In satellite communications, the satellite is very distant from earth-base stations. When the satellite is a geostationary satellite, the distance reaches 35,900 km. Therefore, radiowaves received by receivers are quite weak. Those filters which process these weak waves are required to pass them with low attenuation. Waveguide filters having high selectivity Q have been frequently used as less lossy filters. Also, the transmission station transmits waves with large power and so the energy that is dissipated while being transmitted may be converted into thermal energy, which heats the transmitter equipments. Therefore, waveguide filters exhibiting less lossy properties have been often employed in the same manner as in receivers.
The fundamental structure of a conventional waveguide filter is shown in FIG. 5, where a waveguide 20 of a rectangular cross section is partitioned into a plurality of waveguide resonators by shunt inductor plates 21-28. Thus, a bandpass filter is formed, and this is called a waveguide filter of the shunt inductor type. More specifically, the inductor plates 21-28 form inductor windows. The portion of the waveguide 20 which is defined by the inductor plates 21, 22, 25, 26 forms one waveguide resonator. The portion of the waveguide 20 which is defined by the inductor plates 22, 23, 26, 27 forms another waveguide resonator. Also, the portion of the waveguide 20 which is defined by the inductor plates 23, 24, 27, 28 forms a further waveguide resonator. In this way, the waveguide filter, or bandpass filter, is made up of these three stages of waveguide resonators. The center resonant frequency and the passband width are determined by the width and height of the tube constituting the waveguide 20, the distance between the successive inductor plates, and the width or size of the induction windows formed by the inductor plates 21-28.
The structure of the waveguide filter already proposed by the present applicant is now described in detail by referring to FIGS. 6 and 7. FIG. 6 is an exploded perspective view of this filter. FIG. 7 is a perspective view of the filter, and in which the filter has been assembled. Shown in these figures are waveguides 29-33, inductor plates 34-37 provided with induction windows 40-43, respectively, flanges 38, 39, and a support 44. The waveguides 30-32 are so machined that their dimensions are matched to the center frequency of the bandpass filter. The inductor plates 34-37 are provided with rectangular induction windows 40-43, respectively, which have a width permitting the filter to act as a bandpass filter. Since the waveguides 29 and 33 have no relation to the frequency of the bandpass filter, their length is set to any desired value. When the filter is assembled, the waveguides 29 and 33 are first placed at both ends. Then, the waveguides 29-33 and the inductor plates 34-37 are alternately arranged such that following the waveguide 29 come the inductor plate 43, the waveguide 30, the inductor plate 35, and so on. Subsequently, they are placed on the support 44 which is slightly shorter than the total length of the arrayed waveguides 29-33 and inductor plates 34-37. Then, flanges 38 and 39 are caused to bear against both ends of the support 44, and are firmly fastened to the support with screws to intimately connect the waveguides 29-33 and the inductor plates 34-37.
The waveguide 30 shown in FIGS. 6 and 7 is shown in FIG. 8 to an enlarged scale. The inside of the waveguide 30 is machined usually by drilling or electric discharge. At the four corners, the neighboring sides never intersect at right angles. Therefore, the cross section at the four corners tends to draw an arc-shaped form having a radius of about 0.2 to 0.5 mm.
FIG. 9 is a perspective view of a stub-type adjusting circuit having means for finely adjusting the center frequency of the aforementioned waveguide. This circuit comprises a waveguide 45 of a rectangular cross section. A metal screw 46 is mounted at the longitudinal center of the waveguide 45 and extends at right angles to the longitudinal direction so as to be moved into, or withdrawn from, the waveguide. The screw 46 is locked by a lock nut 47. The screw 46 is appropriately inserted into the waveguide 45 to control the intensity of the electric field, for adjusting the center frequency of the passband.
The inside of the waveguide filter already proposed by the present applicant is difficult to machine with ordinary machining techniques, beause it is cylindrical in shape as mentioned previously. The corners tend to assume an arc-shaped form of a radius of about 0.2 to 0.5 mm. Therefore, the width and height of the waveguide tend to deviate from the designed values. This causes the paths that electric currents flowing on the surface follow vary in length, elevating the center frequency. For example, a waveguide filter having a center frequency of 50 GHz has an inside width of 4.78 mm and an inside height of 2.39 mm. If the four corners are shaped into an arc-shaped form of a radius of about 0.2 to 0.5 mm, then the center frequency is shifted upward by approximately 500 MHz. In this way, the pipes of the waveguides 29-33 are difficult to machine and are not adapted for mass production.
Referring next to FIG. 10, there are show a waveguide 48 and an inductor plate 49 that covers one longitudinal end of the waveguide 48. The waveguide 48 assumes a U-shaped cross section when taken at right angles to the longitudinal direction. Similarly, the inductor plate 49 takes a U-shaped form to form an induction window. Thus, the waveguide 48 is easy to machine. As a result, none of the corners of the waveguide are shaped into arc-shaped form.
However, as shown in FIG. 11, when waveguides 48 and inductor plates 49 constructed as shown in FIG. 10 are mounted on a support 44, it is impossible to press the inductor plates 49 against the support 44, because the inductor plates 49 are very thin. Therefore, the inductor plates 49 do not make good contact with the support 44 at locations 62. This impedes the flow of surface currents, increasing the loss in the passband. Further, the passband width varies from product to product.
The metal screw 46 of the adjusting circuit shown in FIG. 9 is screwed into the waveguide 45 as shown in the enlarged cross section of FIG. 12. When the screw 46 is rotated for adjustment, the point at which the screw 46 is in contact with the waveguide 45 changes discontinuously from n to m. This abruptly changes the paths 63 that surface currents follow, making continuous adjustment impossible. Hence, it is difficult to make fine adjustment. For this reason, the screw 46 of the adjusting circuit for the waveguide filter whose center frequency of the passband lies in 50 GHz band has a small diameter of 1.2 to 1.5 mm and a pitch of 0.3 to 0.4 mm, in order to minimize the interval between the points at which the screw 46 makes contact with the waveguide 45, with unsatisfactory result. Another problem arises from the fact that the crests of the screw 46 form a part of the paths 63 that surface currents follow. That is, the paths 63 are uneven, presenting large resistances at high frequencies. This results in a large loss in the passband.