Microwave filter

Abstract: A microwave filter is disclosed in which open-ended lines (14, 15, 16) are connected at three or four points of a main line (13) having input and output terminals (11, 12) the open-ended lines (14, 15, 16) having a length about one fourth the wavelength of an image signal frequency, to produce a band-stop filter characteristic at an image frequency; all or parts of the open-ended lines (14, 15, 16) constitute open-ended parallel-coupled lines arranged in parallel to the main line (13), and the intervals of connection of the open-ended lines (14, 15, 16) with the main line (13) are properly selected. The insertion loss of the filter within the pass band of a radio frequency signal is reduced and the filter attenuation within a stop band of an image signal is increased, thus providing a compact band-­ stop filter which has a steep rise characteristic.

Description: FIELD OF THE INVENTION The present invention relates to a microwave filter using a strip line or a micro-strip line, or more in particular to a microwave filter configuration with a pass-band frequency higher than a stop-band frequency and both the pass-band and stop-band frequencies limited in bandwidth. DESCRIPTION OF THE PRIOR ART In a mixer supplied with a radio frequency signal (f s in frequency) and a local oscillation signal (f ℓ in frequency) different in frequency from the radio frquency signal for producing an intermediate frequency signal (f s - f ℓ in frequency; f s > f ℓ ) making up a frequency component representing the difference between the former two signals, a filter (hereinafter called the "signal-pass image-rejection filter) for passing the radio frequency signal without loss but stopping an image signal (with frequency f m = 2f ℓ - f s ) having a frequency (2f ℓ - f s ) twice the local oscillation signal (2f ℓ ) less the frequency (f s ) of the radio frequency signal, is inserted in a main line for transmitting the radio frequency signal to a mixer diode. Further, a local band-pass filter (hereinafter called the "local BPF") for selectively passing a local oscillation signal alone is interposed between an input terminal for the local oscilaltion signal and the mixer diode. Upon application of a radio frequency signal and a local oscillation signal to a mixer diode making up a non-linear element, a side band or a high harmonic of mf s ± nf ℓ (m, n: Integers) in frequency are generated. The waves of the image signal frequency f m and the sum frequency f s + f ℓ in these spectra contain a radio frequency component. By returning the image signal, in particular, out of these signals to the mixer diode through a signal-pass image-rejection filter and mixing it with the local oscillation signal again, therefore, it is possible to produce a reconverted intermediate frequency signal and thereby to reduce the conversion loss of the mixer. further, the signal-pass image-rejection filter is capable of preventing an interference wave signal having the same frequency as the image signal frequency from entering the frequency band of the intermediate frequency signal by way of the radio frequency signal input terminal. Especially, a single-ended mixer using only one mixer diode has the performance thereof greatly affected by the manner in which the image signal generated in the mixer diode is processed. The impedance as viewed from a diode terminal is normally set to be reactive against the image signal frequency. A signal-pass image-rejection filter and a local BPF for rejecting an image signal thus constitute indispensable elements for configuring a single-end mixer. The signal-pass image-rejection filter is provided on or in coupling with a main line for transmitting a radio frequency signal to the mixer diode, and therefore the characteristics of the signal-pass image-rejection filter have a direct effect on the mixer performance. In other words, it is not too much to say that the mixer performance is determined by the characteristics of the signal-pass image-rejection filter. The performance described below is required of such a signal-pass image-rejection filter. (1) A minimum insertion loss against a radio frequency signal. (2) Characteristics to reject an image signal sufficiently. (3) A pass bandwidth and a rejection bandwidth