Waveguide filter with coaxial/waveguide mode conversion

Abstract: In order to improve filter loss of a waveguide filter having coaxial/waveguide mode conversion parts (14) at its ends, conductor posts (40) bridging a space between two long sides of the waveguide (10) are provided between the parts and the ends. The conductor posts (40) have sufficient thickness and remain close enough to each other and to short ends (18) of the wafeguide that a partition plane including axes of the conductor posts (40) can be regarded as an imaginary short plane. The conductor posts are located so that the partition plane is positioned at a distance λ/4 × n (n = 1, 3, 5 ...) from the coaxial/waveguide mode conversion (14).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide filter provided with coaxial/waveguide mode conversion parts for converting a transmission mode between a coaxial and a waveguide, in one or both ends thereof. 2. Description of the Related Art A waveguide filter one or both ends of which are to be coupled through coaxial lines to other circuit elements is provided with coaxial/waveguide mode conversion parts in one or both ends thereof. In an end of the waveguide filter where the coaxial/waveguide mode conversion part is provided, the waveguide is sealed by inserting a metal block into an open end of the waveguide and by screwing the waveguide on the metal block. Though a cross section of the metal block is made so as to fit a cross section of a space bounded by the waveguide in order to prevent leakage of electric waves, gaps tend to be formed between the metal block and inner walls of the waveguide because of deflection of the walls of the waveguide that are not secured by screws, and filter loss tends to be increased because of leakage of the electric wave through the gaps. Though conductive adhesive or copper foil has been used to fill the gaps, attenuation of the leaked electric waves is limited to about 60dB. Thus, it has been difficult and costly to reduce the leakage to a desired level. US-A-2 588 103 describes a frequency selective circuit comprising a pair of spaced coaxial lines, and a low loss waveguide connecting said lines and forming with them a network having frequency selective characteristics. The waveguide comprises end waveguide sections and an intermediate waveguide telescoping the end sections. The end sections each have a coaxial to waveguide junction comprising a transverse terminal probe terminating the inner conductor of each coaxial line, with pistons in the end guide sections located symmetrically beyond the probe at positions providing an impedance mismatch at the junctions. The length of the waveguide is adjustable to cause the reflections from the mismatched junction to be out of phase, and the pistons are adjustable to vary the selectively of the circuit. DE-A-0 251 058 describes an arrangement for de-coupling two orthogonal, linearly polarised waves from a waveguide by means of coupling pins inserted into the waveguide and supported in isolation from the waveguide wall, one coupling pin being provided for each polarisation direction. The coupling pins are arranged perpendicularly to each other and at least one short circuit element is provided which has a distance approximately λ g /4 from a transverse plane in which the adjacent coupling pin is located. SUMMARY OF THE INVENTION It is an object of the present invention to provide a waveguide filter with coaxial/waveguide mode conversion parts, wherein leakage of an electric wave from ends of the waveguide filter can be effectively reduced. In accordance with the present invention, there is provided a waveguide filter, comprising: a waveguide having two opposite long sides and two opposite short sides, and a coaxial/waveguide mode conversion part provided at at least one end of the waveguide, for converting a transmission mode between a coaxial mode and a waveguide mode, characterized by more than one conductor post each bridging a space between the two opposite long sides and located between the coaxial/waveguide mode conversion part and the end at which the coaxial/waveguide mode conversion part is provided. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a conventional waveguide bandpass filter having coaxial input/output ports; Figure 2 is a perspective view for explaining an operation of a waveguide bandpass filter; Figure 3 is a perspective view explaining a conventional method of sealing ends of a waveguide filter; Figures 4A to 4C are elevational views of the conventional waveguide filter for explaining a problem thereof; Figure 5A is a perspective view of an end of a waveguide filter according to an embodiment of the present invention; Figure 5B is a longitudinal sectional view of the end; Figure 5C is a plan view of the end; Figure 6A is a longitudinal sectional view of an end of a waveguide filter according to another embodiment of the present invention; Figure 6B is a plan view of the end; Figure 6C is a transverse sectional view of the end; Figure 7 is a side view of an end of a waveguide filter according to another embodiment of the present invention; Figure 8 is a side view of a waveguide filter according to another embodiment of the present invention; Figure 9 is a side view of a waveguide filter according to another embodiment of the present invention; Figure 10A is a plan view of the waveguide filter of Fig. 9 showing a detailed construction thereof; and Figure 10B is a side view of the waveguide filter of Fig. 10A. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the preferred embodiments according to the present invention, examples of the related art are given with reference to the accompanying drawings. Figure 1 is a perspective view of a conventional waveguide bandpass filter having coaxial input/output ports. The waveguide filter includes a waveguide 10 and coaxial/waveguide mode conversion parts 12 and 14 for inputting and outputting signals through coaxial cables. A space bounded by the waveguide 10 is divided by a plurality of partitions each composed of a plurality of conductor posts 16 and longitudinally spaced at prescribed intervals, to form a multiple-stage bandpass filter. In the example of Fig. 1, a space bounded by the waveguide 10 is divided by three partitions each composed of three conductor posts 16 and longitudinally spaced at intervals of λ/2 × n (n = 1, 2, 3 ...) where λ is a wavelength of a signal allowed to pass through. The conductor posts 16 are thinner than the thinnest conductor posts that would make susceptance of the partitions a value corresponding to imaginary short planes with respect to the signal allowed to pass through, in order to allow leakage of that signal. Thus, the signal having a wavelength λ forms a standing wave as shown in Figure 2, but signals having a wavelength not equal to λ cannot form the standing wave, and thus the waveguide 10 functions as a bandpass filter. As shown in Figure 3, the ends 18 of the waveguide 10 are sealed by inserting metal blocks 20, and the metal block 20 is fixed to the waveguide 10 by screwing the side walls of the waveguide 10 to the inserted metal block 20 with screws 22 engaging with screw holes 24 formed in the metal block 20, through holes 26 of the side walls of the waveguide 10. As shown in Figs. 4A and 4B, though a cross section of the metal block 20 is made so as to fit a cross section of a space bounded by the waveguide 10, gaps 28 and 30, or 32 and 34, tend to be formed between the metal block 20 and the waveguide 10 because of deflection of the side walls that are not secured by screws, and the gaps thus formed increase filter loss. Although the gaps may be filled with conductive adhesive or copper foil 36 as shown in Fig. 4C, attenuation of the leaked electric waves is limited to about 60dB, and it has been difficult and costly to reduce the leakage to a desired level. The preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Figures 5A to 5C show one end of a waveguide bandpass filter according to a first embodiment of the present invention, where Fig. 5A, Fig. 5B and Fig. 5C are a perspective view, a longitudinal sectional view, and a plan view, respectively. More than one, for example, three conductor posts 40 bridging a space between two opposite long sides of the waveguide 10 are provided between the coaxial/waveguide mode conversion part 14 and the end 18 of the waveguide 10. The conductor posts 40 are located so that a partition defined by a plane including axes of the conductor posts 40 is positioned at a distance λ/4 × n (n = 1, 3, 5 ...) from an inner conductor 42 of the coaxial/waveguide mode conversion part 14, as shown in Fig. 5B. In addition, the conductor posts 40 have sufficient thickness and remain sufficiently close to each other and to the short sides of the waveguide so that susceptance of the partition represents a value corresponding to an imaginary short plane with respect to the signal allowed to pass through the filter. Thus, the signal having a wavelength λ is prevented from passing through by the partition composed of the conductor posts 40, and leakage of electric waves from the open end 18 is remarkably reduced. Figures 6A to 6C show one end of a waveguide filter according to another embodiment of the present invention, where Figs. 6A, 6B and 6C are a longitudinal sectional view, a plan view, and a transverse sectional view at a position of the conductor posts 40, respectively. Similarly to that of Figs. 5A to 5C, the waveguide filter shown in Figs. 6A to 6C includes three conductor posts 40 for preventing the signal allowed to pass through the filter from leaking out from the end. The conductor posts 40 are laterally positioned at equal intervals of W/4 as shown in Fig. 6C, where W is a width of the waveguide. In addition, the waveguide filter of Figs. 6A to 6C includes a metal block 44 for sealing the end of the waveguide filter. Since electric waves leaking out from the end are reduced by the conductor posts 40 by about 70 - 80 dB, sealing with the metal block 44 is not essential for preventing the leakage of the electric wave but preferable from the point of view of prevention of deterioration of filter characteristics caused by invasion of dust or disturbance by external electric waves. Nevertheless, filling material as shown in Fig. 4C is not necessary. Figure 7 shows a waveguide