c. All of the fields are within the waveguide.
d. The only current flow in waveguide is on the inside surface of the
walls just as it is with resonant cavities.
This is one reason why
waveguide is so efficient.
e. We stated earlier that transmission lines have losses, due to skin
effect, dielectric, and radiation.
These losses are very slight with
waveguide. Let's find out why.
12. Why waveguide has very low loss due to skin effect.
a. When we transmit RF energy along a wire, the current flows on the
outer edge of the wire. As we increase the applied frequency, the current
flows more and more on just the outer edge (or circumference) of the wire.
We call this action skin effect. This means that, although the wire may be
two inches in diameter, only one small section of the wire is actually
carrying current.
b. The action of skin effect is the same
as if we reduced the diameter
of the wire or added resistance to it.
This results in heat being
generated, and the electric power dissipated
in the form of heat is loss.
This is the reason, then, that wire has high
losses due to skin effect at
higher RF frequencies.
c. In waveguide, current flows only along the inside surface of the
guide, but remember that waveguides have four solid walls to carry the
current. Therefore, the current-carrying area is much greater in waveguide
than in wire. This greater area has very little resistance, so losses due
to skin effect are very low.
d. The inside walls of waveguide are sometimes silverplated or gold-
plated as they are in resonant cavities.
Plating the walls reduces any
resistance the walls have and further decreases losses due to skin effect.
13. Why waveguide has very low dielectric loss.
a. You know that any two-wire or coaxial type transmission line has
some kind of insulation to keep the two wires apart.
Any insulating
material, such as rubber, plastic, or even air is actually a dielectric
between the two conductors.
So, there's always some slight current flow
through the dielectric.
We call this slight current flow between the two
conductors leakage current, and the resulting loss of energy is called
dielectric loss.
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