required for a radio frequency signal and an image signal respectively. (4) The more steep the out-of-band characteristics, the closer the frequencies of the radio frequency signal and the image signal to each other. A conventional signal-pass image-rejection filter used with a mixer is disclosed in JP-A-63-10601. This signal-pass image-rejection filter is shown in Fig. 5. In Fig. 5, an input terminal 1 and an output terminal 2 for a radio frequency signal are connected by a main line 3 configured of a strip line. Open-ended stubs 4, 5, 6 having lengths of ℓ₁, ℓ₂, ℓ₃ respectively at equal intervals of ℓ₀ sequentially are connected in shunt with the main line 3. The lengths ℓ₁, ℓ₂, ℓ₃ of the open-ended stubs 4, 5, 6 are selected as equal or near to one fourth of the wavelength of the image signal so that poles of attenuation are placed within or in the vicinity of the image signal band. The length, ℓ₁, ℓ₂, ℓ₃ and the intervals ℓ₀ of the open-ended stubs 4, 5, 6 are also determined in such a manner as to hold the relations of both ℓ₂ < ℓ₁ < ℓ₀ < 2ℓ₂ and ℓ₂ < ℓ₃ < ℓ₀ < 2ℓ₂ at the same time or the relations ℓ₂ < ℓ₁ = ℓ₃ < ℓ₀ < 2ℓ₂, while the length ℓ₀ is selected at a value about 1.5 times one fourth of the wavelength of the radio frequency signal. Numerals 7, 8 designate input and output lines connected to the input and output terminals 1 and 2 respectively. The forementioned signal-pass image-rejection filter with the open-ended stubs 4, 5, 6 projected in the directions perpendicular to the main line 3 has disadvantages in that: (1) The fact that the open-ended stubs 4, 5, 6 are mounted in the form projected in the directions perpendicular to the main line 3 easily causes radiation, thereby increasing an insertion loss within the pass band of a radio frequency signal. (2) The open-ended stub 5 has poles of attenuation on high-frequency side as compared with the stubs 4, 6. If the characteristic impedance of the open-ended stub 5 is increased, a filter having a comparatively steep rise characteristic would be obtained. Since there is only one open-ended stub with poles of attenuation on high frequency side, however, it is impossible to produce a filter having a steep rise characteristic. (3) In view of the fact that the open-ended stubs 4, 5, 6 are projected in the directions perpendicular to the main line, the filter is widened for an increased filter size. It has also been proposed to provide filters in which spur lines are coupled in parallel with the main line, see for example IEEE Transaction on Microwave Theory and Techniques, Vol. MTT-24, No. 5, May 1976, pages 242 to 248 and Electronic Engineering, Vol. 50, No. 604 (1978.04), pages 39 to 41. SUMMARY OF THE INVENTION The present invention has been developed in order to obviate these disadvantages, and the object thereof is to provide a compact microwave filter small in the insertion loss caused by radiation within the pass band of a radio frequency signal and having a steep rise characteristic. According to a first aspect of the present invention there is provided a microwave filter comprising: a main line having an input terminal and an output terminal, first, second and third open-ended lines, each connected to said main line at equal or nearly equal consecutive intervals l₀, l₀′ (l₀≈l₀′), said lines having lengths of l₁, l₂ and l₃, respectively; characterised in that said first and third lines are open-ended lines arranged and coupled parallel to the main line, said second line (15) is an open-ended stub arranged in shunt with said main line, the lengths l₁, l₂ and l₃ of said first, second and third lines are selected to be equal to the 1/4 wavelength of a stop-band frequency such that the poles of attenuation thereof are positioned within a stop band, the lengths l₀, l₀′, l₁, l₂ and l₃ are selected to satisfy the conditions l₁ < l₂ < l₀≈l₀′ < 2l₁ and l₃ < l₂ < l₀≈ l₀′ < 2l₃ or to satisfy the condition l₁ = l₃ < l₂ < l₀≈ l₀′ < 2l₁, and the characteristic impedance of at least the first and third lines is selected to be higher than that of the input and output lines connected to the input and output terminals, and the interval l₀, l₀′, between the first, second and third open-ended lines is selected to be longer than 5/16 wavelength of a pass-band frequency but