Microwave comb-line band-pass filters

Abstract: There is described a comb-line band-pass filter with circular cross section made up of internal rods (A1 to A5) constituting the resonators of the filter and brought to resonance by the use of tuning screws (S1 to S5) and alternating with coupling screws (V1 to V4). In the filter the rods (A1 to A5) are inserted in such a manner that each one is rotated in relation to the preceding one by an angle of 0° < α < 90° while the coupling screws (V1 to V4) are placed at an intermediate angle. For equal electrical characteristics the length of the filter is also a function of the rotation angle of the rods (A1 to A5)

Description: The present invention relates to the field of microwave filters and more specifically to a comb-line band-pass filter. A prevalent if not exclusive application of comb-line band-pass filters is in the accomplishment of microwave branching systems where the requirement for reducing the size and cost of said filters, especially when working in the L (from 1 to 2 GHz) frequency range and S (from 2 to 4 GHz) frequency range wherein the filter structures are developed mainly lengthwise, is growing. It is known that a comb-line band-pass filter is made with a wave guide having a rectangular or circular cross section of a size such that the working frequencies are always less than the cutoff frequency. In the guide is inserted a series of parallel rods arranged in transverse planes and one-eighth wave in length and alternating with coupling screws parallel to the rods. The rods constitute the resonators of the filter and their number depends on the frequency response characteristics of the filter. Each rod is fixed to the guide at one end while at the opposite end a tuning screw is drawn near thereto to create a concentrated capacitive load The tuning screws regulate the tuning frequency while the screws placed between the resonators regulate the coupling and hence the band width, i.e. for greater penetration there is a correspondingly wider band. The magnetic component of the field generated near the rod brings about coupling with the following rod. Coupling depends on the distance between the rods and in particular increases as distance decreases. Narrow bands passing through the filter are obtained with strong decouplings between the rods with the inevitable drawback of lengthening of the structure. A technical solution is known for excessive filter length, e.g. described in the article (or paper) of E.G. Cristal: "Data for partially decoupled round rods between parallel ground planes", MTT Microwave Technique and Technology, May 1968. According to said known solution, to reduce the distance between the rods and hence the length of the filter, between the rods are placed separating baffles which bring about decoupling increasing with their size. The baffles are introduced in slots made by cutting the filter widthwise generally at the coupling screws. Two possible main designs are achieved: (a) with complete baffles consisting of aligned pairs of baffles which leave uncovered more or less narrow bands in the middle of the guide, or (b) with half baffles, consisting of individual baffles rather than pairs, which leave uncovered in addition to the central bands the specular segments not covered by the other baffle. Both of said designs have the drawback of increasing production time and costs because additional manufacturing operations are necessary to cut the guide, construct the baffles, introduce them, position them and weld them in place in the guide. Assembly problems also increase considerably. Indeed the cutting operation considerably weakens the structure at the cuts, especially with complete baffles in which the cut is made through a large part of the cross section. This can lead to axial deformation and bending mainly during welding of the baffles because it involves heating of the guide. Accordingly the object of the present invention is to obviate the above drawbacks and this is accomplished by a comb-line band-pass filter according to claim 1. The coupling screws are placed at an intermediate angle in relation to the tuning screws or the rods. Two variant designs are possible. A first variant calls for the rotation angle to remain unchanged as to modulus but change sign between one rod and the next so as to obtain two alternating rows of parallel rods. A second variant calls for the rotation angle to remain unchanged as to modulus and sign so as to obtain a rod pattern describing a helix on the guide surface. The decoupling between the rods and hence also the length of the filter depend on the value of the rotation angle modulus. To achieve these goals the object of the present invention is a comb-line band-pass filter as described in claim 1 and particularly in the two embodiment variants described in claims 2 and 3. Additional goals and advantages of the present invention will be made clear by the following detailed description of an example of embodiment of one of the variants thereof given only as a nonlimiting example with reference to the annexed drawings wherein:- FIGS. 1 and 2 show longitudinal cross sections along planes