Practical_Antenna_Handbook_0071639586

450 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
• Radiation losses increase dramatically because the spacing between the wires is
a greater fraction of a wavelength. Figure 20.3B shows the electric fields
surrounding the conductors. The fields add algebraically (either constructively
or destructively), resulting in pinching of the resultant field along one axis and
reinforcement along the other.
Now consider a quarter-wavelength shorted stub. As we saw in Chap. 4, a short
circuit is transformed to an open circuit by a quarter-wavelength of any transmission
line. Thus, the “looking-in” impedance of such a stub is infinite, so when it is connected
in parallel across a transmission line (Fig. 20.4A) the stub has no electrical effect on
passing waves for which the stub length is l/4. In other words, at its resonant frequency,
the stub is a metallic insulator and can be used to physically support the transmission
line.
Similarly, we can connect a second l/4 stub in parallel with the first across the same
points on the transmission line (Fig. 20.4B) without loading down the line. This arrangement
effectively forms a half-wavelength pair. The impedance is still infinite, so
no harm is done. But if we can do this at one
point along the transmission line, we can do it at
Two wire
transmission
line
Junction
many (Fig. 20.4C). Ultimately, if we do it everywhere,
we have totally surrounded the original
parallel-wire transmission line with a metal
skin! The waveguide is analogous to an infinite
number of center-fed “half-wave pairs” of
Âquarter-wave shorted stubs connected across
the line. The result is the continuous metal pipe
structure of the common rectangular waveguide
(Fig. 20.2).
/ 4
Short
circuit
Figure 20.4A Quarter-wave stub analogy.
{
/ 4
{
/ 4
Figure 20.4B Quarter-wave stub analogy
extended.
/ 2
/ 4
Figure 20.4C Quarter-wave stub analogy
extended even further.