shorter than 7/16 wavelength of said pass-band frequency. According to a second aspect of the present invention there is provided a microwave filter comprising: a main line having an input terminal and an output terminal, and first, second, third and fourth open-ended lines, each connected to main line at equal or nearly equal consecutive intervals l₀, l₀′ (l₀≈l₀′), and having lengths of l₁, l₂, l₂′ and l₃, respectively, characterised in that the second and third lines of equal or substantially equal lengths, l₂, l₂′ are open-ended stubs arranged in shunt with the main line, extending across said main line on both sides of said main line at the same position, the first and fourth lines are open-ended lines arranged and coupled in parallel to the main line, the lengths l₁, l₂, l₂′, and l₃ of the first, second, third and fourth lines are selected to be the 1/4 wavelength of a stop-band frequency such that the poles of attenuation thereof are positioned within a stop band, the lengths l₀, l₀′, l₁, l₂, l₂′ and l₃, are selected to satisfy the conditions l₁≈l₂≈l₂′ < l₀≈ l₀′ < 2l₁ and l₃≈l₂≈l₂′ < l₀≈l₀′ < 2l₃ or to satisfy the condition l₁ = l₃≈l₂≈l₂′ < l₀≈l₀′ < 2l₁, and the characteristic impedance of at least the first and fourth lines is selected to be higher than that of the input and output lines (17, 18) connected to the input and output terminals. According to a third aspect of the present invention there is provided a microwave filter comprising: a main line having an input terminal and an output terminal, and first, second, third and fourth open-ended lines, each connected to said main line at equal or nearly equal consecutive intervals l₀, l₀′, (l₀≈l₀′) respectively and having lengths of l₁, l₂, l₂′ and l₃ respectively, characterised in that the second and third lines of equal or substantially equal lengths l₂, l₂′ respectively are open-ended line arranged and coupled in parallel to said main lines, the first and fourth lines are open-ended lines arranged and coupled in parallel to the main line, said first and second lines are arranged in corresponding opposed relationship to each other on the opposite sides of said main line, said third and fourth lines are arranged in corresponding opposed relationship to each other on the opposite sides of said main line, the lengths l₁, l₂, l₂′ and l₃ of the first, second, third and fourth lines are selected to be the 1/4 wavelength of a stop-band frequency such that the poles of attenuation thereof are positioned within a stop band, the lengths l₀, l₀′, l₁, l₂, l₂′ and l₃ are selected to satisfy the conditions l₁≈l₂≈l₂′ < l₀≈l₀′ < 2l₁ and l₃≈l₂≈l₂′ < l₀≈l₀′ < 2l₃ or to satisfy the condition l₁ = l₃≈l₂≈l₂′ < l₀≈l₀′ < 2l₁′ and the characteristic impedance of at least the first and fourth lines is selected to be higher than that of the input and output lines connected to the input and output terminals. In one form of the present invention, a microwave filter comprises open-ended lines at three of four points on a main line, in which the length of the open-ended lines is selected at approximately one fourth the wavelength of an image signal thereby to provide band-stop filter characteristics with an image signal frequency. In the case where open-ended lines are used at three points on the main line, first, second and third open-ended lines are sequentially connected at equal or substantially equal intervals, and the length of the open-ended lines is selected to be almost equal to one fourth the wavelength of the image signal, thus producing a band-stop filter characteristics with an image signal frequency. The first and third lines are configured of open-ended parallel-coupled lines in parallel to the main line, and the intervals between the first, second and third lines are selected at a value longer than one fourth and shorter than one half the wavelength of the image signal. Especially by selecting an interval about 1.5 times one fourth the wavelength of a radio frequency signal, band-pass filter characteristics are obtained with the frequency of the radio frequency signal. When open-ended lines are connected at four points on the main line, on the other hand, the filter is configured of a main line and first, second, third and fourth open-ended parallel-coupled lines with one end of each