On the Design of In-Line Pseudo-Elliptic Helical Filtering Structures Implementing Non-Resonating Nodes
Kosmopoulos, S.
Space Engineering

Helical resonator filters [1] are in wide use both in ground based UHF mobile communication systems as well as in UHF transponders developed for space applications [2]. Such filters exhibit reasonable Q and excellent in-band performance over a wide temperature range, offering less volume and mass as compared to conventional coaxial cavity filters operating in the same band. Helical resonator filters, being produced economically in mall quantities, are best suited in applications where conventional lumped-element filters are very small but are too lossy (lower Q), while coaxial resonator filters (higher Q) are too big and unpractical. The most common filter functions realized with helical resonator technology are either Butterworth or Chebyshev without prescribed transmission zeros as extensively summarized in [1]. Low order (i.e., n=4) cross coupled filter, used in dual degenerate mode, gives further size and mass reduction with slightly lower Q and reduced spurious clean window. Furthermore, cross-polarization stray couplings in dual-mode configurations prevents the realization of complicated and high performance filter functions; e.g., 8-pole with internal group delay equalization using many transmission zeros. To be underlined that the alterative use of a circulator coupled external equalizer with a dual mode filter, often leads into unnecessary increase in mass and volume.

The actual implementation of the cross or bypass coupling in helical filters may be either physical or modal. In the case of physical cross coupling, a physical element such as a coupling aperture in waveguide filters may be used. This technique has the advantage that the coupling element is a physically identifiable and adjustable element. Alternatively, the use of other propagating or evanescent modes, as separate paths for energy flow, may be adopted. Early designs based on this technique used higher order modes in waveguide cavities to generate the transmission zeros (TZs) needed for a pseudo-elliptic response. Such a scheme is believed to be limited to filtering functions with TZs only above the pass-band although no rigorous proof of this statement has been reported. On the other hand, it has been shown recently [3] that the introduction of non-resonating nodes (NRNs) eliminates some of the difficulties associated to cross coupling realization either physically and/or via the intra-cavity approach. This is particularly true in the case of evanescent mode filters; e.g., intererdigitally loaded cavity-based filters, helical filters etc. An NRN is simply a node that is connected to ground by a frequency-independent reactance, which may be an open circuit in certain cases. The total number of NRNs in a given filter is arbitrary since it does not affect the order of the filter. Furthermore, the introduction of NRNs in an in-line configuration allows the design of sophisticated pseudo-elliptic filters, being disassociated to the difficulties related to cross or intra-cavity couplings.

In this paper, a high order helical resonator filter is investigated to achieve a pseudo-elliptic response performance (see Fig. 1), required for a UHF communication satellites link been specified by a very sharp rejection response. To do so the experience acquired during the realization of the SRA UHF Diplexer [2], on-board the METOP satellite (see Fig. 2), has been extensively used leading into the development of an in-line 6th order pseudo-elliptic configuration implementing various NRNs. Finally, to be underlined that adequate electromagnetic simulations and optimizations have been applied both for the single helical-loaded cavity and the overall filter structure (see Fig.3).

References [1] Peter Vizmuller, "Filters with Helical and Folded Helical Resonators", Artech Hose, Inc. Norwood, MA, 1987. [2] V. Crinò, S. A. Kosmopoulos, "THE METOP SATELLITE: The SRA Diplexer Design", Technical Report RPT/METOP/00002/SE, Rome, Italy, Jan. 12, 1999. [3] G. Macchiarella, "Synthesis of an in-line prototype with two transmission zeros without cross-coupling," IEEE Microw. Wireless Compon. Lett., vol. 14, no. 1, pp. 19–21, Jan. 2004. Fig. 1: The 6th Order Pseudo-Elliptic Required Filter Fig. 2: The SRA UHF DPLX on-board the Mask. METOP Satellite Fig. 3: The proposed a) single Helical Cavity and b) the overall 6TH Order In-Line Helical Filter