диафрагмированные волноводные фильтры / 94b2ba89-c7eb-4645-86e5-87e11b21f71c

L. Accatino and G. Bertin

AC Consulting

I-10098 Rivoli (TO) – Italy

Email: ac@acconsult.it

Abstract—The paper presents a novel approach for realizing a compact filtering structure composed of single-mode low-loss rectangular waveguide cavities able to implement high-selectivity transfer functions of elliptic type. The creation of transmission zeros is obtained by disposing the cavities in a suitable geometrical configuration and exploiting the properties of the selected resonant mode (TE102). The proposed approach is employed in the design of a four-pole elliptic filter at Ka-band. This is the basic building block for the extension to a 6-pole filter with 2 transmission zeros that can be used in low-loss high-power and high-selectivity diplexers required by modern multibeam payload of last generation satellites operating at Ka-band and above.

Index Terms—Bandpass filters, elliptic function filters, modal cross-coupling, Transmission Zeros (TZs)

I. INTRODUCTION

Modern telecommunication satellites using multibeam antennas at Ka-band and above, require many filtering devices possessing high compactness, high power handling and stringent selectivity specifications. This is particularly the case for diplexers that are used in several applications, such as frequency combination and splitting and also antenna sharing.

A typical example of these applications can be found in in the specification of a Ka-band high-power diplexer contained in a recent ITT (Invitation to Tender) issued by the European Space Agency [1].

When the selectivity specification is stringent it cannot be satisfactorily achieved by all-pole filters, because the high number of poles required leads to insertion loss values that cannot be tolerated. It is therefore necessary to resort to elliptic or pseudo-elliptic filter transfer functions that at the same time allow a reduction of the number of poles (lower losses) and an increase of the selectivity.

Various solutions are shown in the literature for designing waveguide filters with transmission zeros, either exploiting the cross-coupled configuration [2-3] or resorting to the extractedpole synthesis [4]. It should be however remarked that the considered application requires specialized filtering types, where, in addition to selectivity, also compactness and light weight represent a primary requirement. A solution often adopted in space communications is represented by dual- (or multi-) mode filters [5]. This type of filter typically requires a large number of screws for the proper alignment of the frequency response. Unfortunately, such screws may introduce some drawbacks like power handling reduction and PIM gen-

G. Macchiarella

Politecnico di Milano

I-20133 Milano – Italy

Email: giuseppe.macchiarella@polimi.it

eration. Filtering structures not requiring a post-alignment process (tuning-less) would then be preferable.

More recently, new types of rectangular-cavity filters including transmission zeros and making use of the well-known trisection or triplet concept, have been presented [6-11]. The trisections are implemented using the technique of side placing or vertically staking the required bypass cavity.

In this paper we present a novel arrangement of singlemode rectangular cavities where, unlike as in previously cited works [10], [11], the cavities are not only vertically staked, but are properly arranged in orthogonal position in the upper layer, and H-plane displaced in the bottom layer (patent pending). This specific configuration allows the implementation of a quadruplet possessing a pair of symmetrical transmission zeros, one placed below the passband and one above. The resulting assembly is characterized by a high degree of compactness. Moreover, the proposed cavity arrangement lends itself to a tuning-less realization (provided that a sufficiently accurate fabrication process is adopted).

The proposed filtering solution has been validated through the design of a 4-pole filter with 2 transmission zeros, in quasi in-line configuration; it is also shown a possible higher-order configuration (6-pole, 2 zeros) using the proposed solution, that can be employed in high selectivity diplexers [1].

II. DESCRIPTION OF PROPOSED APPROACH

The rectangular cavities all resonate with the TE102 mode and are arranged in such a way to obtain a common wall between two cavities, where the magnetic fields on opposite sides of the wall circulate in opposite directions (Fig. 1a,b).

Fig. 1 a) 3D view of the 4-pole filter assembly b) Routing scheme of the filter; solid lines represent the main couplings and the dashed line is the negative cross coupling determining a pair of symmetrical zeros

Opening a coupling window in this wall will cause a negative coupling between the corresponding resonators, which is only due to the geometrical configuration. The negative coupling enables the implementation of transfer functions with transmission zeros symmetrically placed both above and below the filter passband.

Unlike previously cited works, that consider either H-plane or E-plane configuration, our filtering structure exploits simultaneously both these configurations in order to maintain a high degree of compactness.

The basic elliptic four-pole assembly here presented can be easily extended to a 6-pole pseudo elliptic filter with 2 transmission zeros by adding a further cavity in cascade with the input and output cavities of the four-pole structure.

The configuration of cavities and the coupling mechanism between the resonators are examined in further details in the next section.

III. BASIC 4-POLE RECTANGULAR CAVITY ARRANGEMENT

The four-pole cavity arrangement is visible in the 3Dpicture in Fig. 1a. The first cavity (lower left) is coupled to the input guide through an ordinary double-centered input widow.

