Stripline dual mode ring resonator and band-pass filter composed thereof.

Abstract: A strip dual mode loop resonator (31) consists of a loop-shaped strip line (33) having a pair of straight strip lines arranged in parallel, an electric length of the loop-shaped strip line being equivalent to a wavelength of a microwave circulated in the loop-shaped strip line in two difference directions according to a characteristic impedance of the loop-shaped strip line, and the straight strip lines being coupled to each other in electromagnetic coupling to change the characteristic impedance of the loop-shaped strip line. The microwave is transferred from an input strip line (32) to the loop-shaped strip line through electromagnetic field induced by the microwave. Thereafter, the microwave is reflected in the straight strip lines of the loop-shaped strip line to produce reflected microwaves circulated in opposite directions. Thereafter, the reflected waves are resonated and filtered in dual mode in the loop-shaped strip line. Thereafter, the microwave formed of the reflected waves is transferred from the loop-shaped strip line to an output strip line (34) through electromagnetic field induced by the microwave.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION: The present invention relates to a strip ring resonator utilized to resonate waves in two different modes (dual mode) in frequency bands ranging from an ultra high frequency (UHF) band to a super high frequency (SHF) band, and relates to a band-pass filter composed of a series of resonators which is utilized as a communication equipment or measuring equipment. 2. DESCRIPTION OF THE RELATED ART: A half-wave length open end type of strip ring resonator has been generally utilized to resonate microwaves ranging from the UHF band to the SHF band. Also, a one-wave length strip ring resonator has been recently known. In the one-wave length strip ring resonator, no open end to reflect the microwaves is required because an electric length of the strip ring resonator is equivalent to one-wave length of the microwaves. Therefore, the microwaves are efficiently resonated because electric energy of the microwaves resonated is not lost in the open end. In addition, in cases where a band-pass filter is composed of a plurality of strip ring resonators arranged in series, a strip dual mode ring resonator functioning as a two-stage filter is required to efficiently filter the microwave in the band-pass filter. 2-1 PREVIOUSLY PROPOSED ART: A first conventional resonator is described. Fig. 1A is a plan view of a one-wave length strip ring resonator in which no open end is provided. Fig. 1B is a sectional view taken generally along the line I-I of Fig. 1A. Each of constitutional elements of the ring resonator shown in Fig. 1A is illustrated in Fig. 1B. As shown in Fig. 1A, a one-wave length strip ring resonator 11 conventionally utilized is provided with an input strip line 12 in which microwaves are transmitted, a closed ring-shaped strip line 13 in which the microwaves transferred from the input strip line 12 are resonated, and an output strip line 14 to which the microwaves resonated in the strip ring 13 are transferred. As shown in Fig. 1B, the input and output strip lines 12, 14 and the ring-shaped strip line 13 respectively consist of a strip conductive plate 15, a dielectric substrate 16 surrounding the strip conductive plate 15, and a pair of conductive substrates 17a, 17b sandwiching the dielectric substrate 16. The ring-shaped strip line 13 has an electric length equivalent to one wavelength of the microwave. The electric length of the ring-shaped strip line 13 is determined by correcting a physical line length of the ring-shaped strip line 13 with a relative dielectric constant ε r of the dielectric substrate 16. The input strip line 12 is arranged at one side of the strip ring 13 and is coupled to the ring-shaped strip line 13 in capacitive coupling. That is, when the microwaves transmit through the input strip line 12, an electric field is induced in a gap space between the input strip line 12 and the ring-shaped strip line 13. Therefore, the intensity of electric field in the ring-shaped strip line 13 is also increased at a coupling point P1 adjacent to the input strip line 12 to a maximum value. The output strip line 14 is arranged at an opposite side of the strip ring 13. In other words, the output strip line 14 is spaced 180 degrees (a half-wave length of the microwaves) in the electric length apart from the input strip line 12. In this case, the intensity of the electric field in the ring-shaped strip line 13 is maximized at a coupling point P2 adjacent to the output strip line 14 because the output strip line 14 is spaced 180 degrees in the electric length apart from the input strip line 12. Therefore, the output strip line 14 is electrically coupled to the ring-shaped strip line 13 in capacitive coupling. In the above configuration, when microwaves are transmitted in the input strip line 12, an electric field is induced at a gap portion between the input strip line 12 and the ring-shaped strip line 13 by the microwaves. Therefore, the intensity of the electric field in the ring-shaped strip line 13 is maximized at the coupling point P1 adjacent to the input strip line 12. Thereafter, the electric field induced at the coupling point P1 is diffused into the ring-shaped strip line 13 as traveling waves. In other words, the microwaves are transferred from the input strip line 12 to the ring-shaped strip line 13. In this case, a part of the travelling waves are transmitted in a clockwise direction, and a remaining part of the travelling waves are transmitted in a counterclockwise direction. In cases where the wavelength of the microwaves is equivalent to the electric length of the ring-shaped strip line 13, the microwaves are resonated in the ring-shaped strip line 13. Therefore, the intensity of the microwaves in the ring-shaped strip line 13 is amplified. Thereafter, the intensity of the electric field in the ring-shaped strip line 13 is maximized at the coupling point P2 adjacent to the output strip line 14 because the output strip line 14 is spaced 180 degrees in the electric length apart from the input strip line 12. Therefore, the electric field is induced at a gap space between the ring-shaped strip line 13 and the output strip line 14. As a result, the microwave resonated in the ring-shaped strip line 13 is transferred to the output strip line 14. Accordingly, the strip ring resonator 11 functions as a resonator of the microwaves. In this case, the microwaves can be resonated in the strip ring 13 even though the electric length of the ring-shaped strip line 13 is an integral multiple of the wavelength of the microwaves. The strip ring resonator 11 is often utilized to estimate the dielectric substrate 16 because a resonance frequency (or a central frequency) of the microwaves is shifted according to a physical shape of the dielectric substrate 16 and the relative dielectric constant ε r of the dielectric substrate 16. The strip ring resonator 11 is described in detail in the literature "Resonant Microstrip Ring Aid Dielectric Material Testing", Microwaves & RF, page 95-102, April, 1991. 2-2 ANOTHER PREVIOUSLY PROPOSED ART: A second conventional resonator is described. Fig. 2 is a plan view of a strip dual mode ring resonator functioning as a two-stage filter. As shown in Fig. 2, a strip dual mode ring resonator 21 conventionally utilized is provided with an input strip line 22 in which microwaves are transmitted, a one-wave length strip ring 23 electrically coupled to the input strip line 22 in capacitive coupling, and an output strip line 24 electrically coupled to the strip ring 23 in capacitive coupling. The input strip line 22 is coupled to the strip ring 23 through a gap capacitor 25, and the output strip line 24 is coupled to the strip ring 23 through a gap capacitor 26. Also, the output strip line 24 is spaced 90 degrees (or a quarter-wave length of the microwaves) in the electric length apart from the input strip line 22. The strip ring 23 has an open end stub 27 in which the microwaves are reflected. The open end stub 27 is spaced 135 degrees (or 3/8-wave length of the microwaves) in the electric length apart from the input and output strip lines 22, 24. In the above configuration, the action of the strip dual mode ring resonator 21 is qualitatively described in a concept of travelling waves. When travelling waves are transmitted in the input strip line 22, electric field is induced in the gap capacitor 25. Therefore, the input strip line 22 is coupled to the strip ring 23 in the capacitive coupling, so that a strong intensity of electric field is induced at a point P3 of the strip ring 23 adjacent to the input strip line 22. That is, the travelling waves are transferred to the coupling point P3 of the strip ring 23. Thereafter, the travelling waves are circulated in the strip ring 23 to diffuse the electric field strongly induced in the strip ring 23. In this case, a part of the travelling waves are transmitted in a clockwise direction and a remaining part of the travelling waves are transmitted in a counterclockwise direction. An action of the travelling waves transmitted in the counterclockwise direction is initially described. When the travelling waves transmitted in the counterclockwise direction reach a coupling point P4 of the strip ring 23 adjacent to the output line 24, the phase of the travelling wave shifts by 90 degrees. Therefore, the intensity of the electric field at the coupling