MICROWAVE FILTER

Abstract: Microwave filter formed by one or more transverse partitions mounted behind each other in a waveguide (1), and provided with parallel slots (2) of different lengths. The length of the longest slot is equal to about half the wave­ length of the centre frequency of the pass band, whereas the lengths of the other slots are determined for each par­ tition by the largest possible attenuation in a stop band.

Description: Microwave filter. The invention relates to a microwave filter formed by a transverse partition mounted in a waveguide, which partition is provided with some parallel slot-shaped apertures of different lengths. A microwave filter of this type is known from the British patent specification 649,301. The known filter relates to a polarization filter and therefore the lengths of the slots are not significant. The known filter is only useful for waves which are orthogonally polarized. Microwave filters generally consist of elements which have a capacitive or an inductive behaviour. These elements can be: - A local narrowing of the waveguide (the long or a-dimension smaller), which has an inductive effect. - A local lowering of the waveguide (the short or b-dimension smaller), which has a capacitive effect. - Pins transversely mounted in the waveguide, which has an in ductive effect, and - Transverse partitions with a slot or with a round hole, which has a capacitive and an inductive effect. By mounting two similar elements at a distance of 2 X in a waveguide a resonant cavity is formed. This resonant cavity can be compared with a resonant circuit, so a filter consisting of one circuit. In case several resonant cavities are arranged in a waveguide at a distance of 43 X between them, this involves that certain desirable properties can be attributed to the filter. Filters consisting of inductive and/or of capacitive elements have a band-pass character. A drawback of these filters is that a band-pass behaviour can also occur higher up in the frequency band. This is an undesirable circumstance in many cases when said filters are made use of, e.g. when in a system the transmitting and receiving frequencies have to be separated. When in a receiving system the parasitic pass band is at the transmitting frequency, the entrance of transmission signals into the receiver can be prevented by mounting band-stop filters. The band-stop filters generally used consist of resonant circuits coupled in parallel to a waveguide. These filters are voluminous and making them is costly. The object of the invention is to provide a filter which has on the one hand a band-pass character, e.g. for a receiving band, and on the other hand a band-stop character for a transmitting band. This is achieved because of the fact that the length of the longest slot is equal to about half the wavelength of the centre frequency of the pass band. The longest slot yields the band-pass character with Åa low Q. The short slots have a length of half the wavelength for a frequency higher than the band to be stopped. The entire combination of slots gives a resonance phenomenon at an interjacent frequency, which results in a stop-band attenuation with a maximum of c. 50 dB. The filter according to the invention can often be a very good substitute for the band-stop filters of the conventional type. The use of the filter according to the invention disposes of the drawbacks attended with the conventional filters. A very striking aspect is that the cost of production is considerably lower. The filter according to the invention can be utilized in a special way when the transverse partition forms the wall between two coupled waveguides. Stopping the undesirable overflow of signals can now take place in the coupling area of the connection. The invention will be elucidated below by means of some examples of the utilization of the invention elaborated with the help of the drawing; in which Fig. 1 illustrates an output system at the beginning of a re ceiving circuit; Fig. 2 shows a side-view of a filter consisting of two partitions; Fig. 3,4 show views of the two partitions; Fig. 5 represents a dB-f diagram for the pass band, and Fig. 6 represents a dB-f diagram for the stop band. The output system according to Fig. 1 is part of the waveguide circuit used for Orbital Test Satellite (OTS) experiments. The wall of a round waveguide 1 is provided with four slot combinations 2, to which the waveguides 3 are connected, which in their turn are connected to a receiving system. One end 4 of the waveguide 1 is connected to an aerial; the other end 5 is connected to a transmitter. The part 6 between the slot combinations 2 and the end 5 is made narrow and forms a short-circuit for the received signals. The filter according to Figs. 2 - 4 comprises two partitions 7 and 8. It has a pass-band attenuation of 0.5 dB in the frequency band of 11.35 - 11.8 GHz and a stop-band attenuation of 50 dB in the frequency band of 14.1 - 14.85 GHz. This filter can also be used in the situation according to Fig. 1, in which each slot combination 2 is then formed by a partition 7, and a partition 8 is mounted in each of the four waveguides 3. This configuration is chosen when a broad transmission band has to be stopped. The partition 7 has a long slot 9 and two short slots 10 and 11. The partition 8 also has a long slot 12 and two short slots 13 and 14. The slots 9 and 12 are of the same length. The short slots of each partition are of the same length, but those of the partition 8 are somewhat shorter than those of the partition 7. The distance between the partitions 7 and 8 amounts to 50 mm. The dimensions of the waveguide, those of the slots and the position of the slots can be read in Figs. 3 and 4. The length of the slots 9 and 12 has to do with the pass band. The centre frequency of the band of 11.35 - 11.8 GHz is 11.57 GHz. At this frequency the wave 300 length 13 575 = 25.6 mm Now the length of the slot is 25.6:2 = 12.8 mm. The other dimensions are determined experimentally for the greater part. Fig. 5 shows the connection between the attenuation and the frequency for each of the partitions 7 and 8 (curves 15 and 16) and for the combination of the two partitions (curve 15 + 16)in the pass range of the band of 11035 - 11.8 GHz In this band the attenuation is only very little larger than for each separate partition. Fig. 6 also shows the connection between the attenuation and the frequency for the partitions 7 and 8 separately (curves 17 and 18) and for the combination of the two partitions (curve 17 + 18) in-the range around the stop frequency of 14.551 GHz. It appears that for each separate partition the attenuation at the stop frequency amounts to at least 20 dB, whereas the combination of the two partitions yields an attenuation of 50 dB. As a third possibility of utilizing the invention can be mentioned a band-stop filter with more than two partitions arranged behind each other, in consequence of which every passage in a large stop range will be completely impossible because of the high pass-band attenuation. As already stated the filter according to the invention is, in addition to its better properties, much more attractive than the usual band filters with resonant cavities, which are difficult to make.

Claims: Claims: 1. Microwave filter formed by a transverse partition mounted in a waveguide (1), which partition is provided with some parallel slotshaped apertures (2) of different lengths, characterized in that the length of the longest slot is equal to about half the wavelength of the centre frequency of the pass band. 2. Microwave filter according to claim 1, characterized in that some partitions (7,8) are mounted behind each other, and in that the lengths of the other slots (10,11,13,14) are determined for each partition (7 and 8, resp.) by the largest possible attenuation in the stop band.

Inventors: HULSBERGEN, MARTINUS BAREND
Applicants: STAAT DER NEDERLANDEN (STAATSBEDRIJF DER POSTERIJEN, TELEGRAFIE EN TELEFONIE)
Application year: 1980
Main classification: H01P1
IPC codes: H01P1