3. The cross-sectional area of the inner conductor in a coaxial line is considerably smaller than that of the outer
conductor. Therefore, skin effect makes the effective resistance of the inner conductor much higher than that of
the outer conductor. The removal of the center conductor in a coaxial line eliminates a major cause of skin-
effect loss. The inner surface of a waveguide is large enough to considerably reduce the skin-effect loss.
4. As a result of these advantages, the waveguide is a very efficient transmission line for RF energy above
1,000 MHz.
LEARNING EVENT 3: DIMENSIONS
1. In any system of transmission, the ability to handle high power is usually limited by the distance between
the conducting surfaces and the type of dielectric used. If the diameter of a coaxial line is too small for a given
transmitted power, the energy will arc over from the center conductor to the outer or ground conductor.
2. A waveguide of the same diameter will handle much higher power than will the coaxial line because the
distance of the arc-over path is twice as long.
3. Despite these advantages, the waveguide has not entirely replaced the coaxial line. The size of the
waveguide is determined by the wavelength of the energy to be transmitted. Unlike other transmission lines,
waveguides have a limiting frequency below which they cannot transmit energy. This is known as the cutoff
frequency. The rectangular waveguide is the most commonly used, and its cutoff frequency varies inversely
with the dimensions of the waveguide. (See Figure 2-1.) This relationship is such that waveguides are practical
only at and above the microwave frequency range.
Figure 2-1. Waveguide transformer.
LEARNING EVENT 4: BENDS
1. Waveguide plumbing. To have energy move from one end of a waveguide to the other without reflections
or standing waves, the size, shape, and dielectric material of the waveguide must be constant throughout its
entire length. Any abrupt change in the size
2-3
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