Recently, as a mobile telecommunication system and telecommunication devices have been rapidly digitalized, it is commonly required to employ a telecommunication system having a speedy and powerful data processing capacity. Since the telecommunication system is required to process and communicate a great deal of information, its frequency band has been expanded from a microwave band to a millimeter wave band. In a wireless telecommunication system using the millimeter wavelength, it was usually manufactured as a hybrid type module. Recently, as semiconductor technology makes great strides, the wireless telecommunication system has been developed to employ a single chip of a monolithic microwave integrated circuit (MMIC). Although the hybrid type, as a conventional type, is less competitive than the MMIC in terms of price and mass production, it may be advantageously adopted to manufacture the wireless telecommunication system in small lots.
Nowadays, many attentions have been paid to a NRD waveguide since it can be easily manufactured as compared with the hybrid type and can transmit signals in a longitudinal-section magnetic (LSM) mode with a low transmission loss.
FIGS. 1 and 2 show the structure of a conventional signal receiving device adopting a NRD waveguide multi-layer type. The signal receiving device has an upper conductive plate 1 and a lower conductive plate 2 which is positioned in parallel to the upper conductive plate 1. Dielectric lines 3 and 4 are arranged between the upper and lower conductive plates 1 and 2. A radio frequency signal and a local oscillating signal are inputted into the signal receiving device through the dielectric lines 3 and 4. A horn antenna (not shown) is attached to a rod antenna 10 for receiving/transmitting the signals. When the radio frequency signal is inputted into the rod antenna 10, a bias signal is applied to a Gunn diode (not shown) mounted in a diode mount 7 so that the local oscillating signal is generated. At this time, a longitudinal-section electric (LSE) mode is created. However, the LSE mode is suppressed by a mode suppressor 8. Then, the radio frequency signal passes through a dielectric resonator 9 so that a transferring gain increases at a predetermined frequency band and an intermediate frequency (IF) signal is outputted from two ports 16 and 17 through a 3-dB coupler having a bend shape. The signals outputted from the two ports 16 and 18 are introduced into Schottky diodes of a pair of balanced mixer mounts 5 and 6 and are inputted into an intermediate frequency (IF) terminal 12. Each of the Schottky diodes receives a bias 13 having a predetermined voltage and is grounded by a ground 14 so that a closed circuit is formed.
FIG. 1 represents the typical structure of the signal receiving device using the NRD waveguide. The coupler is fabricated by bending the dielectric lines based on the principle of a parallel dielectric line coupler. In designing bending angles of the dielectric lines, an established database with respect to proper widths and the bending angles of the dielectric lines are used.
However, in fabricating the dielectric coupler having the above structure in a small size, it is not a good choice to reduce lengths of the bend dielectric lines. Accordingly, it is inevitable to bend the dielectric lines much more, but this choice may cause a large error range in fabricating them. A bend dielectric line may cause a transmission loss at a bending portion if the width of the dielectric line is not adaptively reduced with respect to respective bending angles corresponding frequencies. In the fabrication, the reducing of the width without causing a large error is very difficult. Furthermore, it is also difficult to precisely design and fabricate the bending angles, the distance between the dielectric lines and the isolation degree between ports. In addition, if the dielectric coupler is fabricated in a small size with a light weight, the width of the bending portion has to be reduced to enlarge the bending angle. However, it is difficult to precisely reduce the width of the dielectric line which is usually made of Teflon.