point P4 is minimized. Accordingly, the output strip line 24 is not coupled to the strip ring 23 so that the travelling waves are not transferred to the output strip line 24. Thereafter, when the travelling waves reach the open end stub 27, the phase of the travelling wave further shifts by 135 degrees as compared with the phase of the travelling wave reaching the coupling point P4. Because the open end stub 27 is equivalent to a discontinuous portion of the strip ring 23, a part of the travelling waves are reflected at the open end stub 27 to produce reflected waves, and a remaining part of the travelling waves are not reflected at the open end stub 27 to produce non-reflected waves. The non-reflected waves are transmitted to the coupling point P3. In this case, because the phase of the non-reflected waves transmitted to the coupling point P3 totally shifts by 360 degrees as compared with that of the travelling waves transferred from the input strip line 22 to the coupling point P3, the intensity of the electric field at the coupling point P3 is maximized. Therefore, the input strip line 22 is coupled to the strip ring 23 so that a part of the non-reflected waves are returned to the input strip line 22. A remaining part of the non-reflected waves are again circulated in the counterclockwise direction so that the microwaves transferred to the strip ring 23 are resonated. In contrast, the reflected waves are returned to the coupling point P4. In this case, the phase of the reflected waves at the point P4 further shifts by 135 degrees as compared with that of the reflected wave at the open end stub 27. That is, the phase of the reflected wave at the point P4 totally shifts by 360 degrees as compared with that of the travelling waves transferred from the input strip line 22 to the coupling point P3. Therefore, the intensity of the electric field at the coupling point P4 is maximized, so that the output strip line 24 is coupled to the strip ring 23. As a result, a part of the reflected wave is transferred to the output strip line 24. A remaining part of the reflected wave is again circulated in the clockwise direction so that the microwave transferred to the strip ring 23 is resonated. Next, the travelling waves transmitted in the clockwise direction is described. A part of the travelling waves transmitted in the clockwise direction are reflected at the open end stub 27 to produce reflected waves when the phase of the travelling waves shifts by 135 degrees. Non-reflected waves formed of a remaining part of the travelling waves reach the coupling point P4. The phase of the non-reflected waves totally shifts by 270 degrees so that the intensity of the electric field induced by the non-reflected waves is minimized. Therefore, the non-reflected waves are not transferred to the output strip line 24. That is, a part of the non-reflected waves are transferred from the coupling point P3 to the input strip line 22 in the same manner, and a remaining part of the non-reflected waves are again circulated in the clockwise direction so that the microwave transferred to the strip ring 23 is resonated. In contrast, the reflected waves are return to the coupling point P3. In this case, because the phase of the reflected waves at the coupling point P3 totally shifts by 270 degrees, the intensity of the electric field induced by the reflected waves are minimized so that the reflected waves are not transferred to the input strip line 22. Thereafter, the reflected waves reach the coupling point P4. In this case, because the phase of the reflected waves at the coupling point P4 totally shifts by 360 degrees, the intensity of the electric field induced by the reflected waves is maximized. Therefore, a part of the reflected waves are transferred to the output strip line 24, and a remaining part of the reflected waves are again circulated in the counterclockwise direction so that the microwaves transferred to the strip ring 23 are resonated. Accordingly, because the microwaves can be resonated in the strip ring 23 on condition that a wavelength of the microwaves equals the electric length of the strip ring 23, the strip dual mode ring resonator 21 functions as a resonator and a filter. Also, the microwaves transferred from the input strip line 22 are initially transmitted in the strip ring resonator 23 as the non-reflected waves, and the microwaves are again transmitted in the strip ring resonator 23 as the reflected waves shifting by 90 degrees as compared with the non-reflected waves. In other words, two orthogonal modes formed of the non-reflected waves and the reflected waves independently coexist in the strip ring resonator 23. Therefore, the strip dual mode filter 21 functions as a dual mode filter. That is, the function of the strip dual mode filter 21 is equivalent to a pair of a single mode filters arranged in series. In addition, a ratio in the intensity of the reflected waves to the non-reflected waves is changed in proportional to the length of the open end stub 27 projected in a radial direction of the strip ring resonator 23. Therefore, the intensity of the reflected microwave transferred to the output strip line 24 can be adjusted by trimming the open end stub 27. The strip dual mode ring resonator 21 is proposed by J.A. Curtis "International Microwave Symposium Digest", IEEE, page 443-446(N-1), 1991. 2-3 PROBLEMS TO BE SOLVED BY THE INVENTION: However, there are many drawbacks in the strip ring resonator 11. That is, it is difficult to manufacture a small-sized strip ring resonator 11 because a central portion surrounded by the ring-shaped strip line 13 is a dead space. Also, the electric length of the ring-shaped strip line 13 cannot be minutely adjusted after the ring-shaped strip line 13 is manufactured according to a photo-etching process or the like. In this case, the resonance frequency of the microwaves depends on the electric length of the ring-shaped strip line 13. Therefore, the resonance frequency of the microwaves cannot be minutely adjusted. In addition, in cases where a plurality of strip ring resonators 11 are arranged in series to compose a band-pass filter, it is difficult to couple the ring-shaped strip lines 13 to each other because the ring-shaped strip lines 13 are curved. Also, there are many drawbacks in the strip ring resonator 21. That is, a central frequency of the microwaves filtered in the strip ring resonator 21 cannot be minutely adjusted because the central frequency of the microwaves depends on the width of the open end stub 27 extending in a circumferential direction of the strip ring 23. Therefore, the central frequency of the microwaves manufactured does not often agree with a designed central frequency. As a result, a yield rate of the strip ring resonator 21 is lowered. Also, because a resonance width (or a full width at half maximum) can be adjusted only by trimming the length of the open end stub 27, the resonance width cannot be enlarged. In other words, in cases where the width of the open end stub 27 in the circumferential direction is widened to enlarge the resonance width, the phase of the reflected waves reaching the output strip line 24 undesirably shifts. As a result, the intensity of the microwaves transferred to the output strip line 24 is lowered at the central frequency of the microwaves resonated. Accordingly, in cases where a plurality of strip ring resonators 21 are arranged in series to compose a band-pass filter, the filter is limited to a narrow passband type of filter. SUMMARY OF THE INVENTION A first object of the present invention is to provide, with due consideration to the drawbacks of such a conventional strip ring resonator, a strip dual mode loop resonator in which the central frequency of the microwave is minutely adjusted and the resonance width is widened, and to provide a band-pass filter composed of the resonators. Also, a second object is to provide a small-sized strip dual mode loop resonator in which the resonance frequency is easily and minutely adjusted and the resonance width is narrow, and to provide a band-pass filter composed of the resonators. The above objects are achieved by the provision of a strip ring resonator as set out in claim 1 and a band-pass filter as defined in claim 15. Advantageous further developments are as set out in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The objects, features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which: Fig. 1A is a plan view of a conventional one-wave length type of strip ring resonator in which no open end is provided; Fig. 1B is a sectional view taken generally along the line I-I of Fig. 1A; Fig. 2 is a plan view of a conventional strip dual mode ring resonator functioning as a two-stage filter; Fig. 3A is a plan view of a strip dual mode ring resonator according to a first embodiment of a first concept; Fig. 3B is a sectional view taken generally along the line III-III of Fig. 3A according to the first embodiment; Fig. 3C is a sectional view taken generally along the line III-III of Fig. 3A according to a modification of the first concept; Fig. 4 shows frequency characteristics of the microwaves filtered in the strip dual mode ring resonator shown in Fig. 3; Fig. 5 is a plan view of a strip dual mode ring resonator according to a second embodiment of the first concept; Fig. 6 is a plan view of a strip dual mode loop resonator according to a third embodiment of the first concept; Fig. 7 shows frequency characteristics of the microwaves resonated in the strip dual mode ring resonator shown in Fig. 6; Fig. 8 is a plan view