A common strategy to reduce the cost of a filter is to use a manufacturing technique with an intermediate accuracy and use tuning elements to compensate for the manufacturing errors. However, including tuning elements in the final EM simulations, and tuning the filters after manufacturing, can be quite challenging. In this context, therefore, we review in this paper two powerful filter design procedures based on Aggressive Space Mapping (ASM), and two semi-automatic tuning techniques. As a validation, we describe in detail the design and tuning of two types of filters that are commonly used for both space and ground applications, namely, a six-pole inductive filter in rectangular waveguide, and a more complex five-pole filter based on dual-mode resonators.

A continuously tunable Ku-band waveguide diplexer is proposed in this paper. This diplexer is based on a hybrid coupler topology and is composed of an input filter centered at 11.9 GHz with a 200 MHz bandwidth, a pair of ladder-type hybrid couplers, and a pair of tunable filters. This diplexer can provide two bandpass channels (channels 1 and 2) that can continuously go from a state where the bandwidths of channels 1 and 2 are maximum (up to 140 MHz) and minimum (down to 40 MHz), respectively, to another configuration where the bandwidth of channel 1 is the largest and the bandwidth of channel 2 is the narrowest. We propose a tunable filter based on TE012 mode circular cavities that are perturbed by low-loss dielectric inserts to obtain such performance. The resonant frequency of these cavities can be continuously tuned between 11.8 and 12 GHz due to the linear movement of the dielectric perturber. The design process for these components is presented and a breadboard device has been manufactured and measured to prove the concept. Different measured configurations of the diplexer are demonstrated, showing that the 200 MHz operating bandpass between 11.8 and 12 GHz can be efficiently separated into two tunable channels. The measured bandwidth of channels 1 and 2 in the manufactured diplexer can be tuned from 135 to 40 MHz (and vice versa) while maintaining an average guard band between the channels of approximately 26 ± 7 MHz. The obtained bandwidth tuning ratios are 3.2 for the highest channel and 3.1 for the lowest channel.

In this work, a new structure for the implementation of inline pseudoelliptic waveguide filters using non-resonating modes, based on asymmetric iris coupled transverse rectangular ridge resonators has been proposed. The basic structure is comprised of a ridge that is always centered and transverse to the waveguide center axis and has the ability to create a transmission zero (TZ) above or below the passband without involving any rotation of the ridge, thus enabling quicker design of the resulting filters by allowing the use of more efficient analysis tools like FEST3D, in addition to the more general purpose methods like HFSS. A centered ridge enables the ease of manufacturing through machining. The proposed structure makes use of rectangular waveguide's dominant TE10 mode as a non-resonating mode to create an alternate energy path from source to load, thus realizing a TZ, while the asymmetric irises excite the ridge resonator, enabling the overall structure to act as a singlet capable of producing both a pole and a TZ. A third-order filter with one TZ and a fifth-order filter with two TZs have been designed and manufactured. Measured results show good consistency with the simulated data, validating the viability of the proposed structure.

Ultra-compact band-pass filters with Nth-order pseudoelliptic response are presented allowing dramatic volume and mass savings (up to about 90%) compared to conventional filters at the price of a modest Q-factor reduction. TM dual-mode cavities loaded with high-permittivity (εr > 30) ceramic cylinders are employed, while non-resonating modes are used to create N transmission zeros symmetrically or non-symmetrically located in the pass-band. A single cavity behaves as a generalized doublet generating two transmission poles and two transmission zeros and is used as a basic building block for N-pole and N-zero filters with symmetric or asymmetric response. To prove the effectiveness of the approach, a very compact fourth-order filter with 4.35 GHz center frequency, four transmission zeros has been designed, fabricated, and tested showing an unloaded Q close to 2000. It is finally shown that the filter response can be shifted in frequency by simply replacing the dielectric material with another having different εr, so that the same metal enclosure can be used to realize various filters operating at different frequency bands.

A new class of compact rectangular waveguide filters is proposed in this paper. The key element is a square ridge resonator realized within a rectangular waveguide supporting the fundamental TE10 mode. The square ridge can support two modes resonating along orthogonal directions in the gap between the ridge and the opposite waveguide wall. This structure enables dual-mode degeneracy without the need of oversized cavities, thus allowing for considerable size reduction with respect to the conventional dual-mode implementations. Different filter structures employing one or more ridges within short waveguide sections are proposed as attractive solutions for pseudo-elliptic band-pass or dual-band filters. The experimental results of a manufactured fourth-order pseudo-elliptic pass-band filter validate the feasibility of the proposed class of filters.