RU2806696C1 - Waveguide ultranarrow microwave filter - Google Patents

Abstract

FIELD: radio engineering.

SUBSTANCE: relates to the filters. The waveguide filter contains a regular section of a rectangular waveguide, an inductive diaphragm and a cylindrical inductive pin in the center of the waveguide, as well as two additional cylindrical inductive pins located between the central pin and the ends of the waveguide symmetrically with respect to the central pin. All three pins are located on the same axis along the direction of propagation of the main electromagnetic wave of the rectangular waveguide, the radius of the central pin is greater than the radii of the peripheral pins, and the distance from one of the side walls of the rectangular waveguide to the center of each pin is less than the same distance to the other side wall. In addition, all three pins have rectangular ribs, regular along the height of the waveguide and located along the longitudinal axis passing through the centers of the pins, and the central pin has two symmetrical ribs along this axis, and the peripheral pins have one rib each located opposite the first or second rib of the center pin, respectively.

EFFECT: reduction of the longitudinal size of the filter to a value of 2.3λ, providing a relative bandwidth of less than 0.1% and improving the matching in the bandwidth, as well as the ability to select the center frequency without tuning elements.

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Description

The invention relates to devices for ultra-narrowband filtering of electromagnetic (EM) signals in the microwave range and is intended for use in radio-electronic systems of terrestrial and space communications, as well as satellite navigation. The invention has a high level of reliability when operating under conditions of strong external influences and can be used, for example, as part of signal multiplexing devices.

In the output multiplexers of onboard radio-electronic complexes of satellite communication and global positioning systems, microwave filters are most often used, made on standard rectangular waveguides (RW) or on volumetric rectangular or cylindrical resonators with inhomogeneities in the form of diaphragms, metal pins or ribs with tuning elements [microwave -filters and multiplexers for space communication systems / Ed. V.P. Meshchanova - M.: Radio engineering, 2017]. Moreover, they can have both waveguide and coaxial coupling elements with external microwave circuits. The presence of waveguide elements for input and output of electromagnetic (EM) energy is the most important factor for the successful use of such microwave filters in output multiplexers of space communication systems. Several technical solutions for such highly selective filters are known.

For example, the design of a microwave filter with waveguide coupling elements is known [US Patent No. 5012211. Low-loss wide-band microwave filter / F.A. Young, R.K. Rikimaru, 1991], implemented in the form of a cascade connection of several cylindrical resonators with metal diaphragms between them having a cross-shaped aperture. As shown in [Cogollos S., et al. A systematic design procedure of classical dual-mode circular waveguide filters using an equivalent distributed model // IEEE Transactions on Microwave Theory and Techniques. 2012. V.60. N 4. P. 1006-1017], the two-cavity design of this filter demonstrates a passband of 27 MHz near the center frequency of 10.5 GHz. Although such a filter has a high quality factor, to adjust its amplitude-frequency response (AFC), it is necessary to use special metal pins provided in its design. In addition, to match cylindrical resonators with waveguide coupling elements, additional resonant diaphragms are required.

Another design of a narrowband X-band filter based on a standard PRV with cross-sectional dimensions ( a × b = 23 × 10 mm, where a and b are the dimensions of the wide and narrow waveguide walls, respectively) described in [Shang X., Lancaster MJ, Dimov S. Microwave waveguide filter with broadside wall slots // Electronics Letters. 2015. V.51. N 5. P. 401-403] with six inductive diaphragms and a longitudinal slot 2 mm wide, cut in the wide wall of the PrV, has a relative bandwidth of 2%. This filter is simple to implement and has good electrodynamic characteristics, but when transmitting high-power signals, spurious radiation may occur due to the longitudinal slot, which requires additional shielding of the devices to prevent.

Another structure of a narrowband microwave filter for output multiplexers with a relative bandwidth of 0.47% and a voltage standing wave ratio VSWR=1.9 and insertion losses of the order of 1 dB at a center frequency of 11.483 GHz is described in [Qian L., et al. A narrowband 3-D printed invar spherical dual-mode filter with high thermal stability for OMUXs // IEEE Transactions on Microwave Theory and Techniques. 2022. V.70. N 4. P. 2165-2173]. Despite its compact size and very narrow bandwidth, the filter does not provide the required level of matching and loss.

Narrowband filter on PRV WR90 ( a × b=23 × 10 mm) 68 mm long with several cylindrical pins of the same radius developed for the X-band frequencies in [Tomassoni S., Sorrentino R. A new class of pseudo-elliptic waveguide filters using resonant posts // IEEE/MTT-S International Microwave Symposium Digest. 2012. Montreal, Canada. P. 1-3] with a center frequency of 8.5 GHz showed a matching level of VSWR = 1.1 and insertion loss of 0.4 dB, as well as a relative bandwidth of 2.3%. At the same time, some of the pins are capacitive, which limits the level of operating power due to the risk of breakdown in the gaps formed by the waveguide wall and the pins.

