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Previous microwave distributed bandstop filters have had their second harmonic response centered at no more than three times the fundamental bandstop mid-band frequency, due to the use of quarter-wave resonators. This limitation has now been removed by the use of compound resonators having shorter electrical length. Some configurations incorporate lumped capacitors, resulting in additional design degrees of freedom and wider passbands. The new theory has been developed to apply to both wide and narrow stopbands. Example filters having upper passband widths of up to six times the fundamental bandstop center frequency are presented. Index Terms--Bandstop filters, filters, inhomogeneous filters, microwave filters, mixed lumped distributed, noncommensurate lines, redundant lines.

A structure consisting of dielectric-loaded feed fines (such as surface-wave fines similar to Goubau lines) and below-cutoff air-filled cavities can be used to form essentially L-C sections. The capacitance is due to electric-field coupling from the feed-line dielectric medium into the below-cutoff section. The inductance results from combining the inductors in the inductive tee equivalent circuit for such below-cutoff sections. Dielectric loading is used to shorten the guide wavelength at the input to the evanescent section, increasing the effective input inductance. The dielectrically loaded feed lines can comprise microstrip, coplanar waveguide (CPW), coplanar stripline (CPS), Gouban fines (surface-wave structures), waveguide, etc. The resulting resonant elements are usable at frequencies below 1 GHz, with small dimensions. If connected to the common ground plane, these L-C sections act as a transmission zero. If 'floated,' i.e., connected in the 'hot' line rather than to the ground plane, the sections form bandpass circuits (transmission poles). The air-filled below-cutoff sections (evanescent mode) are placed in a supporting low dielectric-constant medium (air, Teflon, or similar) with the open end in proximity to the dielectric portion of the feed line and are, thus, termed 'suspended.' The individual L-C sections can be coupled together using microstrip, surface-wave fine, CPW, CPS, finline, waveguide, or lumped elements. Such combinations can be chosen to implement Chebychev, Butterworth, quasi-elliptic, etc. responses. These applications will be covered at a later time. Index Terms--Bandpass, bandstop, below cutoff, dielectrically loaded, evanacent, Goubau line, resonators, surface wave, suspended line.

A survey of the major techniques used in the design of microwave filters is presented in this paper. It is shown that the basis for much fundamental microwave filter theory lies in the realm of lumped-element filters, which indeed are actually used directly for many applications at microwave frequencies as high as 18 GHz. Many types of microwave filters are discussed with the object of pointing out the most useful references, especially for a newcomer to the field. Index Terms--Bandpass, cavity, ceramic, coaxial, combline, diplexers, evanescent mode, filters, hairpin line, high-pass, high temperature, interdigital, low-pass, lumped element, microstrip, microwave, multiplexers, parallel coupled line, planar, stripline, superconducting, waveguide.

A quasi-elliptic notch-filter response is defined as one in which the ultimate stopband depth is sacrificed in favor of a faster transition from the passband to a specified minimum equiripple stopband attenuation level (i.e., steeper attenuation slope) with no increase in filter order (as compared to a Chebyshev or similar transfer function design). Such notch-filter responses can be achieved if the topology synthesized includes arbitrary frequency invariant impedances and line lengths in the main series line of filters, otherwise similar to those in [1]. Using lumped approximations to the series coaxial sections of [1], but with the series sections not at 50 [Ohms] (for a 50-[Ohms] filter), it is possible to improve upon the Chebyshev-based results of [1]. The resulting network provides lower passband insertion loss, with no reduction in minimum notch depth or power handling, improved temperature performance and additional harmonic reduction. The improvements are due to the implementation of the quasi-elliptic response defined above, and are made possible by the larger range of main line impedances and line lengths available using inductors and capacitors rather than lengths of transmission line. In principle, the technique can be extended to achieve full-elliptic responses. Index Terms - Band reject, bandstop, elliptic, filter, notch.

A new class of compact in-line filters with pseudoelliptic responses is presented in this paper. The proposed filters employ a new type of mixed-mode resonator. Such a resonator consists of a cavity loaded with a suspended high permittivity dielectric puck, so that both cavity TE101 mode and dielectric TE \mathrm {01\delta } mode are exploited within the same volume. This structure realizes the transverse doublet topology and it is therefore capable of generating a transmission zero (TZ) that can be either located above or below the passband. Multiple mixed-mode resonators can be used as basic building blocks to obtain higher order filters by cascading them through nonresonating nodes. These filters are capable of implementing TZs that are very close to the passband edges, thus realizing an extreme close-in rejection. As a result of the dielectric loading, the proposed solution leads to a very compact structure with improved temperature stability. To validate the proposed class of filters, a second-order filter with 2.0% fractional bandwidth (FBW) and a fourth-order filter with 2.5% FBW have been designed and manufactured. [ABSTRACT FROM AUTHOR]