2. Description of the Related Art In general, a mobile terminal has a helical antenna formed in an outward protrusion and a whip antenna, When the whip antenna is contained in the mobile terminal, the helical antenna is operational, and when the whip antenna is extended, the whip antenna is operational.

FIGs. 1A and 1B illustrate the operations of antennas in a conventional mobile terminal. When a whip antenna 100 is contained in the terminal as shown in FIG. l A, an RF signal is transmittedlreceived through a helical antenna 102 formed within a cap 106 which protrudes outward from a terminal 104. When the whip antenna 100 is pulled out as shown in FIG. 1B, the RF signal is transmitted/received through the whip antenna 100.

As shown in FIGs. 1 A and 1B, the helical antenna is comprised of a winding coil and a cap covering the winding coil. The diameter of a top part TP equals that of a bottom part BP in the winding coil. Since the cylindrical winding coil is near to the outer surface of the cap, the helical antenna is significantly affected when it makes contact with a human body.

When the helical antenna operates, a high voltage is induced to the top part TP of the winding coil, and therefore this high voltage contacts cap 106 adjacent to the top part TP and a human body greatly influences the antenna's characteristics.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a helical antenna structure, where the influence of a human body on the antenna's characteristics is prevented in a mobile terminal.

To achieve the above object, there is provided a helical antenna structure of a mobile terminal, which is minimally influenced by a human body. In the helical antenna structure, a cap protrudes from the upper end of the mobile terminal, and a winding coil is formed within the cap, which is spaced from an outer surface of the cap substantially the same distance from a bottom part of the cap to a top part of the cap.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which : FIGs. IA and 1B illustrate the operating state of a conventional antenna in a mobile terminal ; FIG. 2 illustrates a helical antenna structure in a mobile terminal according to an embodiment of the present invention, FIG. 3 is a graph showing the impedance matching state of the conventional helical antenna formed into a cylindrical winding coil ; FIG. 4 is a graph showing the impedance matching state of the helical antenna formed into a conical winding coil, according to the embodiment of the present invention, FIG. 5 is a graph showing the omni-directional radiation characteristic of the conventional helical antenna formed into a cylindrical winding coil ; FIG. 6 is a graph showing the omni-directional radiation characteristic of the helical antenna formed into a conical winding coil according to the embodiment of the present invention ; FIG. 7 is a graph showing the impedance matching state of the conventional helical antenna formed into a cylindrical winding coil when it contacts a human body ; and FIG. 8 is a graph showing the impedance matching state of the helical antenna formed into a conical winding coil, according to the embodiment of the present

invention when it contacts a human body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described hereinbelow with reference to the accompanying drawings, In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

FIG. 2 illustrates a cone-shaped helical antenna for a mobile terminal, according to an embodiment of the present invention. Referring to FIG. 2, a helical antenna 200, according to the embodiment of the present invention, is comprised of the cap 106 protruding outward from an upper end of the terminal 104 and a winding coil with a diameter decreasing from a bottom part BP to a top part TP to increase the distance between the winding coil and the outer surface of the cap 106, so that the distance or space between the outer surface of the cap 106a and the coil 200 remains substantially the same along the length of coil 200. In accordance with the embodiment of the present invention, the winding coil is formed into a cone shape so that the diameter of the coil is decreased at every turn from the bottom part BP to the top part TP, thereby maximizing the distance between the top part TP where a high voltage is induced and the outer surface of the cap 106. Therefore, the antenna's characteristics caused by contact between a human body and the outer surface of the cap 106, especially near the top part of the cap 106, is minimized, FIGs. 3 and 4 are graphs respectively showing the impedance matching states in free space of the conventional helical antenna formed into a cylindrical winding coil (FIG. 3) and the helical antenna formed into a conical winding coil according to the embodiment of the present invention (FIG. 4). As noted from FIGs. 3 and 4, both the antennas have VSWRs of 2 or below, which are indicative of excellent antenna characteristics.

FIGs. 5 and 6 are graphs respectively showing the omni-directional radiation characteristics of the conventional helical antenna formed into a cylindrical winding coil (FIG. 5) and the helical antenna formed into a conical winding coil, according to the embodiment of the present invention (FIG 6). As noted from FIGs. 5 and 6, both the antennas are almost the same in terms of antenna radiation characteristics in free space.

That is, there is little difference between the antenna gains of the two antennas.

FIGs, 7 and 8 are graphs respectively showing the impedance matching states of the conventional helical antenna formed into a cylindrical winding coil (FIG. 7) and the helical antenna formed into a conical winding coil, according to the embodiment of the present invention (FIG, 8) when a user touches the caps of the helical antennas with his finger during a call, Referring to FIG. 7, the VSWR of the conventional helical antenna is about 6.

This implies that little impedance is matched and thus the conventional antenna's characteristics to operate reliably have deteriorated significantly, On the contrary, the VSWR of the helical antenna, according to the embodiment of the present invention, is 3 or below. Therefore, the deterioration of antenna characteristics caused by contact between the helical antenna and a human body is minimized or prevented in the embodiment of the present invention.

As described above, the helical antenna formed into a conical winding coil according to the embodiment of the present invention is similar to the conventional helical antenna formed into a cylindrical winding coil in terms of impedance matching states and antenna radiation characteristics in free space. Even when a human body contacts the cap of the helical antenna during a call, the helical antenna of the present invention maintains the antenna characteristics as in free space, as compared to the conventional helical antenna, which exhibits significant deterioration of its operational characteristics.

While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.