a  to pass through the upper plate  230   a  in a thickness direction. An arrangement direction of the upper connection holes  232  may be perpendicular to an arrangement direction of the slots  231 . 
     Lower connection holes  212  may also be formed in the lower plate  210   a  at positions aligned in a direction in which the upper end portions  241   a  of the shorting strips  240   a  pass through the upper connection holes  232 . 
     The upper end portions  241   a  of the short circuit portions  241  are inserted into the upper connection holes  232  formed in the upper plate  230   a , and the lower end portions  241   b  of the short circuit portions  241  are inserted into the lower connection holes  212  formed in the lower plate  210   a . Here, each of the inserted portions may be fixed by welding. 
     The upper plate  230   a  of the second embodiment also is short-circuited with respect to the lower plate  210   a  through the short circuit portions  241  of the shorting strips  240   a . Here, the short circuit portions  241  are formed of electrically conductive materials, circuit lines or circuit patterns not only for physically connecting the upper plate  230   a  and the lower plate  210   a  but also for electrically connecting them. 
     The upper plate  230   a  is configured with a first substrate  233  supported by the shorting strips  240   a , formed in a circular shape, and configured to serve as a dielectric, and a circular patch portion  234  attached to an upper surface of the first substrate  233 . Particularly, the circular patch portion  234  has a feeding pattern connected to the metal pole  220  and a radiation pattern connected to the feeding pattern to convert electrical signals into electromagnetic waves. Here, the feeding pattern and the radiation pattern may be determined to correspond to antenna characteristics and may not be limited to a particular pattern. 
     The lower plate  210   a  is configured with a second substrate  214  disposed separately from a lower side of the upper plate  230   a  by the shorting strips  240   a  and a ground surface  213  attached to a lower surface or upper surface of the second substrate  214  and electrically connected to the short circuit portions  241  of the shorting strips  240   a.    
     Referring to  FIG. 8 or 9 , the main body  200   a  of the second embodiment is manufactured with the first substrate  233  and the second substrate  214  in the form of a printed circuit board (PCB) while applying components of the antenna thereto, to be operated even at an unlicensed frequency in a frequency band from 900 to 940 MHz. 
     Particularly, the main body  200   a  may be very easy to be mounted in or applied to an existing manhole cover by making a cavity therein because the main body  200  may be made as small as 1.2 cm in thickness T and implemented in a very small size compared to typical manhole covers. 
       FIG. 10  is a graph illustrating frequency characteristics of an antenna when the main body illustrated in  FIG. 8  is applied to a manhole cover. 
       FIG. 10  shows results of frequency characteristics when an antenna manufactured with the main body structure of  FIG. 8 or 9  is applied to a manhole cover. 
     The main body of the manhole cover type omnidirectional antenna is manufactured smaller than a manhole diameter M in consideration of a typical sluice valve manhole diameter M. When looking into return loss with respect to frequency, the manhole cover type omnidirectional antenna having the main body described above is well operated with bandwidths of about 14 MHz and 20 MHz with respect to a center frequency 920 MHz. 
       FIGS. 11 to 14  are graphs illustrating radiation characteristics associated with antenna gains and radiation patterns of manhole cover type omnidirectional antennas depending on manhole diameters. 
       FIGS. 11 and 12  are the cases in which the manhole diameter M of  FIG. 10  is 20 cm, and an antenna gain dB and a radiation pattern corresponding to electric field strength E θ  of a vertical plane exhibit omnidirectional characteristics. 
       FIGS. 13 and 14  show that, even when the manhole diameter M of  FIG. 10  is 30 cm, an antenna gain dB which is very suitable degree for a wireless sensor network or a wireless wide area network is achieved and a radiation pattern also is exhibiting omnidirectional characteristics. 
       FIG. 15  is a graph for describing a formation shape of a radiation pattern of a conventional antenna according to a comparative example of the present invention, and  FIG. 16  is a graph for describing a formation shape of a radiation pattern of the manhole cover type omnidirectional antenna illustrated in  FIG. 2  or  FIG. 8 . 
     Referring to  FIG. 15 , the shorting strips  53  of a comparative example according to a conventional technology are disposed symmetrically to a metal pole  52  of the comparative example. Such a comparative example relates to a radiating portion without having slots with technical features such as those in the first embodiment or the second embodiment. In the comparative example, when the main body is installed in a cavity  51  of a manhole cover  50  to perform an antenna function, based on the horizontal plane or the X-Y plane, there occurs a problem in that omnidirectional characteristics are not exhibited because the main radiation shape  54  of the comparative example forms not an omnidirectional shape but an 8 shape. The main radiation shape  54  is formed in an 8 shape in a direction perpendicular to an arrangement direction of the shorting strips  53  of the comparative direction. Here, this is because, in a current distribution of the radiating portion of the comparative example, much mutual coupling occurs with cavity edges of the manhole at edges of the perpendicular direction. 
     On the other hand, referring to  FIG. 16 , to resolve the above-described problem and to realize omnidirectional characteristics, the slots  231  are symmetrically disposed in a direction perpendicular to the shorting strips  240 . As described with  FIG. 3 , positions of the slots  231  (for example, a separation distance from the feeding point to the slots) and lengths of the slots  231  are adjusted until a radiation shape  235  exhibits omnidirectional characteristics by the embodiments of the present invention. 
     As in  FIG. 16 , adjusting the positions and lengths of the slots  231  may form the radiation shape  235  having omnidirectional characteristics. 
     Distribution of a surface current at an edge of the upper plate serving as the radiation portion becomes uniform due to the slots  231 , the surface current of the edge of the upper plate serving as the radiation portion mutually couples with an edge of the cavity of the manhole, and thereby the omnidirectional characteristics can be exhibited. Particularly, the positions and lengths of the slots are changed to correspond to the positions and lengths of the shorting strips  240 , and thereby the radiation shape  235  may be changed. 
