диафрагмированные волноводные фильтры / a7eaf17e-1a0d-47d1-b300-6b82fe4f3c6d

Waveguide filters using T-shaped resonators

D. Budimir, O. Glubokov and M. Potrebic´

An enhanced waveguide E-plane bandpass filter using T-shaped resonators is presented. Characteristics of the proposed filter structure are analysed and investigated. The proposed filter with 4.5 % bandwidth is designed at 9.45 GHz, fabricated and tested. Its performance is evaluated through measurements, the experimental results obtained show that the proposed bandpass filter structures improve selectivity and size compared with those obtained using a conventional E-plane filter structure. A size reduction of more than 12 % was achieved and selectivity performance improved by more than 60 dB.

Introduction: Waveguide bandpass filters are generally preferred and are used extensively for transmitter/receiver front-end applications when lowloss and high power handling is required. Filter structures with sophisticated transfer functions such as elliptic and pseudo-elliptic filters are used and enable realisation of very sharp rejection skirts and are also relatively compact. They are based on well-established techniques such as the use of cross-couplings between non-adjacent resonators [1], use of the extracted pole cavities [2], use of the non-resonating node (NRN) concept [3], and frequency-dependent couplings [4].

These entire filter structures require an increase of waveguide transverse sections with respect to the conventional direct-coupled cavity filter. When waveguide filters require to maintain a uniform rectangular section, several solutions can be used [4–8], however, to fabricate the filter in separate sections. Ridged waveguide structures of various types have been a favourite topic for researchers, and are currently enjoying renewed interest with very interesting results. E-plane technology together with the ridged waveguide offer a very convenient way of realising an E-plane ridged waveguide structure. In this Letter, we therefore present a solution based on a replacement of the homogeneous rectangular waveguide section in the resonators of E-plane filters with a single T-shaped strip introduced in the fin. We demonstrate through experiment and simulation a third-order bandpass filter. Finally, we propose a compact version of this type of filter.

Proposed filter structures: The layout of the proposed filter configuration is shown in Fig. 1. The filters are constructed on a Teflon (PTFE) substrate with 1r ¼ 2.1, thickness h ¼ 0.116 mm and metallisation thickness t ¼ 0.035 mm. The bandpass component configuration is similar to the standard direct-coupled half-wavelength E-plane resonator filter, but instead of having a homogeneous waveguide of length Lresi between the two septa of length di (i ¼ 1. . .4), a single T-shaped waveguide of length Lrwg1 forms the resonant section.

Fig. 1 Layout of proposed bandpass filter

Fig. 2 Unit cell and its coupling scheme

The structures reveal significant size reduction compared with the conventional E-plane filters, as well as stopband improvement. This can be derived from interaction of the T-shaped resonator and conventional E-plane resonator, which results in obtaining two widely separated resonant modes and transmission zero between them. The feature can be

most easily understood by studying the unit cell containing the E-plane resonator and an embedded T-shaped strip introduced in the fin, which is shown in Fig. 2 with the corresponding coupling scheme.

In Fig. 2, resonators 1 and 2 represent the E-plane resonator and the T-shaped resonator, respectively. The T-shaped ridge is considered as a l/4-type stepped-impedance resonator grounded to the top wall of the waveguide. The coupling scheme of the structure can be represented as the corresponding 4-by-4 coupling matrix. Performance of the doublet in the frequency domain can be characterised using the approach presented in [9]. Taking into account the geometrical symmetry of the unit cell (i.e. MS1 ¼ M1L and MS2 ¼ M2L), and assuming for simplicity of analysis that the resonance frequency of resonator 1 is equal to zero (i.e. M11 ¼ 0), the positions of poles VP and transmission zero VZ of the structure can be calculated analytically, in terms of lowpass prototype normalised frequency, as:

From (1) it is evident that the gap between the two resonant modes depends on the self-resonance frequency of the ridge and the coupling coefficient between the E-plane resonator and the T-shaped ridge, and strong coupling M12, regardless of the sign, is required to create a wide gap between modes. Adjusting the resonance frequency of the ridge, it is possible to move the lower resonance to the left-hand side. Equation (2) suggests that the position of the transmission zero depends on the external couplings of the resonators. Owing to the distant position of the ridge from the source and load, it can be concluded that the value of MS2 tends to zero; therefore the position of the transmission zero is also in the vicinity of zero in terms of the lowpass prototype frequency.

Simulation and experimental results: To demonstrate improvement in both size and selectivity, a three-resonator X-band conventional E-plane bandpass filter and third-order proposed waveguide bandpass filter have been designed. The initial and final dimensions (see Table 1) of the designed filters have been obtained with the aid of full-wave simulators (HFSS and CST Microwave Studio). Fig. 3 shows the comparison between the performances of a conventional E-plane filter and a proposed filter. The improved selectivity of the proposed filter is evident from Fig. 3. To validate the argument, a threeresonator X-band waveguide bandpass filter has been fabricated and tested.

Fig. 3 Comparison between performance of proposed filter and conventional E-plane filter

Table 1: Dimensions (in millimetres) of conventional and proposed filters

Fig. 4 shows the simulated and measured responses of the designed filter. The measured insertion loss at centre frequency is 1.6 dB. Measured return loss in the passband better than 19 dB is achieved.

Fig. 4 Comparison between simulated and measured response for fabricated filter

To demonstrate the compactness of the proposed filter structure, a comparison with an equivalent structure containing a conventional E-plane bandpass filter with the same filter order, passband and ripple, is made. The total length of the conventional E-plane and proposed filter are 66.41 and 58.40 mm, respectively. A size reduction of more than 12 % is achieved.

Conclusion: A novel waveguide E-plane filter using a T-shaped resonator with size reduction and improved selectivity is presented. Improvement has been achieved upon loading the resonators of standard E-plane filters with a T-shaped strip introduced in the fin. The structure is compatible with the split block housing and metal insert E-plane technology, thus maintaining low cost and mass-producible characteristics. A size reduction of more than 12 % has been achieved. Experimental and simulation results are presented to validate the argument.

Acknowledgment: This work is partly supported by the Serbian Ministry of Science.

# The Institution of Engineering and Technology 2011

20 October 2010

doi: 10.1049/el.2010.2958

D. Budimir and O. Glubokov (Wireless Communications Research Group, School of Electronics and Computer Science, University of Westminster, 115 New Cavendish Street, London W1W 6UW, United Kingdom)

E-mail: d.budimir@wmin.ac.uk

M. Potrebic´ (School of Electrical Engineering, University of Belgrade, Bulevar kralja Aleksandra 73, 11120 Belgrade, Serbia)

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