Shaping frequency characteristics of wideband direct-coupled resonator filters by means of electric and magnetic couplings

This paper discusses influence of resonators coupling nature on frequency characteristics of wideband direct-coupled resonator filters. Pure electric and pure magnetic couplings are used to illustrate this phenomenon. Use of electric field for resonators coupling increases filter attenuation in the low frequency stopband of the bandpass filter. Use of magnetic field for resonators coupling increases filter attenuation on the high frequency side of the filter. This property can be employed to shape filter frequency characteristics, especially to design asymmetric filters or diplexers. Mixing of both types of couplings in a single filter is useful when filter symmetry has to be preserved, e.g. complementary couplings filter. This phenomenon is of general application to any filter, wideband and narrowband, but in wideband filters the asymmetry is non-negligible. To introduce this effect several example filters have been designed and manufactured. These include simple LC bandpass filters with 50 - 200 MHz bandwidth and 3.1 - 4.85 GHz ultra-wideband novel combline filter structures.