The structure of a waveguide bandpass filter and a number of its modifications with a central frequency of 10 GHz proposed in [Zhao P., Wu K. Waveguide filters with central-post resonators // IEEE Microwave and Wireless Components Letters. 2020. V.30. N 7. P. 657-660] includes three resonators based on the standard PrV WR90, one of which has a capacitive rectangular pin formed by inductive diaphragms. Having compact dimensions x × y × z = 41 × 60 × 10 mm and a bandwidth of 200 MHz, this filter, however, does not have the necessary electrical strength to transmit powerful microwave signals due to the presence of a capacitive pin in its design. In addition, the 200 MHz bandwidth does not allow it to be classified as a highly selective filter.

The structure of an ultra-narrowband waveguide filter is known [Fedi G. et al. Design of cylindrical posts in rectangular waveguide by neural network approach // IEEE Antennas and Propagation Society Symposium Digest. 2000. Salt-Lake City. USA. P. 1054-1057], implemented on a PrV with two inductive pins of large radius, oriented along the longitudinal axis of the waveguide and shifted to one of its side walls. Being small in size, such a filter allows one to obtain one high-Q resonance, which forms its frequency response, but to adjust the central frequency, all its internal dimensions have to be changed.

The closest to the claimed device can be considered a five-cavity bandpass filter on a standard waveguide WR112 ( a × b = 28.5 × 12.2 mm) with metal inserts in the E-plane [Microwave filters and multiplexers for space communication systems / Ed. V.P. Meshchanova - M.: Radio engineering, 2017]. The filter is designed for transmitting high-power signals in the frequency range 7949.5...7535 MHz (relative bandwidth 0.5%) as part of a three-channel output multiplexer. The signal reflection in the passband does not exceed 17.8 dB, and the signal attenuation is 1.7 dB. The total length of the filter is 190 mm. To ensure the necessary electrodynamic characteristics, the prototype design provides for the presence of adjustment elements in the form of metal pins. The main disadvantages of the prototype are: the large longitudinal size of the filter, amounting to 4.57λ, where λ is the wavelength at the central frequency, high reflection in the passband (VSWR = 1.3), high level of signal attenuation and the presence of tuning elements.

The objective of the invention is to create a compact waveguide microwave filter without adjustment elements with an improved level of matching and transmission of high-power microwave signals in a narrower frequency band.

The technical result of the claimed invention is to reduce the longitudinal size of the filter to a value of 2.3λ, provide a relative bandwidth of less than 0.1% and improve matching in the passband, as well as the ability to select the center frequency without adjustment elements.

This task is achieved by the fact that in a waveguide filter, which includes a regular section of a rectangular waveguide, an inductive diaphragm and a cylindrical inductive pin in the center of the waveguide, as well as two additional cylindrical inductive pins located between the central pin and the ends of the waveguide symmetrically relative to the central pin. According to the proposed solution, all three pins are located on the same axis along the direction of propagation of the main electromagnetic wave of the rectangular waveguide, the radius of the central pin is greater than the radii of the peripheral pins, and the distance from one of the side walls of the rectangular waveguide to the center of each pin is less than the same distance to the other side wall. In addition, all three pins have rectangular fins, regular along the height of the waveguide and located along a longitudinal axis passing through the centers of the pins, with the central pin having two symmetrical ridges along this axis, and the peripheral pins each having one ridge located opposite the first or second fin the central pin accordingly.

The proposed set of solution features allows us to excite two high-Q oscillations and form a Chebyshev frequency response corresponding to a second-order ultra-narrowband filter with low reflection losses in the passband and high losses in the reflection band. In this case, the radius of the central pin can be determined by the ratio: 0.34 a ≤R≤0.36 a , where a is the size of the wide wall of the PrV, and the radius of the peripheral pins - by the ratio: 0.28 a ≤r≤0.29 a . The distance between the axes of the peripheral pins along the direction of propagation of the electromagnetic wave PrV can be: 0.651≤s≤0.75Z, L is the length of the filter. The width of each of the two elements of the inductive diaphragm in the direction perpendicular to the direction of wave propagation can be determined from the relationship: 0.2 a ≤h≤0.22 a . The distance from the side wall of the waveguide to the line on which the axes of all three inductive pins with radii r and R are located can be the value d, determined from the ratio: 0.52 a ≤d≤0.54 a . In general, the metal ribs of all three pins can be made of different sizes: w ₁ ≠ w ₂ ≠ w ₃ , and determined from the relations:

The diaphragm can be made with a thickness t, determined from the ratio: 0.03≤t≤0.04.