L1 and L2 respectively of the comb-line filter which is the object of the present invention, FIG. 3 shows a cross section along plane X1 thereof, FIG. 4 shows a perspective view of a longitudinal cutaway thereof, and FIGS. 5 and 6 show graphs of some characteristic parameters of the filter. With reference to FIGS. 1 and 2 there is shown a longitudinal cross section along planes L1 and L2 respectively of a central portion of wave guide G which permits accomplishment of a comb-line band-pass filter. The two ends of the guide bearing the input and output connections of the filter are not shown because they are of known type. The guide G has a circular cross section X1 as shown in FIG. 3. In FIGS. 1, 2 and 3, to which reference will be made jointly below, the same elements are indicated by the same symbols. In addition in FIGS. 1 and 2 is indicated plane of cut X1 which provides the cross section shown in FIG. 3 while FIG. 3 shows planes of cut L1 and L2 of the longitudinal cross sections shown in FIGS. 1 and 2. A1, A2, A3, A4 and A5 indicate identical rods with a circular cross section. One end of said rods is closed to create a short circuit and is pivoted on the surface of the guide G while the other end, inside the guide, has a recess. S1, S2, S3, S4 and S5 indicate tuning screws which screw in a known manner into the guide and are locked by nuts B1, B2, B3, B4 and B5 respectively at points diametrically opposite those at which the rods are pivoted. The tuning screws, one for each rod, are adjusted in such a manner as to draw near to and optionally penetrate the recess of the corresponding rod end without touching it. The recess in the rods makes adjustment of the tuning screws less critical. Indeed if it were desired to obtain high capacities without a recess the screws would have to be drawn too near to the rods with the risk of touching them. But with the screws able to penetrate the recess the increase in capacity is obtained by increasing the screw surface area facing the recess. V1, V2, V3 and V4 indicate coupling screws which, in the same manner as the tuning screws, screw in a known manner into the guide and are locked by nuts B6, B7, B8 and B9 respectively fixed in holes in the surface of the guide G. FIG. 3 shows two successive rods, e.g. A3 and A2, and their tuning screws S3 and S2 respectively rotated to an angle of 0° < α <90° (e.g. α = 80°) while the intermediate coupling screw, e.g. V2, is rotated to an angle of β = α/2 in relation to the tuning screw S3. The rod A1 is rotated in relation to A2 to an angle equal to -α as is A3 in relation to A4. Thus each rod and hence each tuning screw is rotated in relation to the preceding one by an angle which remains unchanged in modulus and alternating in sign while the coupling screws are rotated by an angle equal to half of the angle between the two adjacent tuning screws. FIG. 4 shows another perspective view of a longitudinal cutaway of the comb-line filter in which the various component parts are indicated by the same symbols used in FIGS. 1, 2 and 3. It can be seen in the above figures that the outlets of the tuning screws on the surface of the guide describe two parallel longitudinal rows (S1, S3, S5 and S2, S4) alternating with an intermediate longitudinal row of coupling screws (V1 - V4). The decoupling between the rods and hence the length of the filter depend on the value of the modulus of the rotation angle α. This result is born out by laboratory tests performed on prototypes with different values of α compared with a filter prototype of known type having parallel rods but no baffles. As a nonlimiting example it was sought to accomplish a filter operating in the frequency range around 1.5GHz. For this purpose a guide G with a circular cross section and an internal diameter D of 26mm and corresponding cutoff frequency of 6.8GHz was chosen. First the prototype of a filter having parallel rods but no baffles was made, then making settings for different band widths B and finding for each value of B an average penetration of the coupling screws in the guide expressed as a percentage of the internal diameter D of said guide: Ip = 100 I/D. I indicates the average length of the portions of the coupling screws inside the guide. Then three prototypes of filters taken from the one with parallel rods, only rotating the rods by angles α of 50°, 80° and 85° respectively, were made and the same measurements were repeated, thus determining new band widths b < B. In this manner the graph shown in FIG. 5 was plotted to establish the relation between the band variation per cent Bp = 100(B-b)/B according to Ip assuming α as the parameter. FIG. 5 shows the experimental points measured and the curves obtained by interpolation for the prototypes accomplished with the rotation angles α = 50°, 80° and 85°. From the curves shown in FIG. 5 is obtained the curve shown in FIG. 6 which, for an average penetration per cent Ip as small as desired, establishes the relation between the band variation per cent Bp and the angle of rotation α. For a predetermined value of α FIG. 6 supplies the parameter B/b which espresses the relation between band B of a parallel rod