of a band-pass filter in which two strip dual mode ring resonators shown in Fig. 3 are arranged in series according to a fourth embodiment of the first concept; Fig. 9 is a plan view of a strip dual mode ring resonator according to a first embodiment of a second concept; Fig. 10A is a sectional view taken generally along the line X-X of Fig. 9; Fig. 10B is a sectional view taken generally along the line X-X of Fig. 9 according to a modification of the second concept; Fig. 11 is a plan view of a strip dual mode ring resonator according to a second embodiment of the second concept; Fig. 12 is a plan view of a strip dual mode ring resonator according to a third embodiment of the second concept; Fig. 13 is a plan view of a strip dual mode ring resonator according to a fourth embodiment of the second concept; Fig. 14 is a plan view of a band-pass filter in which three strip dual mode ring resonators shown in Fig. 9 are arranged in series according to a fifth embodiment of the second concept; DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a strip dual mode ring resonator or strip ring resonator resonating a microwave in two different modes, respectively, and a band-pass filter composed of the resonators according to the present invention are described with reference to drawings. Fig. 3A is a plan view of a strip dual mode ring resonator according to a first embodiment of a first concept. Fig. 3B is a sectional view taken generally along the line III-III of Fig. 3A. As shown in Fig. 3A, a strip dual mode ring resonator 31 comprises an input strip line 32 in which microwaves are transmitted, a loop-shaped strip line 33 having a uniform line impedance in which the microwaves transferred from the input strip line 32 are resonated, an output strip line 34 to which the microwaves resonated in the loop-shaped strip line 33 are transferred, an input coupling capacitor 35 for coupling the input strip line 32 to the loop-shaped strip line 33 in capacitive coupling to transfer the microwaves from the input strip line 32 to the loop-shaped strip line 33, and an output coupling capacitor 36 for coupling the loop-shaped strip line 33 to the output strip line 34 in capacitive coupling to transfer the microwaves from the loop-shaped strip line 33 to the output strip line 34. As shown in Fig. 3B, the loop-shaped strip line 33 comprises a strip conductive plate 37, a dielectric substrate 38 having a relative dielectric constant ε r and surrounding the strip conductive plate 37, and a pair of conductive substrates 39a, 39b sandwiching the dielectric substrate 38. Therefore, when the microwaves transmit through the loop-shaped strip line 33, an electromagnetic field is induced in the dielectric substrate 38 between the strip conductive plate 37 and the conductive substrates 39a, 39b. That is, the loop-shaped strip line 33 is formed of a balanced strip line. Also, the input and output strip lines 32, 34 are composed of the strip conductive plate 37, the dielectric substrate 38, and the conductive substrates 39a, 39b in the same manner as the loop-shaped strip line 33. The first concept is not limited to the balanced strip line. That is, it is allowed that the input and output strip lines 32, 34 and the loop-shaped strip line 33 be respectively formed of a microstrip line shown in Fig. 3C. As shown in Fig. 3C, each of the strip lines 32, 33, and 34 comprises a strip conductive plate 37m, a dielectric substrate 38m mounting the strip conductive plate 37m, and a conductive substrate 39m mounting the dielectric substrate 38m. An electric length of the loop-shaped strip line 33 is equivalent to a resonance wavelength λ o , and the electric length of the loop-shaped strip line 33 is determined by correcting a physical line length of the loop-shaped strip line 33 with the relative dielectric constant ε r of the dielectric substrate 38. In this specification, the length of the loop-shaped strip line 33 equivalent to the resonance wavelength λ o is called 360 degrees in the electric length for convenience because the microwaves are resonated in the strip line 33 in cases where the microwaves have a resonance angular frequency ω o relating to the resonance wavelength λ o . The loop-shaped strip line 33 has a pair of straight strip lines 33a, 33b arranged in parallel to each other. Also, a width of the loop-shaped strip line 33 is W, and a height of the loop-shaped strip line 33 is H. The straight strip lines 33a, 33b are spaced a distance S apart from each other. Therefore, the straight strip lines 33a, 33b are coupled to each other in electromagnetic coupling according to a relative width W/H and a relative distance S/H. In other words, a first electromagnetic field induced by the microwaves transmitting through the straight strip line 33a and a second electromagnetic field induced by the microwaves transmitting through the straight strip line 33b exert an influence on each other. Accordingly, a characteristic impedance of the loop-shaped strip line 33 differs from that of a ring-shaped strip line in which no straight strip lines arranged in parallel to each other are provided. The input and output coupling capacitors 35, 36 are respectively formed of a plate capacitor having a lumped capacitance Cc. One end of the input coupling capacitor 35 is connected to an input point A of the straight strip line 33a, and one end of the output coupling capacitor 36 is connected to an output point B of the straight strip line 33b. The output point B is spaced 90 degrees (or a quarter-wave length of the microwaves) in the electric length apart from the input point A, and the input and output points A, B are symmetrically arranged each other with respect to a middle line M positioned between the straight strip lines 33a, 33b. In the above configuration, when microwaves having various wavelengths around the resonance wavelength λ o are transmitted in the input strip line 32, an electric field is strongly and locally induced in the the loop-shaped strip line 33 adjacent to the input strip line 32 because lumped electric field is induced in the input coupling capacitor 35 by the microwaves. Therefore, the microwaves in the input strip line 32 are transferred to the strip line 33. Thereafter, to diffuse the electric field locally induced in the loop-shaped strip line 33, the microwaves transmit through the strip line 33 in clockwise and counterclockwise directions in the strip line 33 having the uniform line impedance. In this case, because the straight strip lines 33a, 33b of the strip line 33 are coupled to each other in the electromagnetic coupling, a part of the microwaves are reflected in the straight strip lines 33a, 33b to produce reflected waves. The reflected waves are circulated in the strip line 33 in the clockwise and counterclockwise directions. In cases where the wavelength of the microwaves agrees with the resonance wavelength λ o , the microwaves are resonated in the strip line 33 according to the characteristic impedance of the strip line 33. The characteristic impedance of the strip line 33 is determined according to the uniform line impedance of the strip line 33 and the electromagnetic coupling between the straight strip lines 33a, 33b of the strip line 33. In contrast, in cases where the wavelength of the microwaves does not agree with the resonance wavelength λ o , the microwaves are disappeared or extinguished, respectively, in the strip line 33. The resonance wavelength λ o is intrinsically determined according to the electric length of the strip line 33. In this case, a resonance width (or a full width at half maximum) of the microwaves resonated in the strip line 33 is adjusted by changing the intensity of the electromagnetic coupling between the straight strip lines 33a, 33b. The intensity of the electromagnetic coupling depends on the relative dielectric constant ε r of the dielectric substrate 38, the relative width W/H, and the relative distance S/H. Thereafter, intensity of the electric field in the loop-shaped strip line 33 adjacent to the output strip line 34 is maximized by the reflected waves. Therefore, the microwaves in the strip line 33 is transferred to the output strip line 34 because the strip line 33 is coupled to the output strip line 34 according to the capacitive coupling. Accordingly, because the microwaves are resonated in the strip line 33 on condition that the wavelength of the microwaves agrees with the resonance wavelength λ o , the strip dual mode loop resonator 31 functions as a resonator and filter. Also, the microwaves transferred from the input strip line 32 are initially transmitted in the loop-shaped strip line 33 as non-reflected waves, and the microwaves are again transmitted in the loop-shaped strip line 33 as the reflected waves shifting by 90 degrees as compared with the non-reflected waves. In other words, two orthogonal modes formed of the non-reflected waves and the reflected waves independently coexist in the strip dual mode ring resonator 31. Therefore, the strip dual mode ring resonator 31 functions as a two-stage filter in the same manner as the conventional strip dual mode ring resonator 21. Next, frequency characteristics of the microwaves filtered in the strip line 33 are described to show a relationship between the resonance width of the microwaves resonated in the strip line 33 and the relative distance S/H. Fig. 4 shows frequency characteristics of the microwaves filtered in the strip dual mode ring resonator 31 shown in Fig. 3. As shown in Fig. 4, the intensity of the microwaves filtered in the strip dual mode ring resonator 