thereof connected sequentially to the main line at intervals of ℓ₀, ℓ₀′ (ℓ₀ ≅ ℓ₀′) respectively. The first, second, third and fourth parallel-coupled lines, which have the length of ℓ₁, ℓ₂, ℓ₂′ and ℓ₃ respectively, are parallel-coupled with the main line. The lengths ℓ₁, ℓ₂, ℓ₂′ and ℓ₃ are selected to be equal to one fourth the wavelength of the stop-band frequency in such a manner that poles of attenuation thereof are placed within a stop band. At the same time, the lengths ℓ₀, ℓ₀′, ℓ₁, ℓ₂, ℓ₂′ and ℓ₃ are selected to satisfy the conditions ℓ₁≈ℓ₂≈ℓ₂′ < ℓ₀≈ℓ₀′ < 2ℓ₁ and ℓ₃≈ℓ₂≈ℓ₂′ < ℓ₀≈ℓ₀′ < 2ℓ₃ or to satisfy the condition ℓ₁=ℓ₃≈ℓ₂≈ℓ₂′ < ℓ₀≈ℓ₀′ < 2ℓ₁. The present microwave filter has a feature in that a main line is arranged in opposed relationship with a pair of first and second parallel-coupled lines and a pair of the third and fourth parallel-coupled lines, or in that the characteristic impedance of at least the parallel-coupled open-ended first and fourth lines is selected at a value higher than that of the input and output lines connected to an input or output terminals. The fact that each parallel-coupled line is arranged in parallel and coupled with a main line reduces the radiation loss of the filter due to the radiation from the open ends of the parallel-coupled lines, with the result that the filter insertion loss is decreased within the pass-band of the radio frequency signal and the filter attenuation is increased within the stop-band of the image signal. The characteristic impedance of a parallel-coupled line with poles of attenuation thereof on the side nearer to the pass band of the radio frequency signal is set higher than the characteristic impedance of input and ouput lines, whereby the quality factor (Q) within the stop band of the parallel-coupled line is increased while at the same time filter characteristics including a steep rise characteristic are obtained due to the fact that the poles of attenuation are comprised of two parallel-coupled lines. The present invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: Fig. 1 is a diagram showing a pattern of a microwave filter circuit configured of a strip line according to a first embodiment of the present invention. Fig. 2 is a characteristic diagram showing a specific example of the frequency characteristic of insertion loss of the filter circuit shown in Fig. 1. Fig. 3 is a pattern diagram showing a microwave filter circuit configured of a strip line according to a second embodiment of the present invention. Fig. 4 is a pattern diagram showing a microwave filter circuit configured of a strip line according to a third embodiment of the present invention. Fig. 5 is a pattern diagram showing a microwave filter circuit configured of a conventional strip line. DESCRIPTION OF THE PREFERRED EMBODIMENTS A signal-pass image-rejection filter according to a first embodiment of the present invention is shown in Fig. 1. An input terminal 11 and an output terminal 12 for a radio-frequency signal are connected by a main line 13 composed of a strip line. Open-ended parallel-coupled lines 14 and 16 having the length of ℓ₁ and ℓ₃ respectively are parallel-coupled to the main line 13, and the open-ended stub 15 having the length of ℓ₂ is connected in shunt with the main line 13. Further, the parallel-coupled line 14, the open-ended stub 15 and the parallel-coupled line 16 are connected in that order to the main line 13 at the interval of ℓ₀, ℓ₀′ (ℓ₀ ≅ ℓ₀′). The lengths ℓ₁, ℓ₂ and ℓ₃ of the open-ended parallel-coupled line 14, the stub 15 and the parallel-coupled line 16 are selected to be one fourth or substantially one fourth the wavelength of the image signal in such a manner that poles of attenuation thereof are placed within or in the vicinity of the band of the image signal. The lengths ℓ₁, ℓ₃ of the parallel-coupled lines 14 and 16, the length ℓ₂ of the open-ended stub 15 and the interval ℓ₀, ℓ₀′ are selected in such a way as to satisfy the conditions of both ℓ₁ ≅ ℓ₂ < ℓ₀ ≅ ℓ₀′ < 2ℓ₁ and ℓ₃ ≅ ℓ₂ < ℓ₀ ≅ ℓ₀′ < 2ℓ₃ at the same time or the condition ℓ₁ = ℓ₃ ≅ ℓ₂ < ℓ₀ ≅ ℓ₀′ < 2ℓ₁. On the other hand, the length ℓ₀, ℓ₀′ are determined at a value about 1.5 times one fourth of the wavelength of the radio frequency signal, and the characteristic impedance of the parallel-coupled