Filters are important components in many high-frequency systems, from commercial communications to electronic warfare (EW). A wide range of filters are useful at RF/microwave frequencies, including  coaxial, cavity, stripline, microstrip, and waveguide configurations. While they tend to be large, waveguide filters offer high-quality-factor (high-Q) performance with low loss. Evanescent-mode waveguide filters can provide similar high-Q performance as propagating-mode waveguide filters, but in somewhat smaller structures. What follows is a method to design these filters without the need for time-consuming iterative methods.1-4

1. This simple block diagram shows a classical T network.

Download this article in .PDF format
This file type includes high resolution graphics and schematics.

Figure 1 shows a T-coupled structure used in evanescent-mode waveguide filters, for which the equations can be written as:

For the coupling coefficient (k) between the two loops of the T-coupled structure, Eq. 2 can be applied:

2. This block diagram represents a classical Π network.

Figure 2 shows a Π-coupled structure. Using a delta-star transformation yields the relationships of Eq. 3:

Substituting the definitions of Eq. 3 into Eq. 2 yields Eq. 4:

Download this article in .PDF format
This file type includes high resolution graphics and schematics.