31 is varied according to a frequency F(GHz) of the microwaves. Also, the resonance width Δω of the microwaves is varied depending on the shape of the strip dual mode ring resonator 31 and the relative dielectric constant ε r of the dielectric substrate 38. The shape is specified by the relative distance S/H and the relative width W/H. In cases where the relative dielectric constant ε r =10 and the relative width W/H=1.0 are satisfied, a central frequency ω o (or a resonance frequency ω o relating to the resonance wave length λ o ) of the microwaves is fixed to 2 GHz. Also, the resonance width Δω of the microwaves is narrowed in proportion as the relative distance S/H is increased. For example, a relative band width Δω/ω o defined by a ratio of the resonance width Δω to the central frequency ω o ranges from 0.02 to 0.1 when the relative distance S/H is changed from S/H=5 to S/H=1. Accordingly, the resonance width Δω of the microwaves can be suitably adjusted by changing the shape of the strip dual mode ring resonator 31 specified by the relative distance S/H and the relative width W/H. Next, a second embodiment of the first concept according to the present invention is described. Fig. 5 is a plan view of a strip dual mode ring resonator according to a second embodiment of the first concept. As shown in Fig. 5, a strip dual mode ring resonator 51 comprises the input strip line 32, a rectangle-shaped strip line 52 in which the microwaves transferred from the input strip line 32 are resonated, the output strip line 34, the input coupling capacitor 35, and the output coupling capacitor 36. Parts of four corners in the rectangle-shaped strip line 52 are cut off. Therefore, each of the four corners cut off functions as a parallel capacitor, a uniform line, or a series inductor, depending on the shape of the four corners cut off. In the above configuration, the microwaves are resonated and filtered in the strip dual mode ring resonator 51 in the same manner as the strip dual mode ring resonator 31 shown in Fig. 3. Accordingly, the resonance width of the microwaves resonated can be adjusted by changing the shape of the four corners. Next, a third embodiment of the first concept according to the present invention is described. Fig. 6 is a plan view of a strip dual mode ring resonator according to a third embodiment of the first concept. As shown in Fig. 6, a strip dual mode ring resonator 61 comprises the input strip line 32, the loop-shaped strip line 33 having the straight strip lines 33a, 33b, the output strip line 34, the input coupling capacitor 35, the output coupling capacitor 36, and a feed-back capacitor 62 for changing a characteristic impedance of the loop-shaped strip line 33. The feed-back capacitor 62 has a lumped capacitance Cw. One end of the feed-back capacitor 62 is connected to the straight strip line 33a at a first connecting point C, and another end of the feed-back capacitor 62 is connected to the straight strip line 33b at a second connecting point D. The connecting point C is spaced 90 degrees (or a quarter-wave length of the microwaves) in the electric length apart from the input point A at which the input coupling capacitor 35 is connected to the straight strip line 33a. Also, the connecting point D is spaced 90 degrees in the electric length apart from the output point B at which the output coupling capacitor 36 is connected to the straight strip line 33b. In the above configuration, microwaves having various wavelengths around the resonance wavelength λ o are transferred to the strip line 33 in the same manner as in the resonator 31 shown in Fig. 3. Thereafter, to diffuse the electric field locally induced in the loop-shaped strip line 33, the microwaves transmit through the strip line 33 in the clockwise and counterclockwise directions in the strip line 33 having the uniform line impedance. In this case, because the straight strip lines 33a, 33b of the strip line 33 are coupled to each other in the electromagnetic coupling, a part of the microwaves are reflected in the straight strip lines 33a, 33b to produce reflected waves. The reflected waves are circulated in loop-shaped the strip line 33 in the clockwise and counterclockwise directions. Also, intensity of electric field in the loop-shaped strip line 33 is maximized at the connecting point D by the remaining part of microwaves not reflected in the straight strip lines 33a, 33b because the connecting point D is spaced 180 degrees (or a half-wave length of the microwaves) in the electric length apart from the input point A. Therefore, the intensity of the electric field at the connecting point C is maximized because the connecting points C, D are connected with each other through the feed-back capacitor 62. As a result, feed-back waves are generated at the connecting point C. The feed-back waves are circulated in the loop-shaped strip line 33 in the clockwise and counterclockwise directions. In cases where the wavelength of the microwaves agrees with the resonance wavelength λ o , the microwaves formed of the reflected waves and the feed-back waves are resonated in the strip line 33 according to the characteristic impedance of the strip line 33. The characteristic impedance of the strip line 33 is determined according to the uniform line impedance of the strip line 33, the electromagnetic coupling between the straight strip lines 33a, 33b of the strip line 33, and the lumped capacitance Cw of the feed-back capacitor 62. In contrast, in cases where the wavelength of the microwaves does not agree with the resonance wavelength λ o , the microwaves are disappeared in the strip line 33. In this case, a resonance width (or a full width at half maximum) of the microwaves resonated in the strip line 33 is adjusted by changing the intensity of the electromagnetic coupling between the straight strip line 33a, 33b or the lumped capacitance Cw of the feed-back capacitor 62. The intensity of the electromagnetic coupling depends on the relative dielectric constant ε r of the dielectric substrate 38, the relative width W/H, and the relative distance S/H. Thereafter, intensity of the electric field in the loop-shaped strip line 33 adjacent to the output strip line 34 is maximized by the reflected waves. Also, intensity of electric field in the loop-shaped strip line 33 adjacent to the output strip line 34 is maximized by the feed-back waves because the output point B is spaced 180 degrees in the electric length apart from the connecting point C. Therefore, the microwaves in the strip line 33 are transferred to the output strip line 34 because the strip line 33 are coupled to the output strip line 34 in the capacitive coupling. Accordingly, even though the relative width W/H and the relative distance S/H of the strip dual mode ring resonator 61 are fixed, the resonance width Δω can be adjusted by changing the lumped capacitance Cw of the feed-back capacitor 62. Next, frequency characteristics of the microwaves resonated in the strip dual mode ring resonator 61 is described. Fig. 7 shows frequency characteristics of the microwaves resonated in the strip dual mode ring resonator 61 shown in Fig. 6. As shown in Fig. 7, the intensity of the microwaves resonated in the strip dual mode ring resonator 61 is varied according to a frequency F(GHz) of the microwaves. That is, in cases where the relative dielectric constant ε r =10, the relative width W/H=1.0, and the relative distance S/H=1 are satisfied, a central frequency ω o (or a resonance frequency relating to the resonance wavelength λ o ) of the microwaves is 2 GHz. Also, the resonance width Δω of the microwaves in the strip dual mode ring resonator 61 is narrowed as compared with in the strip dual mode ring resonator 31 because the microwaves are transferred from the loop-shaped strip line 33 to the output strip line 34 by the action of the feed-back capacitor 62. Also, the resonance width Δω of the microwaves is narrowed in case of the relative distance S/H=3 (not shown) and in case of the relative distance S/H=5 (not shown) as compared with in the strip dual mode ring resonator 31. Also, the resonance width Δω of the microwaves is widened by changing the lumped capacitance Cw of the feed-back capacitor 62. Accordingly, the resonance width Δω of the microwaves can be suitably adjusted by adding the feed-back capacitor 62. Next, a fourth embodiment of the first concept according to the present invention is described. Fig. 8 is a plan view of a band-pass filter in which two strip dual mode ring resonators 31 shown in Fig. 3 are arranged in series according to a fourth embodiment of the first concept. As shown in Fig. 8, a band-pass filter 81 according to the fifth embodiment comprises the input strip line 32, the input coupling capacity 35, the loop-shaped strip line 33 arranged in a first-stage, an inter-stage coupling capacitor 82 to which microwaves are transferred from the first-stage loop-shaped strip line 33, an inter-stage strip line 83, an inter-stage coupling capacitor 84 to which the microwaves are transferred from the capacitor 82 through the strip line 83, the loop-shaped strip line 33 arranged in a second-stage, the output coupling capacitor 36, and the output strip line 34. In the above configuration, each of the loop-shaped strip lines 33 functions as a resonator and filter in the dual modes, and the loop-shaped strip lines 33 are arranged in series. Therefore, the band-pass filter 81 functions as a four-stage filter. Accordingly, because a central hollow portion of each of the resonators 33 is minimized, and because the central hollow