диафрагмированные волноводные фильтры / 8f2e2343-cf1c-4507-8508-0ffd032e42b4

Abstract—A novel structure, namely the stubbed waveguide cavity (SWC), is introduced in this paper. The SWC consists of a main TE201 mode cavity which is loaded with a pair of E-plane waveguide stubs. The stubs behave as two embedded half-mode cavities supporting a TM110 and TE011 mode, respectively. Three poles and two transmission zeros can be controlled with high design flexibility by properly dimensioning the various structural design parameters. The SWC is particularly suitable for the implementation of symmetric transmission zeros extremely close to the passband edges. The experimental results of a K-band prototype prove the feasibility of this new waveguide cavity.

Keywords—cavities, elliptic functions, microwave filters, transmission zeros, waveguide.

I. INTRODUCTION

Waveguide components represent the primary choice for microwave engineers seeking the highest performance in terms of power handling and insertion loss. Antenna feed systems operating at the higher portion of the microwave spectrum are indeed often making use of complex waveguide assemblies. Filtering structures, such as multiplexers, diplexers, or more simple stand-alone filters, are the key components to prevent interference, provide noise reduction, and ultimately allow us to share the precious electromagnetic spectrum by ensuring proper transmission and reception on selected frequency channels.

Rectangular or circular cavities operating in certain resonant modes are the basic elements for the design of waveguide filters. In its simplest form, a waveguide cavity provides a single pole of resonance, and several cavities are usually needed to obtain higher order filters with steep transition bands. Waveguide cavities with enhanced capabilities can be obtained by employing established multimode approaches [1]-[3] as well as by adopting the more recent nonresonating mode techniques [4]-[8]. In both cases, a single cavity can provide multiple poles and/or transmission zeros, thus lowering the number of cavities that are required for achieving a certain rejection requirement.

Several cavity structures have been proposed over the years with the aim of realizing an increasing number of poles and transmission zeros. As far as rectangular cavities are concerned, some of the most effective implementations are reported in [3]- [8]: the cavities proposed in [4] and [5] are each capable of generating one pole and one transmission zero; the cavity proposed in [3] can generate an additional pole at the expense of virtually doubling the cavity size; finally, the cavity proposed in [6] and [7] can generate two poles and two transmission zeros at the expense of a more complex arrangement requiring advanced design capabilities.

Fig. 1. Perspective view of a stubbed waveguide cavity (input and output feeding waveguides are also included).

To further enhance the capabilities provided by a single cavity structure, this paper introduces a new type of waveguide cavity, namely the stubbed waveguide cavity (SWC). A total of three poles and three transmission zeros can be generated with a single SWC. By properly dimensioning the various structural parameters, two of these transmission zeros can be controlled with high design flexibility for the implementation of challenging third order filtering functions having a pair of symmetric transmission zeros extremely close to the passband edges. The experimental results of a SWC prototype working at K-band is presented at the end of this paper to validate the proposed cavity structure.

II. BASIC CAVITY STRUCTURE

The SWC consists of a main TE201 mode cavity which is loaded with a pair of E-plane waveguide stubs. The whole cavity structure (including the feeding input and output waveguides) is shown in Fig. 1. The two stubs extend from opposite broad walls of the main cavity (one from the top and the other from the bottom), and they have orthogonal polarizations with respect to each other. The top stub is oriented along the width of the main cavity (x axis), while the bottom stub is oriented along the cavity length (z axis). As is for the structure in Fig. 1, the width of the bottom stub can be conveniently set to be coincident with the main cavity length.

The E-plane stubs behave as two embedded half-mode cavities supporting a TM110 and TE011 mode, respectively, which are in addition to the main cavity TE201 mode. The electric fields of these three modes are depicted in Fig. 2 (Ansys HFSS, Eigenmode solver). The resonant frequencies of the modes can be controlled almost independently by the

Fig. 2. Electric fields of the SWC modes: (a) TM110 in the top stub; (b) TE201 in the main cavity; (c) TE011 in the bottom stub.

dimensions of certain structural parameters. More specifically, the TE201 mode resonant frequency is controlled by the width and length of the main cavity, while the resonant frequencies of

the TM110 and TE011 modes are essentially controlled by the length (along the y axis) of their respective stubs (around

quarter-wave long for half-mode cavities).

