BACKGROUND ART As well known to those skilled in the art, in order to increase an amount of transmitted data for data communication, a usable frequency is heightened. However, the heightened usable frequency causes various problems such as a transmission loss due to shortened wavelength. Generally, a MIC (Microwave Integrated Circuit) or a MMIC (Microwave Monolithic Integrated Circuit) technique has been used to deal with wave band over microwaves band. However, in case the usable frequency is approximately (50) GHz, the transmission loss is about (60) dB per lm, thus it is also difficult to build practical circuit using this technique.

Therefore, a NRD guide has been recently introduced. In addition this NRD Guide is non-radiative so that it presents a lower transmission loss than microstrip lines.

Further, the manufacturing of the NRD Guide transmission line is easier than waveguides.

Therefore, the NRD Guide attracts attention as a transmission line for microwaves, especially millimeter waves over 30 GHz.

As illustrated in Figure 1, the NRD Guides (2) proportional to the wave length of the usable frequency is arranged between two parallel metal plates (l). And the distance between the two parallel metal plates (1) is set to be not more than the half of the wavelength of the usable frequency.

The size of the NRD Guides (2) is obtained by two formulas, as follows.

<Formula 1> a/. == 0.45 <Formula 2>

Once the distance between the two parallel metal plates (l) is set depending on the usable frequency, this usable frequency is smoothly transmitted due to the lower transmission loss. However, if the usable frequency is changed, the transmission of the new frequency is impossible.

Herein, sr denotes the relative permittivity of the NRD Guide (2) and denotes a wavelength of the usable frequency. Further, a and b denote a height and a width of the NRD Guide (2).

In order to set the size of the NRD Guide, the usable frequency should be determined. That is, since the size of the NRD Guide is set by the frequency to be used, the usable frequency is fixed thereto. In order to use a designated frequency, the size of the NRD Guide and the distance between the two parallel metal plates are set.

That is, the sizes of the NRD Guide interposed between the two parallel metal plates and other circuit elements should be set to be a designated value.

Since the distance between the two parallel metal plates is fixed, it is difficult to comprise different-sized NRD Guides or different-sized elements. Therefore, only a single frequency can be transmitted through this NRD Guide. Thus, it is impossible to manufacture the large number of high frequency circuits using various frequencies, thereby limiting the utility of the NRD Guide.

In order to solve the above-described problem, a NRD Guide with a multi- layered structure is required. Korean Patent Application No. 99-20294 (filed on June 2, 1999) and 99-24090 (filed on June 24,1999) disclose this NRD Guide with a multi- layered structure.

Figure 2 illustrates a related art NRD Guide with a multi-layered structure, which is capable of transmitting frequencies with different bands. As illustrated in Figure 2, the related art NRD Guide comprises multi-layered metal plates (10,11) and a plurality of NRD Guides (20,21) for propagating various frequencies. Herein, al and bl denote a width and a thickness of the first NRD Guide (20), and a2 and b2 denote a width and a thickness of the second NRD Guide (21).

In accordance with the related art NRD Guide with the multi-layered structure, the parallel metal plates (10, 0, 11) with multi-layered structure are stacked, thereby varying the distance between the two parallel metal plates.

Thus, it is made possible to build the NRD Guide structure that has various size of NRD Guide for transmitting a number of frequencies and space in accordance with the size of the NRD Guide.

A strip resonator or a reed-type resonator is used to interconnect the circuits using different frequencies, thereby transmitting these frequencies.

Further, Korean Patent Application No. 99-24090 (filed on June 24,1999) discloses an oscillation circuit using the NRD Guide.

DISCLOSURE OF THE INVENTION Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a receiving apparatus using plural different-sized NRD Guides, in which a GUNN diode and mixed substrates are mounted, thereby being capable of receiving modulated millimeter waves with various frequency.

In accordance with the present invention, the above and other objects can be accomplished by the provision of a receiving apparatus using NRD Guides. In order to accomplish the object of the present invention, the receiving apparatus using NRD Guides comprises two parallel metal plates disposed at a designated distance, that inner space of the metal plates is composed of a number of layers; an oscillation diode mount interposed between the two metal plates, thereby oscillating a predetermined frequency; a straight NRD Guide connected to the oscillation diode mount with a strip resonator, that transmits the oscillated frequency; a curved NRD Guide spaced from the straight NRD Guide by a designated distance and curved outward at a constant degree of curvature, that outputs mixed signal of the oscillated frequency and the frequency received from outside; and a balance mixer for mixing frequency transmitted via the straight NRD Guide and frequency received via the curved NRD Guide.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 illustrates a perspective view of a related art NRD Guide.

Fig. 2 illustrates a perspective view of a related art NRD Guide with a multi- layered structure, which is capable of transmitting plural frequencies.

Fig. 3 illustrates a schematic view of a receiving apparatus using NRD Guides in accordance with the present invention.

Fig. 4 illustrates an enlarged perspective view of a portion A in the receiving apparatus of Fig. 3.

Fig. 5 illustrates an enlarged perspective view of a portion B in the receiving apparatus of Fig. 3.

Fig. 6 is a graph that illustrates transmitted power and coupled power depending on a distance (D) between the straight NRD Guide and the curved NRD Guide.

Fig. 7 is a graph that illustrates coupling characteristics of a coupler of the present invention.

Fig. 8 is a graph that illustrates conversion loss of a mixer of the present invention according to frequency.