filter according to another embodiment of the present invention, wherein a band elimination filter (BEF) is formed in the end of the band pass filter (BPF). In Fig. 7, the conductor posts 40' are provided between a coaxial/waveguide mode conversion part 14 and a metal block 44 at a distance L1 equal to λ/4 × n (n = 1, 3, 5 ...) from the inner conductor 42 of the coaxial/waveguide mode conversion part 14, similar to the conductor posts 40 of Figs. 6A to 6C. However, the conductor posts 40' are made slightly thinner than the thinnest conductor posts that would make a perfect imaginary short plane, and the distance L2 between the conductor posts 40' and the metal block 44 is made equal to λ'/2 × n (n = 1, 2, 3 ... ) where λ' is a wavelength of an electric wave to be eliminated, for example, that of a local frequency deviating from the pass band frequency by 70 MH z . Thus, an electric wave of λ' included in electric waves having leaked through the conductor posts 40' is absorbed into a resonator having a length of L2 (= λ'/2 × n (n = 1, 2, 3 ... )), to thereby eliminate the undesirable electric wave. Since the distance Ll between the inner conductor 42 and the conductor posts 40' is λ/4 × n (n = 1, 3, 5 ...), the BEF portion is regarded as an open end with respect to a center frequency of the BPF, and therefore, the BEF does not affect the characteristics of the BPF. Figure 8 shows an example of the waveguide filter according to the present invention, wherein the BEF is formed in one end of the BPF. Figure 9 shows another example of the waveguide filter according to the present invention wherein two BEF's are formed in both ends of the BPF. If the distances L2 and L2' between the conductor posts 40' and the metal blocks 44 are equal to each other, in other words, if the center frequencies of the two BEFs are equal to each other, greater elimination of an unnecessary frequency is obtained. If the distances L2 and L2' are different from each other, elimination of two different frequencies is obtained. In addition, if the distances L2 and L2' are set so that the elimination frequencies are close to each other, a BEF having a desired elimination band is obtained. Figures 10A and 10B show a detailed construction of the waveguide filter of Fig. 9, where Fig. 10A is a plan view and Fig. 10B is a side view. In Figs. 10A and 10B, adjusting screws 50 for varying center frequencies of the BEF are shown. Screws 52 are provided for adjusting the center frequency of the BPF and screws 54 are provided for adjusting a degree of coupling. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A waveguide filter, comprising: a waveguide (10) having two opposite long sides and two opposite short sides; and a coaxial/waveguide mode conversion part (12, 14) provided at at least one end of the waveguide, for converting a transmission mode between a coaxial mode and a waveguide mode; characterized by more than one conductor post (40) each bridging a space between the two opposite long sides and located between the coaxial/waveguide mode conversion part and the end at which the coaxial/waveguide mode conversion part is provided. A waveguide filter as claimed in claim 1, wherein the conductor posts have sufficient thickness and remain sufficiently close to each other and to the short sides of the waveguide so that susceptance of a partition plane including axes of the conductor posts represents a value corresponding to an imaginary short plane with respect to an electric wave allowed to pass through the waveguide filter. A waveguide filter as claimed in claim 2, wherein the conductor posts are located so that the partition plane is positioned substantially at a distance λ/4 × n from the coaxial/waveguide mode conversion part, where λ is wavelength of the electric wave allowed to pass through the waveguide filter, and n = 1, 3, 5 ... A waveguide filter, as claimed in claim 1, 2 or 3, further comprising a conductor block (44) inserted into the end at which the coaxial/waveguide mode conversion part is provided, to thereby seal the end. A waveguide filter, as claimed in claim 4, wherein the distance between the partition plane defined by the conductor posts and the conductor block is substantially λ'/2 × n, where λ' is the wavelength of an electric wave to be eliminated, and n = 1, 2, 3 ... A waveguide filter, as claimed in claim 5, wherein the distances between the partition planes and the conductor blocks at both ends are substantially equal to each other. A waveguide filter, as claimed in claim 5, wherein the distances between the partition planes and the conductor blocks at both ends are substantially different from each other. A waveguide filter as claimed in claim 5, 6 or 7, further comprising adjusting screws (50) for varying the wavelength of the electric wave to be eliminated.

Inventors: FUKUDA MASANORI, KUDO KENICHI, SHIMIZU TAKEHIRO, SOGO HIROYUKI, FUKUDA, MASANORI, KUDO, KENICHI, SHIMIZU, TAKEHIRO, SOGO, HIROYUKI, Fukuda, Masanori, c/o FUJITSU LIMITED, Kudo, Kenichi, c/o FUJITSU LIMITED, Shimizu, Takehiro, c/o FUJITSU LIMITED, Sogo, Hiroyuki, c/o FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H01P1
IPC codes: H01P5, H01P1