portion is efficiently utilized to couple the straight strip lines 33a, 33b, an area occupied by the filter 81 can be minimized. In the fourth embodiment, two resonators 31 according to the first embodiment are substantially arranged in series to manufacture the filter 81. However, the number of the resonators 31 is not limited to two. Also, it is preferred that a plurality of resonators 51 or 61 be arranged in series to manufacture a band-pass filter. Also, it is preferred that various types of resonators selected from the group consisting of the resonators 31, 51, and 61 be combined. Also, it is preferred that the filter 81 comprise a multilayer type of resonators in which a plurality of resonators 31, 51, or 61 are arranged in a tri-plate structure. In the first to fourth embodiment of the first concept, the strip lines (or balanced strip lines) are utilized to manufacture the resonators 31, 51, and 61 and the filter 81. However, it is preferred that microstrip lines generally utilized be utilized to manufacture the resonators 31, 51, and 61, and the filter 81. Next, a first embodiment of a second concept according to the present invention is described. Fig. 9 is a plan view of a strip dual mode ring resonator according to a first embodiment of a second concept. Fig. 10A is a sectional view taken generally along the line X-X of Fig. 9. As shown in Fig. 9, a strip dual mode ring resonator 91 comprises an input strip line 92 in which microwaves are transmitted, a loop-shaped strip line 93 having a uniform line impedance in which the microwaves transferred from the input strip line 92 are resonated, an output strip line 94 to which the microwaves resonated in the loop-shaped strip line 93 are transferred, an input coupling capacitor 95 for coupling the input strip line 92 to the loop-shaped strip line 93 in capacitive coupling to transfer the microwaves transmitted in the input strip line 92 to the loop-shaped strip line 93, an output coupling capacitor 96 for coupling the loop-shaped strip line 93 to the output strip line 94 in capacitive coupling to transfer the microwaves resonated in the loop-shaped strip line 93 to the output strip line 94, a line-to-line coupling capacitor 97 having a lumped capacitance Cw for changing a characteristic impedance of the loop-shaped strip line 93, and a variable capacitor 98 having a variable lumped capacitance Cf for changing the characteristic impedance of the loop-shaped strip line 93 in cooperation with the line-to-line coupling capacitor 97. As shown in Fig. 10A, the loop-shaped strip line 93 comprises a strip conductive plate 101, a dielectric substrate 102 having a relative dielectric constant ε r and surrounding the strip conductive plate 101, and a pair of conductive substrates 103a, 103b sandwiching the dielectric substrate 102. Therefore, when the microwaves transmit through the loop-shaped strip line 93, electromagnetic field is induced in the dielectric substrate 102 between the strip conductive plate 101 and the conductive substrates 103a, 103b. That is, the loop-shaped strip line 93 is formed of a balanced strip line. Also, the input and output strip lines 92, 94 are composed of the strip conductive plate 101, the dielectric substrate 102, and the conductive substrates 103a, 103b, in the same manner as the loop-shaped strip line 93. The second concept is not limited to the balanced strip line. That is, it is allowed that the input and output strip lines 92, 94 and the loop-shaped strip line 93 be respectively formed of a microstrip line shown in Fig. 10B. As shown in Fig. 10B, each of the strip lines 92, 93, and 94 comprises a strip conductive plate 101m, a dielectric substrate 102m mounting the strip conductive plate 101m, and a conductive substrate 103m mounting the dielectric substrate 102m. An electric length of the loop-shaped strip line 93 depends on the relative dielectric constant ε r of the dielectric substrate 102, and the electric length of the strip line 93 is equivalent to a resonance wavelength λ o . Therefore, the length of the strip line 93 is 360 degrees in the electric length. The loop-shaped strip line 93 has a pair of straight strip lines 93a, 93b arranged in parallel to each other. Therefore, the straight strip lines 93a, 93b are coupled to each other in electromagnetic coupling. In other words, first electromagnetic field induced by the microwaves transmitting through the straight strip line 93a and second electromagnetic field induced by the microwaves transmitting through the straight strip line 93b exert an influence on each other, in the same manner as in the strip dual mode ring resonator 31 shown in Fig. 3. The input and output coupling capacitors 95, 96 are respectively formed of a plate capacitor having a lumped capacitance Cc. One end of the input coupling capacitor 95 is connected to an input point A of the straight strip line 93a, and one end of the output coupling capacitor 96 is connected to an output point B of the straight strip line 93b. The output point B is spaced 90 degrees (or a quarter-wave length of the microwaves) in the electric length apart from the input point A, and the input and output points A, B are symmetrically arranged each other with respect to a middle line M positioned between the straight strip lines 93a, 93b. The line-to-line coupling capacitor 97 is formed of a plate capacitor or a chip capacitor, and the variable capacitor 98 is formed of a plate capacitor. Both ends of the capacitor 97 are connected to the straight lines 93a, 93b at connecting points C, D which are spaced 1 degrees apart from the input and output points A, B. The degree 1 ranges up to 135 degrees (or a 3/8-wave length of the microwaves) in the electric length. One end of the capacitor 98 is connected to the strip line 93 at a connecting point E which is positioned at equal intervals (or 135 degrees in the electric length) from the input and output points A, B, and another end of the capacitor 98 is grounded. The variable lumped capacitance Cf of the variable capacitor 98 can be minutely adjusted by cutting plates of the variable capacitor 98 after the strip dual mode ring resonator 91 is manufactured. In the above configuration, when microwaves having various wavelengths around the resonance wavelength λ o are transmitted in the input strip line 92, electric field is strongly and locally induced in the straight strip line 93a adjacent to the input strip line 92 because lumped electric field is induced in the capacitor 95 by the microwaves. Therefore, the microwaves in the input strip line 92 are transferred to the strip line 93. Thereafter, to diffuse the electric field locally induced in the loop-shaped strip line 93, the microwaves transmit through the strip line 93 in clockwise and counterclockwise directions in the strip line 93 having the uniform line impedance. In this case, because the straight strip lines 93a, 93b of the strip line 93 are coupled to each other in the electromagnetic coupling, a part of the microwaves are reflected in the straight strip lines 93a, 93b to produce reflected waves. The reflected waves are circulated in the strip line 93 in the clockwise and counterclockwise directions. In cases where the wavelength of the microwaves agrees with the resonance wavelength λ o , the microwaves ares resonated in the strip line 93 according to the characteristic impedance of the strip line 93. The characteristic impedance of the strip line 93 is determined according to the uniform line impedance of the strip line 93, the electromagnetic coupling between the straight strip lines 93a, 93b, the lumped capacitance Cw of the line-to-line capacitor 97, and the lumped capacitance Cf of the variable capacitor 98. In other words, a remaining part of the microwaves not reflected in the straight strip lines 93a, 93b are reflected by the the variable capacitor 98, or the phase of the remaining part of the microwaves are varied by the line-to-line capacitor 97. In contrast, in cases where the wavelength of the microwaves does not agree with the resonance wavelength λ o , the microwaves are disappeared in the strip line 93. In this case, a central frequency ω o (or a resonance frequency relating to the resonance wavelength) of the microwaves resonated in the strip line 93 is adjusted by changing the lumped capacitance Cw of the line-to-line capacitor 97 and the lumped capacitance Cf of the variable capacitor 98. Also, a resonance width of the resonated microwaves is adjusted by changing either the lumped capacitance Cw of the line-to-line capacitor 97 or the lumped capacitance Cf of the variable capacitor 98. Thereafter, intensity of the electric field in the loop-shaped strip line 93 adjacent to the output strip line 94 is maximized by the reflected waves. Therefore, the microwaves in the strip line 93 are transferred to the output strip line 94 because the strip line 93 are coupled to the output strip line 94 according to the capacitive coupling. Accordingly, because the microwaves are resonated in the strip line 93 on condition that the wavelength of the microwaves agrees with the resonance wavelength λ o , the strip dual mode ring resonator 91 functions as a resonator and filter. Also, the microwaves transferred from the input strip line 92 are initially transmitted in the strip line 93 as non-reflected waves, and the microwaves are again transmitted in the strip line 93 as the reflected waves shifting by 90 degrees as compared with the non-reflected waves. In other words, two orthogonal modes formed of the non-reflected waves and the reflected waves independently coexist