The present invention is illustrated by drawings. In FIG. 1 shows the three-dimensional structure of the filter, and FIG. 2 - filter dimensions in the xz plane. Fig. 3 shows one filter design with hollow cylinders as inductive pins instead of solid cylinders.

Reference numbers in FIG. 1 are marked:

1 - regular segment PrV;

2 - central inductive pin;

3 - metal ribs of pins;

4 - peripheral pins;

5 - metal ribs of pins;

6 - inductive diaphragm.

Letter designations in Fig. 2:

r is the radius of the peripheral pin;

w is the width of the pin edge;

l is the length of the pin edge;

h is the length of the inductive diaphragm; t is the width of the inductive diaphragm;

d is the distance from the narrow wall of the waveguide to the center of the metal pins;

L - filter length;

S is the distance between the centers of the peripheral pins;

a is the length of the wide wall of the waveguide.

The waveguide filter includes a regular section of a rectangular waveguide 1, a central inductive pin 2 with metal fins 3, two peripheral inductive pins 4 with metal fins 5 located on the same axis with the central inductive pin 2, and an inductive diaphragm 6. In this case, the peripheral inductive pins 4 are made identical and located on the same axis along the direction of propagation of the main wave of the PrV with a central inductive pin 2 at a distance from the side wall not equal to half the size of the wide wall of the rectangular waveguide. The central inductive pin 2 and the inductive diaphragm 6 are located in the center of the PrV. The central inductive pin 2 has two metal fins 3 symmetrical relative to its vertical axis and located in line with the peripheral pins 4, each having one metal fin 5.

The inventive device operates as follows. At the input of the filter, the main wave Hu of rectangular waveguide 1 propagates, which, after multiple reflections from inhomogeneities in the form of inductive pins 2 and 4, as well as inductive diaphragm 6, arrives at the output, exciting two oscillations in the filter, the resonances of which form the Chebyshev frequency response. In this case, to adjust the central frequency of the filter, it is enough to make ribs with dimensions determined by the ratios indicated earlier.

To confirm the achievement of the technical result, numerical modeling was carried out using the finite element method of the proposed filter based on the standard WR112 waveguide ( a = 28.5 mm, b = 12.6 mm) and the geometric parameters of the device were established, determined by the relations: r = 0.2877 a , R = 0.3509 a , s=0.7L, L=2.807 a , h=0.2105 a , d=0.5263 a , s=1.965 a , t=0.035a (Fig. 4). In this case, the wavelength at the central frequency: l = 1.2208a, and the longitudinal size of the filter along the z axis was: L = 2.3L In this case, we considered a special case of a filter design whose inductive pins have no ribs: w ₁ =w ₂ =w ₃ =0, As a result of numerical analysis, it was shown that such a filter provides matching in a relative frequency band of 0.06% (5.17 MHz) at a VSWR level of 1.17 and attenuation losses no worse than 0.9 dB.

Finite element modeling of another modification of the filter of the same size with metal fins on inductive pins: w _1…3 =0.03509 a , showed (Fig. 5) an increase in the central frequency by 82.9 MHz. Moreover, this filter demonstrates a maximum value of VSWR = 1.12 in the passband, the relative value of which is 0.059%, and the attenuation at the central frequency is no worse than 0.96 dB, that is, its electrodynamic parameters remain practically unchanged.

Thus, the claimed technical solution is characterized by improved electrodynamic characteristics compared to the prototype, provides high reliability during operation under the influence of strong external influences and transmission of high-power microwave energy in an ultra-narrow frequency band, and also has a simple design without adjustment elements and is more compact than the prototype has dimensions.

Claims (4)

1. A waveguide ultra-narrowband microwave filter, including a regular section of a rectangular waveguide, a central inductive pin with two rectangular fins, two peripheral inductive pins, each with one fin, oriented along one line in the direction of propagation of the electromagnetic wave in the waveguide, symmetrically relative to the central pin, and a central inductive diaphragm, characterized in that the radius of the central inductive pin is greater than the radii of the peripheral pins, the distance from one of the side walls of the waveguide to the center of each pin is less than the same distance to the other side wall, and the ribs of the peripheral pins are located opposite the ribs of the central pin. 2. Waveguide ultra-narrowband microwave filter according to claim 1, characterized in that all three inductive pins included in its design are made in the form of hollow cylinders. 3. Waveguide ultra-narrowband microwave filter according to claim 1, characterized by the absence of metal fins of all three inductive pins included in its design. 4. Waveguide ultra-narrowband microwave filter according to claim 2, characterized by the absence of metal fins of all three inductive pins included in its design.