In order to avoid unnecessary intra-coupling mechanisms and to realize a specific coupling scheme among the various modes, the locations of the stubs with respect to the main cavity are properly preset. The top stub is located at the center of the main cavity, where for symmetry reasons no coupling occurs

between the TM110 mode in the stub and the TE201 mode in the main cavity. On the other hand, the TE011 mode in the bottom

stub is meant to be coupled with the TE201 mode in the main cavity. In order to limit and properly control this coupling, the

bottom stub is always located at around quarter-wave distance from the main cavity center. Finally, because of the orthogonal

polarization of the stubs, the TM110 and the TE011 modes are always isolated with respect to each other.

The SWC can be excited at the input and output by means of simple inductive irises as shown in the structure of Fig. 1. The location of the input/output irises with respect to the main

cavity is properly designed so that the TE201 and the TM110 modes are both excited by the feeding waveguides fundamental

Fig. 3. Equivalent topology and modal coupling scheme of the SWC.

mode. To this purpose, the irises are always offset with respect to the cavity center. Moreover, in order to avoid a potential spurious coupling from the input/output to the TE011 mode, the offset of the irises is always directed away from the side where the bottom stub is located.

The resulting coupling scheme is depicted in the topology of Fig. 3. Observe that the negative coupling on the left side of the TE201 mode represents the 180 degrees phase difference at the output port between this mode and the TM110 mode. The direct coupling between the input and output (dashed line between source and load) represents the contribution of the nonresonating TE10 mode which is bypassing the other modes inside the cavity. In contrast with all the other coupling coefficients, such an input-to-output coupling must be considered as a spurious coupling whose value cannot be exploited as an operative degree of freedom for design purposes. As a result, although the topology in Fig. 3 and the corresponding SWC structure do generate filtering functions with three poles and three transmission zeros, only two of the transmission zeros can be properly controlled with high design flexibility. In fact, two transmission zeros can be located extremely close to the sides of the passband edges by properly dimensioning prescribed structural parameters, while the third transmission zero will always fall above the passband at a relatively far distance depending on the frequency, bandwidth, and other structural parameters upon which only a limited control can be exercised.

The main structural design parameters for coupling control are the dimension and location of the input and output irises, as well as the location of the bottom stub. More specifically, the dimension of the input iris is used to increase or decrease the coupling KS1 and KS2 (both at the same time), while the offset of the iris with respect to the main cavity center is effectively used to change the ratio between KS1 and KS2. The same considerations hold true for the output iris and the coupling K1L and K2L. The coupling K23 is efficiently controlled by adjusting the location of the bottom stub: as the stub location approaches quarter-wave distance from the main cavity center, K23 tends to zero (the coupling between the TE201 and the TE011 vanishes due to symmetry reasons); on the other hand, as the stub gets closer to the cavity center or closer to the cavity side wall (past the quarter-wave distance) K23 increases significantly.

Fig. 4. Coupling matrix response and HFSS simulation of the designed SWC.

III. RESULTS

A cavity structure working at f0=21.5 GHz with 1.33% fractional bandwidth and a pair of very close transmission zeros has been designed and optimized in a standard WR42 according to the following normalized coupling matrix:

As previously discussed, MSL is not an operative degree of freedom, and therefore its value is usually preset according to a prior full-wave simulation where the actual input-to-output coupling occurring in the structure is computed.

The coupling matrix response along with the HFSS simulation of the structure are shown in Fig 4. The agreement between the curves, especially at the passband region where the two close transmission zeros are located, validates the proposed coupling scheme. As expected, a third transmission zero is also generated at a relatively far distance above the passband.

The SWC structure has been easily manufactured using a conventional H-plane split-block design (silver plated aluminum) as shown in Fig. 5. The experimental results are presented in Fig. 6. The measured insertion loss is 0.125 dB at f0, which corresponds to an experimental Q-factor of about 4100. This number is consistent with a typical 60-70% efficiency with respect to the theoretical unloaded Q-factor,

which for this design would be 6600 for the TE201 and 5900 for the TM110 and TE011 modes.

IV. CONCLUSION

A new type of waveguide cavity named SWC has been introduced and experimentally validated. The SWC can be considered as a basic building block for the realization of higher order pseudo-elliptic filtering functions. Future investigations must be devoted to the design and implementation of actual filters employing multiple SWCs or a combination of SWCs with conventional waveguide cavities.

Fig. 5. Manufactured prototype: main cavity and bottom stub are machined in the housing (right); top stub is machined in the cover (left).

Fig. 6. Measurement and HFSS simulation of the manufactured prototype.

REFERENCES

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