in the strip dual mode ring resonator 91. Therefore, the strip dual mode ring resonator 91 functions as a two-stage filter in the same manner as the conventional strip dual mode ring resonator 21. Also, the central frequency of the resonated microwaves can be adjusted by changing the lumped capacitance Cw of the line-to-line capacitor 97 and the lumped capacitance Cf of the variable capacitor 98. Moreover, the central frequency of the resonated microwaves can be minutely adjusted by changing the lumped capacitance Cf of the variable capacitor 98 after the strip dual mode ring resonator 91 is manufactured. Also, because the resonance width of the resonated microwaves can be adjusted by changing either the lumped capacitance Cw of the line-to-line capacitor 97 as a coupling element arranged between the electromagnetically coupled strip lines or the lumped capacitance Cf of the variable capacitor 98, the resonance width can be enlarged. Also, even though the straight strip lines 93a, 93b are connected to each other through a lumped capacitor such as the line-to-line coupling capacitor 97 having the lumped capacitance Cw, the characteristic impedance of the strip line 93 can be changed. Also, even though the input and output strip lines 92, 94 are coupled to the strip line 93 in the capacitive coupling through impedance elements such as the input and output coupling capacitors 95, 96 respectively having a lumped impedance, the microwaves can be transferred between the strip line 93 and the input and output strip lines 92, 94. In addition, because the central frequency and the resonance width of the resonated microwaves can be adjusted after the resonator 91 is manufactured, a yield rate of the resonator 91 can be increased. Next, a second embodiment of the second concept according to the present invention is described. Fig. 11 is a plan view of a strip dual mode ring resonator according to a second embodiment of the second concept. As shown in Fig. 11, a strip dual mode ring resonator 111 comprises an input strip line 112 in which microwaves are transmitted, a loop-shaped strip line 113 having a uniform line impedance in which the microwaves transferred from the input strip line 112 are resonated, an output strip line 114 in which the microwaves resonated in the loop-shaped strip line 113 are transmitted, an input gap capacitor 115 having a distributed capacitance Cc for coupling the input strip line 112 to the loop-shaped strip line 113 in capacitive coupling, an output gap capacitor 116 having the distributed capacitance Cc for coupling the loop-shaped strip line 113 to the output strip line 114 in capacitive coupling, a line-to-line gap capacitor 117 having a distributed capacitance Cw for changing a characteristic impedance of the loop-shaped strip line 113. and an open end stub 118 for changing the characteristic impedance of the loop-shaped strip line 113 in cooperation with the line-to-line gap capacitor 117. The electric length of the loop-shaped strip line 113 agrees with a resonance wavelength λ o , and the loop-shaped strip line 113 has a pair of straight strip lines 113a, 113b arranged in parallel to each other. Therefore, the straight strip lines 113a, 113b are coupled to each other in electromagnetic coupling in the same manner as the straight strip lines 93a, 93b. In addition, projecting portions 113c, 113d facing to each other inwardly extend from the straight strip lines 113a, 113b to form the line-to-line gap capacitor 117. Because the distance between the projecting portions 113c, 113d is narrower than that between the straight strip lines 113a, 113b, the projecting portions 113c, 113d are strongly coupled to each other according to the capacitive coupling. The input gap capacitor 115 is formed by approaching the input strip line 112 to the straight strip line 113a. The output gap capacitor 116 is formed by approaching the output strip line 114 to the straight strip line 113b. A coupling portion A of the straight strip line 113a adjacent to the input strip line 113 is spaced 90 degrees in the electric length apart from a coupling portion B of the straight strip line 113b adjacent to the output strip line 114. The input and output strip lines 112, 114 are symmetrically arranged each other with respect to a middle line M positioned between the straight strip lines 113a, 113b. The open end stub 118 is arranged at equal intervals (or 135 degrees in the electric length) from the coupling portions A, B of the straight strip lines 113a, 113b. In the above configuration, microwaves having various wavelengths around the resonance wavelength λ o are transferred from the input strip line 112 to the loop-shaped strip line 113 because the input strip line 112 is coupled to the strip line 113 by the action of the gap capacitor 115. In the strip line 113, the microwaves are reflected in the straight strip lines 113a, 113b, the projecting portions 113c, 113d, and the open end stub 118 to produce reflected waves. Therefore, the characteristic impedance of the strip line 113 is determined according to the uniform line impedance of the strip line 113, the electromagnetic coupling between the straight strip lines 113a, 113b, the distributed gap capacitance Cw of the line-to-line gap capacitor 117, and a length of the open end stub 118 outwardly extending. Thereafter, the reflected waves are circulated in the loop-shaped strip line 113. In cases where the wavelength of the microwaves agrees with the electric length of the strip line 113, the reflected waves are resonated in the strip line 113. In contrast, in cases where the wavelength of the microwaves does not agree with the electric length of the strip line 113, the reflected waves are disappeared in the strip line 113. In this case, the intensity of the microwaves reflected in the open end stub 118 is varied by trimming the open end stub 118. Also, the intensity of the microwaves reflected in the line-to-line gap capacitor 117 depends on both a gap distance between the projecting portions 113c, 113d and a gap width of the projecting portions 113c, 113d. Thereafter, intensity of electric field in the strip line 113 adjacent to the output strip line 114 is maximized by the microwaves resonated in the strip line 113. Therefore, the microwaves resonated are transferred to the output strip line 114. Accordingly, even though the straight strip lines 113a, 113b are connected to each other through a distributed impedance element such as the line-to-line gap capacitor 117 having a distributed constant, the characteristic impedance of the strip line 113 can be changed. Also, because the input and output strip lines 112, 114 are coupled to the strip line 113 in the capacitive coupling, the microwaves can be transferred between the strip line 113 and the input and output strip lines 112, 114. Also, the resonance width of the resonated microwaves can be adjusted by trimming the open end stub 118. Also, not only the resonance width of the resonated microwaves but also the central frequency of the resonated microwaves can be adjusted by trimming the open end stub 118 and the projecting portions 113c, 113d. Next, a third embodiment of the second concept according to the present invention is described. Fig. 12 is a plan view of a strip dual mode ring resonator according to a third embodiment of the second concept. As shown in Fig. 12, a strip dual mode ring resonator 121 comprises an input strip line 122 in which microwaves are transmitted, the loop-shaped strip line 93 in which the microwaves transferred from the input strip line 122 is resonated, an input magnetic coupling line 123 arranged in parallel to the strip line 93 for coupling the input strip line 122 to the strip line 93 in magnetic coupling (or inductive coupling) by inducing magnetic field therein, an output strip line 124 to which the microwaves resonated in the loop-shaped strip line 93 are transferred, an output magnetic coupling line 125 arranged in parallel to the strip line 93 for coupling the output strip line 124 to the strip line 93 in magnetic coupling (or inductive coupling) by inducing magnetic field therein, and a line-to-line coupling inductor 126 having a lumped inductance Lw for changing a characteristic impedance of the loop-shaped strip line 93. A coupling portion A of the straight strip line 93a adjacent to the input magnetic coupling line 123 is spaced 90 degrees in the electric length apart from a coupling portion B of the straight strip line 93b adjacent to the output magnetic coupling line 124. One end of the input magnetic coupling line 123 is connected to the input strip line 122, and another end of the input magnetic coupling line 123 is grounded. A line width of the input magnetic coupling line 123 is narrow so that magnetic field is dominantly induced around the input magnetic coupling line 123 when the microwaves are transmitted therein. Therefore, the input strip line 122 is coupled to the loop-shaped strip line 93 in the magnetic coupling. Also, one end of the output magnetic coupling line 125 is connected to the output strip line 124, and another end of the output magnetic coupling line 125 is grounded. A line width of the output magnetic coupling line 123 is narrow so that magnetic field is dominantly induced around the output magnetic coupling line 123 when magnetic field induced by the microwaves is increased at the coupling portion B. Therefore, the output strip line 124 is coupled to the loop-shaped strip line 93 in the magnetic coupling. Both ends of the line-to-line coupling inductor 126 are connected to the straight