lines 14 and 16 at a value higher than the characteristic impedance Z₀ (normally 50 Ω) of the input and output lines 17, 18 connected to the input and output lines 11 and 12. According to the first embodiment, the fact that the parallel-coupled lines 14, 16 high in characteristic impedance are parallel-coupled to the main line 13 reduces the radiation loss of the filter due to the radiation from the open ends of the parallel-coupled lines 14, 16, with the result that the insertion loss of the filter is reduced within the pass band of a radio frequency and that the filter attenuation is increased within the stop band of the image signal. Also, since the characteristic impedance of at least two parallel-coupled lines 14 and 16 is set at a high level, the quality factor Q within the stop band of the parallel-coupled lines 14 and 16 is increased, thus producing a filter having a steep rise characteristic. The present embodiment is especially effective as a filter for image rejection used with a mixer having radio frequency signal and an image signal comparatively close to each other, thus realizing superior mixer performance. Also, the filter dimensions are reduced as the parallel-coupled lines 14, 16 are parallel-coupled to the main line. Fig. 2 shows filter characteristics based on the assumption that the dielectric constant of a dielectric substrate of the strip line is 2.6, the thickness thereof 0.6 mm, the characteristic impedance of the parallel-coupled lines 14, 16 is 120 Ω, the characteristic impedance of the open-ended stub 15 is 50 Ω, and the length ℓ₀ (= ℓ₀′), ℓ₁ (= ℓ₃) and ℓ₂ are 6.3 mm, 5.5 mm and 5.6 mm respectively. This computation takes into account the effect of the fringing capacitance which is caused by the edge effect at the open ends of the stub 16 and the parallel-coupled lines 14, 16. The filter shown in Fig. 2 has characteristics including a VSWR (voltage standing-wave ratio) less than two in the frequency range from 11.4 to 14.0 GHz and an attention more than 30 dB in the frequency range from 8.2 to 9.9 GHz. As a result, a filter having the characteristics shown in Fig. 2 sufficiently satisfies the performance required of a signal-pass image-rejection filter for a mixer with an image signal of a frequency from 8.2 to 9.9 GHz and a radio frequency signal from 11.4 to 14.0 GHz. Further, if the distance ℓ₀, ℓ₀′ between the parallel-coupled lines and the open-ended stub is selected as a value longer than 5/16 and shorter than 7/16 the wavelength of the radio frequency signal, a filter having an especially superior characteristic is configured. The filter having the characteristic shown in Fig. 2 is selected to have a size satisfying these conditions. Fig. 3 shows a signal-pass image-rejection filter according to a second embodiment of the present invention. The same component parts as in Fig. 1 will be explained by attaching the same reference numerals as in Fig. 1 respectively. Numerals 25, 25′ designate open-ended stubs having the same or substantially the same line lengths ℓ₂, ℓ₂′ selected to be one fourth or almost one fourth the wavelength of the image signal so that poles of attenuation are positioned within or in the vicinity of the band of the image signal. These stubs are connected at corresponding positions on the opposite sides of the main line 13. Assume that the characteristic impedances of the open-ended stubs 25, 25′ are Z₂₅ and Z₂₅′, that the characteristic impedance of the open-ended stub in Fig. 1 is Z₁₅, and that the lengths of the open-ended stubs 15, 25 and 25′ are selected so that all the frequencies of the poles of attenuation of the open-ended stubs 15, 25 and 25′ are coincident with each other. Then, the filter characteristics in Figs. 1 and 3 coincide with each other as far as the relationship holds that 1/Z₁₅ = 1/Z₂₅ + 1/Z₂₅′. In the second embodiment, in addition to the effect of the embodiment shown in Fig. 1, the lengths ℓ₂, ℓ₂′ of the open-ended stubs 25, 25′ are set slightly different from each other, so that there are two poles of attenuation due to the open-ended stubs 25, 25′, thereby making it possible to distribute the positions of poles of attenuation over an image signal band, with the result that the amount of attenuation in an image signal band may be averaged