SECTION I. WAVEGUIDE PRINCIPLES
LEARNING EVENT 1: TRANSMISSION OF ENERGY
1. The transmission of energy at the microwave frequencies requires special care to avoid loss of energy.
Transmission lines used at these frequencies are considerably different, physically and electrically, from those
used for lower frequencies.
2. The reason for the special transmission lines is that the radiation loss in a two-wire conductor increases as
the frequency increases. This radiation effect increases to a point where most of the energy will be radiated into
space and practically none will reach the output end.
3. A coaxial line may be used for transmission at these frequencies; even though the inner conductor radiates
energy, all of this energy is kept within the confines of the outer and inner conductors. The energy cannot
escape into space because the outer conductor prevents this. The outer conductor of a coaxial line controls the
energy more than the inner conductor. If the inner conductor is not needed, it can be removed. When this is
done, the resulting transmission line is called a cylindrical, or circular, waveguide. When a hollow rectangular
conductor is used, the transmission line is called a rectangular waveguide.
4. Moving electrical energy consists of magnetic and electric fields, and ordinary current and voltage are
incidental phenomena that are results of these fields. When a piece of hollow conductor is used as a
transmission line, it is difficult to discuss it in terms of current and voltage, so the electromagnetic wave concept
becomes more useful.
LEARNING EVENT 2: ADVANTAGES
1. Waveguides have several advantages over ordinary conductors for transmitting energy at microwave
frequencies. At these frequencies, ordinary conductors radiate most of the energy applied to them. In
waveguides, radiation losses are almost zero because all the energy is confined inside the waveguide.
2. In a coaxial line, leakage in the dielectric used to support the inner and outer conductors causes considerable
signal attenuation. The frequency of the transmitted energy determines the amount of dielectric loss. As the
frequency increases, the amount of electromagnetic energy absorbed by the dielectric also increases. Dielectric
loss of energy is eliminated in a waveguide because there is no center conductor requiring a solid dielectric
support.
SS0031
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