The second cavity is placed on top of the first cavity and orthogonally turned left (upper left in Fig. 1a). The coupling between the first and second cavity (k12 in Fig 1b) is obtained through a double-iris configuration.

The two irises are symmetrically placed with respect to the longitudinal axis of the first cavity in order to avoid the excitation of unwanted modes and for the same reason are also are symmetrically placed with respect to the longitudinal axis of the second cavity.

The third cavity is placed side-by-side with the second cavity (upper right in Fig. 1a) and is coupled to it through a side window.

The fourth cavity is placed below the third cavity in a displaced position with respect to the first cavity and externally aligned with the end of the third cavity. The third and fourth cavities are coupled by the same double-iris configuration described for the coupling between the first and second cavities and work in the same manner. Finally, the fourth cavity is coupled to the output guide through an ordinary doublecentered input widow.

As pointed out above, the simple addition of a further cavity at input and output ports of the 4-pole leads a 6-pole 2-zero pseudo-elliptic filter, depicted in Fig. 2, and usable as channel filter of a high-selectivity diplexer for space communications [1].

In order to have a better understanding about how the negative coupling is achieved the magnetic field inside each cavity is examined in more details. Reminding that the resonant mode is the TE 102, a couple of loops circulating in opposite directions represents the magnetic field behavior.

A picture of the magnetic field into all four cavities is reported in Fig. 3, where for better clarity the upper and lower cavities are separately shown.

Fig. 2 3D view of a 6-pole 2-zero pseudo-elliptic filter obtained from the basic 4-pole building block

The numbers represent the resonators order, the letters A – A and B – B remind the overlapped sections where there is the dual-iris coupling and the coupling coefficients, as shown in Fig. 1b, are indicated.

Fig. 3. Magnetic coupling mechanism

Starting from resonator 1 and supposing that the first magnetic loop excited by the input coupling window is rotating counterclockwise, the second loop (A section into lower cavi-

ty 1) is rotating clockwise. This clockwise rotation is coupled by the dual-iris arrangement (k12) into the upper A section of resonator 2 and results in a counterclockwise rotation in the left part of resonator 2.

The side-coupling slot (k23) between resonators 2 and 3 maintains the same direction of the fields on each side of the slot, resulting in a clockwise rotation in the left B section of resonators 3 and a counterclockwise rotation in the right part of the same resonator.

This B section of resonator 3 overlaps the B section of resonator 4, so that the same clockwise rotation is maintained in the lower B section of resonator 4. The rotation in the upper part of resonator 4 is obviously counterclockwise.

It can be now observed that the B section of resonator 4 and the A section of resonator 1 share a common portion of wall, into which we can cut a slot obtaining a negative coupling (k14), since the magnetic fields on each side of the slot rotate in opposite directions.

Though being completely different owing to the singlemode rectangular cavities used and configured, the way of achieving a negative coupling by a geometrical arrangement of cavities is conceptually similar to that developed many years ago for dual-mode circular filters [2]

The assembled filter has been subjected to a further optimization cycle in order to achieve a satisfactory level of return loss.

The computed S-parameters, transmission and reflection, are reported in Fig. 4 and the group delay is shown in Fig. 5

The bandwidth of the optimized filter results slightly lower than the design value, being about 210 MHz; the center frequency has been satisfactorily achieved.

This filter is currently under manufacturing; the measured performance will be presented at the Conference.

IV. DESIGN OF A FOUR-POLE TEST FILTER

A filter with the 4 cavities arranged as above described (fig. 1a) has been designed for validating the proposed solution. The center frequency and bandwidth are respectively 19.82 GHz and 220 MHz; the minimum return loss in the passband is 22 dB; the transmission zeros are placed in order to get at least 24 dB in the stopband.

The four intercavity couplings of the filter have been separately optimized using the HFSS simulation tool and input/output couplings have been accurately designed using mode-matching techniques in order to fit the requirements.

Fig. 4 Computed transmission and reflection of the four-pole elliptic filter

Fig. 5 Computed group delay response of the four-pole elliptic filter

V. CONCLUSION

A novel assembly for quasi-elliptic filters in rectangular waveguide has been presented. The proposed configuration is particularly suited for satellite applications, where reduced overall size and light weight are key factors in the selection of the most convenient filtering structures. From this point of view, even if the proposed configuration adopts single-mode cavities, the optimized arrangement may be comparable (in terms of footprint) with the classical solution based on dualmode cavities.

Also the achievable unloaded Q of the selected mode (TE102), around 7500, is not too far from what can be achieved with dual-mode cavities. Finally, the proposed configuration makes it possible a very accurate modeling (e.g. using modematching techniques), opening the way to a possible tuningless implementation.

REFERENCES

[1]ARTES 5.1 - Lightweight and Compact Diplexer for Multibeam Payloads” ESA-ESTEC Document AO/1-7447/13/NL/EM (ITT)

[2]A.E. Williams “A four-cavity elliptic waveguide filter” IEEE Trans. Microw. Theory Tech., vol. 18, no. 12, pp. 258–265, Dec. 1970.

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