out. If a line of a low characteristic impedance is required for the open-ended stub 15 in the embodiment of Fig. 1, an effectively low characteristic impedance may be easily attained by dividing into two open-ended stubs 25, 25′ as shown in the second embodiment. In addition, since the line width of the open-ended stubs 25, 25′ is kept small, the formation of the stubs, which otherwise might have a wider line, is facilitated in connecting the main line 13 and the open-ended stubs 25, 25′. Furthermore, a filter of especially superior characteristics may be configured by selecting an interval ℓ₀, ℓ₀′ longer than 5/16 and shorter than 7/16 the wavelength of the radio frequency signal. A signal-pass image-rejection filter according to a third embodiment of the present invention is shown in Fig. 4. An input terminal 31 and an output terminal 32 for a radio frequency signal are connected by a main line 33 constituting a strip line. Open-ended parallel-coupled lines 34, 35, 35′, 36 having lengths ℓ₁, ℓ₂, ℓ₂′, ℓ₃ respectively are coupled in parallel to the main line 33. The parallel-coupled lines 34, 35 (or 35′) and 36 are connected in that order to the main line 33 with intervals (ℓ₀ ≅ ℓ₀′). The parallel-coupled lines 34, 35 are disposed in opposed relationship at corresponding positions on the side of a main line portion 33, and the parallel-coupled lines 35′, 36 in opposed relationship at corresponding positions on the side of the other main line portion 33. The parallel-coupled lines 34, 35, 35′, 36 are selected at lengths ℓ₁, ℓ₂, ℓ₂′, ℓ₃ respectively which are one fourth or substantially one fourth the wavelength of the image signal to secure poles of attenuation at positions within or in the vicinity of the image signal band. The lengths ℓ₁, ℓ₂, ℓ₂′, ℓ₃ and the interval ℓ₀, ℓ₀′ of the parallel-coupled lines 34, 35, 35′, 36 are also determined in such a manner as to satisfy the conditions ℓ₁ ≅ ℓ₂ ≅ ℓ₂′ < ℓ₀ ≅ ℓ₀′ < 2ℓ₁ and ℓ₃ = ℓ₂ ≅ ℓ₂′ < ℓ₀ ≅ ℓ₀′ < 2ℓ₃ at the same time, or the conditions ℓ₁ = ℓ₃ ≅ ℓ₂ ≅ ℓ₂′ < ℓ₀ ≅ ℓ₀′ < 2ℓ₁, while selecting the length ℓ₀ at about 1.5 times one fourth the wavelength of the radio frequency signal. Further, the parallel-coupled lines 34, 36 are selected to have a characteristic impedance higher than the characteristic impedance Z₀ (normally 50 Ω) of the input and output lines 37 and 38 connected to the input and output terminals 11 and 12. According to the third embodiment, in view of the fact that the parallel-coupled lines 34, 35, 35′, 36 are coupled in parallel to the main line 33, it is possible to reduce the radiation loss of the filter caused by the radiation from the open ends of the parallel-coupled lines 34, 35, 35′, 36, with the result that the insertion loss of the filter is decreased within the pass band of the radio frequency signal, thereby increasing the amount of attenuation of the filter within the rejection band of the image signal. Also, the characteristic impedance of at least two parallel-coupled lines 34, 36 is set high, so that the quality factor (Q) in the stop band of the parallel-coupled lines 34, 36 is high and a filter with a steep rise characteristic is obtained. Especially, an effective and superior mixer performance are realized as a filter for image rejection used with a mixer having a radio frequency signal and an image signal comparatively close to each other. Further, the main line 33 is connected only with the parallel-coupled lines arranged in parallel thereto, and therefore the filter width can be greatly reduced for a smaller filter size. The small filter width works effectively especially when the filter is housed in a case in cut-off region to reduce the radiation effect. Also, the parallel-coupled lines 35, 35′ are set to slightly different lengths ℓ₂, ℓ₂′, so that there are two poles of attenuation due to the parallel-coupled lines 35, 35′. This disperses the positions of the poles of attenuation for the filter as a whole in the image signal band, resulting in a uniform amount of attenuation in the image signal band. In addition, a filter with especially superior characteristics is configured, if a length longer than 5/16 and shorter than 7/16 the wavelength of the radio frequency signal is selected as the interval ℓ₀, ℓ₀′ of the parallel-coupled lines.