BEST MODE FOR CARRYING OUT THE INVENTION Fig. 3 to Fig. 5 illustrate a receiving apparatus using NRD Guides in accordance with the present invention. The receiving apparatus using NRD Guides of the present invention comprises two parallel plates (30,35), an oscillation diode mount (40), a strip resonator (45), a mode suppressor (47), a straight NRD Guide (50), a diode mount formed on the straight NRD Guide (55), a first mixer substrate (57), a curved NRD Guide (60), a diode mount formed on the curved NRD Guide (65), and a second mixer substrate (67).

The two parallel metal plates (30,35) are disposed at a designated distance.

The inner space of the metal plates (30,35) are multi-layered. One end of the straight NRD Guide (50) is connected to the oscillation diode mount (40) with the strip resonator (45). A GUNN diode (40a) is mounted on the oscillation diode mount (40). Herein, bias voltage is supplied to the GUNN diode (40a) through a cable inserted into an inner surface of the oscillation diode mount (40) and a 7J4 choke (40b) is patterned on a side of source of the oscillation diode mount (40).'The a/4 choke (40b) serves to prevent millimeter waves from being leaked.

The mode suppressor (47) is inserted into a H-plane of the end of the straight NRD Guide (50). The mode suppressor (47) is inserted in order to suppress unnecessary

mode (such as LSE mode, TEM mode) which may be generated in case the strip resonator (45) is connected to the straight NRD Guide.

Therefore, millimeter waves, in which waves of the unnecessary modes are removed by the mode suppressor (47), are transmitted to the straight NRD Guide (50).

The curved NRD Guide (60) is spaced from the straight NRD Guide (50) by a designated distance. The curved NRD Guide (60) is curved outward at a constant degree of curvature. One end of the curved NRD Guide (60) is protruded from the two parallel metal plates (30,35), thereby receiving millimeter waves from the outside.

Although one end of the curved NRD Guide (60) of the present invention is not protruded from the metal plates (30,35) and the whole body of the curved NRD Guide (60) is enclosed by the metal plates (30,35), millimeter waves are capable of being received from the outside.

A balance mixer is formed on the other ends of the straight NRD Guide (50) and the curved NRD Guide (60), respectively. The balance mixer includes the diode mount formed on the straight NRD Guide (55), which is spaced from the other end of the straight NRD Guide (50) at a designated distance and connected to the other end of the straight NRD Guide (50) with the first mixer substrate (57) and the diode mount formed on the curved NRD Guide (65), which is spaced from the other end of the curved NRD Guide (60) at a designated distance and connected to the other end of the curved NRD Guide (60) with the second mixed substrate (67).

The straight NRD Guide (50) and the curved NRD Guide (60) act as a 3dB coupler. That is, the straight NRD Guide (50) and the curved NRD Guide (60) are used as the 3dB coupler by regulating the distance between the straight NRD Guide (50) and the curved NRD Guide (60). Millimeter waves oscillated by the GUNN diode (40a) are used as local oscillation frequency of the receiving apparatus of the present invention. These oscillation waves are inputted via the straight NRD Guide (50). Then, half of power of the oscillation waves are transite into the curved NRD Guide (60) at an area of the straight NRD Guide (50) adjacent to the curved NRD Guide (60).

On the other hand, the curved NRD Guide (60) serves as means for receiving millimeter waves from the outside. Millimeter waves from the outside are received via the curved NRD Guide (60). Then, half of power of the received waves are also transite to the straight NRD Guide (50) at the area of the curved NRD Guide (60) adjacent to the straight NRD Guide (50).

As described above, about half of power of both the straight NRD Guide (50) and the curved NRD Guide (60) are transite to each other. The amount of the transite power of the straight NRD Guide (50) and the curved NRD Guide (60) is determined by the distance (D) between the straight NRD Guide (50) and the curved NRD Guide (60).

Fig. 6 is a graph that illustrates transmitted power and coupled power depending on a distance (D) between the straight NRD Guide and the curved NRD Guide. As illustrated in Fig. 6, the amounts of the transite power and transmitted power vary depending on the distance between straight NRD Guide (50) and the curved NRD Guide (60).

Fig. 7 is a graph that illustrates coupling characteristics of the 3dB coupler of the present invention. The 3dB coupler of Fig. 7 is designed to make transmitted power and coupled power equal by maintaining the determined distance between the straight NRD Guide (50) and the curved NRD Guide (60).

The millimeter waves transite by the above-described process are inputted to the straight NRD Guide (50) and the curved NRD Guide (60). The transite waves and original waves are mixed by the first mixer substrate (57) and the second mixer substrate (67). The mixed waves are outputted as an intermediate frequency.

Fig. 8 is a graph that illustrates conversion loss of the mixer according to frequencies, that is, receive sensitivity of the intermediate frequency. From the graph of Fig. 8, it is noted that the intermediate frequency characteristic is constant through bandwidth over 800MHz INDUSTRIAL APPLICABILITY As apparent from the above description, the present invention provides an receiving apparatus using different-sized plural NRD Guides, thereby forming a high- performance miniaturized millimeter waves integrated circuit. Further, the receiving apparatus of the present invention can be effectively used in a short-distance broad-band communication system and very high-speed integrated communication system.

Also, the preferred embodiments of the present invention have been disclosed for illustrative purposes and should not be construed as limiting the scope of the present invention. Those skilled in the art will appreciate that various modifications, additions and substitutions are possible. However, those modifications, additions and substitutions should be considered to be in the scope and spirit of the present invention.