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 reduce the skin-effect loss considerably.
d. As a result of these advantages, the waveguide is a very efficient transmission line for RF energy
above 1,000 MHz.
a. 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.
b. 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.
c. 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. This relationship is such that waveguides are practical only at and above the
Section II. WAVE PROPAGATION
2-4. WAVEGUIDE CONSTRUCTION
The mechanics of a waveguide can be explained in terms of the two-wire transmission line theory. A
shorted quarter-wave stub has a high impedance and can be used as an insulated support. We can construct a two-
wire line rigidly supported above and below by a large number of quarter-wave stubs and still use the line
successfully. The resemblance between a rectangular waveguide and a two-wire transmission line is shown in
figure 62. Part A is a single quarter-wave stub support. In part B, many stubs extending both ways from the two-
wire line have been added, and they still do not affect the propagation of the desired frequency. In part C, the
stubs have been joined into a rectangular tube that represents a waveguide.
2-5. PROPAGATION RULES
Before we start discussing how energy can be moved through a waveguide, let's look at these rules:
a. Energy propagated in space consists of magnetic and electric lines at right angles to each other and at
right angles to the direction of propagation.