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Since one side of the conductor is exposed to air, these transmission lines can only
exist on top and bottom PCB layers. The trace impedance is influenced by both the
dielectric of the substrate material and the air above it.

STRIPLINE

Unlike microstrip, stripline transmission lines are fully contained within a substrate
which is sandwiched between two ground planes.

Figure 38-15. Stripline - transmission line
Due to the substrate impregnated nature of stripline, these transmission lines can
only exist on internal routing layers and require a minimum of 3 board layers (2
ground planes and a routing layer).

WAVEGUIDES

Any system of conductors and insulators assembled for the purpose of directing an
electromagnetic wave can be referred to as a waveguide. Ordinary coaxial cable or
parallel line is a waveguide. The designation for transmission line, for example,
RG8U means "Radio Guide number 8U".

It is usual though to use metallic rectangular or circular pipes as waveguides. At
frequencies above 1GHz waveguides are a better alternative than coaxial or parallel
lines because the latter start to become very lossy at such frequencies.

Pipe of any cross section could be used as a waveguide however it makes sense to
use simple cross sections like a rectangle or a circle. Whatever cross section we
choose we are going to need a system of bends, joiners and other specialty
components that we can insert into the waveguide.

Figure 38.16 shows the basic construction of rectangular and circular waveguides.
We can inject an electromagnetic signal into the waveguide using an antenna or a
coupling loop just like we do with resonant cavities.

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