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ARTICLE | JUNE 29, 2022




FILTER BASICS PART 6: WAVEGUIDES AND TRANSMISSION LINES

Source: Knowles Precision Devices DLI

In Part 5, we covered the basics of distributed element filter construction and
gave an overview of two important methods for propagating electromagnetic waves
around a circuit – waveguides and transmission lines. At a high level, the main
difference between these two methods is the number of conductors involved and
the types of electromagnetic modes supported. Let’s explore how each of these
three types of electromagnetic modes work and how they are supported, or not
supported, by waveguides and transmission lines.

THREE TYPES OF ELECTROMAGNETIC MODES

When we say “modes”, what we are referencing is the different solutions to the
electromagnetic field equations for the particular structure we are referring
to. These three modes include transverse magnetic (TM), transverse electric
(TE), and transverse electro-magnetic (TEM).

A TM mode solution involves the electric (E) field having components in the
z-direction, which is along the direction of propagation, so that the magnetic
(H) field is transverse, or at right angles to the z-direction (Figure 1).



Figure 1. The E and H fields for TM mode moving in a rectangular waveguide.

On the other hand, TE mode has E field components at right angles to the
z-direction as shown in Figure 2.



Figure 2. The E and H fields for TE mode moving in a rectangular waveguide.

 TEM mode has E field and H field components at right angles to the Z direction.
Figure 3a shows an example of an electromagnetic wave propagating through space
in TEM mode while Figure 3b shows how TEM mode works in a cross-section of a
coax cable.

A.



 

 

B.

Figures 3a and 3b. These diagrams show examples of how the E and H field move in
TEM mode.

THE DIFFERENT MODES SUPPORTED BY TRANSMISSION LINES AND WAVEGUIDES

When we look at transmission lines and waveguides in terms of the
electromagnetic modes supported, we can make clear distinctions between the two.
A waveguide is a hollow tube made of a single conducting surface, which means it
cannot support TEM mode. We usually see waveguides designed as a metal tube, but
in recent years, the development of substrate-integrated waveguide (SIW)
technology is changing this. An SIW is basically a rectangular waveguide in
which the single conductor walls are formed by plated surfaces and vias. In this
format, the wave uses TE mode to propagate in high dielectric-constant
materials. A depiction of an SIW is shown in Figure 4.



Figure 4. A representation of an SIW.

Since a transmission line is a two-conductor structure, it can carry
electromagnetic waves using TEM mode. A common example of a transmission line,
although it is sort of going out of style now, is the coax cable shown in Figure
3b, which carries DOCSIS signals to cable TV boxes. Additional transmission line
examples, the modes one would usually see, and their traditional performance
characteristics compared to what you would see with a waveguide are all outlined
in Table 1.



Table 1. With all other qualities of the application being equal, this Table
provides an overview of how waveguides and different transmission line methods
compare.

To learn how modern approaches to filter design are changing some of these
traditional limitations, read part 1 and part 2 of our article published in
Microwaves & RF.

To summarize, the biggest thing to remember when it comes to waveguides versus
transmission lines is that a waveguide = one conductor (usually a tube) and a
transmission line = more than one conductor, such as the structure seen in a
coax cable.

In part 7, we will shift gears a bit and cover the different ways you can think
about Q factor.








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