       FIG. 17  is a three-dimensional graph resulting from a radiation characteristics experiment for a manhole cover type omnidirectional antenna installed in a manhole cover. 
     Referring to  FIG. 17 , the antenna and the manhole cover according to the embodiment of the present invention manufactured as a prototype using features of manufacturing and design methods of the antenna described in detail as above have a diameter of a typical sluice valve manhole and exhibits an omnidirectional radiation shape as shown in the experimental result of  FIG. 17  even when the antenna and the manhole are installed on an X-Y plane which is the Earth&#39;s surface. 
     From the experimental result of  FIG. 17 , it is confirmed that the present invention provides sufficiently reliable radiation quality to meet requirements of a wireless sensor network. 
     As described above, the present invention according to the second embodiment and the first embodiment can be very suitable for a wireless sensor network or a wireless wide area network for remotely collecting and managing sensing information from various sensors in an underground space. 
     That is, when a main body, that is, an antenna is manufactured and installed in a manhole cover according to the descriptions of the present embodiments, wireless communication up to a ground position at a long distance from the manhole is possible. Sensing information inside the manhole at a long distance can be collected and managed by a wireless network. A large area information network can be formed over a network. 
     By applying the manhole cover type omnidirectional antenna according to the embodiments of the present invention in a planar-type multi-plate structure provided with the upper plate and the lower plate in parallel with the metal pole and the shorting strips interposed therebetween to the manhole cover, wireless communication up to a ground position at a long distance from the manhole is possible, thereby helping collect and manage the sensing information collected from a plurality of sensors inside the manholes at a long distance by forming a wireless sensor network or a wireless wide area network. 
     The manhole cover type omnidirectional antenna according to the embodiments of the present invention having a small angle between the main beam direction and the Earth&#39;s surface and having omnidirectional characteristics can relatively enhance actual communication distance with respect to a distance between the main body and a gateway, thereby providing an effect of forming a large area information network over a network including a wireless sensor network operated with small power and a wireless wide area network. 
     The manhole cover type omnidirectional antenna according to the embodiments of the present invention can wirelessly acquire sensing information without needing to directly approach manholes at locations on roads of a downtown area etc. which are not easy to approach, thereby having an advantage in terms of safety. 
     The manhole cover type omnidirectional antenna according to the embodiments of the present invention allows the main body to be installed inside a manhole cover at an equivalent level with the Earth&#39;s surface, has frequencies and bandwidth that enable seamless communication, has a relatively small radiation angle formed with respect to the Earth&#39;s surface compared to conventional technologies, can stably convert electrical signals into electromagnetic waves between the wireless transmitter connected to sensors in an underground space and a gateway on the ground, thereby having a very suitable advantage of forming a network between the underground space and the ground space by a wireless sensor network or a wireless wide area network. 
     The manhole cover type omnidirectional antenna according to the embodiments of the present invention is horizontally placed inside the cavity of the manhole cover, is smoothly operated inside the manhole cover formed of a metal because of being protected by the radome inserted into the cavity to be at the same level as the upper surface of the manhole cover, thereby having an advantage of being used as a product that is relatively long in actual communication distance or has great antenna gain. 
     The manhole cover type omnidirectional antenna according to the embodiments of the present invention has an advantage of excellent applicability and usability even when an installation height of a gateway installed at a position spaced apart from a manhole cover installed at an arbitrary position is almost close to the Earth&#39;s surface because the antenna is installed and assembled in the cavity of the manhole cover to have omnidirectional characteristics, the angle formed between the main beam direction and the Earth&#39;s surface is relatively small compared to an existing product, and the main body is relatively small in diameter and thickness compared to a diameter and a thickness of a typical manhole cover. 
     The above description of embodiments is merely for describing technical sprit of the present invention, and those having ordinary skill in the art should understand that various changes and modifications may be made therein without departing from the spirit and features of the present invention. Accordingly, the above described embodiments of the present invention should be considered in a descriptive sense only and not in a limitative sense. The scope of the present invention is not limited by the above-described embodiments. The scope of the present invention should be interpreted only according to the attached claims, and it should be understood that all technical ideas within an equivalent scope thereof should be interpreted as being included in the scope of the present invention. 
     REFERENCE NUMERALS 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 100: MANHOLE COVER 
                 110: CAVITY 
               
               
                   
                 120: CABLE HOLE 
                 200, 200a: MAIN BODY 
               
               
                   
                 210, 210a: LOWER PLATE 
                 220: METAL POLE 
               
               
                   
                 230, 230a: UPPER PLATE 
                 240, 240a: SHORTING STRIP 
               
               
                   
                 300: RADOME 
                 400: CONNECTOR 
               
               
                   
                 500: GATEWAY 
                 600: WIRELESS TRANSMITTER 
               
               
                   
                 700: SENSOR 
                 800: CABLE