strip lines 93a, 93b at connecting points C, D. The connecting point C is spaced 1 degrees in the electric length apart from the coupling portion A. In the same manner, the connecting point D is spaced 1 degrees in the electric length apart from the coupling portion B. In the above configuration, when microwaves having various wavelengths around the resonance wavelength λ o is transmitted in the input strip line 122, the input magnetic coupling line 123 is coupled to the loop-shaped strip line 93 in the magnetic coupling. That is, magnetic field is locally induced in the loop-shaped strip line 93 adjacent to the input magnetic coupling line 123. Therefore, the microwaves are transferred to the loop-shaped strip line 93. Thereafter, to diffuse the magnetic field locally induced in the strip line 93, the microwaves are transmitted in the strip line 93 according to the characteristic impedance of the strip line 93. The characteristic impedance is determined according to the uniform line impedance of the strip line 93, the electromagnetic coupling of the straight strip lines 93a, 93b and the line-to-line coupling inductor 126. Therefore, the microwaves are reflected at the straight strip lines 93a, 93b and the line-to-line coupling inductor 126 to produce reflected waves. Thereafter, the reflected waves are circulated in the strip line 93 in the clockwise and counterclockwise directions. In this case, when the wavelength of the microwaves agrees with the resonance wavelength λ o , the microwaves are resonated in the strip line 93. Also, intensity of magnetic field in the strip line 93 adjacent to the output magnetic coupling line 125 is maximized by the reflected waves on condition that the wavelength of the microwaves agrees with the resonance wavelength λ o . Therefore, the strip line 93 adjacent to the output magnetic coupling line 125 is coupled to the output strip line 124 in the magnetic coupling by the action of the output magnetic coupling line 125. This is, the microwaves in the strip line 93 are transferred to the output strip line 125. Accordingly, the strip dual mode ring resonator 121 functions as a filter and resonator because the microwaves are resonated in the strip line 93 in cases where the wavelength of the microwaves agrees with the resonance wavelength λ o . Also, because two orthogonal modes formed of the non-reflected waves and the reflected waves shifting by 90 degrees as compared with the non-reflected waves independently coexist in the strip dual mode ring resonator 93, the strip dual mode loop resonator 121 functions as a two-stage filter in the same manner as the strip dual mode ring resonator 91. Also, even though the input and output strip lines are coupled to the strip line 113 in the magnetic coupling, the microwaves can be transferred between the strip line 93 and the input and output strip lines 122, 124. Also, even though the straight strip lines 93a, 93b are connected to each other through a lumped inductor such as the line-to-line coupling inductor 126 having the lumped inductance Lw, the characteristic impedance of the strip line 93 can be changed. Also, even though the characteristic impedance is adjusted by changing the lumped inductance Lw of the line-to-line coupling inductor 126, the resonance width of the resonated microwaves can be adjusted. Next, a fourth embodiment of the second concept according to the present invention is described. Fig. 13 is a plan view of a strip dual mode ring resonator according to a fourth embodiment of the second concept. As shown in Fig. 13, a strip dual mode ring resonator 131 comprises an input coupling line 132 in which microwaves are transmitted, the loop-shaped strip line 93 in which the microwaves transferred from the input coupling line 132 are resonated, a gap capacitor 133 having a distributed capacitance Cc for coupling the input coupling line 132 and the strip line 93 in capacitive coupling, the line-to-line coupling inductor 126, an output coupling line 134 to which the microwaves resonated in the loop-shaped strip line 93 are transferred, and a magnetic coupling line 135 arranged in parallel to the strip line 93 for coupling the output coupling line 134 to the strip line 93 in magnetic coupling. The gap capacitor 133 is formed by approaching the input coupling line 132 to the loop-shaped strip line 93. A coupling portion A of the straight strip line 93a adjacent to the input coupling line 132 is spaced 180 degrees (a half-wave length of the microwaves) in the electric length apart from a coupling portion B of the straight strip line 113b adjacent to the output magnetic coupling line 135. One end of the line-to-line coupling inductor 126 is connected to the straight strip lines 93a at a connecting point C, and another end of the line-to-line coupling inductor 126 is connected to the straight strip lines 93b at the coupling portion B. The connecting point C is spaced 90 degrees in the electric length apart from the coupling portion A. In the above configuration, when microwaves having various wavelengths around the resonance wavelength λ o transmit through the input coupling line 132, intensity of electric field is maximized at the strip line 93 adjacent to the input coupling line 132 by the action of the gap capacitor 133. Therefore, the microwaves are transferred to the strip line 93. Thereafter, to diffuse the electric field, the microwaves are transmitted in the clockwise and counterclockwise directions. In this case, because the characteristic impedance of the strip line 93 is determined according to the uniform line impedance of the strip line 93, the electromagnetic coupling of the straight strip lines 93a, 93b, and the line-to-line coupling inductor 126. Therefore, the travelling waves are reflected at the straight strip lines 93a, 93b and the line-to-line coupling inductor 126 to produce reflected waves. The reflected waves are circulated in the strip line 93 in the clockwise and counterclockwise directions. In cases where the wavelength of the microwaves agrees with the resonance wavelength λ o , the microwaves formed of the reflected waves are resonated in the strip line 93, and the intensity of the magnetic field induced by the reflected waves is maximized at the coupling portion B. Therefore, the output coupling line 134 is coupled to the strip line 93 in the magnetic coupling by the action of the magnetic coupling line 135 so that the microwaves resonated in the strip line 93 are transferred to the output coupling line 134. Accordingly, the strip dual mode ring resonator 131 functions as a filter and resonator because the microwaves are resonated in the strip line 93 in cases where the wavelength of the microwaves agrees with the resonance wavelength λ o . Also, because two orthogonal modes formed of the non-reflected waves and the reflected waves shifting by 90 degrees as compared with the non-reflected waves independently coexist in the strip dual mode ring resonator 93, the strip dual mode ring resonator 131 functions as a two-stage filter in the same manner as the strip dual mode ring resonator 91. Also, even though the input and output coupling lines 132, 134 are coupled to the strip line 93 in different types of impedance coupling such as the capacitive coupling and the magnetic coupling, the microwaves can be transferred between the strip line 131 and the input and output coupling lines 132, 134. Next, a fifth embodiment of the second concept according to the present invention is described. Fig. 14 is a plan view of a band-pass filter in which three strip dual mode ring resonators 91 shown in Fig. 9 are arranged in series according to a fifth embodiment of the second concept. As shown in Fig. 14, a band-pass filter 141 according to the fifth embodiment comprises a series of three strip dual mode ring resonators 91. That is, the strip dual mode ring resonator 91 in a first stage is connected with the strip dual mode ring resonator 91 in a second stage through an inter-stage coupling capacitor 142. Also, the strip dual mode ring resonator 91 in the second stage is connected with the strip dual mode ring resonator 91 in a third stage through an inter-stage coupling capacitor 143. In the above configuration, each of the strip lines 93 in the strip dual mode ring resonators 91 functions as a resonator and filter in dual modes or two different modes, respectively. Therefore, the band-pass filter 141 functions as a six-stage filter. Accordingly, because central hollow portions of the resonators 91 are minimized, and because the central hollow portions are efficiently utilized to couple the straight strip lines 93a, 93b, an area occupied by the filter 141 can be minimized. In the fifth embodiment, three resonators 91 according to the first embodiment is utilized to manufacture the filter 141. However, the number of the resonators 91 is not limited to three. Also, it is preferred that a plurality of resonators 111, 121, or 131 be arranged in series to manufacture a band-pass filter. Also, it is preferred that various types of resonators selected from the resonators 91, 111, 121, and 131 be combined. Also, it is preferred that the filter 141 comprise a multilayer type of resonators in which a plurality of resonators 91, 111, 121, or 131 are arranged in a tri-plate structure. In the first and fifth embodiment, the strip lines (or balanced strip lines) are utilized to manufacture the resonators 91, 111, 121, and 131 and the filter 141. However, it is preferred that microstrip lines be utilized to manufacture the resonators 91, 111, 121, and 131 and the filter 141 .