Claims: A microwave filter comprising: a main line (13) having an input terminal (11) and an output terminal (12), and first, second and third open-ended lines (14, 15, 16), each connected to said main line at equal or nearly equal consecutive intervals l₀, l₀′ (l₀≈l₀′), said lines having lengths of l₁, l₂ and l₃, respectively; characterised in that said first and third lines (14, 16) are open-ended lines arranged and coupled in parallel to the main line, said second line (15) is an open-ended stub arranged in shunt with said main line, the lengths l₁, l₂ and l₃ of said first, second and third lines are selected to be equal to the 1/4 wavelength of a stop-band frequency such that the poles of attenuation thereof are positioned within a stop band, the lengths l₀, l₀′, l₁, l₂ and l₃ are selected to satisfy the conditions l₁ < l₂ < l₀≈l₀′ < 2l₁ and l₃ < l₂ < l₀≈ l₀′ < 2l₃ or to satisfy the condition l₁ = l₃ < l₂ < l₀≈ l₀′ < 2l₁, and the characteristic impedance of at least the first and third lines is selected to be higher than that of the input and output lines (17, 18) connected to the input and output terminals, and the interval l₀, l₀′, between the first, second and third open-ended lines is selected to be longer than 5/16 wavelength of a pass-band frequency but shorter than 7/16 wavelength of said pass-band frequency. A microwave filter comprising: a main line (13) having an input terminal (11) and an output terminal (12), and first, second, third and fourth open-ended lines (14, 25, 25′, 16), each connected to said main line at equal or nearly equal consecutive intervals l₀, l₀′ (l₀≈l₀′), and having lengths of l₁, l₂, l₂′ and l₃, respectively, characterised in that the second and third lines (25, 25′) of equal or substantially equal lengths, l₂, l₂′ are open-ended stubs arranged in shunt with the main line, extending across said main line on both sides of said main line at the same position, the first and fourth lines (14, 16) are open-ended lines arranged and coupled in parallel to the main line, the lengths l₁, l₂, l₂′ and l₃ of the first, second, third and fourth lines are selected to be the 1/4 wavelength of a stop-band frequency such that the poles of attenuation thereof are positioned within a stop band, the lengths l₀, l₁, l₂, l₂′ and l₃ are selected to satisfy the conditions l₁≈l₂≈l₂′ < l₀≈ l₀′< 2l₁ and l₃≈l₂≈l₂′ < l₀≈l₀′ < 2l₃ or to satisfy the condition l₁ = l₃≈l₂≈l₂′ < l₀≈l₀′ < 2l₁′ and the characteristic impedance of at least the first and fourth lines is selected to be higher than that of the input and output lines (17, 18) connected to the input and output terminals. A microwave filter comprising: a main line (33) having an input terminal (31) and an output terminal (32), and first, second, third and fourth open-ended lines (34, 35, 35′, 36), each connected to said main line at equal or nearly equal consecutive intervals l₀, l₀′ (l₀≈l₀′) respectively and having lengths of l₁, l₂, l₂′ and l₃ respectively, characterised in that the second and third lines (35, 35′) of equal or substantially equal lengths l₂, l₂′ respectively are open-ended lines arranged and coupled in parallel to said main lines, the first and fourth lines (34, 36) are open-ended lines arranged and coupled in parallel to the main line, said first and second lines (34, 35) are arranged in corresponding opposed relationship to each other on the opposite sides of said main line, said third and fourth lines (35′, 36) are arranged in corresponding opposed relationship to each other on the opposite sides of said main line, the lengths l₁, l₂, l₂′ and l₃ of the first, second, third and fourth lines are selected to be the 1/4 wavelength of a stop-band frequency such that the poles of attenuation thereof are positioned within a stop band, the lengths l₀, l₀′, l₁, l₂, l₂′ and l₃ are selected to satisfy the conditions l₁≈l₂≈l₂′ < l₀≈l₀′ < 2l₁ and l₃≈l₂≈l₂′ < l₀≈l₀′ < 2l₃ or to satisfy the condition l₁ = l₃≈l₂≈l₂′ < l₀≈l₀′ < 2l₁, and the characteristic impedance of at least the first and fourth lines is selected to be higher than that of the input and output lines (37, 38) connected to the input and output terminals. A microwave filter according to claim 2 or 3, wherein the intervals l₀, l₀′ are selected to be longer than 5/16 wavelength of a pass-band frequency but shorter than 7/16 the wavelength of said pass-band frequency.

Inventors: SAKA HIROSHI, SAKA, HIROSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: H01P1
IPC codes: H01P1