Compact Low-Pass Waveguide Filter with Wide Rejection Band
Arregui, I¹; Arnedo, I¹; Lopetegi, T¹; Laso, M¹; Marcotegui, A^2
1Public University of Navarre; ²CONATEL, S.L.

In order to achieve low-pass waveguide filters with spurious bands removed up to third harmonic suitable for the front-end of current communication satellites, several techniques have been developed such us waffle-iron waveguide low-pass filters [1] or classical high power E-plane corrugated low-pass filters cascaded with Bragg reflectors. The latter was recently proposed by the authors in [2] and has the advantage of allowing simultaneously high power handling as demanded by the satellite output multiplexers.

In this paper, a new technique to design compact low-pass filters with spurious-free frequency responses in rectangular waveguide technology is proposed. The technique uses sinusoidally-shaped λ/4 stubs as stop elements to achieve the required rejection band and windowing to get good return losses in the pass-band. Moreover, high compactness is also achieved because no distance is required following this technique between the stubs, unlike classical design methods based on coupled resonators [3].

In order to show the potential of this technique, we have fabricated a prototype fulfilling commercial specifications: return losses better than 14 dB from 8.2 to 12.5 GHz and a rejection band with an attenuation of at least 40 dB from 16.4 GHz to 37.5 GHz. A photograph of the filter is given in Fig. 1 showing standard WR-90 ports. Its frequency response (simulation and measurement) is given in Fig. 2 along with the commercial specifications (shaded area). The device total length (78.5 mm) is, however, half of those of commercial alternatives.

It is important to point out that the proposed technique can be generalised to other requirements fulfilling arbitrary stringent performances and it is simultaneously compatible with compact devices (as in this example) or high-power handling. During the conference, a low-pass filter with sharp cut-off frequency response, wide rejection band and high-power handling capabilities (large E-plane gap and smooth profile) will be presented. [1] Caputo, J., and Bell, F., "Waffle-iron harmonic suppresion filter," IEEE Trans. Microwave Theory and Techniques, vol. 13, pp. 701-703, September 1965. [2] I. Arnedo, J. Gil, N. Ortiz, T. Lopetegi, M.A.G. Laso, M. Sorolla, M. Thumm, D. Schmitt and M. Guglielmi, "Ku-band high-power lowpass filter with spurious rejection," Electronics Letters, vol. 42, no. 25, December. 2006. [3] G. Matthei, L. Young, and E. M. T. Jones, Microwave Filters, Impedance Matching Networks, and Coupling Structures. Norwel, MA: Artech House, 1980.