Claims: A strip ring resonator, comprising: a loop-shaped strip line (33, 52, 93, 113) for resonating a microwave in two different modes, the electric line length of the loop-shaped strip line being equivalent to one wavelength of the microwave to resonate the microwave circulated in the strip line according to the line impedance of the loop-shaped strip line; an input strip line (32, 92, 112, 123, 132, 152) electromagnetically coupled to an input point of the loop-shaped strip line for inputting the microwave to the input point of the loop-shaped strip line; and an output strip line (34, 94, 114, 125, 135, 154) electromagnetically coupled to an output point of the loop-shaped strip line for outputting the microwave resonated in the loop-shaped strip line from an output point of the loop-shaped strip line, characterized in that the loop-shaped strip line includes a pair of electromagnetically coupled straight strip lines sections (33a; 33b, 93a; 93b, 113a; 113b) arranged in parallel to each other to electromagnetically couple the electromagnetically coupled straight strip lines of the loop-shaped strip line to each other, the line impedance of the loop-shaped strip line being dependent on the electromagnetic coupling between the coupling partial lines. A resonator according to claim 1, further comprising a coupling element (62, 97, 113c; 113d, 126) arranged between the electromagnetically coupled straight strip lines of the loop-shaped strip line for changing the line impedance of the loop-shaped strip line, a first electric line length between the input point of the loop-shaped strip line and one end of the coupling element connected to one of the electromagnetically coupled straight strip lines being equal to a second electric line length between the output point of the loop-shaped strip line and the other end of the coupling element connected to the other of the electromagnetically coupled straight strip lines, and the output point of the loop-shaped strip line being spaced a quarter of the wavelength of the microwave apart from the input point of the loop-shaped strip line. A resonator according to claim 2, in which the first and second electric line lengths are equal to a quarter of the wavelength of the microwave. A resonator according to claim 1, in which the loop-shaped strip line has a rectangular shape, and four corners of the loop-shaped strip line are cut off. A resonator according to claim 1, further comprising a capacitor (98) having a variable capacitance for changing the line impedance of the loop-shaped strip line, one end of the capacitor being connected to a connecting point (E) of the loop-shaped strip line spaced a three-eighth of the wavelength of the microwave apart from the input (A) and output (B) points of the loop-shaped strip line, the other end of the capacitor (98) being grounded, and the output point (B) of the loop-shaped strip line being spaced a quarter of the wavelength (90°) of the microwave apart from the input point (A) of the loop-shaped strip line. A resonator according to claim 1, further comprising an open end stub (118) for reflecting the microwave to change the line impedance of the loop-shaped strip line, the open end stub being spaced a three-eighth of the wavelength of the microwave apart from the input and output points of the loop-shaped strip line, and the output point of the loop-shaped strip line being spaced a quarter of the wavelength of the microwave apart from the input point of the loop-shaped strip line. A resonator according to claim 1, further comprising an input coupling capacitor (35, 95) for coupling the input strip line to the loop-shaped strip line in capacitive coupling, and an output coupling capacitor (36, 96) for coupling the output strip line to the loop-shaped strip line in capacitive coupling. A resonator according to claim 1, further comprising an input magnetic coupling line (123) for coupling the input strip line to the loop-shaped strip line in magnetic coupling, and an output magnetic coupling capacitor (125) for coupling the output strip line to the loop-shaped strip line in magnetic coupling. A resonator according to claim 2, in which the coupling element is a capacitor (62 , 97) having a lumped capacitance. A resonator according to claim 2, in which the coupling element is a coupling capacitor (113c; 113d) having a distributed capacitance. A resonator according to claim 2, in which the coupling element is an inductor (126) having a lumped inductance. A resonator according to claim 1, in which the loop-shaped strip line and the input and output strip lines are respectively a microstrip. A resonator according to claim 1, in which the loop-shaped strip line and the input and output strip lines are respectively a balanced strip line. A resonator according to claim 1, further comprising a coupling element (126) arranged between the electromagnetically coupled straight strip lines of the loop-shaped strip line for changing the line impedance of the loop-shaped strip line, one end of the coupling element connected to one of the electromagnetically coupled straight strip lines being spaced a quarter of the wavelength of the microwave apart from the input point of the loop-shaped strip line, and the other end of the coupling element connected to the other of the electromagnetically coupled straight strip lines being positioned to the output point of the loop-shaped strip line, and the output point of the loop-shaped strip line being spaced a half of the wavelength of the microwave apart from the input point of the loop-shaped strip line. A band-pass filter for filtering a microwave, comprising: a plurality of loop-shaped strip lines (33, 52, 93, 113) arranged in series, each of the loop-shaped strip lines having an input point, an output point, and including a pair of electromagnetically coupled straight strip lines sections (33a; 33b, 93a; 93b, 113a; 113b) arranged in parallel to each other, the electric line length of each of the loop-shaped strip lines being equivalent to a wavelength of the microwave to resonate the microwave circulated in each of the loop-shaped strip lines according to a line impedance of each of the loop-shaped strip lines, the electromagnetically coupled straight strip lines of each of the loop-shaped strip lines being electromagnetically coupled to each other, the line impedance of each of the loop-shaped strip lines being dependent on the electromagnetic coupling between the electromagnetically coupled straight strip lines, and the output point of each of the loop-shaped strip lines being spaced a quarter of the wavelength of the microwave apart from the input point thereof; an input strip line (32, 92, 112, 123, 132, 152) for inputting the microwave to the input point of the loop-shaped strip line arranged in a first stage in electromagnetic coupling; a plurality of inter-stage coupling elements (142, 143) which are each arranged between an adjacent pair of upper and lower stage loop-shaped strip lines, one end of each of the inter-stage coupling elements being coupled to the output point of the upper stage loop-shaped strip line, and the opposite end of each of the inter-stage coupling elements being coupled to the input point of the lower stage loop-shaped strip line; and an output strip line for outputting the microwave resonated in the loop-shaped strip lines from the output point of the loop-shaped strip line arranged in a final stage in electromagnetic coupling. 16. A resonator according to claim 1, the loop-shaped strip line further comprising a coupling element (62, 97, 113c; 113d, 126) arranged between the electromagnetically coupled straight strip lines of the loop-shaped strip line for changing the line impedance of the loop-shaped strip line, a first electric line length between the input point of the loop-shaped strip line and one end of the coupling element connected to one of the electromagnetically coupled straight strip lines being equal to a second electric line length between the output point of the loop-shaped strip line and the other end of the coupling element connected to the other of the electromagnetically coupled straight strip line.

Inventors: FUJIMURA MUNENORI, HASEGAWA MAKOTO, MAKIMOTO MITSUO, TAKAHASHI KAZUAKI, FUJIMURA, MUNENORI, HASEGAWA, MAKOTO, MAKIMOTO, MITSUO, TAKAHASHI, KAZUAKI, FUJIMURA, MUNENORI MIYAMATSU-RYO 14587-1
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H01P1
IPC codes: H01P7, H01P1