filter without baffles of length L and the band b of the filter having the same L but rods rotated to angle α. Therefore if L′ <L indicates the maximum allowable length of the filter to be sized, b′ the required through band and B′ the band of the corresponding parallel rod filter of length L′, the following proportionality relation applies: B : b = B′ : b′ (1) From (1) it follows that if b′ is the band actually required the filter with rotated rods must be designed for a virtual band: B′ = b′ (B/b) (2) In (2) B/b may be considered the corrective parameter to be used in sizing the central section of the filter while the design criteria of the input and output couplings remain unchanged. In FIG. 6 it is seen that the parameter B/b increases as α increases so that the inclination between the rods can be increased appropriately until there is obtained an ever greater virtual band B′ and thus ever shorter filtering structures compatible with the maximum longitudinal dimensions required by the design specifications. Analyzing the results, in the filter accomplished as explained above and for a through band B = 12MHz and return loss = 25dB, there is obtained a percentage shortening of length LC as compared with the known filter without baffles and with parallel rods of 10% for α = 50°, 35% for α = 80° and 45% for α = 85°. The above observations also apply according to internal diameter of the guide provided the cutoff frequency of the guide is always at least two octaves above the working frequency. If this were not true there could occur undesired couplings between nonadjacent resonators because of the poor attenuation which the electromagnetic wave undergoes when it propagates inside the guide. The above description makes clear the advantages of making comb-line band-pass filters in accordance with the present invention. In particular there is obtained a strong structure easy to build and not excessively costly. To reduce its length it is thus unnecessary to introduce additional production operations to modify the guide as is the case when cutting to introduce baffles and weld them. There are no particular assembly problems and the structure is not weakened. The length of the filter is no longer a parameter linked exclusively to the electrical characteristics of the filter but becomes a magnitude which can be varied even on the basis of mechanical requirements, appropriately selecting the mutual inclination of the rods. It is clear that numerous variants of the example of embodiment described are possible without thereby exceeding the scope of the innovative principles contained in the inventive idea. In particular the angle of rotation α between the rods can be held constant in modulus and sign to obtain an arrangement of the rods and hence of the tuning and coupling screws in which the outlets on the surface of the guide describe a helix. The coupling screws are still always rotated to an angle equal to half that between the two adjacent tuning screws.

Claims: Comb-line band-pass filter for microwaves consisting of a wave guide with a circular cross section in which there are inserted resonators consisting of rods (A₁...A₅) and tuning screws (S₁...S₅) alternating with coupling means, said resonators and said coupling means being arranged on transverse parallel planes of the guide, characterized in that each resonator (A1, S1 to A5, S5) is rotated in relation to the adjacent one by a constant modulus angle of 0° < α < 90°. Comb-line band-pass filter in accordance with claim 1 characterized in that said angle α is of alternating sign between successive resonators. Comb-line band-pass filter in accordance with claim 1 characterized in that said angle α is constant in sign between successive resonators. Comb-line band-pass filter in accordance with claim 1 characterized in that said coupling means consist of coupling screws and characterized in that each coupling screw (V1 to V4) is placed at an intermediate angle in relation to the adjacent tuning screws. Comb-line band-pass filter in accordance with claim 4 characterized in that said intermediate angle is β = α/2. Comb-line band-pass filter in accordance with any of the above claims, in which said resonators comprise λ/8 rods and tuning screws, each rod being fixed at one end to the guide and there being positioned near to the other end the tuning screw at the axis of the rod characterized in that said rods (A1 to A5) have a recess at the end facing the related tuning screw (S1 to S5) such that the latter may optionally penetrate the recess without touching it. Comb-line band-pass filter in accordance with any of the above claims characterized in that for given characteristic electrical parameters of the filter as said angle α increases the distance between said resonators decreases on the basis of a parameter B/b where b is the through band of the filter with a given α and B is the band which there would be for α = 0.

Inventors: DE STENA NICOLO, MARCONI FRANCO, DE STENA, NICOLO, MARCONI, FRANCO, De Stena, Nicolò
Applicants: SITS SOC IT TELECOM SIEMENS, SIEMENS TELECOMUNICAZIONI S.P.A.
Application year: 1989
Main classification: H01P1
IPC codes: H01P1